Early in the evening of September 27, 2007, a Delta II blasted off from Space Launch Complex 17B at the Cape Canaveral Air Force Station. On board was Dawn, an enormous unmanned spaceship designed to visit two asteroids: Vesta,and Ceres. Vesta is the fourth asteroid ever discovered and the second most massive of the asteroids. Ceres is both the first asteroid ever discovered as well as the largest. In fact it is so large that it is now classified as a dwarf planet.
Weighing 2800 pounds and bearing two solar panels that stretch 65 feet from tip to tip, Dawn is powered not by chemical rockets, but by an ion propulsion system first tested on the Deep Space 1 probe that launched in October, 1998. Deep Space 1 flew past asteroid Braille and comet Borelly. Its mission was completed by the end of 2001.
On a normal spaceship, the chemical propellant (usually either liquid oxygen and liquid hydrogen, or liquid oxygen and kerosene) makes up the bulk of the weight. For example, when the space shuttle rockets into orbit, it burns more than two million pounds of solid rocket propellant in the twin solid rocket boosters within two minutes. Meanwhile, the large external tank carries more than 1.5 million pounds of liquid oxygen and hydrogen, every drop of which is used in the eight and a half minutes it takes the shuttle to climb to orbit, going from zero to 18,000 miles per hour in order to reach an altitude barely 150 miles up. It thus burns through more than 16,000 pounds of solid rocket fuel per second and nearly 3000 pounds of liquid fuel each second. The space shuttle’s twin solid rocket engines alone put out nearly six million pounds of thrust.
Dawn’s ion propulsion, in contrast, is only a tiny fraction as powerful as the chemical engines used on a shuttle. The amount of thrust put out by Dawn’s engines is the equivalent of what your hand feels holding a single sheet of 8½ by 11 inch paper. But while a chemical rocket dumps its whole load of several million pounds of propellant in just a few minutes, an ion engine can burn its propellant continuously, non-stop, for years. Dawn was sent out of Earth orbit using standard chemical propulsion—but once out in interplanetary space, the ion engines took over. After burning for 11 days non-stop, just to check to make sure they were functioning properly, Dawn began its long-term cruise state on December 17, 2007. The engines then burned non-stop until October 31, 2008, when it was by then on course for its gravity assisted flyby of Mars on February 17, 2009, which sped it on its way into the asteroid belt. During those 270 days of continual thrusting, it used only 158 pounds of propellant. During its entire mission—flying to Vesta, orbiting Vesta, leaving Vesta and then going to Ceres and orbiting Ceres, with its engines firing continuously for years—it will use a grand total of barely 900 pounds of propellant. Contrast that with the space shuttle which uses more than three and a half million pounds of propellant in less than nine minutes just going to orbit, 150 miles up.
If automobiles got the sort of mileage Dawn gets, you’d fill up your car once when you bought it and your grandchildren’s grandchildren would still be driving around on that same tank of fuel. Of course, it would take you a full day to get from zero to sixty miles per hour.
What exactly is ion propulsion? The propellant is Xenon, a colorless, heavy, odorless noble gas that exists in trace amounts in Earth’s atmosphere. Electricity from Dawn’s large solar panels breakdown the gas and send its ions zooming from the nozzle of the engine. Though the ions don’t weigh much, their speed is high. Thus it makes for an efficient, if low power, thrust that builds up over time, ultimately giving the spacecraft a very high velocity.
On July 16, 2011, Dawn settled into orbit around Vesta. Escape velocity from Vesta is only 78 miles per hour, compared to 25,000 miles per hour for Earth. Vesta averages only about 330 miles in diameter, with a surface gravity so weak that an astronaut standing on its surface would still feel weightless. Your average baseball pitcher would be able to hurl a fastball from Vesta on an escape trajectory. If he aimed it just right, he could hurl it all the way back to Dodger Stadium. In fact, an astronaut would have to be very careful: if he jumped too hard he would never come down again. He’d escape Vesta’s gravity and float away into interplanetary space.
Vesta is not quite round, but it’s close. After spending a year orbiting, mapping and studying Vesta, Dawn fired up its engines and left orbit on September 5, 2012. Dawn is now en route to Ceres, with a scheduled arrival in February, 2015 (the same year that the New Horizons spacecraft will reach Pluto). Unlike Vesta, Ceres, is actually round. It is nearly 600 miles in diameter and is thought to have an enormous amount of water ice on and in it. In fact, the evidence at this point suggests that Ceres might actually have more fresh water than Earth does, albeit not in liquid form.