That rescue was completed in time to give the restored Deep Space 1 a chance to encounter comet Borrelly in September 2001. The probe must use its ion propulsion system to shape its orbit in just the right way that it and the comet will be at the same place (well, *almost* the same place!) at the same time as they travel in their separate orbits around the Sun.

This requires DS1 to fire its advanced ion drive at certain times and coast at others. The spacecraft operated so much better than expected after the complex and rushed rescue that it actually got ahead of the plan, and so, as described in the October mission log, beginning on October 18 it throttled down to what we call impulse power. But now the time that it would have to start thrusting at high power again is approaching.

Rather than wait until the mathematically perfect time to resume, we commanded DS1 to start early. In that way, we keep the craft a little ahead of the needed thrust profile, so if a problem occurs, DS1 will not immediately begin falling behind, thus jeopardizing the meeting with Borrelly.

The recipe for determining when to thrust at high throttle involves sophisticated navigation analyses combined with complex trajectory optimization and encounter retargeting computations as well as human judgment. And in this human's opinion, that last ingredient is a large contributor to the fun of this work!

January 2 was selected as the day to switch from impulse power to high thrust, and the spacecraft dutifully throttled up, while exclamations in Scottish accents could be heard in the Deep Space 1 control center at JPL.

The highest achievable throttle level depends upon the spacecraft's distance from the Sun. Bodies in orbit generally move along elliptical paths, in which the distance from the source of gravitational attraction alternately increases and decreases as the orbit is followed.

In November, DS1 reached its maximum distance from the Sun, and for a few more months its orbit will continue to take it closer (although it will not get as close to the Sun as Earth is).

As the distance from the Sun changes, the intensity of the Sun's light reaching the probe changes, and the amount of electrical power produced by DS1's solar concentrator arrays varies accordingly.

These sophisticated arrays were tested as part of the primary mission, which ended in September 1999. Although the testing is complete, the arrays have continuedto work flawlessly, powering all of the probe's systems, including its ion engine. (By the way, if all goes well, before the end of this month the ion propulsion system will have accumulated 365 days of operation in space.)

To remain stable while it is firing its ion engine, the spacecraft fixes its gaze on a carefully selected reference star, known as a thrustar. This process is part of what was invented last year to compensate for the loss of the star tracker. Residents of solar systems in the constellation Pisces will be honored to know that right now the thrustar is Eta Piscium.

Although the system that maintains the lock on a star has performed far beyond even our most optimistic predictions, we remain responsibly cautious. The spacecraft keeps taking pictures of the star and processes them for use in stabilizing its orientation.

If the probe drifts so much that the star moves out of the camera's view, the pictures will continue to be taken, but when they are analyzed the star will not be found. There are many built-in means to recover, but there is always the chance that they won't be sufficient to relocate the star.

So when the spacecraft is thrusting, how can we monitor the system's performance? While pointing its ion engine in the required direction, DS1 cannot simultaneously point its main antenna (referred to as the "high-gain antenna" by engineers and by those who don't really want to disseminate helpful information) to Earth.

About once a week it turns to direct that antenna to Earth, but what about the rest of the time? We would like to have some insight into the space vehicle's behavior at least once between these main communications sessions.

The spacecraft is outfitted with other, weaker antennas ("low-gain antennas") that might allow communications even when the main antenna is not Earth-pointed, but in most cases they are just too weak to return data to distant Earth unless the largest of NASA's Deep Space Network antennas (70 meters, or 230 feet, in diameter) are being used for reception. And because there are only three of those behemoths on the planet, one is not always available for DS1's use.

Sometimes other antennas (still not exactly small, at 34 meters, or 112 feet, in diameter) are used, and as remarkably sensitive as they are, they are not up to the task of detecting data from the tiny probe practically one million times farther away than the International Space Station.

The ever-resourceful DS1 team developed a clever method to let controllers know something about the status of the lock to the thrustar during the so-called mid-week tracking session even when a weak antenna on the spacecraft and one of the smaller Deep Space Network antennas are used.

A timer running in the computer on the spacecraft keeps track of how long it has been since a picture of the reference star was processed and used for stabilizing the orientation. If the lock is lost, the pictures will not reveal the thrustar, so the timer will count up. It will continue to increase until the star is regained, at which point it restarts counting from zero.

If the timer ever reaches 90 minutes, a special response on board is triggered that accomplishes several functions. One of them is the activation of the signal to help alert controllers during the next scheduled contact with the Deep Space Network that this event occurred.

Although normal data cannot be sent, the spacecraft transmits one frequency if the timer has not tripped and a different one if it has. Either of these frequencies can be detected by the Deep Space Network antennas even when they cannot receive the more complex signals that encode data.

You can understand this by imagining trying to understand something that someone at a great distance is shouting to you. By the time the sounds reach you, they may simply be too faint for you to catch what is being said; you simply can't extract anything meaningful from the sound.

But suppose instead that your distant (and now hoarse) friend plays only one of two musical notes, one high and one low. You would probably have much less trouble deciphering that simple sound. And if you had previously agreed on what the notes meant, you could be successful in communicating some limited information. That is the idea of the system used on DS1.

If the Deep Space Network detects a high frequency, it means that the timer has not reached 90 minutes. If it is ever a low frequency, then the timer did reach 90 minutes and that is an indication the spacecraft is having difficulty.

In that case, we will be alerted before the next weekly communications session with the main antenna that there may be a problem on board, and we can respond more promptly. So far, the spacecraft has only transmitted the high frequency, the principal response to which is simply the gentle murmur of renewed contentment in mission control.

By coincidence, Deep Space 1 and comet Borrelly now are separated by nearly the same distance as DS1 and Earth are, although of course Earth and the comet are in different directions from the spacecraft.

After reaching its maximum distance from Earth two months ago, the remote traveler and its home planet will slowly get closer. The space probe and the comet will converge much more quickly, as they close in on each other for their September appointment.

Deep Space 1 is now 350 million kilometers or almost 218 million miles from comet Borrelly. DS1 is almost 2.3 times as far from Earth as the Sun is and nearly 900 times as far as the moon. At this distance of 342 million kilometers, or over 212 million miles, radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.

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