The cosmic rescue merits special mention in the history of space exploration. And now that the hardy craft has resumed normal operations, it has added still another record to its long of accomplishments: the longest operating time for a propulsion system in space. But before getting to the details of that, let's find out what DS1 has been up to.

The tenacious little probe spends most of its time with its new software keeping it pointed at a preselected star. Taking pictures with its camera, it analyzes the images to make sure the star shows up in the right place.

Whenever the spacecraft drifts a bit the star appears to move, and the new on-board system then computes how to reorient the ship to bring the star back to the desired region within the camera's view. When ground instructions tell it to turn to a different star, it uses its laser gyros for the period that no star is in the camera's sights.

Then when it has completed the turn, it looks for the new star it is supposed to track and locks on. This complex new system is doing a superb job of keeping the craft stable, compensating for the absence of the star tracker, which failed in November.

The amazing rescue of Deep Space 1, reported in recent mission logs with accounts interpreted according to local customs (with intriguing results) by DS1 enthusiasts throughout much of the observable universe, is one of NASA's many great triumphs in space.

Since it locked to a star on its first try on June 12, DS1 has maintained its hold on the desired stars extremely well. On only one occasion did it lose a star and fail to find again it on its own.

But in that case the spacecraft still did what it was supposed to: it continued pointing in that general direction using its remaining sensors (a capability that was built into the new software) until it was time to point back to Earth two days later.

Then when it was in contact with Earth, the operations team helped it find a new star. The loss of several days to go through the process of locking to a new star, transmitting information so controllers could make a positive identification of that star, and getting the spacecraft back to its routine was stressful for the team but not significant for the progress of the overall mission.

The restoration of the probe has given it a new chance to travel to a distant and daring encounter with Comet Borrelly in September 2001. There are many risks ahead, as the spacecraft pushes well beyond what it was designed to do. But the opportunities for new scientific information and for exciting exploration make the bold undertaking worth attempting.

To reach the correct point in space and time to greet the comet as it streaks around the Sun, DS1 will need to thrust with its advanced ion propulsion system for about 8 months. It has now completed over a month of that thrusting, since resuming powered flight at the end of June.

Each week now DS1 follows a routine established during its primary mission, which ended in September of last year. Most of the time is spent with the ion thruster steadily propelling the craft toward the encounter, as it remains locked to a star (which we call a "thrustar").

Once a week, following stored commands, the spacecraft turns from the thrustar to a star which is chosen by controllers so that when the camera is pointed at it, Deep Space 1's main antenna faces distant Earth.

The spacecraft then radios data to reveal how it has performed during the preceding week, and any new instructions can be sent to it. After about 6 or 8 hours of contact with its planet of origin, Deep Space 1 turns back to the thrustar to resume its long journey to the comet.

Until the middle of September, DS1 will use a thrustar in the constellation Sagittarius which many terrestrial readers can see on these warm summer nights.

The thrustar is near the handle of the "teapot" asterism that is the most familiar part of that constellation. When you go out to enjoy the lovely view of your planet's nighttime sky, you might take a moment to reflect upon the pleasant job DS1 has of gazing at a distant star as it quietly and gently thrusts with its ion propulsion system.

That fantastically efficient propulsion system uses only about 100 grams of xenon propellant each day (or about one pound every 4 days). The exact rate depends upon how much electrical power the spacecraft can devote to thrusting and, therefore, how far it is from the Sun whose light is transformed to electricity by DS1's sophisticated solar concentrator arrays.

To get a sense of how violent the thrusting is in a reasonably safe way, rest a sheet of paper on your hand. The paper pushes on your hand about as hard as the ion engine pushes on the spacecraft. In the frictionless environment of space, a day of thrusting is sufficient to increase the spacecraft's speed by about 7 meters/second, or a modest 16 miles/hour.

This may not sound like much, but day after day, week after week, the speed steadily increases and ultimately two things occur: 1) with patience the spacecraft can reach much much higher speeds than could one with conventional chemical propulsion, and 2) analogies with tortoises and hares become nearly inevitable.

Ion propulsion systems have been used in tests and for more limited applications on other spacecraft, but DS1's is the first to be used actually to take the spacecraft to its destination. And now DS1 holds the record for the spacecraft with the longest running time for a propulsion system of any kind in space.

Today, the ion propulsion system has logged 195 days of operation. The previous record also belonged to a spacecraft with ion propulsion. The Space Electric Rocket Test II, which was launched in 1970 to test an earlier version of this technology in Earth orbit, accumulated just under 162 days of operation. DS1 continues adding to its operating time every day as it makes its way through the solar system.

Deep Space 1 is now over 2.2 times as far from Earth as the Sun is and more than 865 times as far as the moon. At this distance of 332 million kilometers, or more than 206 million miles, radio signals, traveling at the universal limit of the speed of light, take nearly 37 minutes to make the round trip.