Dawn on an asteroid
Ken Tapping, August 24, 2011
One of the greatest quests in science is to trace the path between the Big Bang and what we see around us today: a universe with galaxies, stars and planets. A major step in this is to find out exactly what was delivered to the “building site,” from which our Solar System formed.
We are very unlikely to ever find identifiable “left over construction material” lying around anywhere on Earth. Plate tectonics and erosion have erased it. The Earth’s surface is being continually recycled, so there are few really old rocks left. Even though in Canada and Australia we have found rocks dating back almost to the formation of the Earth, some 4.5 billion years ago, it seems some processing had happened even back then. Furthermore they have changed since, so they are not dependable samples of the original material.
The Moon might seem to be a better bet. The mountains on the Moon are the result of impacts, not plate motions, and there are no signs of the recycling processes we see on Earth. Today the Moon seems to be solid all the way through, and has been for a long time, which rules out plate motions. However, temperatures ranging from hotter than boiling water during the lunar day to far below zero at night have powdered the lunar surface layers, along with billions of years of micrometeor impacts. This allowed the layers to dry out and then huge lava flows changed the surface, following major impacts. However, the Moon is not a trustworthy source of the original material either. We have to rule out all the other planets for similar reasons. Pluto is a possibility, but is very hard to get to. Fortunately, there is a viable alternative – the asteroid belt.
Thanks to the gravity of Jupiter, the largest planet in the Solar System, the planet that should have formed between Mars and Jupiter never did. The material remained in the form of countless lumps, the largest of which are less than a thousand kilometres across – most of them are much smaller. They never got big enough for volcanic action to start, and they are far enough from the Sun to remain completely frozen. The material of which the asteroids are made is probably as close as we will ever get to the construction material used to make the Solar System without venturing out into the dark, frigid realm beyond Pluto. Of course, to be sure, we would have to visit more than one asteroid. Using conventional spacecraft would be difficult, but the Dawn spacecraft is different.
For most of the history of spaceflight, getting somewhere meant giving something a huge shove and then letting it free-fall to its destination. This works well when the numbers and sizes of in-flight course corrections are small. However, for manoeuvring around asteroids, this does not work so well. Fortunately, Dawn uses an ion drive. Although only producing a small thrust, by using a small amount of fuel and ejecting it as an extremely fast jet, the ion drive is a highly economical and effective means to visit lots of destinations. Dawn was launched in 2007 and went into orbit around the asteroid Vesta in July 2011. It will leave Vesta in July 2012 and arrive at Ceres, its next destination and the largest asteroid, in 2015.
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