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Deep Space One Keeps Ion Burning
By Dr Marc Rayman
 Pasadena - September 9, 1999 - Deep Space 1 has been spending most of its time recently with its ion propulsion system gently but relentlessly pushing it along to bring it to its appointments with Comet Wilson-Harrington in January 2001 and Comet Borrelly in September 2001.

Each day of thrusting is enough to change the spacecraft's speed by about 15 miles/hour. Every Monday, the autonomous navigation system stops the ion engine so that the spacecraft can photograph asteroids and stars to use in determining its location in the solar system.

It then points the main antenna at Earth to transmit information on the spacecraft's performance during the preceding week. That opportunity is used by controllers to radio new commands to assure it stays in good health or to conduct tests.

In addition, although it has accomplished over 100 days of powered flight with the ion engine since launch last October, over 400 days of thrusting lie ahead. Some minor adjustments are still being made to prepare the ship for that journey. Following the communications session, thrusting resumes.

On Monday, August 23, a bug in the complex AutoNav software prevented it from completing the normal weekly task of determining its location, and the spacecraft's computer had to restart itself to clear the problem and then await further instructions from Earth.

Early that morning, when it came time for the communications session with the Deep Space Network, mission controllers discovered what had occurred.

But there was still another surprise awaiting them. DS1 always carries 3 copies of the software, so if there is a problem with one, it can use another.

When AutoNav's problem occurred, it caused one copy of the software to be partially overwritten and, therefore, not useable. The computer detected this and used a different copy of the software, but the one it used is an old version.

Although that version is reliable, it does not have all the features needed to conduct the activities planned for DS1, and it does not have the improvements made in later versions of the software.

So after establishing what had caused the problem, the operations team restarted the computer once more, this time forcing it to use the newest version of the software. (Also, this replaced the damaged copy of the software with a pristine one.)

Following that, the spacecraft was restored to its normal operational configuration, and thrusting with the ion propulsion system resumed Tuesday evening. Now that the bug in the software has been found, it is easily avoided by altering some commands for AutoNav.

Most of the next two months will be devoted to using the ion engine to propel the spacecraft. DS1 will not thrust for most of November and will conduct other activities that will be described in future recordings.

Deep Space 1 is now about 43% farther away from Earth than the Sun is and 560 times as far as the moon. At this distance of 214 million kilometers, or 133 million miles, radio signals, traveling at the universal limit of the speed of light, take almost 24 minutes to make the round trip.

Thanks again for calling! And now let's learn the answer to the question we've all been asking: How many messages are waiting?

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