A week ago, just after midnight, the Dawn spacecraft arrived in the vicinity of the major asteroid Vesta.
It will spend several months there, and will then blast off for Vesta's larger cousin, Ceres.
This is the first spacecraft designed to spend quite a bit of time orbiting two different celestial bodies. The two asteroids —I insist on including Ceres in the asteroid family, even though it's also been designated a “dwarf planet” — are very different. Ceres is dark and icy; Vesta is bright and dry. They need close study.
But today we'll focus on the craft that will be studying them. When someone says “spacecraft,” you may think of the Space Shuttle, rocketing off its pad in Florida to the accompaniment of very loud noises and gigantic flames. Dawn is quite a different beast.
Admittedly, it was launched into Earth orbit with a conventional Delta rocket back in 2007. But since then it has been propelled by a new kind of engine, called an ion-propulsion engine.
This is not a chemical rocket; nothing is burned. Instead, the engine expels tiny amounts of Xenon gas from its engine at very high speeds.
First, the gas is electrically charged. Then electrical forces spew it out of the rocket nozzle. Ion propulsion is a much more efficient system than chemical rockets. It's also decidedly cheaper.
It is slow to work and quite weak, however, when compared to conventional rockets. The thrust of one of the Space Shuttle's solid-rocket boosters reached a level of 2.8 million pounds of force. The Saturn V first-stage engines produced 1.5 million pounds apiece … and there were five of those babies firing simultaneously!
In contrast, Dawn's engine can push with a force of no more than a third of an ounce! The Space Shuttle reached an orbital speed of 17,500 miles per hour in a few minutes; Dawn could go from 0 to 60 mph in about four days.
The key difference: Dawn's engine almost never stops. Most planetary probes, starting from near-Earth orbit, use conventional chemical rocket engines to leave orbit. They burn nearly all their fuel in a few minutes, then coast for months out to Mars, or Jupiter, or wherever they're headed.
Dawn's engine thrusts continuously, except for a few hours per week during which the spacecraft points its antenna at the Earth for data transmission and instrument calibration.
The thrust is small, but it mounts up as time goes by. A ship like Dawn, given enough time, could go anywhere in the solar system.
One thing that makes the engine so efficient is the speed of the gas coming out. Those Xenon atoms (actually ions: electrically charged atoms) are zipping out at 10 times the speed of the gas coming out of the Space Shuttle's main engines.
There's not a lot of it coming out, of course. Where the Shuttle used up over a million pounds of fuel in two exciting minutes, Dawn uses only about 10 ounces in 24 hours, even at maximum thrust. The spacecraft's tank held only a bit over 900 pounds of Xenon propellant when it left. But that's enough to keep the probe accelerating for 2,000 days.
It does take a lot of power to generate even that small amount of push. Dawn's solar panels make it as long as a tractor-trailer, even though the main spacecraft is only as long as a motorcycle. This is the longest wingspan for any probe we've sent out.
Near the Earth, this array can generate up to 10,000 watts of electrical power, which is used to ionize the Xenon gas and to push it out the back end (and to power the various instruments). At Vesta, sunlight is weaker, but even there the two giant arrays will suffice to provide power for several years.
Dawn will be using dozens of scientific sensors to study Vesta at various orbital altitudes. You may be reading about its discoveries in the news over the next few months (in this column if nowhere else.) Next summer: on to Ceres (which will take another three years: this is not your grandfather's rocket).
South Jersey Newspapers and nj.com accept no responsibility or liability whatsoever with regard to the information in this article.