Unlike most interplanetary probes, Dawn is not propelled by a chemical rocket engine, but by a solar-electric ion propulsion system, particularly suitable for long-distance flights. This innovative propulsion concept was first used in 1998 on NASA’s Deep Space 1 mission to Comet Borrelly. In 2004 the Europeans successfully tested their own ion propulsion system on the Smart 1 lunar probe. Both missions were important technical demonstrations for the further development of electrical space propulsion systems.
In an ion propulsion system, electrons are fired into a magnetic field containing the noble gas xenon. When a xenon atom is hit, it loses one of its negatively charged electrons and turns into a positively charged ion. The magnetic field then accelerates the xenon ions out of the propulsion jet. The resulting thrust is no greater than the pressure of a piece of paper on the outstretched palm of the hand – tiny in comparison to chemical rocket engines, but effective all the same.
A little fuel for a long haul
The secret lies in the endurance of the ion technology. While chemical propulsion systems operate for only a few minutes, generating their thrust in this short period, electrical engines can run for weeks and even months, gradually accelerating a spacecraft in small increments. On a long interplanetary trip, their thrust can even theoretically exceed that of the most powerful launcher.
The key to the endurance of ion-propelled spacecraft lies in their low fuel consumption. Dawn only requires 250 grams of xenon to fire for 24 hours. At the end of the mission, the engines will have been in operation for 50,000 hours and will only have used 425 kilograms of xenon gas. Each kilogram of fuel will then have produced 10 times as much thrust as a kilogram of hydrogen and oxygen in a conventional rocket engine.
En route to Vesta, Dawn achieved the greatest velocity increase that any interplanetary spacecraft has ever reached. All acceleration and braking manoeuvres taken together, down to the lowest orbit round Ceres, will amount to about 2000 days of operation for Dawn's three ion engines; the total change in velocity of the speed of the spacecraft during this time will be 38,620 kilometres per hour – that, in comparison to the conventional technology of chemical engines, is almost the thrust of a US Delta II rocket.
The fuel economy of the ion drive enables Dawn to follow an ambitious trajectory, which would not be possible with a chemical engine while still remaining within the cost limits. For the first time in the history of spaceflight, the spacecraft will enter orbit around two celestial bodies consecutively. Vesta, and subsequently Ceres will, at the beginning, guide the spacecraft into an orbit at a distance of several thousand kilometres. Firing its propulsion system, Dawn will then spend several weeks closing in on its target object in an inward spiral.