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OUR OWN EYES SHOW US FOUR STARS (TRAVELING AROUND JUPITER)

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BEFORE

1543 Nicolaus Copernicus proposes a theory of a sun-centered cosmos, but proof is needed because Earth does not seem to move. 1608 Dutch eyeglass-makers develop the first telescopes.

AFTER

1656 Dutch scientist Christiaan Huygens builds ever-bigger telescopes that are capable of detecting more detail and fainter objects. 1668 Isaac Newton produces the first reflecting telescope, an instrument that is far less affected by the distortion of chromatic aberration. 1733 The first flint glass/crown glass achromatic lens is made. This greatly improves the potential image quality of refracting telescopes.

Galileo Galilei’s effective use of a telescope marked a watershed in the history of astronomy. There have been other turning points—such as the introduction of photography, the discovery of cosmic radio waves, and the invention of the electronic computer—but the invention of the telescope was fundamental to the advancement of the subject.

“The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.” Galileo Galilei

Limits of the naked eye

Before Galileo, the naked eye was all that was available to observe the sky. The naked eye is limited in two main ways: it is unable to record detail, and it can only detect objects that are reasonably bright.

When looking at a full moon, the lunar diameter subtends (spans) an angle of 1⁄2º at Earth’s surface. This means that two lines extending from opposite sides of the moon meet at the eye to make an angle of 1⁄2º. However, the naked eye can only detect separate objects that are more than about 1⁄60º apart. This is the eye’s resolution, and determines the level of detail it can detect. Looking at the full moon with the naked eye, the lunar diameter is resolved into only 30 picture elements, analogous to individual pixels in a digital photograph. Dark lunar seas and lighter lunar highland are discernible, but individual mountains and their shadows are beyond detection.

Looking up at the night sky on a cloud-free, moonless night in Galileo’s Italian countryside, 2,500 stars would be visible above the horizon. The Milky Way—the disk of the solar system seen side-on—looks like a river of milk to the naked eye. Only a telescope shows that the Milky Way seems to be made up of individual stars; the bigger the telescope, the more stars are visible. By turning his new telescope to the night sky, Galileo would become one of the very first people to appreciate the true nature of this band of stars across the sky.

The resolution of the naked eye is about 1⁄60°. The moon subtends an angle of 1⁄2° seen from Earth, meaning that the lunar diameter can be resolved into 30 picture elements.
Galileo demonstrates his telescope to Leonardo Donato, the Doge of Venice. Like other astronomers of his time, Galileo relied on patronage to fund and legitimize his work.

Building a telescope

Galileo did not invent the telescope himself. The idea of combining two lenses—a large one at the front of a tube to collect the light, and a small one at the back to magnify the image—had come from the Dutchmen Hans Lipperhey, Jacob Metius, and Sacharias Janssen in around September 1608. (It had taken over 300 years to progress from the invention of reading glasses to the invention of a telescope.) After hearing about this new instrument, Galileo had resolved to make one for himself.

A telescope does two important things. Its resolution (the detail a telescope can detect) is proportional to the diameter of the objective lens—the large lens at the front that collects the light. The larger the objective lens, the better the resolution. An eye that has fully adapted to the dark has a pupil that is about ¼ in (0.5 cm) across, and a resolution of around 1⁄60º. Put the eye at the back of a telescope with an objective lens of 1, 2, or 4 cm diameter, and the resolution improves to 1⁄120º, 1⁄240º, and 1⁄480º respectively. Details then spring into view. Jupiter, for example, looks like a disk and not just a point.

A telescope also acts as a “light bucket.” Every time the diameter of the objective lens is doubled, the light gathered increases by a factor of four, and objects of similar light output can be detected if they are twice as far away. Objective lenses of 1, 2, and 4 cm enable the eye to discern 20,000, 160,000, and 1,280,000 stars respectively.

Galileo was not satisfied with his first instrument, which only magnified three times. He realized that a telescope’s magnification was directly related to the ratio of the focal length of the objective lens to the focal length of the eyepiece. A longer-focus convex lens for the objective, or a shorter-focus concave lens for the eyepiece was required. Since these were not available, Galileo taught himself to grind and polish lenses and made them for himself. Living in northern Italy, the glassmaking center of the world at the time, helped him considerably. He eventually developed a new telescope with 33 times magnification, and it was with this improved instrument that he discovered the Jovian (“of Jupiter”) moons.

“My dear Kepler, what would you say of the learned who … have steadfastly refused to cast a glance through the telescope?” Galileo Galilei

“Three little stars”

Galileo discovered the moons of the planet Jupiter on the night of January 7, 1610. At first, he thought he was looking at distant stars, but he quickly realized that the new bodies were moving around Jupiter, At the time, Galileo was a 45-year-old professor of mathematics at the University of Padua near Venice. When he published his pioneering telescopic observations, he wrote: “Through a spyglass, Jupiter presented himself. And since I had prepared for myself a superlative instrument, I saw (which earlier had not happened because of the weakness of other instruments) that three little stars were positioned near him—small but yet very bright. Although I believed them to be among the number of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and parallel to the ecliptic ….”

Refracting telescopes suffer from a problem known as chromatic aberration. The different wavelengths of light are brought to slightly different foci, so the final image is surrounded by a halo of color.

“Galileo had the experience of beholding the heavens as they actually are for perhaps the first time.” I Bernard Cohen

Repeated observations

Galileo’s unexpected discovery fascinated him. As he observed Jupiter night after night, it soon became clear that the new stars were not beyond Jupiter, in the distant heavens. They not only accompanied the planet as it moved along its path across the sky, but also moved around the planet.

Just as the moon orbits Earth every month, Galileo realized that there were four moons in orbit around Jupiter, staying with it as it orbited the sun. The more distant moons took longer to complete their orbits than the closer ones. The time to complete one orbit from the inner to the outer moon is 1.77, 3.55, 7.15, and 16.69 days, respectively. The Jovian moon system looked like a small model of the sun’s planetary system. It was proof that not everything in the cosmos orbited Earth, as had been thought in pre-Copernican days. The observation of these four moons was a boost to the theory of the sun-centered cosmos.

Galileo quickly published his discovery in his book Siderius Nuncius (The Starry Messenger), published on March 10, 1610. In the hope of advancement, Galileo dedicated the book to a former pupil of his who later became the Grand Duke of Tuscany, Cosimo II de’ Medici. He named the moons the Medicean Stars in honor of the four royal Medici brothers. This political thoughtfulness won him the position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. However, the name did not catch on.

At first, many were sceptical, suggesting that the moons were no more than defects in the telescope lens. However, other pioneering telescopic astronomers such as Thomas Harriot, Joseph Gaultier de la Vatelle, and Nicolas-Claude Fabri de Peiresc confirmed their existence when Jupiter returned to the night sky later in 1610, after passing behind the sun.

Galileo’s telescope had a concave lens as an eyepiece. When viewing a celestial object a great distance away, the distance between the two lenses would equal the focal length of the objective lens minus the focal length of the eyepiece.
Kepler’s telescope, developed soon after, had a convex lens as an eyepiece. The length of the telescope was equal to the objective focal length plus the focal length of the eyepiece.

Disputed priority

In 1614, German astronomer Simon Marius published Mundus Iovialis, in which he described Jupiter’s moons and claimed to have discovered them before Galileo. Galileo would later accuse Marius of plagiarism, but it is now generally accepted that he made his discovery independently at around the same time. Marius named the moons Io, Europa, Ganymede, and Callisto after the Roman god Jupiter’s love conquests, and these names are still used. They are now known collectively as the Galilean moons.

REFRACTING TELESCOPES

There were two kinds of early refracting telescope: the Galilean, and the Keplerian, developed in 1611 by Johannes Kepler. They both had a long-focus, large diameter lens at the front, called the objective. This collected the light and brought it to a focus. The image at the focus was magnified using the smaller, short-focus eyepiece lens.

The magnification of the instrument is equal to the focal length of the objective lens divided by the focal length of the eyepiece. A flatter convex objective lens reduced chromatic aberration, gave a longer focal length, and, for a fixed eyepiece, greater magnification. For this reason, telescopes became longer in the 17th century. The minimum focal length of eyepieces at the time of Galileo and Kepler was about 1–1½ in (2–4 cm). This meant that, for a magnification of x30, an objective lens with a focal length of 24– 48 in (60–120 cm) was needed. Built in 1888, the huge James Lick Telescope on Mount Hamilton, California (above), has a 36-in (90-cm) lens and a focal length of 57 ft (17.37 m).

A Jovian clock

Galileo carefully studied the changing positions of the Jovian moons from day to day. He concluded that, like the planets, their positions could be calculated in advance. Galileo saw that, if this could be done accurately, the system would act as a universal clock and could solve the problem of measuring longitude at sea. To establish longitude requires the ability to tell the time, but in Galileo’s day, there were no timepieces that would work on a boat. Because Jupiter is at least four times farther away from Earth than the sun, the Jovian system looks the same from anywhere on Earth, so a “Jovian clock” would work from anywhere. The longitude problem was finally solved with the introduction of accurate chronometers by the English clockmaker John Harrison around 1740. This was well before the orbits of Jupiter’s moons had been worked out in detail.

Galileo’s discovery of four satellites around Jupiter had another interesting consequence. When Jonathan Swift published Gulliver’s Travels in 1726, he predicted, in the chapter on Laputa, that Mars would have two moons simply because Earth had one and Jupiter four. In 1877, this prediction was fortuitously proved to be correct when Asaph Hall discovered Mars’s two small moons, Phobos and Deimos, using a new 26-in (66-cm) refracting telescope at the US Naval Observatory in Washington.

Starting with the closest to Jupiter, the Galilean moons are, from left to right, Io, Europa, Ganymede, and Callisto. Ganymede is larger than the planet Mercury.

Support for Copernicus

In Galileo’s time, there was still a heated debate between believers of the old biblical theory that Earth was stationary at the center of the cosmos and Copernicus’s new idea that the Earth was in orbit around the sun. The geocentric (Earth-centered) idea stressed the uniqueness of the planet, while the heliocentric (sun-centered) proposal made Earth just one of a family of planets. The assumption that Earth does not occupy a privileged place in the cosmos is now known as the Copernican principle.

The challenge now was to find observations to prove that one theory was correct and the other false. The discovery of moons around Jupiter was great support for a sun-centered system. It was now clear that everything did not orbit around a central Earth, but there were still unanswered questions. If the sun-centered system was correct, Earth must be moving. If Earth had to travel around the sun every year, it had to have an orbital speed of 20 miles/sec (30 km/sec). In Galileo’s time, the exact distance from Earth to the sun was not known, but it was clearly far enough that Earth would need to be moving quickly, and humans cannot apprehend this movement. Also, this orbital motion should make the stars appear to swing from side to side every year in a phenomenon called stellar parallax. This again was not observed at the time. Galileo and his contemporaries did not suspect that the typical distance between stars in the Milky Way was about 500,000 times larger than the distance between Earth and the sun, which makes stellar parallax so small that it is difficult to measure. It was not until the mid-19th century that vastly improved instruments made it possible to detect this swing.

Despite these questions, Galileo considered that his findings had proved Copernicus correct beyond reasonable doubt. His discoveries also included the phases of Venus, which are best explained if the planet is in orbit around the sun, and the fact that the sun is spinning, shown by the movement of sunspots. By 1619, Galileo’s pugnacious defense of Copernicus had drawn him into conflict with the Church, which had declared in 1616 that heliocentricism was heretical. In 1633, he appeared before the Inquisition. His books were banned, and he spent the last 10 years of his life under house arrest.

“The Bible shows the way to go to heaven, not the way the heavens go.” Galileo Galilei

New moons

Jupiter only had four known moons for 283 years. A fifth satellite, Amalthea, was discovered by the American astronomer E. E. Barnard in 1892, using the 36-in (91-cm) refractor at the Lick Observatory in California. It was the last solar system satellite to be discovered by direct observation. Subsequently, satellites have been found by the meticulous examination of photographs. The number of known Jupiter satellites had crept up to 12 by the mid-1950s, and has now reached 67. Many smaller moons may be found in the future.

GALILEO GALILEI

Galileo Galilei was born in Pisa, Italy on 15 February 1564. He was appointed to the Chair of Mathematics at the University of Pisa in 1589, moving to the University of Padua in 1590. Galileo was an astronomer, physicist, mathematician, philosopher, and engineer, who played a pivotal role in the process of intellectual advances in Europe now known as the Scientific Revolution.

He was the first person to effectively turn the refractor telescope on the heavens. During 1609–10, he discovered that the planet Jupiter had four moons, Venus underwent phase changes, the moon was mountainous, and the sun was spinning round once in about a month. He was a prolific writer and made his findings accessible to a wide audience.

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Planets

A LABORATORY ON MARS

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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

Wanderers

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.

EXOMAR

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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Planets

WE CHOOSE TO GO TO THE MOON

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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

GENE KRANZ

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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Solar system

IT HAS TO BE SOME NEW KIND OF STAR

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In the late 1950s, astronomers across the world started to find mysterious, compact sources of radio signals in the sky without any corresponding visible objects. Eventually a source of these radio waves was identified—a faint point of light, which became known as a quasar. In 1963, Dutch astronomer Maarten Schmidt discovered a quasar that was hugely distant (2.5 billion light-years away). The fact that it was so easily detected meant it must be pouring out energy.

Searching for quasars

By the mid 1960s, many radio astronomers were searching for new quasars. One such figure was Antony Hewish, part of a radio astronomy research group at Cambridge University. Hewish had been working on a new technique in radio astronomy based on a phenomenon called interplanetary scintillation (IPS), which is a “twinkling,” or fluctuation, in the intensity of radio emissions from compact radio sources. The twinkling of sources of visible light, such as stars, is caused by disturbances in Earth’s atmosphere that the light has to pass through. The twinkling of radio sources, however, is caused by streams of charged particles emanating from the sun. As radio waves pass through this “solar wind,” they are diffracted, meaning that the waves spread out, making the radio source appear to twinkle.

Hewish hoped that IPS could be used to find quasars. Radio waves coming from a compact source, such as a quasar, twinkle more than radiation from a less compact source, such as a galaxy, and so quasars should twinkle more than other radio sources. Hewish and his team built a large radio telescope designed specifically to detect IPS. It covered an area of nearly 4.5 acres (2 hectares), took two years to construct, and required more than 120 miles (190 km) of cable to carry all the signals.

Members of the Cambridge radio astronomy group built the new telescope themselves. Among them was a Ph.D. student named Jocelyn Bell. When the telescope started operating in July 1967, Bell was made responsible for operating it and analyzing the data, under the supervision of Hewish. Part of her job was to monitor output data from the telescope, made by pen recorders on chart-recorder printouts. Examining about 100 ft (30 meters) of chart paper every day, Bell quickly learned to recognize scintillating sources.

Little Green Man 1

About two months into the project, Bell noticed an unusual pattern of signals, which she described as “scruff.” It looked far too regular and had too high a frequency to be coming from a quasar. Checking back through her records, she found it had appeared in the data before and always came from the same patch of sky. Intrigued, Bell started making more regular chart recordings of the same area of sky. At the end of November 1967, she found the signal again. It was a series of pulses, equally spaced and always 1.33 seconds apart.

Bell showed the signal, dubbed “Little Green Man 1” (LGM-1), to Hewish. His initial reaction was that a pulse occurring every 1.33 seconds was far too fast for something as large as a star, and the signal must be due to human activity. Together, Bell and Hewish ruled out various human-related sources, including radar reflected from the moon, Earth-based radio transmissions, and artificial satellites in peculiar orbits. A second telescope was also found to pick up the pulses, which proved that they could not be due to an equipment fault, and calculations showed that they were coming from well outside the solar system.

Hewish had to revise his opinion that the signals had a human origin. The possibility that they were being sent by extra-terrestrials could not be ruled out. The team measured the duration of each pulse and found it was only 16 milliseconds. This short duration suggested that the source could be no larger than a small planet. But a planet—or an alien civilization living on a planet— was unlikely, since the signal would show slight changes in frequency, called Doppler shifts, as a planet orbited its star.

This image of a pulsar in the Crab nebula, a well-known supernova remnant, was taken in space by the Chandra X-ray Observatory. The white dot at the center is the neutron star.

Publishing dilemma

Hewish, Bell, and their colleagues were unsure how to publish their findings. While it seemed unlikely that the signals were being sent by an alien civilization, no one had any other explanation. Bell returned to her chart analysis, and soon found another “scruff” in a different part of the sky. She discovered it was due to another pulsating signal, this time slightly faster, with pulses every 1.2 seconds. Now she was reassured that the pulses must have some natural explanation—two sets of aliens in different places would surely not be sending signals to Earth at the same time and at nearly the same frequency.

By January 1968, Hewish and Bell had found four pulsing sources in total, which they decided to call “pulsars.” They wrote a paper describing the first source, suggesting that it might be due to pulsed emissions from a theoretical type of superdense collapsed star called a neutron star. Objects of this type had been predicted as long ago as 1934, but up to that time had never been detected.

“My eureka moment was in the dead of night, the early hours of the morning. But when the result poured out of the charts … you realize instantly how significant this is—what it is you’ve really landed on—and it’s great!” Jocelyn Bell Burnell

Explaining the pulses

Three months later, Thomas Gold, an Austrian−American astronomer at Cornell University in the US, published a fuller explanation for the pulsed signals. He agreed that each set of radio signals was coming from a neutron star, but proposed that the star was rapidly spinning. A star like this would not need to be emitting pulsed radiation to account for the pattern of signals observed. Instead, it could be emitting a steady radio signal in a beam that it swept around in circles, just like a beam of light from a lighthouse. When the pulsar’s beam (or perhaps one of its two beams) was pointing at Earth, a signal would be detected, which would show up as the sort of short pulse that Bell had noticed on printouts. When the beam had passed by Earth, the signal would stop until the beam came around again. Challenged about the pulsation rates, which implied extremely rapid spinning, Gold explained that neutron stars could be expected to behave in this way because of the way in which they form—from the collapse of stellar cores in supernova explosions.

A pulsar is a spinning neutron star with an intensely strong magnetic field. It emits beams of radiation from its north and south poles.

Confirming the hypothesis

Initially, Gold’s explanations were not well received by the astronomy community. However, they became widely accepted after the discovery of a pulsar in the Crab nebula, a well-known supernova remnant. Over subsequent years, many more pulsars were found. They are now known to be rapidly rotating neutron stars with intense electromagnetic fields, which emit beams of electromagnetic radiation from their north and south poles. These beams are often, but not always, radio waves and sometimes other forms of radiation, including in some cases visible light. One reason for the excitement regarding the discovery of pulsars was that it increased the likelihood that another theoretical phenomenon—black holes—might also be detected and proven. Like neutron stars, black holes are objects that could result from the gravitational collapse of a stellar core following a supernova explosion.

In 1974, Hewish and Martin Ryle shared a Nobel Prize: “Ryle for his observations and inventions … and Hewish for his decisive role in the discovery of pulsars.” However, Jocelyn Bell Burnell was told that she would not share the award with them because she had still been a student at the time of her work. She graciously accepted that decision.

A spinning neutron star that is emitting radiation beams can be detected as a pulsar on Earth if, as it spins, one or possibly both of its radiation beams recurrently point in Earth’s direction as they are swept around through space. The pulsar will then be detected as a very regular series of signal “blips.”

JOCELYN BELL BURNELL

Jocelyn Bell was born in 1943 in Belfast, Northern Ireland. After earning a physics degree from Glasgow University in 1965, she moved to Cambridge University, where she studied for a Ph.D. There, she joined the team that built a radio telescope to detect quasars. In 1968, Bell became a research fellow at the University of Southampton and changed her last name to Bell Burnell when she married. She has held astronomy and physics-related positions in London, Edinburgh, and at the Open University, where, from 1991 to 2001, she was professor of physics. From 2008 to 2010, she was President of the Institute of Physics. Bell Burnell has received numerous awards for her professional contributions, including the Herschel Medal of the Royal Astronomical Society in 1989. In 2016, she was visiting professor of astrophysics at Oxford University.

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