exploring other bodies
Comet Borrelly and its
topographical map (right) (Deep Space-1 mission).
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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.)
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.
more ambitious European comet mission, known as Rosetta (because comets
are the Rosetta stones of the Solar System) is scheduled for a January
2003 launch atop an Ariane-5 rocket. Rosetta will pass several asteroids
and then study the comet 46P/Wirtanen for two years, deploying a rover to
its surface. In 1999, NASA cancelled a similar mission known as Deep Space
4/Champollion. But comets remain near the top of NASA's exploration list.