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Feel Those Rays: It's Ion Power
By Dr Marc Rayman
 Pasadena - September 20, 1999 - Deep Space 1 continues to spend most of its time with its ion propulsion system providing a gentle but steady push. Like all the planets, asteroids, comets, and some other spacecraft, Deep Space 1 is orbiting the Sun.

When the ion propulsion system is turned off, the little spacecraft coasts in its orbit; it does not need propulsion to keep going any more than Earth does.

As DS1 participates in the complex choreography of the solar system ballet, it must reshape its orbit to assure that it passes by Comet Wilson-Harrington at just the right time in January 2001 to allow it to continue on to intercept Comet Borrelly in September 2001.

When DS1 was launched nearly a year ago, its rocket propelled it away from Earth and into orbit around the Sun. But the probe uses its ion propulsion system to change that orbit to reach asteroid Braille and then to journey to each of the two comets.

In the first eight months of its flight through the solar system, Deep Space 1 receded from the Sun. For the past three months it has been getting closer to the Sun.

Each day now it moves nearly 240,000 kilometers, or almost 150,000 miles, closer to the brilliant center of the solar system. That means that the spacecraft's solar arrays, which convert sunlight into electricity, can produce a little more power each day.

In addition, as it approaches the Sun, the spacecraft's electrical heaters do not have to work as hard to keep components at their correct operating temperatures. So with the solar arrays generating more power and the spacecraft heaters requiring less, there is more power available to the ion propulsion system.

Each day could bring 3 to 4 watts more for the ion engine. In fact, the ion propulsion control electronics are built to operate at specific, discrete settings. These throttle levels are spaced about 17 watts apart, so about every 5 days the autonomous navigation system commands the ion propulsion system to throttle up one step.

The shaping of its orbit around the Sun requires that DS1 not thrust during November, so that month will be devoted to other activities. Now that the mission has turned from evaluating technologies to focusing on comet science, preparations will begin for the jobs the two science instruments will perform at the comets.

Some of the tests with the camera will help characterize how it behaves with very dim objects. It was the camera's unexpectedly weak response at asteroid Braille, combined with the asteroid's surprisingly dark appearance, that prevented the acquisition of some of the bonus science data, including the close-up pictures. The tests of the science instruments will be explained in future recordings.

Deep Space 1 is now about 50% farther away from Earth than the Sun is and 580 times as far as the moon. At this distance of 222 million kilometers, or 138 million miles, radio signals, traveling at the universal limit of the speed of light, take almost 26 minutes to make the round trip.

The next update to this report will be on October 24.

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