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Opposition In Deep Space

On November 28, DS1 turned to point its main antenna directly at Earth and, like an excited tourist after a successful trip, spent several days reporting the details of its month-long vacation on the far side of the Sun. On December 1 it turned to a new thrustar.
by Marc Rayman
Pasadena - Dec. 6, 2000
Deep Space 1 has completed another challenging phase of its remarkable journey through the solar system: passing on the opposite side of the Sun from Earth.

Ever since it left Earth in October 1998, DS1 has been orbiting the Sun just like a planet or most of the myriad other celestial bodies that form the solar system.

One of the many surprises of the beautiful choreography resulting from what engineers and scientists so unromantically call orbital mechanics is that while objects in more distant orbits travel more slowly than ones closer to the Sun, it takes energy to reach those more remote orbits. DS1's advanced ion propulsion system has not increased the spacecraft's speed but rather has propelled it into an orbit beyond that of Earth.

As a consequence, Earth travels around the Sun faster than its ambassador to the cosmos.

After more than two years of following their separate paths, Earth had moved so far ahead that it finally was on `the opposite side of the Sun from the spacecraft. You can visualize the alignment by imagining the face of a clock.

If the Sun is at the center, then Earth is the same distance as the tip of the hour hand, and DS1, in its more distant orbit, is at the tip of the minute hand.

Last month they were on opposite sides of the center, as if it were 6:00, in an arrangement astronomers call superior conjunction. Now in one way, this alignment is not special.

After all, with so many bodies orbiting the Sun at different speeds, many coincidences of geometry occur.

But it was significant for DS1, because the radio signals that normally take instructions from Earth to the probe and return data from the probe to Earth had to pass very near the Sun.

The unpredictable turbulence of the plasma in the solar corona can cause strong interference, even to the point of completely drowning out these signals.

So DS1's controllers had to assume that communications would be impossible during the time the spacecraft was nearly aligned with the Sun.

Instructions were radioed to the craft in October to configure it for a month without ground assistance. From October 30 to November 28, it did not expect to hear from Earth at all. If any problems had occurred during the period of superior conjunction, DS1's terrestrial colleagues would have been unable to provide assistance. Although the ion propulsion system has proved to be remarkably reliable, glitches elsewhere on the complex craft could have led to an interruption of thrusting.

And without the operations team to restore thrusting, the craft would have fallen behind in its flight plan. One way the mission control team prepared for that was by formulating a flight plan last summer that did not require DS1 to accomplish any ion propulsion system thrusting during conjunction.

The design of the thrust plan was explained in more detail in the last thrilling mission log, which even now is responsible for a new, widespread sense of hopefulness among the previously despondent inhabitants of a rather ordinary planet on the far side of the Virgo supercluster of galaxies. Following the long-distance rescue mission earlier this year, operation of DS1 without its star tracker requires that it lock on to a reference star to keep its orientation stable.

During the entire four weeks that communications were unreliable, the lock remained perfectly solid. This is another impressive demonstration of the success of the very risky and ambitious rescue.

In addition, the ion propulsion system maintained thrust at a low throttle level (I call this "impulse power") to allow the probe to use it to make most of the corrections in its orientation, thus preserving the conventional rocket propellant, called hydrazine, which is on board for that purpose but is now in very short supply.

This was the longest uninterrupted period of thrusting of the entire mission.

The ion propulsion system thrusted for 699.74 hours, or slightly over 29 days, without a break.

The longest thrust marathon prior to this was 335.3 hours, beginning just one month after Deep Space 1's launch.

The ion propulsion system on this cosmic Energizer bunny has now accumulated a total of more than 310 days of operation in space. As it turns out, limited signals were received from DS1 during the conjunction period.

Although the Sun is near the peak of its 11-year cycle of activity, the path of radio signals from DS1 to Earth did not happen to intersect the largest and most intense regions of the solar corona.

The reason signals could be sent at all is that a complex and clever analysis allowed engineers to find an orientation of the spacecraft during conjunction that had the camera fixed on a bright reference star (called a "thrustar") while pointing the ion engine in the correct direction for impulse power thrusting and leaving the main antenna pointing in the general direction of Earth just in case solar interference was not too great. During November the celestial dance took the two partners as far away from each other as they will be at any time during DS1's primary or extended mission. (Just think of the clock: the tips of the hour and minute hands are farthest apart at 6:00.)

According to the latest navigational analysis (the accuracy of which is limited by the sparseness of the data during conjunction and the solar interference on the radio signal used for navigation during that time), late in the afternoon of November 6, DS1 and Earth were separated by about 352,371,000 kilometers or 218,953,000 miles.

That is nearly one million times greater than the orbital altitude of the International Space Station.

And in the morning of November 18, the craft was at its maximum distance from the Sun of about 204,275,000 kilometers or 126,930,000 miles.

The spacecraft's elliptical orbit will take it back to that range again next year, but Earth and DS1 will be somewhat closer to each other then. Now that superior conjunction is over and DS1 continues to sail along so smoothly, it remains ahead of the thrust profile required to reach comet Borrelly in September 2001.

So it will continue to use impulse power until January 2, thus moving closer to the mathematically optimal route to the time and place at which it will encounter the comet.

Two major events will follow the end of impulse power thrusting: DS1 will throttle back up to full power that day, and this log will be updated the following weekend. On November 28, DS1 turned to point its main antenna directly at Earth and, like an excited tourist after a successful trip, spent several days reporting the details of its month-long vacation on the far side of the Sun. On December 1 it turned to a new thrustar.

Now DS1 has its attention fixed on a star in the constellation Cetus known as Eta Ceti or Deneb Algenubi. Deep Space 1 is now more than 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 almost 350 million kilometers, or over 217 million miles, radio signals, traveling at the universal limit of the speed of light, take nearly 39 minutes to make the round trip.

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Deep Space 1 Give Ion Power Extra Impulse
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.



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