Exploring Small Bodies in the Solar System

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Just two decades ago, scientists did not have very much specific information about the small bodies in the solar system, such as comets and asteroids. There was a great deal of speculation about the true nature of a comet's nucleus, and no one had ever seen the surface of an asteroid up close. All that changed very quickly when robot spacecraft missions flew past, imaged, sampled, probed, and even landed on several of these interesting celestial objects. This section of the chapter discusses NASA's Stardust spacecraft and its comet sampling missions, which represents one of the most significant small-body missions that have taken place.

Asteroids and comets are believed to be the ancient remnants of the earliest years of the formation of the solar system, which took place more than four billion years ago. From the beginning of life on Earth to the spectacular collision of comet Shoemaker-Levy 9 with Jupiter (in July 1994), these so-called small bodies influence many of the fundamental processes that have shaped the planetary neighborhood in which Earth resides.

Scientists currently believe that asteroids are the primordial material that was prevented by Jupiter's strong gravity from accreting (accumulating) into a planet-size body when the solar system was born about 4.6 billion years ago. It is estimated that the total mass of all the asteroids (if assembled together) would comprise a celestial body about 932 miles (1,500 km) in diameter—an object less than half the size (diameter) of the Moon.

NASA's Galileo spacecraft was the first to observe an asteroid close up, flying past main-belt asteroids Gaspra and Ida in 1991 and 1993, respectively. Gaspra and Ida proved to be irregularly shaped objects, rather like potatoes, riddled with craters and fractures. The Galileo spacecraft also discovered that Ida had its own moon—a tiny body called Dactyl in orbit around its parent asteroid. Astronomers suggest that Dactyl may be a fragment from past collisions in the asteroid belt.

A comet is a dirty ice rock consisting of dust, frozen water, and gases that orbits the Sun. As a comet approaches the inner solar system from deep space, solar radiation causes its frozen materials to vaporize (sublime), creating a coma and a long tail of dust and ions. Scientists think that these icy planetesimals are the remainders of the primordial material from which the outer planets were formed billions of years ago. As confirmed by spacecraft missions, a comet's nucleus is a type of dirty ice ball, consisting of frozen gases and dust. While the accompanying coma and tail may be very large, comet nuclei generally have diameters of only a few tens of miles (kilometers) or less.

The primary objective of NASA's Discovery class Stardust mission was to fly by the comet P/Wild 2 and collect samples of dust and volatiles in the coma of this comet. NASA launched the Stardust spacecraft from Cape Canaveral Air Force Station, Florida, on February 7, 1999, using an expendable Delta II rocket. Following launch, the spacecraft successfully achieved an elliptical, heliocentric orbit. By midsummer 2003, it had completed its second orbit of the Sun. The spacecraft then successfully flew by the nucleus of comet Wild 2 on January 2, 2004. When Stardust flew past the comet's nucleus, it did so at an approximate relative velocity of 6.1 kilometers per second. At closest approach during this close encounter, the spacecraft came within 155 miles (250 km) of the comet's nucleus and returned images of the nucleus. The spacecraft's dust-monitor data indicated that many particle samples were collected. Stardust then traveled on a trajectory that brought it near Earth in early 2006. The comet material samples that were collected had been stowed and sealed in the special sample storage vault of the reentry capsule carried on board

This artist's rendering shows NASA's Stardust spacecraft encountering comet Wild 2 (in January 2004) and collecting dust and volatile material samples in the coma of the comet. The robot spacecraft collected, stowed, and sealed the comet material samples in the special storage vault of an Earth-return reentry capsule, which was also carried on board Stardust. On January 15, 2006, the sample capsule successfully returned to Earth. (NASA/JPL)

This artist's rendering shows NASA's Stardust spacecraft encountering comet Wild 2 (in January 2004) and collecting dust and volatile material samples in the coma of the comet. The robot spacecraft collected, stowed, and sealed the comet material samples in the special storage vault of an Earth-return reentry capsule, which was also carried on board Stardust. On January 15, 2006, the sample capsule successfully returned to Earth. (NASA/JPL)

the Stardust spacecraft. As the spacecraft flew past Earth in mid-January 2006, it ejected the sample capsule. The sample capsule descended through Earth's atmosphere and was recovered successfully in the Utah desert on January 15, 2006.

In inspecting the samples of comet material obtained by the Stardust spacecraft, scientists were able to confirm some anticipated results but were also surprised. For example, the returned samples show high-temperature materials (like olivine, the green Hawaiian beach sand) from the coolest parts of the solar system. It now appears that comets are really a mixture of materials formed at all temperatures, characteristic of places near the Sun and very far away from it. The olivine components included iron, magnesium, and other elements. The samples from comet Wild 2 included other high-temperature materials that contain calcium, aluminum, and titanium, as well as the silicate mineral forsterite, which can be found on Earth in gemstones called peridot. The preliminary analyses performed by approximately 200 scientists from around the world resulted in a general consensus that many of the comet particles are constructed like loose dirt clods, that is, composed of both large strong rocks as well as fine powdery materials. One of the most exciting preliminary conclusions is the suggestion that the comet is a mix of both stardust grains from other stars as well as materials formed in the solar system. This hypothesis would explain the puzzle of how the collected comet samples could contain some of the "hottest materials found in the coldest places." If this preliminary interpretation of the data is correct, then NASA's selection of the name Stardust for both the spacecraft and mission would have proven very appropriate.

Extraterrestrial Contamination: War of the Microbiological Worlds

In general, extraterrestrial contamination is the contamination of one world by life-forms, especially microorganisms, from another world. Using the Earth and its biosphere as a reference, this planetary contamination process is called forward contamination if an extraterrestrial sample or the alien world itself is contaminated by contact with terrestrial organisms, back contamination if alien organisms are released into the Earth's biosphere.

Here on Earth, an alien terrestrial biological species will usually not survive when introduced into a new ecological system because it is unable to compete with native species that are better adapted to the environment. Once in a while, however, alien species actually thrive because the new environment is very suitable and because indigenous life-forms are unable to defend themselves successfully against these alien invaders. When this "war of biological worlds" occurs, the result might very well be a permanent disruption of the host ecosphere, with severe biological, environmental, and possibly economic consequences.

Of course, the introduction of an alien species into an ecosystem is not always undesirable. Many European and Asian vegetables and fruits, for example, have been introduced successfully and profitably into the North American environment. However, any time a new organism is released in an existing terrestrial ecosystem, a finite amount of risk is also introduced.

Frequently, alien organisms that destroy resident species are microbiological life-forms. Such microorganisms may have been nonfatal in their native habitat, but once released in the new ecosystem, they become unrelenting killers of native life-forms that are not resistant to them. In past centuries on Earth, entire human societies fell victim to alien organisms against which they were defenseless, as were, for example, the rapid spread of diseases that were transmitted to native Polynesians and American Indians by European explorers.

But an alien organism does not have to infect humans directly to be devastating. Who can ignore easily the consequences of the potato blight fungus that swept through Europe and the British Isles in the 19th century, causing a million people to starve to death in Ireland alone?

In the space age, it is obviously of extreme importance to recognize the potential hazard of extraterrestrial contamination (forward or back). Before any species is introduced intentionally into another planet's environment, scientists must determine carefully not only whether the organism is pathogenic (disease causing) to any indigenous species but also whether the new organism will be able to force out native species—with destructive impact on the original ecosystem. The introduction of rabbits into the Australian continent is a classic terrestrial example of a nonpathogenic life-form creating immense problems when introduced into a new ecosystem. The rabbit population in Australia simply exploded in size because of their high reproduction rate, which was essentially unchecked by native predators.

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