NASA’s Dawn mission is en route to the asteroids Vesta and Ceres, the most massive of the protoplanets. It will go into orbit around Vesta first, gathering data on that body for seven months, then move into orbit around Ceres, which it will study for the next five months. When its mission is complete, the probe will remain in orbit around Ceres. It will be the first space probe to orbit two bodies, and also the first to take up residence in the main asteroid belt.
The protoplanets are baby planets whose development was stopped by the gravity of Jupiter, which disturbed the dust in that region and prevented it from forming into full-size planets. What was left were small bodies that could have been the cores of larger planets, if Jupiter hadn’t been there. It is thought that Earth, Venus and Mars started from similar bodies, but continued their evolution into true planets. So the protoplanets are like snapshots from the early history of the solar system, frozen in perpetual babyhood, and it is hoped that by studying them, scientists will be able to learn something about the conditions under which they were formed. Another objective of the mission is to determine the nature of the building blocks from which the terrestrial planets formed, thus increasing our understanding of the process. A third objective is to contrast two bodies which seem to have followed very different paths in their development, in an attempt to understand the evolutionary processes involved.
The mission is timely because astronomers are beginning to detect planetary systems around other stars, and in the near future we should be able to answer one of the most fundamental questions of space exploration: is our solar system typical, or unusual? Are there other planetary systems out there that are similar to ours? If so, why? If not, why not? Understanding the processes that form such systems is obviously fundamental to answering these questions.
Ceres is the largest of the asteroids. It was the first to be discovered, found on New Year’s Day, 1801 by Giuseppe Piazzi of the Palermo Observatory. Its year is 4.6 times as long as Earth’s, and it has a diameter of about 960 km. (600 miles.) Vesta is the brightest of the asteroids and the only one that is ever visible with the naked eye. It was discovered on March 29, 1807 by Heinrich Olbers. Its year is 3.6 terrestrial years long, and it has a diameter of about 520 km. (320 miles.)
Simply giving the diameter of these bodies is a bit misleading, since they are not flat landforms such as we are used to here on Earth, but three-dimensional objects. While many asteroids are really just pebbles whizzing through space, Ceres and Vesta are big enough to be considered true worlds. To date, the largest asteroid to be approached by a space probe is Mathilde, which was studied by the NEAR-Shoemaker probe in 1997. That asteroid had a very irregular shape, with its largest dimension being about 66 km. (41 miles.) Compare that to the size of Ceres and Vesta, and it becomes obvious that they really are worlds. Vesta has a surface area more than three times that of Arizona, while Ceres’ surface is as big as Alaska, Texas and California combined. Ceres is so large that it may have a wispy atmosphere, and there is even the possibility that it may have permanently frozen polar caps composed of water frost.
Dawn’s ambitious itinerary, with all the course adjustments that it involves, would not be possible with old-style chemical propulsion. Dawn was first lifted into space by a Delta rocket, but since leaving that rocket, it has been relying on its ion drive. This is an advancement that was anticipated by science fiction writers for many years, and it works just as well in real life as it did in fiction. First used to great advantage by the Deep Space 1 probe, ion drive allows far greater flexibility and maneuverability than a chemical rocket would. Whereas chemical rockets are able to deliver large amounts of thrust for a short time, the ion drive is able to deliver lower thrust for a much longer time. The system is powered by solar panels. The power from these ionizes the fuel (xenon) and then accelerates it with an electric field between two grids. The resulting jet shoots out the rear of the engine at a speed of 60,000 miles per hour.
The idea of the ion drive has had a long history. Hermann Oberth, one of the pioneers of rocketry, proposed such a system in the 1930′s, but because chemical rockets were easier to design, the development of rocketry went in that direction instead. Now the technology has finally caught up with the vision, and we see that Oberth was right: this is a reliable and efficient propulsion system. Given its excellent performance so far, it will certainly be used on many spacecraft in the future.
In more ways than one, Dawn is the fulfillment of dreams. Many a science fiction story has been written about the settlement and mining of the asteroid belt. With the Dawn mission, we are taking a step toward that dream.
Sources:
Website of the Jet Propulsion Laboratory, California Institute of Technology dawn.jpl.nasa.gov/
Dawn: a Journey to the Begining of the Solar System ssc.igpp.ucla.edu/dawn/
Ambrosiano, Nancy: Dawn Space Mission is a Go in Los Alamos National Laboratory News Bulletin lanl.gov/news/index.php/fuseaction/nb.story/story_id/11526
Dawn Spacecraft at Aerospaceguide.net aerospaceguide.net/spacecraft/dawn.html
Many scientists and space enthusiasts have dreamt of a spacecraft exploring the asteroid belt and it will be great when the Dawn spacecraft arrives there. I hope the ion drive works well. It’s fantastic the technology like the ion drive which allows for more flexibility and maneuverability than a chemical rocket would. In comparison if they used a chemical rocket the spacecraft would have to be mega massive.