The Story of Astronomy (28 page)

The
Phoenix
lander is currently en route, and will search likely sites for signs of water. Two further
Scout
missions are planned to arrive in 2013 and 2018 and they
will eventually provide much better exploration data. The geological survey of the whole of Mars will be complete within a few years, by which time the scientists will attempt much closer surveys of the more interesting areas. Geologists on Earth want a sample of Martian rock and the Mars Science Laboratory, to be launched in 2009, will help to provide one. It is hoped that a sample can be brought back to Earth by 2014.

Venus was the next planet to be explored by probes. The planet had traditionally been thought to be the one most similar to Earth, albeit with a slightly smaller mass and a much warmer climate. The Russian
Venera
probes reached the surface of Venus in the 1970s to early 1980s and were able to determine conditions underneath the thick layer of cloud that obscures the planet. The temperature was a searing 470 °C (880 °F), and the atmosphere was 96 percent carbon dioxide and about 4 percent hydrogen. It was also about 90 times denser than the Earth's atmosphere. None of the
Venera
probes lasted longer than about two hours, succumbing quickly to the extreme heat and pressure at the surface. There had been much volcanic activity on Venus, producing large quantities of sulfur in the atmosphere. Indeed, the dense clouds that enveloped
the planet were found to consist mostly of sulfuric acid. Radar surveys showed many gently rolling hills on the surface, many volcanoes and two “continents.” There were many craters including a huge impact crater on the surface, christened Klenova, which was 88 miles (142 km) across. It was a great disappointment to discover that the surface of the planet of love could hardly have been a more hostile environment.

In 1974 the
Mariner 10
probe reached Mercury and went into an orbit around the planet that enabled three close approaches to be made. The probe mapped a remarkable 45 percent of the surface during these orbits. The surface proved to be heavily cratered like the Moon. Unfortunately the planet was no more hospitable than Venus. The very thin atmosphere consisted of hydrogen, helium, sodium, potassium and a trace of oxygen. As a result of the thin atmosphere the daytime temperatures reached about 350 °C (662 °F), but at night they fell to about 170 below zero. Features examined included the Caloris Basin, about 808 miles (1300 km) in diameter and surrounded by a ring of mountains. It was also possible for the probe to measure the length of the day on Mercury. The year on Mercury has long been known to be the equivalent of 88 Earth days, but the solar day turned out to be 176
Earth days—two Mercurian years! The first fly-by of NASA's
Messenger
mission, launched in 2004, was achieved in January 2008, with two more fly-bys to come before it enters Mercury's orbit in 2011.

The development and launch of the very sophisticated
Voyager
probes in the 1970s meant that exploration of the outer planets of the solar system became possible. The two
Voyager
probes visited Jupiter in 1979 and Saturn in 1980 and 1981, with
Voyager 2
traveling to study both Uranus and Neptune during the late 1980s. These missions were followed by the
Galileo
spacecraft's visit to Jupiter, and
Cassini's
sojourn at Saturn in later decades. To save fuel, these probes were designed to reach the outer reaches of the solar system by using the gravitational pull of the other planets to produce a sort of slingshot effect, helping to propel them on their journey. Very sharp and interesting images of the gas giants and their satellites (moons) were transmitted back to Earth. Clearly visible on the surface of Jupiter—the biggest planet in the solar system—is the Great Red Spot. This feature is so large that it was seen in the 17th century. It is about 15,500 miles (25,000 km) long by 7, 456 miles (12,000 km) wide—large enough to swallow two Earths. It takes about six days to make a rotation, and although it changes shape
over a period of time it has been a permanent feature on Jupiter for over 300 years. It is best described as a colossal hurricane or typhoon. The energy needed to maintain it comes from inside the planet, and it is part of an ever-changing atmosphere around Jupiter.

In 1993 there was great excitement when it was realized that the comet Shoemaker-Levy 9 was approaching Jupiter and that it would strike the planet in 1994. The comet was broken into more than 20 pieces by the gravitational field of Jupiter and the fragments struck the planet between the 16th and the 24th of July 1994. The Hubble Space Telescope and other large telescopes were brought to bear on the event and it was also observed and recorded by the world's astronomers. The impacts punched dark, giant asymmetric holes in Jupiter's atmosphere, which took several months to dissipate.

The
Galileo
probe was able to make a spectral analysis of the atmosphere of Jupiter. It was found to be composed of 86 percent hydrogen and 13 percent helium. The remaining 1 percent consisted of traces of methane, ammonia and water vapor. The
Galileo
spacecraft dropped a probe into the planet, revealing wind speeds of 373 miles per hour (600 km/hr) and small concentrations of helium, neon, oxygen, carbon, water and sulfur. Although the sample was taken at only one point on Jupiter's vast surface, it is likely to be typical of the whole
planet. Jupiter is one of the large planets known as gas giants. It has no solid surface, being composed of gas and liquid, although it has a rocky core.

The close-up views of the planets' satellites often proved to be just as exciting as the planets themselves. In the case of Jupiter, its larger moons Io, Europa, Ganymede and Callisto all have interesting features. Io was found to be extremely active volcanically, with molten lava and great plumes of sulfur ejecting from numerous volcanoes and rising to heights of up to 300 miles (500 km) above the surface. Europa by contrast was found to have an icy surface. The images showed brown streaks on the surface typically about 12.5 to 25 miles (20–40 km) wide and the
Galileo
spacecraft was able to see a cracked surface with the appearance of ice floes, suggesting evidence of liquid water beneath the ice.

Ganymede is the largest satellite of Jupiter and the largest in the solar system, with a diameter greater than that of Mercury. It has an iron-rich core with a permanent magnetic field, a rocky mantle with a thin atmosphere and an ocean of liquid water deep beneath the surface. Ganymede takes only 7.2 days to circle around Jupiter in a synchronous orbit. The probes were able to detect changing magnetic fields and electric currents near
the surface, and the most likely cause of this would be an ocean of salt water. There are deep furrows in the icy crust and these can be explained in terms of a system of tectonic plates similar to the Earth's crust. The evidence for liquid water on Europa is even stronger and the possibility that both Ganymede and Europa could support some form of primitive life has created great excitement.

Callisto is the outermost of the true satellites of Jupiter, and it takes 16.7 days to complete an orbit. It has a thin atmosphere of nitrogen and carbon dioxide, and like Ganymede and Europa it may have liquid water under its icy surface. One of Callisto's most striking features is a set of concentric rings left behind by a huge impact millions, or perhaps billions, of years ago. The crater was named “Valhalla” and the impact was so great that the outer rings are 1864 miles (3000 km) in diameter, and in some places huge spires of rock up to 100 meters (328 ft) in height have been thrown up. The source of heat to retain liquid water in such a cold part of space is probably created by radioactive decay inside the crust and the mantle of the moons. However, the surface of the planet is very cold; probes have measured 155 °K (−118 °C) at noon and 80 °K (−193 °C) at night. With the advantage gained by the use of space probes, scientists believe Jupiter has at least 63 satellites. This number will doubtless rise when more sensitive probes make the journey to Jupiter
and the outer planets. Many of the satellites are not true moons but captured asteroids—identified by the fact that they are irregular ovals rather than true spheres.

The planet Saturn has also revealed some of its secrets to the space probes
Voyager
and
Cassini
. The number of known satellites of Saturn is now 60, but only seven of these are large and spherical. New details of the ring system were revealed, showing the small “shepherding” satellites called Prometheus and Pandora confining the particles on what is known as the F band to a fixed orbit. The rings consist mostly of lumps of ice, but some of the particles have a rocky core. The thickness of the rings proved to be a mere 10 meters (33 ft); they are not visible from the Earth when they are viewed edge on.

Saturn's largest moon is Titan. It was discovered by Christiaan Huygens (1629–95) in 1655. Titan is heavy enough to retain an atmosphere, of which 90 percent is nitrogen, and the rest is mainly methane and other hydrocarbons. The
Cassini
spacecraft showed that the surface of Titan is partially liquid and free of craters, and that it is still undergoing dynamic changes. There has been much discussion about the possibility of Titan supporting life, due to the presence of complex organic molecules on its surface and in its atmosphere, but one of the main
stumbling blocks is the very low surface temperature of only 95 degrees above absolute zero. For the necessary chemical reactions to create life, much higher temperatures are probably needed.

Probes have also visited Uranus and Neptune. In 1986
Voyager 2
found Uranus to be a very featureless world, but the Hubble Space Telescope discovered a system of belts and zones. The rings of Uranus had been discovered in 1977 during an occultation (eclipsing) of a star by the planet.
Voyager
enabled a further study, showing them to be different from those around Jupiter and Saturn, and perhaps formed more recently.
Voyager
discovered ten additional small moons including some small ring-shepherding satellites similar to those of Saturn. The most interesting moon is Miranda; the
Voyager 2
image shows a world that underwent a shattering collision millions of years ago, Miranda has re-formed into a spherical shape, but with deep valleys and cliffs twice the height of Mount Everest.

The planet Neptune also has a ring system. The largest moon is Triton, discovered in 1846 and with a circular but retrograde motion around its planet. The tidal forces on Triton are so great that in the near future it could break into many small pieces—this would create a very spectacular ring around Neptune to rival those around Saturn.

For many years after its discovery in 1930 by Clyde Tombaugh (1906–97), Pluto was considered to be the most distant planet in the solar system. But in 2006 a formal decision was made to downgrade this icy little world with a diameter of 1,490 miles (2,390 km) and a mass of about 2 percent of that of the Earth from the status of a main planet to that of a dwarf planet. This decision was made in the light of the identification of several new solar system bodies similar in density, orbit and size to Pluto. The largest of these, Eris, was discovered in 2005 by a team led by the American astronomer Michael Brown (b. 1965). With an estimated diameter of at least 1,550 miles (2500 km), it is larger than Pluto. Ceres, the largest of the asteroids in the belt that lies between Mars and Jupiter, is also now classified as a dwarf planet. None of these dwarf planets has yet been visited by any space probe, but this will be rectified in 2015, when the
Dawn Mission
visits Ceres, and the
New Horizons
probe reaches Pluto.

One of Pluto's three moons, called Charon, has a diameter of nearly 1243 miles (2000 km) and is only about 12,426 miles (20,000 km) away, so it may claim to be a double planetary system. Pluto's orbit passes inside the orbit of Neptune, and it takes 248 Earth years for Pluto to orbit around the Sun.

Beyond the orbits of Neptune and Pluto are many chunks of rock and ice, numbering several billions. These chunks of rock and ice are called comets. Some of them are located in a ringed area called the Kuiper Belt and others are found in an approximately spherical region called the Oort Cloud. When the orbit of one of these comets takes it near the Sun, the Sun's heat melts the ice, and as it evaporates it forms the familiar spectacular “tail” sometimes visible to the naked eye. The Kuiper Belt was named after the American astronomer Gerard Kuiper (1905–73), who proposed the existence of the belt in 1951. It is believed that there are about 200 million comets in the Kuiper Belt. The Oort Cloud, where most comets are thought to exist, was discovered by the Dutch astronomer Jan Oort (1900–92) in 1950.

The space age has also seen the development of instruments that can be used closer to home but which can be employed to study objects in deep space. In 1990 the space shuttle
Discovery
launched the Hubble Space Telescope (HST). It was a successful launch, but soon afterward there was great dismay when it was discovered that the 2.4-meter (7.8 ft) objective mirror was flawed and a haze surrounded all of the star images. It was three
years before the defect could be corrected, but the mirror was successfully upgraded in 1993 and the telescope was enhanced again in 2002. After the second upgrade the resolution of the telescope was a tenth of a second of arc. This meant that the telescope had the power to see something the size of a 1-centimeter (0.4 in) diameter coin at a distance of 12.4 miles (20 km). The HST is due for a fifth and final upgrade that is planned to extend its life by several more years.