Probing the asteroids

In October 1991, the NASA space probe Galileo took the first detailed photograph of an asteroid while en route to Jupiter. The asteroid photographed, named Gaspra, is an irregularly shaped object measuring about 19 by 12 kilometres. The Galileo probe also passed by another asteroid, Ida, in August 1993. Both Gaspra and Ida are classified as S-type asteroids because they are composed of metal-rich silicates.

Figure 8.4 Path of the Galileo space probe as it passed by the asteroid Gaspra.
Figure 8.5 The asteroid Gaspra as seen by the Galileo space probe. (Photo: NASA)

Both asteroids are probably fragments of larger parent bodies that were broken apart by catastrophic collisions. Ida's surface is more heavily cratered than Gaspra's, but Ida is much older. Ida has its own companion, Dactyl, which is 1.5 km long and orbits Ida at a distance of 100 km.

Figure 8.6 The asteroid Ida and its companion Dactyl, as seen by the Galileo probe. (Photo: NASA)

In 1996, NASA launched the Near Earth Asteroid Rendezvous (NEAR) space probe. This probe flew within 1216 km of the asteroid Mathilde in 1997. This encounter gave scientists the first close-up look of a carbon-rich C-type asteroid. This visit was unique because NEAR was not designed for fly-by encounters. The next year, NEAR flew past the asteroid Eros at a distance of 3829 km, and it went into orbit around Eros in February 2000. In March 2000, the probe was renamed NEAR-Shoemaker in honour of the American astronomer Eugene Shoemaker, who had died not long before. In February 2001, NEAR-Shoemaker became the first spacecraft to land on an asteroid when it landed on Eros.

In October 1998, NASA launched a probe called Deep Space 1. The probe flew within 26 km of the asteroid Braille in July 1999.

The Japan Aerospace Exploration Agency launched the probe Hayabusa on 9 May 2003. The probe's mission was to land on the surface of the asteroid Itokawa, and to collect samples and return them to Earth. The first attempt at landing failed, but the second, in September 2005, was successful. Problems with the probe's engines delayed the return flight and the probe is not expected to return to Earth until June 2011. On its return the probe will release its samples via a re-entry capsule. The capsule should land, using a parachute, near Woomera in South Australia.

In September 2007, NASA launched the DAWN spacecraft on a mission to the asteroid belt. Seeking clues about the birth of the solar system, the craft will encounter Vesta in 2011 and Ceres in 2015. Both these asteroids are believed to have evolved more than 4.5 billion years ago, about the same time Mercury, Venus, Earth and Mars were formed. Scientists believe the growth of these asteroids was stunted by Jupiter's gravitational attraction. Images taken by the Hubble Space Telescope show these two asteroids are geologically diverse, but mysteries abound. DAWN will orbit each asteroid, photographing the surface and studying their interior composition, density and magnetism. A gamma-ray and neutron detector on board will enable the chemical composition to be measured. A spectrometer will detect visible and infrared light to identify surface

Table 8.2 Significant space probes to asteroids


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