A unique spacecraft has been in orbit for the past year around Vesta, a small rock about 330 miles in diameter and the second-largest asteroid in the asteroid belt.

The Dawn mission is a breakthrough, being the first time ion engines have been used to send a spacecraft into orbit around a distant object in the solar system. If all goes as planned, it will also be the first time a spacecraft has gone to a body, orbited it, then left to orbit another.

Dawn will leave Vesta under its own power in September and begin a mission to orbit Ceres, the largest asteroid in the asteroid belt. Dawn was launched from Earth in September 2007. Its ion engines sent it on a slowly accelerating spiral that met up with Mars in February 2009. There it got a gravity boost, spiraling yet farther outward and, after two laps around the sun, into the asteroid belt where it caught up to Vesta in July 2011.

After orbiting Vesta for one year and collecting thousands of high-resolution photos, Dawn will be off on another voyage to the asteroid Ceres, arriving in February 2015.

The mission was made possible by the ion drive, a relatively new propulsion system for rockets in space.

Instead of chemicals reacting to push hot gases of the exhaust, Dawn’s engines use electric fields to accelerate positively charged xenon gas ions, which shoot out of the spacecraft at speeds up to 90,000 mph.

Unlike chemical rockets where the fuel is stored onboard, the energy to propel Dawn’s ions comes from solar panels. This makes the ion engines 10 times more efficient than chemical fuels, akin to getting 300 miles per gallon.

A spacecraft cannot carry enough chemical fuel to burn all the way to its planetary target. It is hoisted to Earth orbit by a multistage, chemically fueled rocket. Each of the multiple stages is jettisoned after expending its fuel. To leave Earth orbit requires propulsion. A chemical burn must be short in duration so as not to waste the precious fuel that is carried as cargo to be used later.

Once a chemical rocket has reached the desired speed and direction, it coasts by inertia. From then on the only burns are for minor course corrections and to slow the craft for landing on the planetary surface.

By contrast, Dawn’s fuel-efficient ion engine imparts a minuscule acceleration from a thrust of one-third of an ounce, about the weight of a piece of paper. The absence of friction in space allows the small but continuous ion thruster to add continual increases in velocity over longer times. The craft spirals outward from Earth orbit with final speeds up to 10 times faster than traditional rockets.

After two years, 1.7 billion miles and two spiraling laps around the sun, Dawn caught up to Vesta’s 43,000 mph orbital speed. Its approach speed was about 60 mph, slow enough for Vesta’s weak gravity to draw it gently into orbit.

Learn more about ion propulsion, Vesta and the Dawn mission at dawn.jpl.nasa.gov.

Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to brill@hawaii.edu.