“...for you are dust, and to dust you will return” reads one of the most emotional quotations from the Bible (NET [New English Translation] Bible, Biblical Studies Press, Richardson, Texas, 2005). And it’s very correct. Today we know that there is some “celestial humus” that, together with a few gases, was the building stuff of Earth and all living beings. The search for it turned into an 11-year, 212-million-dollars project that, surprisingly, managed to obtain, far, far away in the sky, a little of this mythical substance. This is the story of that incredible feat.
In a dark night, if we stare at the popular Orion’s Belt we notice that to one side of it there are three other in-line stars. With binoculars, it is possible to see that the little star in the middle is not actually a star: it’s a gigantic cloud of dust and gases in space, much larger than the entire Solar System.
With big and sophisticated telescopes, one discovers that, inside the cloud, the gases and dust get together (due to the gravitational attraction of their atoms) until they form whirlpools. In the centers of these whirlpools, the cloud compresses itself to incandescence, and so stars appear. It is believed that the nearest star, the Sun, came to be from another cloud like this. And the material that did not fall into the Sun continued to spin around, and gradually began to agglomerate until it formed pebbles, and these joined forming enormous blocks, and these, in turn, joined to form the Earth… with everything on it, including ourselves.
THE DUST AT PRESENT TIME
In 1970, Donald Brownlee, an University of Washington professor, by means of balloons and special high-altitude airplanes, managed to demonstrate that small quantities of this “cosmic humus” still rain from the sky.
But the very best samples would be found in the region of Pluto and beyond. Due to the low temperatures there, the dust would be preserved like in a freezer, frozen with some gases of the ancient cloud, waiting for us. Unfortunately, our current capabilities are insufficient to go and get them from there.
But there is an alternative: due to perturbations in their orbits, many of these big icy blocks fall toward the part of the Solar System where we live. Here, the Sun’s heat melts the ices, and the gases and dust freed in this manner form a cloud that envelopes the body and stretches in the form of a tail. This small body is then known as comet.
In 1986, half-a-dozen ships, made by us, went out to intercept the best known of all comets: Halley’s Comet. They withstood terrible bombardments when they collided with the particles that emanated from the comet. Even though some analysis were performed by the onboard instruments, the spacecraft where not designed to bring particles home.
The desire to bring particles for more detailed studies (mineralogy; petrography; composition; magnetic, molecular and atomic structure; origin and history), in well-equipped laboratories, persisted. Then Don Brownlee and others prepared a proposal. They chose a comet that was new in this region of the Solar System, and has an “easy orbit”: comet 81P/Wild 2. In 1995, NASA decided to fund the project, all for the sake of science.
THE STARDUST SPACECRAFT
In order to catch the particles without damage, technicians chose a novel material: a special silicon gel that is 99 % air. Then, they build a ceramic capsule to bring the samples to Earth.
Meanwhile, the Max Planck Institute in Germany prepared a portable mass spectrometer, for chemical analysis when still at the comet. The University of Chicago provided shields with micro-impacts analyzers. And NASA’s Jet Propulsion Laboratory was responsible for a camera (with a periscope), plus gyroscopes and accelerometers, and a radio transmitter with the capability of showing any off-course deviation of the spacecraft by the comet. All these equipments were mounted on a structure built by Lockheed-Martin, and later a small amount of fuel for maneuvers was added, for a total mass of 385 kg. The craft so constructed was christened with the name Stardust.
With the budget at hand, the project was able to acquire a three-stage, 170-ton Boeing Delta II rocket. It was capable of launching Stardust beyond Mars, but that was not enough for cutting 81P/Wild 2’s orbit. This problem could be solved by letting the spacecraft perform and entire circle around the Solar System, back to Earth, in order to get advantage of our planet’s gravity and speed to “tow” the spacecraft into a new trajectory.
Finally, on February 7, 1999, Stardust lifted off from Cape Canaveral, USA, beginning a 7-year journey.
Once in route to the comet, part of the special catching gel was used to trap grains from a second dust, coming from the constellation Scorpius, which was discovered some years prior by other probes. The spacecraft also photographed the asteroid (5535) Annefrank.
THE COMET ENCOUNTER
In January 2004, members of the project were ready to send the last commands and receive data collected by Stardust (away 22 minutes at the speed of light), through gigantic, 70-m diameter antennas in Australia, Spain and California.
Some 100 days before the encounter, the spacecraft detected the comet. Technicians scheduled 30 communication sessions, 4-hour long each. The spacecraft was sending images once a week. Within 50 days left, it began to send two images per week.
When there were 30 days left, a trajectory correction was performed, and another one when 10 days were left. At nine days from the encounter, the gel capture system was deployed. In the last week, the specialist began to receive images daily, and within two days from the crossing with the comet, they ordered the spacecraft another trajectory correction.
On the last day, technicians programmed the encounter activities into Stardust computers. Communications were now continuous. They were seeing new images each hour, with many details. The specialist performed the last trajectory calculations, and sent the final commands just 8 hours before the encounter. With just 6 hours left, the spacecraft performed its last bearing correction, obediently.
In the 5 final hours, all the instruments were working. The spacecraft entered the cloud and headed toward the nucleus, a further 100 000 km inside.
The most dangerous phase was now beginning. The nucleus appeared, and 20 minutes before the maximum approach, the spacecraft computer began to track it. The camera was taking pictures every 10 seconds.
Then, the minimum-distance phase arrived. During these 8 minutes, the camera was working at its maximum: images every 5 seconds; Stardust was now needing to perform rotations in order to keep it pointed to the nucleus. The technicians were afraid that these maneuvers would cause a drop in the radio signal, so the data was now being stored in the spacecraft memory. From this moment on, they were receiving only vital signs. At 19:41 Universal Time of January 2, 2004 the spacecraft crossed by 81P/Wild 2’s nucleus, and in Pasadena, California, the members of the Stardust project applauded and cheered in joy.
THE RETURN TRIP
After the encounter, the gel was enclosed inside the ceramic capsule and the spacecraft’s health was checked. During the following 50 days, scientist analysed the encounter conditions, and the data that have been stored in the onboard computer’s memory were received.
Stardust headed home. At last, in 2006, the spacecraft arrived at Earth and jettisoned its capsule: at 03:10 in the early morning of January 15, it fell under parachutes to the deserts in Utah, USA, and was recovered by technicians. Mission accomplished!
At the Johnson Space Center in Houston, the minute grains were retrieved, one by one, from the catching gel. Preliminary analysis showed that they are something never seen before by scientists… and so Stardust has taken us a little closer to our own origins.
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Based on a lecture delivered at USP, on 18 August 2001. First published in ABC Color, on 9 April 2006. Artist rendering: Stardust Comet 81P/Wild-2 Encounter. Credit: Courtesy NASA/JPL-Caltech. With permission from Stardust Education and Outreach.