Over the last few years, there have been huge strides in our understanding of comets. Comets are no longer just strange lights that streak across our sky. Now we are learning something about them, and the new information that has been gained is forcing some radical changes in our understanding of the early history of the solar system. Much of this is due to one highly successful NASA mission: the Stardust probe, which encountered the comet Wild 2 in 2004 and actually returned a sample of comet dust to Earth.
Whereas asteroids are protoplanets that were prevented by gravitational disturbance from reaching their maturity, comets come from an even earlier stage. They are actually frozen droplets of ice and dust from the original cloud that surrounded the early sun, from which the planets and all the other bodies in the solar system would later form. Comets are the solar system’s baby pictures, offering a view of the primordial material which grew into all that we see today. And unlike the material on larger bodies, the stuff of comets has not been changed by geological forces. It hasn’t been melted and reformed, as the matter on the planets has. By looking at it, we can get the earliest possible view of where everything else in the solar system came from.
Nasa’s Stardust, launched in February 1999, has been a huge success and a technological tour-de-force. It has already yielded revolutionary data on how and where comets form, and its mission has now been extended into a project called StardustNExT. The probe’s original mission was to capture dust from the comet Wild 2. To do this, it used a groundbreaking piece of technology that will certainly be used again on future missions: aerogel.
Aerogel is a substance that almost isn’t there at all; it is more than 99 per cent empty space. It looks like a faint fog, like your breath on a cold morning, but motionless. When particles of fast-moving dust hit a thick layer of aerogel, they penetrate the surface and get embedded inside, but they are not broken or deformed by the impact. The aerogel becomes a convenient storage medium, holding the dust particle until it is retrieved by scientists.
For the Stardust mission, the collection device was a metal framework resembling an oversized tennis racket, with blocks of aerogel in the squares of the grid. The probe passed through the comet’s coma with this framework extended, allowing bits of comet dust to strike the aerogel and get embedded. The samples obtained this way were sealed in a capsule and returned to Earth, but Stardust stayed in space and continued along its orbit.
This operation was tricky, and it was uncertain whether the samples could be returned to Earth undamaged. Because of aerogel’s fragility, there was a possibility that the substance would simply shatter from the stress of the landing. Tension was high when the capsule came down in the Utah desert, but when it was retrieved, scientists were relieved to find that their precious samples were undamaged.
Before these samples were obtained, scientists thought they knew what comets were made of. The conventional wisdom held that these bodies had formed on the distant edge of the solar system and had remained there ever since, frozen in cosmic cold storage. It was assumed that because of their extreme distance from the sun, comets had never received much heat or light.
But when some of the dust was retrieved from the aerogel and examined, scientists were amazed to find rounded particles called chondrules. These are already well known, as they are a common component of certain meteorites. They are bits of rock that melted, formed round bubbles and then solidified again.
Obviously these dust particles had not spent their entire existence beyond the orbit of Pluto.
Also found in the comet dust were Calcium Aluminum Inclusions (CAIs). These are thought to be the oldest solar system materials, and are composed of exotic substances that can only form under extreme heat.
The region of space where comets exist has always been extremely cold. It is absolutely impossible for chondrules and CAIs to form there. The NASA scientists were forced to admit something that sounded impossible: these particles were formed very close to the sun, under heat that could melt rocks. However, a large part of a comet’s mass is water ice, which obviously would not form in such heat. The inescapable conclusion was that the constituents of a comet did not form all at once, in the same place. Rather, the dust particles formed very near the sun, and then somehow migrated out to the edge of the solar system, where they became encased in ice.
While this result was not expected, even before this there were several theories which suggested that large-scale mixing may have occurred in the early solar system, with rocky particles forming very close to the sun and then migrating out to more distant regions. These results clearly confirm this, and indicate that the constituents of comets were formed in different places, and possibly over a very long period of time.
More recently, Stardust has given us another revelation. In their continuing examination of the collection device and the samples it contains, scientists have now found the amino acid glycine adhering to the aluminum grid that holds the aerogel blocks. Further analysis confirmed that this substance, which is one of the basic consitutents of life, has a different isotopic ratio from glycine that originates on Earth, and so must be of extraterrestrial origin. This lends support to the theory that life on Earth may have been seeded by amino acids from comets.
This is not the last we will hear from Stardust. The mission has now been extended into Stardust NExT, a mission in which the probe will perform a flyby of the comet Tempel 1, taking pictures and gathering other data. This is the comet that was deliberately struck by the Deep Impactor probe in January 2005. By the time Stardust arrives there in 2011, six years will have passed since that encounter, and scientists will be able to see if any surface changes have occurred in that time. They will probably also be able to see the impact crater caused by Deep Impact; this crater was obscured by dust at the time, and was never actually seen. When Stardust encounters Tempel 1, it will be the first time a comet has been visited twice by spacecraft.
Stardust will be telling us things for years to come. The dust samples will continue to be examined and analyzed, and the probe has new places to go. Watch this site for future updates.
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