The planets were the first and most obvious targets for spacecraft exploration. Once they had been visited, scientists turned some of their attention to the smaller bodies of the Solar System, so-called “planetesimals” including comets and asteroids. Most of this exploration did not begin in earnest until the 1990s. But now the United States, Europe, and Japan are devoting increased emphasis to these subjects, because they offer clues to the formation of the Solar System.

There are generally two categories of asteroids, those in the Asteroid Belt and those classified as Near-Earth Objects, whose orbits cross Mars or even Earth's path around the Sun and may pose a collision threat to Earth itself. Both categories are similar in composition—either rocky or metallic—and of interest to scientists. The Asteroid Belt is a region between the orbits of Mars and Jupiter. Unlike in movies and television, real asteroids are not bunched close together, constantly colliding with each other, but spread far apart and one is not visible from another. Only a few thousand are more than a kilometre in diameter.

Unlike asteroids, comets are not rocky or metallic but primarily “dirty snowballs,” are less dense than water, and travel in highly eccentric orbits causing them to sweep into the inner Solar System and then back out into deep space. As they get nearer the Sun (usually inside the orbit of Saturn), their heat causes their surfaces to vaporize and fly off in a long thin gaseous “tail.” They have thus been some of the most enigmatic objects in the sky and have terrified observers for thousands of years. Scientists have been particularly intrigued with them because they represent some of the original building blocks of the Solar System and because comets may be the source of Earth's water as well as its organic matter.

The first visit to an asteroid was made in October 1991, when NASA's space probe Galileo flew past the asteroid 951 Gaspra on its way to Jupiter. In August 1993 Galileo flew past another asteroid, 243 Ida. Galileo discovered that Ida had a spherical 1.5-kilometer-wide satellite later named Dactyl. Later, astronomers observing from the ground discovered that the asteroid known as 4179 Toutatis was actually a double object. Because of the discovery of Dactyl and Toutatis, planetary astronomers now believe that asteroids with their own tiny satellites are common. Some scientists even think that asteroids could have two or three tiny moons in orbit around them, although none have been discovered so far.

 
This is the first full picture showing both asteroid 243 Ida and its newly discovered moon to be transmitted to Earth from the National Aeronautics and Space Administration's (NASA's) Galileo spacecraft --the first conclusive evidence that natural satellites of asteroids exist.

The second mission to fly by an asteroid was not as lucky as Galileo. The small Department of Defence technology demonstration spacecraft Clementine was built for the Ballistic Missile Defence Organization (BMDO) to test advanced systems such as star tracking devices. It conducted a successful lunar mapping mission and was headed toward asteroid 1620 Geographos in 1994 when it malfunctioned. It never reached its target.

The most extensive asteroid mission was made by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft, which was launched in 1996. NEAR successfully flew by the asteroid 253 Mathilde on its way to its primary target, the asteroid Eros. The spacecraft eventually entered orbit around Eros on February 14, 2000, becoming the first spacecraft to orbit around an asteroid. NEAR took spectacular photographs of Eros for a year. Although NEAR had no landing legs and was never intended to land, controllers gradually lowered its orbit and it made a risky “soft-landing” on Eros in February 2001. This was the first landing of a spacecraft on a body other than the Moon, Earth, Venus, or Mars. The mission was successfully ended on March 1, 2001.

Deep Space-1 was a NASA technology demonstration spacecraft launched in October 1998, and intended to test a number of new technologies for planetary exploration, such as an ion drive engine and solar panels equipped with focusing lenses to provide more power. Despite its main mission, DS-1 also managed to conduct some worthwhile science during its journey through the Solar System. In July 1999, it flew past the asteroid 9969 Braille and in September 2001, past the comet Borrelly. It was finally shut off in December 2001, but the technology that it proved is now being incorporated into many more planetary missions. Japan's Muses-C spacecraft will launch in late 2002, rendezvous with asteroid 1998 SF36, and collect surface samples to return to Earth.

During the 1970s, comets were monitored by satellites in Earth orbit. The first robotic spacecraft visit to a comet was made by the NASA spacecraft called International Sun-Earth Explorer 3 (ISEE-3), originally designed to study the interaction of the solar wind and the Earth's magnetic field. ISEE-3 was renamed the International Cometary Explorer (ICE) and sent to the comet Giacobini-Zinner. It flew through the comet's tail in September 1985. It did not have a camera, however, but detected unexpected particle and field events in the tail of the comet and studied the way that the solar magnetic field draped over the comet nucleus.

Six months later, a flotilla of five spacecraft reached the famous Halley's comet, which made an ideal target because its orbit was accurately known and countries could plan missions to visit it long in advance. The spacecraft that visited Halley were the European Space Agency's Giotto, Japan's Suisei and Sakigake, and the Soviet Vega 1 and 2. These spacecraft were able to measure how much water and dust Halley was losing each hour and collect impressive images of the tail and nucleus.

 
Comet Halley as taken with the Halley Multicolour Camera on the ESA mission Giotto.

After that flurry of activity surrounding Halley, there was a long drought before the next comet flyby, but comets will soon come under intense study by a number of spacecraft. The reasons include a change in scientific priorities, improved technology, and cheaper planetary spacecraft built and launched in much less time than before.

The NASA spacecraft Stardust, launched in February 1999, will make three loops around the sun and will spend 150 days collecting particles of interstellar dust. But its ultimate goal is to pass through the tail of comet Wild-2, collecting samples. It will collect these particles in a wispy material known as an aerogel and return them to Earth in a re-entry capsule. They will be carefully isolated and studied. (Another NASA mission, named Genesis and launched in August 2000, will use the same technique to return particles from the solar wind.)

The Comet Nucleus Tour (CONTOUR) mission launched in July 2002. It will slowly fly past comet Encke in November 2003 and comet Schwassmann-Wachmann-3 in November 2003. It may also rendezvous with comet d'Arrest in 2008, or another comet not yet discovered. Deep Impact will launch in January 2004 and rendezvous with comet Tempel 1 in July 2005. It will deploy a small probe that will impact with the surface at high velocity, creating a huge crater and blowing up a cloud of debris that can be analyzed and offering a view below the surface.