NASA's Stardust Mission: Returning Comet Samples to Earth
By Dr. Donald Brownlee
Stardust Principal Investigator

Why Explore Comets?

An artificially colored composite of an image of the nucleus of comet Wild2 superimposed on a longer exposure image of the jets of materials leaving the comet.

Over a lifetime, most people are fortunate enough to see at least a few bright comets grace the night sky. The bright ones, along with their more numerous faint siblings, are mysterious travelers from the frigid and most distant parts of the solar system. When a comet approaches the sun, its surface warms and it dramatically becomes brighter. To an observer on Earth, a bright comet may be seen over a period of weeks or months. It drifts across the constellations, gets brighter from night to night and then slowly fades to invisibility after rounding the sun and heading back into to the depths of space. Over the past half century, Hale-Bopp, West, Hyakutake, Ikeya-Seki and Bennett have all been bright comets that could be easily seen with the naked eye, sometimes even by observers looking into the brightly lit skies of modern cities. With the unaided eye, we see comets as objects with a hazy head and one or two nebulous tails stretching away from the Sun. Only rare comets are truly bright and most appear as mere ghostly visitors, clearly out of place among the familiar sky of stars and planets.

It is ironic that comets are rarely seen because they are actually the most abundant bodies that orbit the sun. They only seem to be rare because we usually only see them when they are in the inner solar system and close to Earth. Over a trillion of them are believed to exist in two distant regions. One region is called the Kuiper Belt, a flattened annular ring just beyond the planet Neptune, and the other is the Oort Cloud, a spherical cloud that extends outwards to a distance roughly 50,000 times larger than distance between the Earth and the Sun. Although these bodies are usually far away they still orbit the sun. An additional irony of comets is that the ones that that come into the inner solar system and become bright and active, are actually near the ends of their long lives as solar system bodies. Even if they survive the rigors of being heated near the sun, they are exposed to gravitational encounters with planets that cause them to be ejected from the solar system. They are ejected and become frigid relics drifting among the stars for a very very long time. Even after the Sun has burned itself out, billions of years from now, vast numbers of our ejected comets will still be wandering our Milky Way Galaxy. Even when all of the stars of the Milky Way have finally burned out, these bodies will still be there wandering the darkness of space.

Stereo image of the comet taken near the closest approach. To view the 4.5 kilometer diameter comet in stereo, use red and blue (or red and magneta) glasses with blue covering the right eye.

In Greek and Roman times, comets were known as "hairy stars". Images taken with modern telescopes show them to be spectacular with a head (called the coma) up to ten times the diameter of Earth and complex tails that sometimes stretch away from the Sun for more than 150 million kilometers, the distance between the Earth and the Sun. The coma and tails are composed by gas and dust that was liberated from the comet by solar heating. The activity of comets that produces the coma and tail, usually increases dramatically as they enter the inner regions of the solar system and their surfaces are baked by the intense heat of the sun. Far from the Sun, comets are just cold inactive bodies. As they approach the sun, their interiors stay cold but their black-as-coal surfaces become quite hot. At the Earth's distance from the sun, you would burn your hand it you touched the sun-facing side of a comet. Solar heating causes water ice and less abundant frozen volatiles such as methanol and carbon monoxide to vaporize and the liberated gas molecules stream outwards from the surface at near supersonic speed. Dust and rocks that were glued together by ice are released and driven outwards by the wind of escaping gas. The rocks and dust travel at low to moderate speeds but the gas soon becomes electrically charged and is blown away from the Sun by the solar wind at speeds of about 0.1% of the speed of light. The gas and dust tails point in somewhat different directions because of the physics that drives them. The gas blows in the direction that it is pushed by the solar wind, and the dust and rocks move in a path controlled by the Sun's gravity and the pressure of its light.

A stereo view of the west edge of comet as the spacecraft was on approach. The large depression near the bottom is Shoemaker Basin.

As beautiful as they are, when you usually see a comet you are not actually seeing it. The tails and coma are not the comet at all, they are what is what is produced when a comet is in the process of being destroyed by the heat of the Sun. An active comet loses tons of matter each second and the escaping debris is what is seen, not the comet itself. Typical comets are small irregular black bodies of rock and ice only a few kilometers across. Even if you have a telescope large enough to detect light directly reflected from the comet's surface, you usually cannot see it because of its enshrouding smog-like coma. Bright comets are bright because of the fluorescence of their escaping gas and sunlight reflecting off escaping dust. When you observe a comet it is somewhat like driving home and seeing your house, unexpectedly from a great distance, because it is on fire and producing great clouds of smoke. When a house burns or a comet produces dust, the total surface area of the smoke or dust is much larger than that of the body that produced it and therefore is much easer to see from a distance. This "amplification effect" is a result of the fact that small particles have large surface areas in comparison to their mass.

Comets are wonderful solar system objects but much of their physical makeup remains a mystery. They contain near-surface ice and volatiles that sublime to gaseous state when close to the Sun. This process also liberates vast numbers of solid materials ranging in size from dust much too small to see with the naked eye to boulder-sized objects. Comets are beautiful and they are mysterious but they are also of extraordinary scientific value. They are cold bodies that formed near the outer edge of the solar system. Some comets are so volatile that they begin to evaporate near the orbit of Saturn (ten times further from the Sun than we are). These primitive bodies have been largely preserved at very low temperatures. At the very minimum, they contain regions that have not been heated above cryogenic temperatures since the formation of the sun and planets 4.5 billion years ago.

Stereo close-up of Shoemaker basin with a large pinnacle rising from its center. The Sun is on the right and the column-like pinnacle casts its shadow on the wall on the left. Mesas, flat topped hills surrounded by clifts, can be seen on both the left and right sides of the picture.

It is widely believed that comets are the best bodies for preserving the very materials that the solar system was built from. Like a fantastic library, they have stored preserved materials and records of our formation. The gaseous emissions can be analyzed from a distance either by telescopes on or orbiting Earth or by instruments on spacecraft sent to comets. The dust and rocks are another matter. They are believed to be samples of the solid building blocks of the solar system, samples of the solids that began the process of collision and sticking in the early history of the solar system that began with dust and produced ever larger bodies, and ultimately whole solid planets like Earth, Mars and Pluto. Some information can be obtained by telescopic study of the dust but the real secrets of the material cannot be examined remotely. On scales from kilometers to microns (a hair is about 100 microns in diameter) primitive materials just look like black charcoal. They are black due to their high content of carbon and other light absorbing materials and most of their components are so small that they cannot be individually seen by the naked eye. Like books in a library, they have to be opened and read but in this case the words are small, very small. So small they can only be read by large microscopes and other instruments that are too heavy, complex and power hungry to be put onto robotic spacecraft.

Stereo view of the flat-floored depression called "left foot". The cliffs are 140 meters high.

The fine dust and rocks in comets are best studied on Earth where complex instruments can be used to fully study the materials at scales approaching atomic spatial resolution. The need for laboratory study of samples from the Solar System's most primitive bodies, was the inspiration for the NASA Stardust mission, the first NASA mission to return samples from space since the Apollo 17 Lunar mission in 1972. Stardust was launched in 1999 with the goal of flying past the comet Wild 2, collecting thousands of dust particles and returning them to Earth. After return, the samples will be studied, in intimate detail, by researchers around the world using the most sensitive and accurate instruments in existence. The samples will be studied to learn about the fundamental nature of interstellar grains (stardust) and other solid materials that assembled to form the solar system 4.6 billion years ago.

The particles are collected by impact into an exotic glass called aerogel. Like sand it is composed of silicon dioxide but unlike other solid materials, it is extremely lightweight. Although it is a solid that can be held in your hand, its density is more similar to air than to normal solid material. During the comet flyby, comet dust hits the crystal-clear aerogel and imbeds deep inside forming tracks shaped like carrots with the captured particle located at the tip. Aerogel is the lowest density solid that has ever been made and one of its nicknames is "frozen smoke".

Encountering a Comet

Stereo view of Rahe, a feature that is believed to be an impact crater.

On January 2, 2004, after five years in space and billions of kilometers of travel, Stardust finally reached its target for a brief but daring encounter. The spacecraft flew within 236 km of the comet Wild 2 and survived the high speed impact of millions of dust particles and small rocks up to nearly half a centimeter across. With its tennis racket shaped collector extended, Stardust captured thousands of comet particles that will be returned to Earth on January 15, 2006.

In addition to collecting samples, Stardust also took pictures during its flyby. The images were startling because they showed a surface that is unlike anything previously seen in space. The camera's mirror tracked the comet during the flyby enabling the camera to take images over a full range of angles. The 4.5 km comet is shaped like a hamburger and the approach and exit images provided edge-on views. Near the closest approach, the broadest face of the comet was seen. The long exposure images showed sunlight reflecting off jets of dust projecting into space. We were expecting one or two jets but we saw 20. Jets are produced when gas escapes from localized regions and carries dust and rocks outwards. The low-density gas is invisible but the entrained dust scatters sunlight making it visible. The large number of jets shows that gas sources on Wild2 are numerous and probably small. Dust detectors on the spacecraft measured bursts of impacts when Stardust flew through jets.

The camera also took short exposure images that were ideal for imaging features on the comet's surface. Before Stardust, only spacecraft had imaged the surfaces of two comets, Halley and Borrelly. Both of these comets had elongated shapes, similar to peanuts, and their surface features were relatively subtle. Unlike the previous comets, Wild 2 was seen to have a slightly flatten shape and its surface was covered with dramatic features. The very first image that was downloaded showed two spectacular depressions, a kilometer across, with steep walls and flat floors. These large depressions resemble sinkholes seen on Earth but their origin is still a mystery. They might be impact craters, they might have formed because of internal processes or they may have formed by surface processes related to the volatilization of ice.

Stereo view of the comet and its jets. Note that two of the jets on the right come from the comet's dark side.

There were two expectations about the nature of the comet's surface. If the surface was billions of years old we expected to see a cratered surface like that of the Moon and asteroids. If the surface was young, the old surface removed by vaporization and solar heating effects, we expected to see a smooth, somewhat featureless surface. One thought was that a young surface on an active comet would be smooth because valleys and hills would be covered by "fall-back material" that had been ejected skyward but then fell back. Old or new, its was also anticipated the comet's surface would be composed of a "rubble pile" of fine sand-like debris loosely held together. What we saw was a very rough surface that was not made of loose material. It has strength and some areas have vertical cliffs and even overhangs. We also saw that much of the comet's surface is covered with depressions. We believe that some of the depressions are impact craters but they are unusual and not a single depression, anywhere on the comet, is similar to typical impact craters seen on the Moon, Mars, satellites and asteroids. Normal impact craters on solar system bodies are bowl-shaped depression surrounded with raised rims. One of the comets features called Rahe, does appear to be an impact crater and it has a bowl-shaped central pit but it is oddly surrounded by a very jagged halo that appears to have been blasted off the comet. This morphology has never been seen or other bodies for such large craters. Other crater-like depressions have flat floors lined with vertical cliffs and some have oddly non-circular shapes.

A major surprise was the observation of ridges, mesas (flat toped hills bounded by cliffs) and pinnacles rising more than 100 meters above their surroundings. It is likely that these features are remnants left after the loss of more than 100 meters of original surface. The pinnacles are column-like features that have never been observed on other solar system bodies, other than Earth.

The flyby was a great success not only in collecting particles from a dangerous environment but also in providing a wealth of information on the comet and its evolution during its long history of travel in the solar system. The remarkable surface of this body is the result of billions of years of residence beyond the orbit of Neptune and a brief recent history inside the orbit of Jupiter.

The sampled comet formed near Pluto at the very edge of the solar system, and it only recently entered the inner solar system where it could be approached for sampling. Comets like Wild 2 are a special interest to astrobiology because they are preserved samples of the fundamental building blocks of the solar system. All planetary systems are likely to also contain cometary bodies composed of ice, organics and rock. For planets like Earth, which formed close enough to a star to be in its warm habitable zone, comets provide a source of organic material and water that continually impact from distant regions of the planetary system. Most of the atoms in our planet and our bodies were at one time contained in particles of the type that are released by comet Wild 2. As stated in a famous song, quite literally "we are stardust".

The Stardust spacecraft is now speeding home and its sample return capsule will land in the desert near Salt Lake City Utah in the early morning of January 15, 2006.

Last updated November 14, 2005