Illustration of CONTOUR mission. Photo: JHU-APL

Comets are some of the oldest objects in the solar system, celestial time capsules preserving a record of the chemical and physical processes at work when the planets began forming more than 4.5 billion years ago. Scientists believe comets may have brought to Earth some of the water in the oceans, some of the gases of our atmosphere, and even the life-generating molecules from which we arose. But we cannot examine these possibilities until we know more about comets and understand how individual comets are alike or different.

CONTOUR will visit and study at least two comets - assessing their diversity, detailing their properties, and tackling some of the many questions about how comets evolve.

The 'heart' of a comet
The heart of each comet is the nucleus: a jagged chunk of ice and rock, often only a few kilometers across. When a comet gets to within several hundred million kilometers of the Sun the nucleus heats up, its ices begin to evaporate, and the released bursts of gases and dust form a large, thin atmosphere called the coma. These conspicuous fuzzy parts of comets can extend thousands of kilometers from the nucleus. Sunlight sweeps the gases and dust back into a tail that can stretch millions of kilometers.

Comas and tails are big enough to study with Earth-based telescopes, but the nucleus is so tiny that we can study it only by getting close. So far, we've gotten close enough to see the nucleus of a comet only twice: the European Space Agency's Giotto spacecraft took color photos of comet 1P/ Halley's nucleus in 1986, and NASA's Deep Space 1 collected detailed images and data on comet 19P/Borrelly in 2001.

Comets are the most numerous sizeable bodies in the solar system; current estimates place their number at a million million. Most comets remain where they formed - in the vast region of deep space that stretches beyond Neptune and Pluto - but some are bumped from their orbits and approach the Sun. Jupiter's gravity can further alter a comet's path, capturing it into an orbit that never extends far beyond the giant planet.

CONTOUR's planned targets - 2P/Encke and 73P/Schwassmann-Wachmann 3 - belong to Jupiter's "family" of comets. Encke and SW3 are among the short-period comets, meaning they take less than 200 years to orbit the Sun. CONTOUR will fly past each at the peak of its activity, taking images and spectral maps of the nucleus while measuring and analyzing the surrounding gas and dust.

CONTOUR's key measurements

• Image the nucleus at resolutions of 4 meters per pixel -10 times better than Deep Space 1 and 25 times better than Giotto - to reveal details of morphology and processes that show how comets work;
• Determine nucleus size, shape, rotation, albedo/color heterogeneity and activity through global imaging (100-500 meters per pixel);
• Map the composition of the nucleus and coma;
• Obtain detailed compositional measurements of gas and dust near the nucleus;
• Assess the level of outgassing through imaging, spectroscopy, and gas and dust measurements.

CONTOUR's comets
CONTOUR's target comets were selected because of their diverse physical characteristics and proximity to Earth at encounter time. The spacecraft visits Encke and Schwassmann-Wachmann 3 when they're close to the Sun and at their most active. The comets will also be relatively close to Earth - less than 50 million kilometers (31 million miles) - and well situated in the night sky for astronomers worldwide to make concurrent observations from the ground.

First spotted in 1786, 2P/Encke has been observed at more apparitions (approaches to the Sun) than any other comet. It has the shortest orbital period of any known comet, circling the Sun every 3.2 years. Encke might be the most famous comet after Halley - American astronomer and CONTOUR scientist Fred Whipple devised the "dirty snowball" model of a comet nucleus after studying Encke more than 50 years ago.

Encke is thought to be about 8 kilometers (5 miles) long, with an average radius of 2.5 kilometers. Encke is an "old" comet that gives off relatively little gas and dust, but it remains more active than scientists expect for a comet that has made thousands of close passes around the Sun.

73P/Schwassmann-Wachmann 3 was discovered in 1930 and considered predictable by most astronomers - until it passed close to the Sun in the mid-1990s and split into several pieces. One fragment has already disappeared; the largest of the remaining sections may be no more than 2 kilometers across. A relatively young and active comet - especially when compared to Encke - SW3 takes about 5.5 years to orbit the Sun. When CONTOUR reaches SW3 in June 2006, mission scientists hope to get a great look at the comet's fresh, unaltered surfaces and evidence of the material inside the nucleus.

The 'new' comet
The mission team can use any Earth swingby (after the Encke encounter) to send CONTOUR toward a new target. This flexibility may allow it to study a newly discovered long-period comet making one of its first passes around the Sun, as the bright, spectacular comet Hale-Bopp did in 1997.

Comparable data
CONTOUR's data will cover much of the ground addressed by Giotto at Halley and Deep Space 1 at Borrelly, providing key insights into the differences between Halley-like comets and shorter-period comets such as Encke and SW3. CONTOUR and NASA's Stardust spacecraft carry the same dust analyzer, offering a direct comparison of the missions' data.

Near their encounter times, Encke and SW3 will be close enough to Earth for astronomers worldwide to observe them from the ground. CONTOUR's high-resolution, near-nucleus observations can be linked with pictures, spectroscopic and thermal data obtained at broader scales by ground-based and Earth-orbiting telescopes.

True to any mission selected through the competitive NASA Discovery Program, the planetary science community played a key role in approving CONTOUR and its science goals. The team will further enhance the involvement of the science community with the early release of all data to NASA's Planetary Data System.