Astronomy 101 Essay, College of Marin Caroline Webb
From an astronomy lecture at the Sonoma State University What Physicists Do Lecture Series
April 3, 2006
[To see this essay with pictures, download the PDF here]
The Stardust Mission
I arrived at the lecture by Dr Donald Brownlee a few minutes after it had started, with no knowledge of what I would be listening to—all I knew was that the talk was about comets. Little did I realize what a ride I was in for. This talk was about a spectacular mission to go out to the furthest reaches of our solar system and collect samples of dust from a comet, as well as from interstellar space. The Stardust Mission, I learned, was the first ‘Sample Return Mission’ from space since Apollo 17 in 1972, which brought back samples of moon rock for analysis in the lab, and its story is an outstanding accomplishment of space scientists and engineers.
In January 2006, Stardust succeeded in bringing back particles to the lab from somewhere beyond Pluto, having made an amazing journey lasting 7 years and covering some three billion miles. Out in the distant region of the solar system called the Kuiper Belt, a comet called Wild 2 had been selected for a visit because it was known to have only recently (1974) become a short-period comet and is therefore in pristine condition. Wild 2 originated in the Oort Cloud, which is a region forming a sphere around the solar system, filled with comets that make very long orbits around the Sun —a million years or more.
Dr Donald Brownlee is an Astronomer working out of Washington University and is a Principal Investigator for Stardust. He said that it was a ground-breaking development for samples of this sort to be brought back to Earth, as they are believed to derive from the Solar Nebula which transformed into our planetary system and Sun. He explained that this material could not be properly examined by robot methods far from Earth. It had to be brought back for highly sophisticated examination in large machines like the truly huge ‘Synchotron’ and in Electron Microscopes, to obtain really good quality information.
We learned that particles traveling at a speed of 6.1 km/second were collected from the comet’s coma and the method was to fly very close to the coma, exposing a special collector to the stream of particles leaving the comet. Shaped like a large ice cube tray, with a grid of receptacles filled with something called Aerogel, the particles were captured and brought to a halt inside this special material which is the least dense solid ever known. Further away from the comet, in a place known to contain particles streaming in from well beyond the solar system, the other side of this collector was used to pick up interstellar particles, giving us the opportunity to compare and contrast the primal matter of our galaxy.
Stardust Mission lifted off into space from Florida on Feb 7, 1999 and made a 7 year journey that involved three loops round the Sun, using gravitational assist from Venus and from Earth to pick up enough speed to get out to the edge of the solar system and rendez-vous with the comet in the Kuiper Belt. The spacecraft cost $40 million, which Brownlee said was a ‘budget’ priced rocket, although its Particle Collector cost a great deal more than that ($200 million). Its total journey, out and back, was 3 billion miles.
On January 2, 2004 Stardust reached its destination and flew past the comet, which is only 4 kilometers across. The mission scientists expected to see a fluffy ball of ice and dust but were very surprised to see that Wild 2 has an irregular shape, with distinct crater-type indentations on its surface. But these craters are flat-bottomed and could not have been formed from an impact with another object, so they are something of a mystery to the scientists. The temperature of the surface was found to be ‘room temperature’ but its interior is very cold.
Apparently, when Stardust first left Earth, it was not known exactly where the comet was. At 1.8 AU from the Sun, the comet was located by an onboard navigation camera, which then used instructions from Earth about star constellations to keep its target in view and maintain a path that led to the successful rendez-vous. The navigation camera came in handy for taking some snapshots of the comet once it arrived.
Brownlee emphasized the great challenge of getting the samples of dust back to Earth. The little capsule entered the Earth’s atmosphere at world record speed (28,000 mph) and landed in a top-secret military base in Utah. Luckily it did not hit any of the pieces of ordinance, which are scattered all over this desert region. So secret is this place that no one was allowed to take pictures of its re-entry but a NASA Ames plane was in the air, filming its passage as a very bright light descending to Earth and the movie is very dramatic.
Brownlee then spoke of the delicate task facing scientists to first of all locate the dust particles inside the gel of the collector, and then to analyze their composition. The particles made tracks in the gel, shaped a bit like carrots, and these tracks have now been sliced up into 50 nm thin slices for analysis of the chemical composition of the particles by a synchotron machine, while electron microscopes are used to look at the atoms. The findings so far have greatly surprised the scientists. Finding olivine, pyroxine and glass with embedded metal and sulfides, it seems as though the comet was formed at high temperatures—which is the opposite of what was expected. It raises the question of how hot the solar nebula was from which our solar system formed and whether this comet might have formed close to the Sun and was then flung out to the far distant Oort cloud or even if it might have formed round another star altogether.
Brownlee said that organic material had been found and that these were being studied by one of the 6 teams involved in particle analysis. Chromium, Vanadium and Titanium had been found and Brownlee said that they expected to find all the elements of the Periodic Table, although Uranium and Iron would be very rare.
Having listened to this fascinating lecture, I went online to the Stardust Mission website and did a lot more reading and downloading of pictures. The more I read, the more impressed I became. This story has a truly engaging dramatic appeal. The appeal comes not only from the detail of its story, which includes an episode of being caught in a solar wind ‘storm’, which for 24 hours or so put all the scientists into a fever of apprehension about their mission, but also from its deeper meaning.
We are seeking out the story of the formation of our solar system. It is somewhat like going back in our imagination to the moment at which our parent’s cells fused to become the fertilized egg from which we grew into existence as individuals. It sends a strange feeling down the spine. The Solar Nebula is a moment of origin for humans and all else that lives and breathes on Earth. We come out of that nebula just as Comet Wild 2 did (or so we think) and in seeking tangible contact with that original matter, we are discovering something about ourselves in the universe. We are exploring the process that gave rise to our existence. There has been no break in that process and there is nothing like going to visit the matrix from which all the planets emerged to really appreciate the immense continuity between ourselves as humans and the cosmos we now explore. We are an evolved form of that same original matter which comet Wild 2 comprises.
Thus, there is room for awe and wonder and profound appreciation in this Stardust Mission. The scientists may think of all this in private but their public pronouncements are strictly scientific. When we step back from that scientific detail and regard the whole story, we are able to see that this is, in a very real sense, the universe itself in pursuit of its history—its story of evolution. We did not get flown in to planet Earth. We evolved out of its body to become something new and something wonderful ourselves. Stardust Mission helps us all to appreciate the universe in a deeper, more personal way. The dust from stars has a new role to play.
©Caroline Webb
April 16, 2006
Web References
http://stardust.jpl.nasa.gov/home/index.html
http://stardustathome.ssl.berkeley.edu/
http://www.planetary.org/programs/projects/stardustathome/facts.html
http://www.berkeley.edu/news/media/releases/2006/01/10_dust.shtml
