Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. December 3, 1998



Explanation: Going gently into the night, Deep Space 1's ion drive has been running smoothly since it was restarted on November 24. How powerful is this high-tech spacecraft's ion propulsion system? At full throttle the engine will consume about 2,100 watts of electrical power generated by solar panels and produce about 1/50th of a pound of thrust. This is roughly equivalent to the force you exert when holding up a single sheet of paper! While clearly not suitable for vehicles which need rapid acceleration, ion propulsion is advantageous to use for missions involving asteroid and comet rendezvous. For these long space voyages which ultimately require a lot of energy, the continuous, gentle nudge of an ion engine easily wins out over brief, powerful, but less efficient blasts from chemical rockets. Pictured here during ground tests, the Deep Space 1 craft is now about three million miles from planet Earth.