by Dr Marc Rayman
Pasadena - October 26, 1999 - Sunday was the first anniversary of Deep Space 1's launch. Since leaving Earth, it has accomplished a wealth of testing of technologies that were deemed important for science missions but too risky to use without testing them on a bold mission like DS1 first.
The tremendous volume of data collected is still being digested by engineers and scientists, but there is no doubt that future missions will indeed benefit from the experiments DS1 conducted.
Some of the technologies are immediately ready for use, while the testing of others revealed unexpected behaviors or even problems that need to be changed before future science missions use them.
In all cases, DS1's objective of reducing the cost and risk of future missions has been a remarkable success. And now that the primary mission is complete, DS1 has turned its attention to conducting an ambitious science mission.
The spacecraft remains healthy as it continues on its way to intercept two comets in 2001 so that it can return important data on these intriguing objects.
To reach these appointments at just the right time and place, the advanced ion propulsion system is used to shape DS1's orbit around the Sun. The spacecraft had been in powered flight since July 30, with a hiatus of just over half a day each week to allow our old friend AutoNav to collect its pictures for navigation and to point the main antenna at Earth.
On Wednesday, October 20, shortly after 4:00 am PDT, the engine had accomplished the desired thrusting for this part of the orbit. Under AutoNav's control, the engine was turned off, and the spacecraft will coast until the middle of December, when a three month thrust period will begin.
In its year of traveling through space, DS1 has journeyed nearly 800 million kilometers, or almost half a billion miles. That sounds pretty impressive, but consider that Earth, carrying some of the listeners to this recording, has traveled almost 950 million kilometers, or a bit less than 600 million miles.
To get an idea of the beautiful physics that explains the reason the spacecraft slowed down when it left Earth, think of the Sun being at the bottom of a hill. Earth is part way up that hill, but DS1's celestial targets are even higher. So as the ion propulsion system has pushed on the spacecraft, its real effect has been to help it climb that hill. But the Sun continues to tug on the spacecraft, so as DS1 ascends, it is slowed by the relentless force of the Sun's gravity.
The ion propulsion system has operated for nearly 5 months of the mission, but in that time it has used less than 22 kilograms, or under 50 pounds, of its xenon propellant.
If the spacecraft were not fighting the Sun's gravity, this small amount of xenon would have been enough to increase its speed by over 1300 meters per second, or around 2900 miles per hour. To achieve the same effective change in speed with conventional propulsion would have required 250 kg, or 550 pounds, of propellants.
Now that DS1 is coasting, the spacecraft will be commanded to conduct more detailed tests and calibrations with the two science instruments that will collect data at the comets.
The Plasma Experiment for Planetary Exploration, affectionately known to the most devoted of listeners as PEPE, was first activated and tested in December. PEPE measures the energy, composition, and direction of movement of the constituents of plasmas, which are collections of charged particles, both electrons and charged atoms, or ions.
PEPE has operated with its electronic sensors set at 8000 volts so far, but when it reaches the comets, to achieve its full measurement capability for the complex ions the comets produce, PEPE may need to be boosted to its maximum of 15,000 volts.
During the next few weeks, the power supplies will be turned up and controllers will measure its performance and watch the instrument carefully to be sure that no problems develop. This week, PEPE is scheduled to be brought up to 11,000 volts.
Also to be tested during this coasting period is the combination visible camera and imaging spectrometer. An imaging spectrometer allows the construction of a picture in which each small element of the picture, known as a pixel, contains information on the spectrum of light; that is, the light is broken into its individual colors, as when you look through a prism.
The imaging spectrometers in DS1 operate in the ultraviolet and infrared, and the resulting data allow scientists to determine, among other things, the chemical composition of objects being viewed. At asteroid Braille for example, bonus science data returned revealed the nature of the minerals making up the surface of the asteroid (although the initial interpretation of the data has been replaced following more detailed analyses).
Traditional spacecraft would have 3 separate devices to accomplish all the functions of this one. But for NASA to launch smaller, more cost-effective missions, it will be important to integrate these functions into small packages.
Thus, DS1 tested a miniature integrated camera spectrometer, which, following the tradition of inspirational naming, is known by its initials as MICAS. During the technology testing phase of the mission, several problems were found with this instrument.
When the ultraviolet spectrometer was discovered not to be working, a number of diagnostic experiments were conducted. Next month further tests will be conducted to investigate new ways to make it operable.
Also, unwanted stray light reaches the cameras, which are not as sensitive as expected, and analysis has shown some distortions in the pictures. So MICAS will take pictures of selected star fields to allow a more comprehensive characterization of its performance to prepare for the comet encounters.
More calibrations will be performed with the infrared spectrometer, and it will collect some distant but valuable scientific observations of Mars.
As the experiments are conducted between now and early December, your loyal correspondent will provide progress reports and other news of the mission.
Deep Space 1 is now about 57% farther away from Earth than the Sun is and 615 times as far as the moon. At this distance of 235 million kilometers, or more than 146 million miles, radio signals, traveling at the universal limit of the speed of light, take over 26 minutes to make the round trip.
DEEP SPACE ONE
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