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Deep Space 1 Give Ion Power
Extra Impulse

DS1 is so far ahead of its optimal thrust plan it must now reduce thrusting, otherwise it will overshoot its flyby target.
by Dr Marc Rayman
Pasadena - Oct. 31, 2000
DS1 Pushes Ever Deeper Into Space As DS1 pushes ever deeper into space, the probe and one of its partners in a lovely celestial dance are about to perform a new step.

As all planets, asteroids, comets, and the very few spacecraft beyond Earth orbit make their journeys around the Sun, many coincidences of alignment occur. One such coincidence will take place very soon as Deep Space 1 and Earth line up on opposite sides of the Sun.

Today if anyone on Earth could see the tiny and distant craft, it would appear less than 3 degrees from the Sun. This small angle is only 6 times the apparent diameter of the solar disk. To picture the arrangement, think of the face of a clock. If the Sun is at the center, Earth orbits it at the tip of the hour hand. DS1's orbit has carried it out to the end of the longer minute hand.

The separation today is about how the hour hand and minute hand would be aligned when it is just 30 seconds before 6:00. (Although this analogy does not correctly depict the relative speed, it may help you visualize the alignment.) From the perspective of terrestrial readers, the spacecraft will slip behind the edge of the Sun on November 11 and emerge on the other side two days later.

This special arrangement, referred to as superior conjunction, is more than just an interesting geometrical oddity. Radio signals traveling between the spacecraft and Earth must pass so close to the Sun that they go through the turbulent plasma of the solar corona, which can introduce powerful interference.

To make matters worse, the Sun is near the peak of its 11-year cycle of activity, so the interference will be especially strong. As a result, from now until late in November, communications with the spacecraft will be difficult and sometimes impossible.

The operations team has programmed DS1 to operate completely on its own throughout that time, with no expectation of conducting the routine weekly communications sessions (although we do hope to get some signals with minimal information at certain times).

Early tomorrow morning DS1 will turn and then lock on to a star in the constellation Cygnus the Swan, and use it as a pointing reference the entire time. Early in the evening sometime this autumn if you can see that constellation, with the familiar asterism known as the northern cross, you and DS1 may be viewing the same star while on opposite sides of the solar system.

But while you are in the company of an entire planet, DS1 obediently makes its way through space alone, completely cut off from Earth -- the ultimate in solitary confinement. The team will be waiting anxiously at the end of November to learn how our distant but dear friend fared. And this log will be updated the first weekend in December.

The mission has continued to go extremely well as the spacecraft tirelessly thrusts with its ion propulsion system. It was not so long ago that the project, now in its extended, or bonus, phase, faced what should have been a catastrophic failure on the spacecraft.

But following a remarkable rescue earlier this year, the flight of DS1 has progressed even more smoothly than we had hoped. This has led to a surprising consequence. To understand it requires some patience as we consider what the effect of the long-term thrusting with the ion propulsion system has been.

DS1 is orbiting the Sun as confidently as any of the planets, and it does not need to propel itself to continue that movement. But the orbit it is in now won't allow the spacecraft to meet its intended target, comet Borrelly.

So the ion propulsion system is gradually modifying the orbit, so that in September 2001 DS1 and the comet will pass each other as they travel their separate paths. This reshaping of the orbit does not call for thrusting at all times. Very complex analyses tell us what the best periods are for thrusting and what the best periods are for coasting in order to keep the cometary appointment.

Now as trillions of loyal readers remember, part of the exciting rush in our efforts to salvage DS1 was the desire to have it ready for thrusting in time to achieve this cosmic encounter. All spacecraft experience glitches, which can range from being completely benign to truly mission-terminating.

Because of the radical extent of the long-distance repair to DS1, we expected that it was more likely than usual to have some problems once thrusting resumed. So we designed a flight plan that could accommodate periods during which thrusting might be interrupted for days or even weeks.

To do this, we started thrusting earlier than necessary, and even when our analyses told us to coast, we continued thrusting. We were, in effect, building up a savings account of orbital energy that we expected to dip into when a problem arose. Engineers call this maintaining mission margin -- anyone who has ever been self-supporting would call it saving for a rainy day.

But by the middle of October, after almost four months of thrusting (the longest thrust period of this, or any other, mission), DS1 simply had not had problems that caused it to miss any thrusting; the rejuvenated craft actually completed more thrusting than we had had any right to expect. In fact, it is so far ahead of the mathematically optimal thrust plan, that we now must reduce thrusting. If we continued running the ion engine so much, the spacecraft would, in effect, overshoot.

Now as we keep that thought in mind, to continue this story, we need to take a moment to consider a different aspect of the mission. How does DS1 hold its orientation in the free-floating conditions of orbital flight?

It carries a small supply of conventional rocket propellant, called hydrazine, which it fires through small thrusters to turn from one orientation to another or to hold a desired orientation. But when DS1 was launched on October 24, 1998, it was embarking on a mission scheduled to last only 11 months, so it was not necessary to carry more hydrazine than was expected to be needed for that assignment.

Of course, as it is now widely known and often recounted in song and story (as well as these recordings), the mission met or exceeded all of its success criteria, and the sturdy craft just keeps going. But its supply of hydrazine is dwindling.

Only if we spend the remaining hydrazine very carefully will there be just barely enough to conduct all the planned activities through the return of data from next year's exceedingly challenging comet encounter.

One of the many clever design features of DS1 is that when the ion propulsion system is thrusting, that fantastically efficient engine, using ionized xenon as propellant, can be used not only for pushing the spacecraft along but also for partial control of the orientation.

There are three sets of directions, or axes, around which objects can be rotated and oriented, and because of the layout of the spacecraft, the single ion engine can be repointed to control two of them while still propelling the spacecraft.

To visualize the three directions, our human readers can take advantage of the design of their heads. You can rotate your head in three ways: around the axis through your ears (as if nodding "yes"), around the axis from the top of your head down to your neck (as if shaking your head "no"), and around the axis through your nose to the back of your head (tipping your head side to side).

DS1's ion thruster can, in effect, get the spacecraft to nod yes or no (although it cannot move it in the third direction). Without this trick of using the ion drive to stabilize two directions of orientation, known as thrust vector control, hydrazine would always have to be used for all three axes.

And in that case, the hydrazine could not have been stretched to last so far beyond the end of the primary mission. Letting the ion engine do some of the work that the hydrazine system is responsible for is a crucial part of our strategy to keep DS1 going.

Shortly after powered flight resumed this summer, we concluded that hydrazine was in such short supply that even when no thrusting is called for to alter the trajectory, we should thrust to preserve that precious commodity.

But remember that right now we don't want to continue reshaping DS1's orbit around the Sun -- we want the ion engine off. On the other hand, we do want to control its orientation without using too much hydrazine, so we want the ion engine on. Yet it is not practical to turn it on and off frequently.

What's a rocket scientist to do? Our solution is to operate the ion engine at a very low throttle level; I like to call this "impulse power."

So beginning October 18, because the thrusting was so far ahead of schedule, DS1 throttled back to a level chosen so that it is just high enough that it can gently nod yes or no whenever asked, but low enough that it will not cause the spacecraft to modify its orbit excessively. The probe will sail along on impulse power throughout conjunction and then undertake maximum thrusting for several more months.

Deep Space 1 is now over 2.3 times as far from Earth as the Sun is and more than 900 times as far as the moon. At this distance of 351 million kilometers, or 218 million miles, radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.

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Driving Blind In Deep Space
Pasadena - Oct. 18, 2000
The most recent telemetry from Deep Space One was on October 9, and a planned carrier-only signal was received on October 12. The spacecraft is thrusting in the attitude required to reach comet Borrelly, and all spacecraft systems are working well. DS1 has accomplished 100% of the thrusting that was planned for the last week, and is set to enter radio silence October 23.


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