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have so many moons that we may not have counted them all. Would you like to know the solar system moon count as of 2010? The tally is up to 167 and counting.

There are different types of moon – satellites. The size and roundness of them vary. Most are formed near their partner planet. Some are asteroids that were captured by a planet’s gravity.

Here are some terms that are used in the moon exploration business that concerns how moons orbit their planets:

Inclination: Describes the orbital path of the satellite compared to the equatorial latitude of the planet.

Prograde: Is orbiting the planet in the way that a planet orbits the Sun while it is circling between 0 and 90 degrees compared to the planet’s equator. The vast majority of moons are prograded. They circle counter clockwise if looking down at the planet’s north pole.

Retrograde: Is orbiting the opposite direction of the way planet travels around the sun and/or has an orbital path that is more than a 90 degree angle than the equator. Most, if not all retrograde moons are captured asteroids.

Irregular: Moons that have elliptical orbits and are still within the 90 degree equatorial angle.

Regular: moons are all prograde, but not all prograde moons are regular. Some are irregular.

All prograde – irregular moons have an “a” at the end of their names. All retrograde moons have names that end in the letter “e”.

In this section, we will look at some of the more interesting moons, which include our own.

Earth’s Moon

The Moon ‘Credit NASA’

We see our moon at night – sometimes even by day. It is big, round and looks a little beat up. The colors are whitish gray, gray and some black. It can also be yellow when close to the horizon and very bright and white when high in the sky.

Our Moon had different names during ancient times when many thought of it as a goddess. Some of the given names were: Diana, Lunea, Cynthia, and Selene. It has been both worshipped and feared. It was an object representing romance. It was a light for harvesting at night. It was thought of as a factor in transforming people into werewolves. It was believed that the Moon could even cause people to become “lunatics.” (Derived from the Latin word luna.)

We have only one moon. In English, we call it the Moon. It is one of many in the solar system. Here are some of additional Moon facts: It has a 2,160 mile diameter at its equator.

The Moon is a little more than one quarter the size of Earth.

Its density is about sixty percent of the density of the Earth.

Its average distance is about 221,423 miles from Earth. Its orbit is elliptical (an oval shaped path). The farthest point the Moon is from Earth is 252,667 miles.

There are tremendous temperature swings on the Moon because there is a very slight atmosphere to hold or release heat and cold.

The Moon travels around the Earth at a rate of nearly 1.5 miles per second!

There is water ice on the Moon.

Evolutionists age the Moon at 4.4 billion years.

It has 17% percent of earth’s gravity.

Moon Tricks

Have you ever noticed that there can be a “halo” around the Moon? It is not what it seems to be and neither is it where it seems to be. The circle is caused by ice crystals high in our sky.

Another phenomenon is known as the “Moon Illusion.” No one is quite sure why, but the Moon looks much bigger when seen at the horizon level.

The Moon has structure. The center, or core, is thought to be solid but could be partly molten. Moonquakes have been detected. They are very weak, leading to the belief that there might be some liquid towards the middle of the core. Moonquakes seem to be related to the tidal forces caused by interacting with Earth’s gravitational forces.

The surface temperature averages 107 degrees Celsius in the sunlight and -135 degrees Celsius in the shadows. Sometimes the temperature drops as low as -249 degrees Celsius, which would rival temperatures found in the Kuiper belt.

Galileo thought that the dark areas on the Moon were oceans. He called them maria, (plural), or mare, (singular). The mare is actually basalt rock lava beds that have cooled from the pouring out of magma, were caused by meteorite impacts cracking the surface. The largest mare is the Mare Imbrium, 1,700 miles across.

The highest areas on the moon are known as “The Highlands” and are some of the oldest surfaces on our Moon formed, during the period of volcanism. As the Moon cooled, there were flows of volcanic basalt, which were most active between 3.2 and 1.2 billion years ago. Igneous (volcanic), rock areas covers almost 80% of the Moon’s surface. Some smaller features exhibit geographical features that appear to be caused by out – gassing. (An explosive leakage from subsurface gas pockets.)

Virtually all of the Moon’s mountains are walls of craters. Very few high hills or mountains are from volcanic episodes, since the Moon has no tectonic plates; no mountains were caused by plate movement.

The Moon is mostly covered with silicon dioxide, magnesium, calcium, glass and dust, which form a layer called the lunar regolith, ranging in depth from 5 to 50 feet. It is created by the smashing of the surface that result in debris from the impacts of meteorites and micrometeorites being strewn over the surface. When brought back to earth and studied, scientists said it had the scent of gunpowder.

The crust is made of 45% oxygen, 21% silicon, 6% magnesium, 13% iron, 8% calcium and 7% aluminum. The mantle: underneath is made up of olivine, orthopyroxen and clinopyroxen. The core of the Moon is only 255 miles in diameter and seems to be made of partly molten iron and nickel. Some of the basalts on the surface also contain titanium.

The Moon is the second least dense moon in the solar system. (Only Io is less dense.) The Moon has a magnetic field that is much less potent than the one form emanating from Earth.

The moon looks a little beat up! In a way it is just that. It has small craters called craterlets, larger craters, some extremely large craters more than a hundred miles across, called “walled plains”. Most of these craters were the result of meteor hits. The larger pieces of material ejected by these collisions caused more craters to be formed. Smaller particles blasted out, called “rays”, formed patterns that stretch out for hundreds of kilometers.

The far side of the moon looks far different from that facing the earth. It does not have large maria, but it does have craters and highlands.

The Moon, (La “Luna” in Spanish), is being hit constantly by space debris. With no atmosphere to protect it, the surface is bombarded by large as well as sand-sized rocks, small meteors called “micrometeorites” they hit the moon at speeds up to 70,000 miles per hour. This makes the moon’s surface a very dangerous place for humans. A small grain could go right through the body of an exploring astronaut!

The micrometeorites affect changes on the surface of the moon: they can cause erosion. However, they are 10,000 times less effective in creating changes than those caused by air and water on the surface of our Earth.

Several years ago a military satellite turning toward the Moon to adjust its instruments, reported back an amazing find: water ice on the Moon! NASA sent a probe, called a “Lunar Prospector”, to investigate and to confirm the findings. It is now thought that ice could have come from comets that have collided with the Moon. The spacecraft Clementine has further confirmed these phenomena. The Indian probe, Chandrayaan, detected large quantities of ice.

The ice is not only in permanently shadowed craters, but it may also be found in the regolith. It seems there are literally millions of tons of ice present. One 10th of 1% of the regolith soil may be frozen water. Scientists looked again at the collected Moon rocks brought to earth by the Apollo 15 astronauts and found that there was water in some of them.

This ice could be used to support a space station on the Moon. The hydrogen and oxygen can be separated, and the elements used for drinking water, breathable air and rocket fuel.

The Moon’s atmosphere is extremely thin but has a very complicated composition. It most likely results from decay and out – gassing. These elements are: sodium, potassium, polonium, argon, oxygen, methane, nitrogen, carbon monoxide and carbon dioxide.

We have sent many “visitors” to our moon, including machine probes and manned vehicles. Later in this book, we will study these explorations.

There have been several recent discoveries, most of which were found by the lunar astronauts who brought back moon rocks. The rocks range in age from 3.1 billion years to 4.4 billion years. The rocks also reveal another important fact: The Moon has a different geologic make up than present day Earth.

The Moon has several measurable effects on Earth. One is that of the lunar eclipse. That’s when the Moon passes between the Sun and the Earth. The shadow of the Moon falls upon the Earth. From Earth, the Sun and Moon look the same size because the Moon blots out the sunlight when aligned with us.

A major Moon influence on Earth is the tidal effect. The Moon’s gravity actually pulls at our oceans. The powerful pull of our natural satellite can change the depth of the coastal waters by many feet in a few hours. (This subject is covered in the Planets / Earth section.)

Where did the Moon come from? How did it form?

There are several theories:

The Fission Hypothesis: The Moon was formed by a chunk of partly molten earth flying off into space and leaving the Pacific Ocean area as a scar.

Capture Hypothesis: The Moon was a free – wandering space object that was capture by our planet’s gravity.

Co-formation Hypothesis: The earth and Moon formed at the same time.

Giant Impact Hypothesis: A large object, perhaps a large as 1/3 of the earth, collided with our very young planet and debris was thrown into space. This debris coalesced into the Moon. This is presently, the most popular scientific theory.

Where is the Moon going? It is currently drifting away from Earth at 1 ½ inches, per year. Theoretically, over billions of years, the Moon could become a planet. But the Moon and Earth, according to science timelines, would already be destroyed by the Sun.

Did you know?

When seen on Earth, the Moon is 25,000 times brighter than the nearest star, other than our Sun. But it only reflects 11% of the sunlight shown on it.

The Moon has a synchronized rotation. A moon day rotation equals the same amount of time it takes the Moon to go around the Earth (27.3 days). Therefore, we can only see one side of the Moon at any given time.



This, the largest moon in our solar system, is also the seventh largest world as well. Titan was discovered in 1655 by Christiaan Huygens. It is the most complicated moon when considering its atmosphere and its interactions within the moon’s surface.

Larger than Pluto and Mercury, Titan is one of the 22 moons of Saturn. It is famous not only for its size but also for its atmosphere. Titan is the only moon with a substantial atmosphere. Other than our own Moon, it is now one of the most studied moons in the solar system. The largest moon in the Saturn system, it orbits Saturn from a distance of about 720,000 miles. It takes 16 earth days to circle Saturn.

Its atmosphere contains 95% nitrogen and 5% methane, plus a multitude of trace gases. Its surface air pressure is actually 50% stronger than that of Earth’s atmosphere. However, differences between temperatures of Titan and Earth are extreme. The average temperature of Titan is -292 Fahrenheit. Methane freezes just a few degrees below that level. Methane, on Titan, is found in frozen, gaseous and solid states. The atmosphere averages 190 miles in thickness. There are continuous strong winds in the upper atmosphere that are as fast as category 3 hurricanes. These winds make the atmosphere move around Titan five times faster than the spin of the moon itself. Its lower atmosphere tends to be calm.

It rains ethane on Titan, in very large drops (1cm). The rain drops to the ground, just like it does on earth. But because there is much less gravity, it can take as much as an hour for them to finally land. Most of the drops evaporate before they reach the ground.

From a distance one cannot see any features on Titan. It is covered with a yellowish brown -thick- haze of methane, ethane, and acetylene compounds. It actually snows on this moon! The snowflakes are made of methane and fall slowly to the ground, due to the light gravity of this world. Every day appears to be gloomy on Titan.

The sky is brown and the sunlight gives little more illumination than our moonlight provides. A day on Titan is 16 Earth days long. Eight are dark and, the rest are gloomily lit.

Did you know?

During the recent fly – by of Titan, by the space probe Cassini, it recorded radar images of lakes! They are filled with liquid hydrocarbons. This makes Titan the only place, other than Earth, that is known to have liquid lakes! Many of the lakes look like they may be filled in impact craters or are a result of volcanism. Others look like a typical filled – in lake on earth. Does this mean there is an equivalent to Earth’s water cycle working on Titan? Time will tell. Saturn takes 29 years to orbit the sun. As observations continue, we will be able to tell whether there is seasonality to these liquid deposits.



Iapetus is the third largest moon in the Saturn system. It is a peculiar world that is very bright on one side, very dark on the other. (One side is ten times brighter than the other side).

Iapetus is the third largest moon that orbits Saturn. It orbits at a distance of about 2.1 million miles from the planet. It takes 79.33 days to complete one orbit.

The light side looks like a typical cratered moon. The other side is as dark as charcoal and has virtually no features. The darkness may have to do with collecting dust from the moon Phoebe or maybe that part of the moon is more exposed to the sun and the ice has melted. The Cassini space probe arrived in 2004 tried to investigate why this situation exists. So far, there are no conclusions. Iapetus has polar water ice. It also has frozen carbon dioxide lying on its surface. This moon is a little less than half the size of our Moon.

Iapetus has a clearly defined equator and is marked by a mountain line around the middle. From a distance, this feature gives the moon a glued together look.

There are other moons that orbit Saturn. Mimas looks like a typical battered moon. Mimas, Rheas, Enceladus, Dione, and Tethys are midsized moons. These worlds, as well as the smaller moons, are airless and icy .



Ariel is nearly all ice. Not only is the surface icy, but so is the interior. It has strange canyons in several areas. Probably it was partially melted in its early years. This moon is about one third the size of our own.



Triton, a moon of Neptune, is a very cold place that averages – 400 Fahrenheit during the day or night. The surface water ice is as hard as granite. There may be liquid nitrogen pools lying below the surface. This type of “ground water” is heated by decaying, radioactive particles and tidal forces from nearby Neptune.

Occasionally the liquid breaks through, and geysers shoot it above the surface. As it falls, it freezes and forms snow on the surface. Methane mixed with liquid falls nearby, leaving purple and black stains near the vents. In essence these vents act as ice volcanoes.

Triton has an interesting and fairly unique feature; it has wind. The air is thin and clear. Sometimes a thin cloud can be seen. The atmosphere is fed by geysers.

Did you know?

There is a theory that a planet near Triton was once orbiting the Sun and was captured by Neptune’s gravity. One reason for this theory is that Triton orbits around Neptune in the opposite direction of the planet’s spin. (Saturn has one moon which does this and Jupiter has four.).


Io moon

This is a world that is so different from the rest of the solar moons. It is a moon of constant change. A violent place, it has extraordinary events happening daily.

Io is the 13th largest space object in the solar system. Discovered by Galileo 63 Galilei, it is close to 30% the size of earth. It flies around Jupiter, making one orbit every 1.8 days! It circles Jupiter at a distance of a little more than 250,000 miles.

When Voyager 1 passed Jupiter in 1979, the cameras turned to it and captured one of the most stunning images ever taken. It was a moon with not one visible meteor crater. The place is orange, yellow, white and brown with touches of black. Some say it looks more like a giant pizza.

After several days of investigation the team of scientist realized they were looking at a moon with many active volcanoes. There are at least 300 hundred of them. These were the first active volcanoes ever seen outside of Earth. It has incredible volcanic lava temperatures that range from 1,400 to 1,700 degrees Fahrenheit. Most likely the magma contains a lot of magnesium, which tends to burn hot. The lava on Io is hotter than the lava on Earth. The vents shoot out both liquid rock and gaseous sulfur. Jets of gas and molten rock spout up as much as two hundred miles above the surface. There are even lakes of liquid sulfur and rivers of molten lava. This moon is the second hottest place in the solar system, second only to the Sun.

Most of Io’s interior is molten. The surface is a weak, brittle and has a thin crust over a hot ball of molten sulfur dioxide. No other moon is like this one. In fact, no other moon even comes close. The gravitational forces of Jupiter and three other moons pull and mix Io into this weird state and make the moon have an elliptical orbit. It is estimated that the side of Io that faces Jupiter can be pulled out into space for as far as 6 miles.

The volcanic activity is so widespread and constant that it is useless to try to map this moon, in 5 to 7 years the whole surface may look different. Every billion or so years the whole moon turns itself completely inside out!

Where the volcanoes are not spewing, there is ice and cold rock. The average temperature on Io’s surface, away from the vents, is -143 degrees Fahrenheit. Magma and ice; Io is a moon of contradictions.



Ice and cold are not the only interesting observation one can make about this world. When pictures of this moon were studied it was easy to compare the surface of Europa with glaciers on our own planet. The folds, crevasses and undulations look the same. It has 70% sunshine reflectivity off the icy surface, making it very visible in the sky. (Remember, our Moon has only 11% reflectivity.).

It’s about 2,000 miles in diameter and orbits Jupiter from a distance of 400,000 miles. Europa takes only 3.6 earth days to orbit its planet.

The icy surface area is estimated to be, on average, 30 million years old. There are some craters, grooves and cracks on its’ surface. The average temperature at the equator is – 260 degrees Fahrenheit. At the poles, the temperature drops to -370 degrees. This was caused by the underlying liquid and possibly tidal forces when it passes near other moons.

There is a very slight atmosphere around Europa. The air is composed of oxygen. When charged particles from the sun hits the surface interacts with water molecules, thereby releasing some oxygen and hydrogen occurs. The hydrogen is lighter and therefore escapes more easily from the moon’s grip.

But what is under all that ice? To quote a line from the movie 2010 it might be: “Something wonderful.” The ice is anywhere from 10 to 100 miles thick. It replenishes itself on a continuing basis. In many ways it acts like our atmosphere; it retains heat, provides protection from cosmic projectiles and blocks out dangerous rays from penetrating the outside, protective layer.

Beneath this ice cover is a sea up to 60 miles deep. It is made up of water. The core is hot rock. Could there be life in this sea? The answer is absolutely… yes! The icy surface on Europa could very well have been liquid when Jupiter and Europa were young. Primeval life may have formed in its oceans and may still be there, under the ice. This likely would be aquatic life with no knowledge of the Universe above the ice, which is not much different from the deep sea life that never breaks the surface of our own oceans. It very well could be that our first alien contact will be with creatures living on a moon circling Jupiter. Today, there may be alien life form near a hot vent of the ore, feeding and may be able to think: “What’s up there?”

There are plans to launch a spacecraft to with lasers and radars to study its’ surface. This would provide additional information about subsurface conditions. It is the next step towards having a probe land on the moon and drill, or melt, the ice in order to see if organisms dwell below the surface.



Ganymede, the largest of Jupiter’s moons and the largest moon in the Solar System, bigger than Mercury, is pocked by many impacts. The moon is mostly composed of ice, with dust and dirt lying about on its surface. The Moon’s has a skin of dirty ice and when objects hit it, they expose a clean ice sheet beneath. The materials from the collisions are thrown out onto the surrounding area, providing some contrast between clean ice and the dirty ice surface. There are many craters on the moon, suggesting that its surface is very old. The ice, over time, reduces the craters by relaxing the walls leaving more rounded edges and eventually flattened – circular scars on the surface. These are called palimpsets.

Ganymede had internal heat at some point. How much is not clear, but there can been seen some effects on the ice. This may be the reason that it has its own magnetic field.

Like Europa, it too probably might have a below – surface, saltwater sea. But the ice on this moon is much thicker than that found on Europa. The core seems to be made of metals and rock.



The outermost of the Galilean, (Jupiter’s) moons, Callisto has been peppered with debris. It is one of the more cratered worlds in our solar system. It is covered in water and carbon dioxide ice. This probably means that its surface and the object itself are very old. The relaxing of crater walls has occurred, as happened on Ganymede.

Our Moon and Callisto resemble each other but with a major difference; Callisto is mostly of water! They have the same strength in gravity and have no atmospheres, but a rock sampling from Callisto would melt at room temperature. The moon consists of a mixture of water ice and dirt. The ice thickness of the moon is several miles deeper than on Europa or Ganymede, making the possible liquid water below the crust impossible to reach.

The surface temperature on Callisto is around -230 Fahrenheit. It is the second largest moon of Jupiter and the 12th largest space object in the solar system. It orbits Jupiter from a distance of 1.2 million miles and completes one orbit of Jupiter in a little over 16.5 earth days. The core is most likely rock, the rest perhaps water ice, slush and liquid salt water.



This moon of Saturn has an orbit distance of 7.5 million miles. The 9th largest moon in Saturn’s system, it is a captured asteroid that has a retrograde orbit. One trip around Saturn takes 550 earth days. This asteroid turned- moon is pockmarked by craters. It has a water and carbon dioxide – ice, covering.



Enceladus is the most reflective of all the moons. About 99% of the sunlight reflects from its surface. It is also the smallest moon with an atmosphere. It has water geysers that provide the inconsistent atmosphere and replenishes a smooth looking surface. The energy producing these actions could be tidal in nature coming from Saturn and a few nearby moons.

The average surface temperature is – 330 degrees Fahrenheit. There are less than 20 smaller craters to be found on the moon.

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In August 2012, the Mars Science Laboratory Rover, better known as Curiosity, landed on Mars. This 2,000-lb (900-kg) wheeled vehicle, which is still roaming the Martian surface, is a mobile laboratory equipped to conduct geological experiments aimed at figuring out the natural history of the red planet. It is the latest robot explorer to reach Mars, and the largest and most advanced in a long line of rovers sent to explore other worlds.

“Mars has been flown by, orbited, smacked into, radar-examined, and rocketed onto, as well as bounced upon, rolled over, shoveled, drilled into, baked, and even blasted. Still to come: Mars being stepped on.” Buzz Aldrin


The potential of rovers in space was clear as far back as 1971, when Apollo 15 carried a four-wheel Lunar Roving Vehicle to the moon. This agile twoseater widened the scope of lunar exploration for the last three Apollo missions. For instance, during the first moon landing in 1969, Neil Armstrong and Buzz Aldrin spent just two and a half hours moonwalking, and the farthest they moved from their lunar module was 200 ft (60 m). By contrast, however, in the final Apollo moon mission, Apollo 17, in 1972, the crew of two—Eugene Cernan and Harrison Schmitt—spent more than 22 hours outside. In their rover, they covered 22 miles (36 km) in total, with one drive taking the pair 4.7 miles (7.6 km) from their spacecraft. The Lunar Roving Vehicle, or moon buggy, was used to collect rocks. The six Apollo missions returned to Earth with 840 lb (381 kg) of them.

Analysis of these rocks revealed much about the history of the moon. The oldest were about 4.6 billion years old, and their chemical composition clearly showed a common ancestry with rocks on Earth. Tests revealed no evidence of organic compounds, indicating that the moon has always been a dry and lifeless world.

Geologist−astronaut Harrison Schmitt collects samples from the lunar surface during the 1972 Apollo 17 mission. He spent many hours exploring the surface on the moon buggy.

Lunokhod 1

The Soviet lunar program, which began in the early 1960s, relied on unmanned probes to explore the moon. Three of the Soviet Luna probes returned with a total of 11.5 oz (326 g) of rock. Then, in November 1970, the Soviet lander Luna 17 arrived at a large lunar plain called the Sea of Rains (many lunar areas are named after the weather conditions they were once thought to influence on Earth). Luna 17 carried the remote-controlled rover Lunokhod 1 (Lunokhod means “moonwalker”). This was the first wheeled vehicle to traverse an extraterrestrial world, arriving about eight months before the first Apollo buggy. The concept behind it was simple—instead of sending moon rocks to Earth, the rover would do the analysis there.

The Soviet Lunokhod 1 rover, seen here in tests on Earth, was the first rover ever to land on an alien world—its predecessor, Lunokhod 0, was launched in 1969 but never reached orbit

Remote-controlled explorer

The Lunokhod rover was 7½-ft (2.3-m) long and resembled a motorized bathtub. The wheels were independently powered so that they could retain traction on the rough lunar terrain. Lunokhod was equipped with video cameras that sent back TV footage of the moon. An X-ray spectrometer was used to analyze the chemical composition of rocks, and a device called a penetrometer was pushed into the lunar regolith (soil) to measure its density. Lunokhod was powered by batteries that were charged by day using an array of solar panels that folded out from the top of the rover. At night, a source of radioactive polonium inside the machine acted as a heater to keep the machinery working. The rover received commands from controllers on Earth about where to go and when to perform experiments. A human might have done a better job, but rovers could stay in space for months on end, and did not require food and water from Earth.

Lunokhod 1 was designed to work for three months, but lasted almost 11. In January 1973, Lunokhod 2 landed in the Le Monnier Crater on the edge of the Sea of Serenity. By June, Lunokhod 2 had traveled a total of 24 miles (39 km), a record that would stand for more than three decades.

“Over time you could terraform Mars to look like Earth … So it’s a fixer-upper of a planet.” Elon Musk

Martian walker

As Lunokhod 1 was exploring the moon, the Soviet space program was eyeing an even greater prize: a rover on Mars. In December 1971, two Soviet spacecraft, code-named Mars 2 and Mars 3, sent modules to land on the red planet. Mars 2 crashed, but Mars 3 made a successful touchdown—the firstever landing on Mars. However, it lost all communications just 14.5 seconds later, probably due to damage from an intense dust storm. Scientists never found out what happened to Mars 3’s cargo: a Prop-M rover, a tiny 10-lb (4.5-kg) vehicle designed to walk on two ski-shaped feet. It was powered through a 50-ft (15-m) umbilical cord, and once on the surface was designed to take readings of the Martian soil. It is unlikely that the Prop-M ever carried out its mission, but it was programmed to operate without input from Earth. A radio signal between the moon and Earth travels in less than 2 seconds, but a signal to or from Mars takes between 3 and 21 minutes to arrive, varying with the planet’s distance from Earth. For a Martian rover to be a successful explorer, it needed to work autonomously.

Bounce down

In 1976, NASA’s two Viking landers sent back the first pictures of Mars. Following this success, many more rovers were planned, but most of these projects never reached their destination, succumbing to what the press dubbed the “Martian Curse.”

NASA eventually had a success with its 1997 Mars Pathfinder mission. In July of that year, the Pathfinder spacecraft entered the Martian atmosphere. Slowed first by the friction of a heat shield and then by a large parachute, the spacecraft jettisoned its outer shielding, and the lander inside was lowered on a 65-ft (20-m) tether. As it neared the surface, huge protective airbags inflated around the lander, and retrorockets on the spacecraft holding the tether fired to slow the speed of descent. The tether was then cut, and the lander bounced across the Martian surface until it rolled to a stop. Fortunately, once the airbags had deflated, the lander was the right side up. The three upper sides or “petals” of the tetrahedral lander folded outward, revealing the 24-lb (11-kg) rover.

During development, the rover was called MFEX, short for Microrover Flight Experiment. However, it was known to the public as Sojourner, meaning “traveler” and chosen for its link to Sojourner Truth, a 19th-century US abolitionist and rights activist.

“We landed in a nice flat spot. Beautiful, really beautiful.” Adam Steltzner

Rolling on Mars

Sojourner was the first rover to take a tour of the Martian surface. However, the Pathfinder mission was really a test for the innovative landing system and the technology that would power larger rovers in the future. The minuscule vehicle traveled just 300 ft (100 m) during its 83-day mission, and never ventured farther than 40 ft (12 m) from the lander. Now named the Carl Sagan Memorial Station, the lander was used to relay data from the rover back to Earth. Most of the rover’s power came from small solar panels on the top. One of the goals of the mission was to see how these panels stood up to extreme temperatures and what power could be generated in the faint Martian sunlight.

The rover’s activities were run from NASA’s Jet Propulsion Laboratory (JPL) in California, and JPL has remained the lead agency in developing Martian rovers. With the time delays inherent in communicating with Mars, it is not possible to drive a rover in real time, so every leg of a journey must be preprogrammed. To achieve this, cameras on the lander were used to create a virtual model of the surface around Sojourner. Human controllers could view the area in 3-D from any angle before mapping a route for the rover.

During its 83 days of operation, the tiny Sojourner rover explored around 2,691 sq ft (250 sq m) of the planet’s surface and recorded 550 images.

Spirit and Opportunity

Despite its limitations in terms of size and power, Sojourner’s mission was a great success, and NASA pressed ahead with two Mars Exploration Rovers (MERs). In June 2003, MER A, named Spirit, and MER B, Opportunity, were ready for launch. They were about the same size as a Lunokhod rover, but were much lighter, at around 400 lb (180 kg). By the end of January the following year, both were traveling across the Martian deserts, hills, and plains, photographing surface features and chemically analyzing rock samples and atmosphere. They sent back the most glorious vistas of the Martian landscape ever seen, enabling geologists to examine the large-scale structures of the planet.

Spirit and Opportunity had landed using the same airbag-and-tether system as Sojourner. Like Sojourner, both relied on solar panels, but the new rovers were built as self-contained units, able to wander far from their landers. Each vehicle’s six wheels were attached to a rocking mechanism, which made it possible for the rovers to keep at least two wheels on the ground as they crossed rugged terrain. The software offered a degree of autonomy so that the rovers could respond to unpredictable events, such as a sudden dust storm, without needing to wait for instructions from Earth.

“Whatever the reason you’re on Mars is, I’m glad you’re there. And I wish I was with you.” Carl Sagan

Low expectations

Nevertheless, expectations for these rovers were low. JPL expected that they would cover about 2,000 ft (600 m) and last for 90 Martian sols (equivalent to about 90 Earth days). During the Martian winter, however, the team did not know whether the solar-powered rovers would retain adequate power to keep working. Of all the solar system’s rocky planets, the seasons of Mars are the most Earth-like, due to the similar tilts of the planets’ rotational axes. Martin winters are dark and bitterly cold, with surface temperatures falling to as low as –225°F (–143°C) near the polar ice caps.

As predicted, Martian winds blew fine dust onto the solar arrays, cutting their generating power; but the wind also blew the panels clean from time to time. As winter drew nearer, the JPL team searched for suitable locations in which the rovers could safely hibernate. To do this, they used a 3-D viewer built from the images taken from the rover’s stereoscopic cameras. They chose steep slopes that faced the rising sun in order to maximize electricity generation and to top off the batteries. All nonessential equipment was shut down so that power could be diverted to heaters that kept the rovers’ internal temperature above –40°F (–40°C).

An artist’s impression portrays a NASA Mars Exploration Rover. Rovers Opportunity and Spirit were launched a few weeks apart in 2003 and landed in January 2004 at two sites on Mars.

Continuing mission

The hibernation worked, and incredibly, JPL has managed to extend the rover missions from a few days to several years. More than five years into its mission, however, Spirit became bogged down in soft soil; all attempts to free it by remote control from Earth failed, and unable to move to a winter refuge, Spirit finally lost power 10 months later. It had traveled 4.8 miles (7.73 km). Opportunity, meanwhile, has avoided mishap and continues to operate. In 2014, it beat Lunokhod 2’s distance record, and by August 2015 it had completed the marathon distance of 26.4 miles (42.45 km). This was no mean feat on a planet located some hundreds of millions of miles from Earth.

In the “Kimberley” formation on Mars, photographed by Curiosity, strata indicate a flow of water. In the distance is Mount Sharp, named after US geologist Robert P. Sharp in 2012.

Curiosity needed

Spirit and Opportunity were equipped with the latest detectors; including a microscope for imaging mineral structures and a grinding tool for accessing samples from the interiors of rocks.

However, Curiosity, the next rover to arrive on the planet in August 2012, carried instruments that not only studied the geology of Mars but also looked for biosignatures—the organic substances that would indicate whether Mars once harbored life. These included the SAM or Sample Analysis at Mars device, which vaporized samples of ground rock to reveal their chemicals. In addition, the rover monitored radiation levels to see whether the planet would be safe for future human colonization.

Considerably larger than previous rovers, Curiosity was delivered to Mars in an unusual way. During the landing phase of the mission, the radio delay (caused by the sheer distance from Earth) was 14 minutes, and the journey through the atmosphere to the surface would take just seven—all on autopilot (not remotely controlled from Earth). This created “seven minutes of terror”: the engineers on Earth knew that by the time a signal arrived informing them that Curiosity had entered the Martian atmosphere, the rover would already have been on the ground for seven minutes—and would be operational or smashed to pieces.

“The Seven Minutes of Terror has turned into the Seven Minutes of Triumph.” John Grunsfeld

Safe landing

As Curiosity’s landing craft moved through the upper atmosphere, its heat shield glowed with heat, while rockets adjusted the descent speed to reach the Gale Crater, an ancient crater caused by a massive meteorite impact. A parachute slowed the craft to about 200 mph (320 km/h), but this was still too fast for a landing. It continued to slow its descent over a flat region of the crater, avoiding the 20,000-ft (6,000-m) mountain at its center. The craft reached about 60 ft (20 m) above the surface and then had to hover, since going too low would create a dust cloud that might wreck its instruments. The rover was finally delivered to the surface via a rocket-powered hovering platform called a sky crane. The sky crane then had to be detached and blasted clear of the area so that its eventual impact did not upset any future exploration.

Having survived the landing, Curiosity signaled to Earth that it had arrived safely. Curiosity’s power supply is expected to last at least 14 years, and the initial two-year mission has now been extended indefinitely. So far, it has measured radiation levels, revealing that it may be possible for humans to survive on Mars; discovered an ancient stream bed, suggesting a past presence of water and perhaps even life; and found many of the key elements for life, including nitrogen, oxygen, hydrogen, and carbon.


In 2020, the European Space Agency, in collaboration with the Russian space agency, Roscosmos, will launch its first Mars rover, ExoMars (Exobiology on Mars), with the goal of landing on Mars the following year. In addition to looking for signs of alien life, the solar-powered rover will carry a ground-penetrating radar that will look deep into Martian rocks to find groundwater. The ExoMars rover will communicate with Earth via the ExoMars Trace Gas Orbiter, which was launched in 2016. This system will limit data transfer to twice a day. The rover is designed to drive by itself; its control software will build a virtual model of the terrain and navigate through that. The rover software was taught how to drive in Stevenage, England, at a mockup of the Martian surface called the Mars Yard (above).

The ExoMars rover is expected to operate for at least seven months and to travel 2.5 miles (4 km) across the Martian surface. It will be delivered to the surface by a robotic platform that will then remain in place to study the area around the landing site.

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In the early 1960s, the US lagged behind the Soviet Union in the “Space Race.” The Soviets had launched the first satellite in 1957, and on April 16, 1961, Yuri Gagarin became the first human in space. In response, in 1961 US President John F. Kennedy publicly committed to landing a man on the moon before the end of the decade. The project was carefully chosen—landing on the moon was so far beyond the capabilities of either protagonist that the Soviets’ early lead might not seem so significant.

Despite the reservations of many at the time regarding a moon landing’s scientific value, especially given the dangers and technical complexities involved, human spaceflight was now the focus of the US space program. NASA managers felt that with enough funding they could put a man on the moon by 1967. NASA administrator James E. Webb suggested another two years be added as a contingency.

In those six years from 1961 to 1967, NASA tripled its workforce, even though most of the planning, designing, and building of the hardware was undertaken by private industry, research institutes, and universities. NASA claimed that only the construction of the Panama Canal and the Manhattan Project to develop the nuclear bomb rivaled the effort and expense of the Apollo program.

“I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth.” John F. Kennedy

Project Mercury astronaut John Glenn enters the Friendship 7 on February 20, 1962. His mission, lasting just under five hours, was the US’s first manned orbital spaceflight.

Which way to the moon?

At the time of Kennedy’s historic announcement, the US boasted a grand total of 15 minutes of human spaceflight. To move from here to a moon landing, many technological hurdles needed to be overcome. One of the first was the method of getting to the moon. Three options, known as mission architectures, were on the table. The direct ascent (DA) profile, or “all-theway,” required an enormous multistage rocket with enough fuel on board to transport the crew back to Earth. This was initially the favored approach. However, it was also the most expensive, and doubts were raised over the feasibility of building such a monster rocket before the 1969 deadline.

In the Earth-orbit rendezvous (EOR) profile, a moon-bound rocket ship would be assembled in space and dock with modules that had already been placed in orbit. Lifting things into space is the most energy-consuming part of any off-Earth mission, but multiple rocket launches would sidestep the need for a single spaceship. This was the safest option, but it would be slow. The real weight-savings came with the lunar-orbit rendezvous (LOR) profile.

Here, a smaller rocket would put a three-part spaceship on course to the moon. At the moon, a command module would remain in orbit with the fuel for the journey home, while a lightweight two-stage lunar lander would be sent to the surface. This quick and comparatively cheap option carried with it the very real risk of leaving a crew stranded in space should anything go wrong. After much debate and lobbying, influential figures, such as Wernher von Braun, director of NASA’s Marshall Space Flight Center, threw their backing behind LOR, and in 1962, LOR was chosen. This was the first of many leaps of faith for Apollo.

Technological hurdles

On February 20, 1962, John Glenn became the first American to orbit Earth, looping three times around the planet in Friendship 7, as part of the US’s first spaceflight program, Project Mercury, which ran from 1958 to 1963. Three more successful Mercury flights followed, but there was a big difference between operations in low Earth orbit and landing on the moon. An entire new fleet of launch vehicles was required. Unlike Mercury spacecraft, which carried a single astronaut, Apollo missions would need a crew of three. In addition, a more reliable power source was needed and much more experience in space. The world’s first fuel cells were built to provide the power.

Project Gemini, NASA’s second human spaceflight program, provided the skills, with endurance spaceflights, orbital maneuvers, and space walks. Scientists also needed to know more about the moon’s surface. A deep layer of dust could swallow up a spacecraft and prevent it from leaving, clog up the thrusters, or cause the electronics to malfunction.

Unmanned fact-finding missions were mounted in parallel with Apollo, but the first wave of robotic explorers dispatched to the moon was an unmitigated failure. Six Ranger landers failed on launch, missed the moon, or crashed on impact, causing the program to be nicknamed “shoot and hope.” Luckily, the final three Rangers were more successful.

Between 1966 and 1967, five Lunar Orbiter satellites were placed in orbit around the moon. They mapped 99 percent of the surface and helped to identify potential Apollo landing sites. NASA’s seven Surveyor spacecraft also demonstrated the feasibility of a soft landing on the lunar soil.

The Saturn V rocket was developed for the Apollo program. Many private corporations were involved in its production, including Boeing, Chrysler, Lockheed, and Douglas.

“From this day forward, Flight Control will be known by two words: “Tough and Competent.”” Gene Kranz

A gamble and a disaster

At 363 ft (110.5 m), Saturn V—the heavy-lift booster that carried the Apollo astronauts out of Earth’s atmosphere—is still the tallest, heaviest, and most powerful rocket ever built. “Man-rating” the rocket (certifying it to carry a human crew) proved particularly troublesome. The mammoth engines generated vibrations that threatened to break the rocket apart. Knowing that the project was behind schedule, NASA’s associate administrator for manned spaceflight, George Mueller, pioneered a daring “all-up” testing regime. Rather than the cautious stage-by-stage approach favored by von Braun, Mueller had the entire Apollo–Saturn systems tested together.

While striving for perfection, the NASA engineers developed a new engineering concept: that of redundancy. Key or critical components were duplicated in order to increase overall reliability. The Mercury and Gemini projects had taught engineers to expect unforeseen risks. A fully assembled Apollo vehicle had 5.6 million parts, and 1.5 million systems, subsystems, and assemblies. Even with 99.9 percent reliability, the engineers could anticipate 5,600 defects. Nevertheless, over its 17 unmanned and 15 manned flights, the Saturn boosters had shown 100 percent reliability. With two partially successful test flights under its belt, Mueller declared that the next launch would carry astronauts.

Until 1967, progress had been smooth, despite the breakneck pace. Then disaster struck. An electrical short-circuit during a launch rehearsal started a fire that incinerated the Apollo 1 crew inside the Command Module. The toxic smoke and intensity of the fire in a pressurized, pure-oxygen atmosphere killed Virgil “Gus” Grissom, Ed White, and Roger Chaffee in less than five minutes. In the wake of this tragedy, the next five Apollo missions were unmanned tests. Modifications were made, resulting in a safer spacecraft with a new gas-operated hatch, a 60–40 oxygen–nitrogen mix in the cockpit, and fireproof wiring throughout.

The Lunar Orbiter satellites took images of potential landing sites. In 1966, Lunar Orbiter 2 sent back this image of Copernicus Crater, one of the first-ever close-up views of the moon.

“Apollo riding his chariot across the sun was appropriate to the grand scale of the proposed program.” Abe Silverstein

Earth’s place in space

Apollo 8 was the first manned spacecraft to leave Earth’s orbit. On Christmas Eve 1968, Frank Borman, James Lovell, and Bill Anders looped around the far side of the moon and witnessed the astounding sight of Earth rising from behind the moon’s surface. For the first time, humans could see their home from space—a startlingly blue world lost in the immensity of the void. As Anders put it: “We came all this way to explore the moon, and the most important thing is that we discovered the Earth.”

The crew was also the first to pass through the Van Allen radiation belts. This zone of charged particles extends up to 15,000 miles (24,000 km) from Earth, and was initially thought to be a serious barrier to human space travel. As it turned out, it resulted in a dosage of radiation only equivalent to a chest X-ray.

Finally, the program was ready for the last step—to take real steps on the moon itself. On July 21, 1969, an estimated global audience of 500 million tuned in to watch Neil Armstrong land the Lunar Module and step out onto the surface of the moon, closely followed by Buzz Aldrin. It was the culmination of nearly a decade of collaborative effort and effectively ended the Space Race.

There were six more missions to the moon following Apollo 11, including the near-disaster of Apollo 13, whose lunar landing in 1970 was aborted after an oxygen tank exploded on board. The crew was returned safely to Earth on the crippled spacecraft in a real-life drama that played out in front of a worldwide television audience.

In 1968, Apollo 8 broadcast live from moon orbit. Images taken from the spacecraft by astronaut Bill Anders included the iconic Earthrise.

Learning about the moon

Before Apollo, much of what was known about the physical nature of Earth’s only natural satellite was speculation but, with the political goals achieved, here was an opportunity to find out about an alien world firsthand. Each of the six landing missions carried a kit of scientific tools—the Apollo Lunar Surface Experiments Package (ASLEP). Apollo’s instruments tested the internal structure of the moon, detecting seismic vibrations that would indicate a “moonquake.” Other experiments measured the moon’s gravitational and magnetic fields, heat flow from its surface, and the composition and pressure of the lunar atmosphere.

Thanks to Apollo, scientists have compelling evidence from analysis of moon rock that the moon was once a part of Earth. Like Earth, the moon also has internal layers and was most likely molten at some point in its early history. Unlike Earth, however, the moon has no liquid water. Since it has no moving geological plates, its surface is not continually repaved, and so the youngest moon rocks are the same age as Earth’s oldest. The moon is not entirely geologically inactive, however, and occasionally has moonquakes that last for hours.

One Apollo 11 experiment remains active and has been returning data since 1969. Reflectors planted on the lunar surface bounce back laser beams fired from Earth, enabling scientists to calculate the distance to the moon to within an accuracy of a couple of millimeters. This gives precise measurements of the moon’s orbit, and the rate at which it is drifting away from Earth (about 1½ in [3.8 cm] per year).

Neil Armstrong took this famous photograph of Buzz Aldrin on the surface of the moon. Armstrong’s reflection, standing next to the lunar module, can be seen in Aldrin’s visor.

Apollo’s legacy

On December 19, 1972, the sonic boom over the South Pacific, as the Apollo 17 capsule thumped into Earth’s atmosphere, sounded the end of the Apollo program. In total, 12 men had walked on the moon. At the time, it was widely assumed that routine flights to Mars would soon be a reality, but in the intervening 40 years, scientific priorities changed, politicians worried about costs, and human space travel has not ventured farther than Earth’s orbit.

For many, the decision to end manned moon missions was a wasted opportunity, caused by a lack of imagination and leadership. However, the end of the acute Cold War competition that gave rise to the Apollo program heralded a new era of international cooperation for NASA, with Skylab, Mir, and the International Space Station.

Gene Cernan, the last man on the moon, predicted that it could be another 100 years before humankind appreciates the true significance of the Apollo missions. One result could be that it may have made the US smarter—the intake for doctoral degrees at American universities tripled during the 1960s, particularly in the field of physics. Apollo contracts also nurtured nascent industries, such as computing and semiconductors. Several employees of the California-based Fairchild Semiconductors went on to found new companies, including Intel, a technology giant. The Santa Clara area where these firms were based has become today’s Silicon Valley. But perhaps Apollo’s real legacy is the idea of Earth as a fragile oasis of life in space. Photos taken from orbit, such as the “Blue Marble” and “Earthrise”, fed into a growing awareness of planet Earth as a single entity, and the need for careful stewardship.

Apollo 11’s command and service module docked with the lunar module in orbit before heading for the moon. Before touchdown, the service module was jettisoned, and only the command module returned to Earth.

“Houston. Tranquility Base here. The Eagle has landed.” Neil Armstrong

On the final three Apollo missions, astronauts explored the surface of the moon on lunar rovers. The rovers were abandoned and can still be seen where they were left behind


Perhaps the embodiment of the NASA spirit is not the heroic astronauts but the legendary Apollo flight director Gene Kranz. Born in 1933, Kranz was fascinated by space from an early age. He served as a pilot with the US Air Force before leaving to pursue rocket research with the McDonnell Aircraft Corporation and then NASA.

Prominent and colorful, with a brutally close-cut flattop hairstyle, Kranz was unmistakable in Mission Control, dressed in his dapper white “mission” vests made by his wife.

Although he never actually spoke the words “Failure is not an option”— they were written for his character in the movie Apollo 13—they sum up his attitude. Kranz’s address to his Flight Control staff after the Apollo 1 disaster has gone down in history as a masterpiece of motivational speaking. In it, he stated the Kranz Dictum—“tough and competent”—that would guide Mission Control. Kranz was awarded the Presidential Medal of Freedom in 1970 for successfully returning Apollo 13 to Earth.

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Before the 17th century, all astronomers were also astrologers. For many, including German astronomer Johannes Kepler, casting horoscopes was the main source of their income and influence. Knowing where the planets had been in the sky was important, but of greater significance for constructing astrological charts was the ability to predict where the planets would be over the next few decades.

To make predictions, astrologers assumed that the planets moved on specific paths around a central object. Before Copernicus, in the 16th century, this central body was thought by most to be Earth. Copernicus showed how the mathematics of planetary prediction could be simplified by assuming that the central body was the sun. However, Copernicus assumed that orbits were circular, and to provide any reasonable predictive accuracy, his system still required the planets to move around a small circle, the center of which moved around a larger circle. These circular velocities were always assumed to be constant.

Kepler supported the Copernican system, but the planetary tables it produced could still easily be out by a day or two. The planets, the sun, and the moon always appeared in a certain band of the sky, known as the ecliptic, but actual paths of individual planets around the sun were still a mystery, as was the mechanism that made them move.

“Kepler was never satisfied by a moderate agreement between theory and observation. The theory had to fit exactly otherwise some new possibility had to be tried.” Fred Hoyle

Kepler’s most productive years came in Prague under the patronage of Holy Roman Emperor Rudolf II (r.1576–1612). Rudolf was particularly interested in astrology and alchemy.

Finding the paths

To improve the predictive tables, Danish astronomer Tycho Brahe spent more than 20 years observing the planets. He next tried to ascertain a path of each planet through space that would fit the observational data. This is where the mathematical abilities of Kepler, Brahe’s assistant, came into play. He considered specific models for the solar system and the paths of the individual planets in turn, including circular and ovoid (egg-shaped) orbits. After many calculations, Kepler determined whether or not the model led to predictions of planetary positions that fit into Tycho’s precise observations. If there was not exact agreement, he would discard the idea and start the process again.

Abandoning circles

In 1608, after 10 years of work, Kepler found the solution, which involved abandoning both circles and constant velocity. The planets made an ellipse— a kind of stretched-out circle for which the amount of stretching is measured by a quantity called an eccentricity. Ellipses have two foci. The distance of a point on an ellipse from one focus plus the distance from the other focus is always constant. Kepler found that the sun was at one of these two foci.

These two facts made up his first law of planetary motion: the motion of the planets is an ellipse with the sun as one of the two foci. Kepler also noticed that the speed of a planet on its ellipse was always changing, and that this change followed a fixed law (his second): a line between the planet and the sun sweeps out equal areas in equal times. These two laws were published in his 1609 book Astronomia Nova.

Kepler had chosen to investigate Mars, which had strong astrological significance, thought to influence human desire and action. Mars took variable retrograde loops—periods during which it would reverse its direction of movement—and large variations in brightness. It also had an orbital period of only 1.88 Earth years, meaning that Mars went around the sun about 11 times in Tycho’s data set. Kepler was lucky to have chosen Mars, since its orbit has a high eccentricity, or stretch: 0.093 (where 0 is a circle and 1 is a parabola). This is 14 times the eccentricity of Venus. It took him another 12 years to show that the other planets also had elliptical orbits.

Studying Brahe’s observations, Kepler was also able to work out the planets’ orbital periods. Earth goes around the sun in one year, Mars in 1.88 Earth years, Jupiter in 11.86, and Saturn in 29.45. Kepler realized that the square of the orbital period was proportional to the cube of the planet’s average distance from the sun. This became his third law and he published it in 1619 in his book Harmonices Mundi, alongside lengthy tracts on astrology, planetary music, and platonic figures. The book had taken him 20 years to produce.

When just one body goes around a larger body undisturbed, the paths it can follow are known as Kepler orbits. These are a group of curves called conic sections, which include ellipses, parabolas, and hyperbolas. The shape of the orbit is defined by a property called eccentricity. An eccentricity of 0 is a circle (A). Eccentricity between 0 and 1 is an ellipse (B). Eccentricity equal to 1 produces a parabola (C), and greater than 1 a hyperbola (D).

Searching for meaning

Kepler was fascinated by patterns he found in the orbits of the planets. He noted that, once you accepted the Copernican system for the cosmos, the size of the orbits of the six planets—Mercury, Venus, Earth, Mars, Jupiter, and Saturn—appeared in the ratios 8 : 15 : 20 : 30 : 115 : 195.

Today, astronomers might look at a list of planetary orbital sizes and eccentricities and regard them as the result of the planetary formation process coupled with a few billion years of change. To Kepler, however, the numbers needed explanation. A deeply religious man, Kepler searched for a divine purpose within his scientific work. Since he saw six planets, he presumed that the number six must have a profound significance. He produced an ordered geometric model of the solar system in which the sun-centered spheres that contained each planetary orbit were inscribed and circumscribed by a specific regular “platonic” solid (the five possible solids whose faces and internal angles are all equal). The sphere containing the orbit of Mercury was placed inside an octahedron. The sphere that just touched the points of this regular solid contained the orbit of Venus. This in its turn was placed inside an icosahedron. Then followed the orbit of Earth, a dodecahedron, Mars, a tetrahedron, Jupiter, a cube, and finally Saturn. The system was beautifully ordered, but inaccurate.

Kepler’s great breakthrough was his calculation of the actual form of the planetary orbits, but the physics behind his three laws did not seem to concern him. Rather, he suggested that Mars was carried on its orbit by an angel in a chariot, or swept along by some magnetic influence emanating from the sun. The idea that the movements were due to a gravitational force only arrived with the ideas of Isaac Newton some 70 years later.

According to Kepler’s second law, the line joining a planet to the sun sweeps out equal areas in equal times. This is also known as the law of equal areas. It is represented by the equal areas of the three shaded areas ABS, CDS, and EFS. It takes as long to travel from A to B as from C to D and from E to F. A planet moves most rapidly when it is nearest the sun, at perihelion; a planet’s slowest motion occurs when it is farthest from the sun, at aphelion.

Wider contributions

Kepler also made important advances in the study of optics, and his 1604 book Astronomiae Pars Optica is regarded as the pioneer tome in the subject. Galileo’s telescope interested him greatly and he even suggested an improved design using convex lenses for both the objective and the magnifying eyepiece. He wrote, too, about the supernova that was first seen in October 1604, today commonly called Kepler’s supernova. Following Tycho, Kepler realized that the heavens could change, contradicting Aristotle’s idea of a “fixed cosmos.” A recent planetary conjunction coupled with this new star led him to speculate about the Biblical “Star of Bethlehem.” Kepler’s fervent imagination also produced the book Somnium, in which he discusses space travel to the moon and the lunar geography a visitor might expect on arrival. Many regard this as the first work of science fiction.

Kepler’s most influential publication, however, was a textbook on astronomy called Epitome Astronomiae Copernicanae, and this became the most widely used astronomical work between 1630 and 1650. He ensured that the Rudolphine Tables (named after Emperor Rudolf, his patron in Prague) were eventually published, and these tables of predicted planetary positions helped him greatly with the well-paid calendars that he published between 1617 and 1624. The accuracy of his tables, proven over a few decades, also did much to encourage the acceptance of both the Copernican sun-centered solar system and Kepler’s own three laws.

“Kepler was convinced that God created the world in accordance with the principle of perfect numbers, so that the underlying mathematical harmony … is the real and discoverable cause of the planetary motion. “ William Dampier

In Harmonices Mundi, Kepler experimented with regular shapes to unlock the secrets of the cosmos. He linked these shapes with harmonics to suggest a “music of the spheres.”


Born prematurely in 1571, Kepler spent his childhood in Leonberg, Swabia, in his grandfather’s inn. Smallpox affected his coordination and vision. A scholarship enabled him to attend the Lutheran University of Tübingen in 1589, where he was taught by Michael Maestlin, Germany’s top astronomer at the time. In 1600, Tycho Brahe invited Kepler to work with him at Castle Benátky near Prague. On Tycho’s death in 1601, Kepler succeeded him as Imperial Mathematician.

In 1611, Kepler’s wife died, and he became a teacher in Linz. He remarried and had seven more children, five of whom died young. His work was then disrupted between 1615 and 1621 while he defended his mother from charges of witchcraft. The Catholic Counter-Reformation in 1625 caused him further problems, and prevented his return to Tübingen. Kepler died of a fever in 1630.

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