FREQUENTLY ASKED QUESTIONS ABOUT
ION PROPULSION
What is ion propulsion?
Ion propulsion is a technology that involves ionizing a gas to propel a craft. Instead of a spacecraft being propelled
with standard chemicals, the gas xenon (which is like neon or helium, but heavier) is given an electrical charge, or ionized.
It is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as
exhaust from a spacecraft, they push the spacecraft in the opposite direction.
What implications does this technology have for space exploration?
If DS1 shows that electric propulsion works as well as is expected, there will be many new missions that will take
advantage of it. Deep Space 4 is expected to use four ion engines to fly alongside a comet in 2004 so that it can land.
Ion propulsion is not of value for missions that require high acceleration, and it often will not be worthwhile for missions
that can be done quickly using conventional propulsion systems (such as missions to the moon). But for a wide variety of
missions with high energy requirements (such as missions to asteroids and comets, Mercury and the inner solar system, and
some to the outer solar system), the low but steady acceleration of ion propulsion wins out over the less efficient bursts
from chemical alternatives.
Why has NASA not used this technology before?
NASA's first ion engine was built by Glenn Research Center in 1960. Since then, there have been many tests of the technology
in the laboratory and some limited tests in space. But no mission has really been willing to use the technology as a
primary propulsion system because of the possibility that it might not work, which would mean the failure of an expensive
mission and the loss of important scientific data. Managers of missions are usually not willing to take on this kind
of risk, and tend to use the tried and true system of chemical propulsion. The whole purpose of the New Millennium program,
of which DS1 is the first flight, is to test whether or not some very important but risky technologies, which can make
a big difference in our ability to explore space less expensively and with more capable spacecraft, will work in deep
space and around Earth. By taking on the risk, New Millennium flights will help lower the cost and risk to future missions
that will use the technologies for the first time for scientific purposes.
How does ion propulsion compare with chemical propulsion? Does it make the spacecraft go any faster?
Under the circumstances for which ion propulsion is appropriate, it can push a spacecraft up to about ten times
as fast as chemical propulsion. Because the ion propulsion system, although highly efficient, is very gentle in
its thrust, it cannot be used for any application in which a rapid acceleration is required. With patience, the
ion propulsion system on DS1 imparts about 3.6 km/s to the spacecraft. To undertake the same mission with a
chemical propulsion system would require a more expensive launch vehicle and a larger spacecraft to accommodate
a large tank for the chemical propellants.
How much time would a spacecraft using ion propulsion take to get to Mars? Can the technology be used for a manned mission to Mars?
Ion propulsion could be used for a manned mission to Mars. The decision on whether that would be the preferred
approach would involve many questions such as which technique might get the crew there the fastest (independent
of how fuel efficient the trip might be) in order to reduce the radiation exposure and effects of long periods
of near weightlessness.
How fast can DS1 go using ion propulsion and how long will it take for it to reach maximum speed? How does the amount of fuel required by an ion engine compare with the amount required by a chemical engine?
The ultimate speed of a spacecraft using ion thrust depends upon how much propellant it carries; indeed, the same
principle applies to chemical propulsion systems, although they are much less efficient. The ion
propulsion system on Deep Space 1 carries about 81.5 kilograms of xenon propellant, and it takes about
20 months of thrusting to use it all. It increases the speed of the spacecraft by about 4.5 kilometers
per second, or about 10,000 miles per hour. If we had the same amount of chemical propellant, it would provide
only one tenth as much velocity increment. If DS1 carried a larger solar array, it certainly would have a slightly
higher acceleration, and if it carried more Xe propellant it could reach a much higher final velocity by simply
thrusting longer. But DS1 is testing ion propulsion solely to find out if it works as well as predicted. Future
missions that use it likely will carry more propellant to achieve still higher speeds.
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