by Dr Marc Rayman
Pasadena - November 14, 1999 - Deep Space 1 has spent most of the last three weeks engaged in activities to prepare for its encounters with two comets in 2001. Meanwhile, it has continued coasting in its orbit around the Sun, with its ion propulsion system turned off.
The combination visible camera and imaging spectrometer, known to DS1 enthusiasts as MICAS, collected a wealth of new data to aid scientists and engineers in understanding details of its abilities to take pictures and spectra under a variety of conditions.
Targets with well known properties, including Mars, Jupiter, and selected stars, were viewed so that the instrument's electronically recorded pictures and spectra could be compared with data collected elsewhere. Some of the experiments included viewing a target with a range of exposure times; in others, the target was placed in different locations within the instrument's field of view.
Analysis of the resulting data will aid in selecting exposure times and controlling MICAS' pointing for the comet encounters and will be of great importance in interpreting the images of those unexplored bodies.
Also, while the visible camera has a relatively large field of view, the infrared spectrometer has a very narrow view, so determining exactly how to point it is difficult.
Thus, many snapshots were taken with the infrared detector as the spacecraft made minute changes in where it was pointed. This will allow engineers to determine the optimum way to point it in the future.
Other esoteric data were collected as well, all contributing to a better characterization of MICAS' performance, including the effects of unwanted stray light and the small, but nonetheless larger-than-expected, distortions of its black and white images.
New data were collected with the ultraviolet detector, which has never functioned properly, as part of a continuing effort to see if it can be made operational.
Like all spacecraft, DS1 is not perfectly stable, but rather turns back and forth slowly in the frictionless environment of space. Small thrusters using conventional rocket propellant fire occasionally to keep it pointed in the direction that has been commanded, so it gradually moves until the sensors determine that it is getting to its acceptable limit, and the computer fires a thruster to turn it back.
After a while it drifts to the opposite limit, and again a thruster fires for a fraction of a second to reverse the direction. This motion is normal, and engineers call it deadbanding.
When a long exposure is taken with the camera, the movement of the spacecraft can reduce the picture quality. As a result, it is difficult to image very very faint targets.
So DS1 is testing a new idea. When the ion propulsion system is thrusting, in addition to pushing the spacecraft along, its famously low thrust provides a more gentle control over the spacecraft orientation.
Thus, the spacecraft will turn more slowly, allowing longer pictures to be taken. So a test was conducted in which the ion engine was on for about four hours, during which pictures of faint stars were taken.
PEPE Up In Deep Space
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.
So far, PEPE has operated with its electronic sensors for ion composition set to range from -8000 volts to +8000 volts, but scientists predicted that when it reaches the comets, to achieve greater measurement capability for the composition of complex ions the comets produce, PEPE may need to be boosted to its maximum range of -15,000 volts to +15,000 volts.
On October 25 the power supplies were turned up gradually, reaching 11,000 volts. To be safe, the instrument was turned back down to 10,000 volts until the next week. In the next phase, on November 1, the power supplies were turned up slowly to 13,000 volts, but at 12,750 volts the positive side unexpectedly dropped to +5500 volts.
The data are still being analyzed, but it appears that PEPE may be limited to operate from -8000 volts to +5500 volts for measuring ion composition. This does not affect the measurements of electrons or of the energy and the direction of ions; but it does mean that some of the heavy comet ions may not be measured.
PEPE's data on the solar wind, the stream of charged particles flowing from the Sun, will be unaffected. In addition, PEPE is still well suited to make exciting measurements of the complex structure and behavior of the cometary tail and coma, or the expanding cloud of gas around the comet.
Because of DS1's great distance from Earth, its small antenna can return data at only a very limited rate. So extra tracking from NASA's Deep Space Network, the worldwide system of antennas for communicating with probes in deep space, is needed to return the large volume of special data that is being collected.
Solving Problems Alone In Deep Space
The star tracker, imaginatively so named because it tracks stars, helps determine the spacecraft's orientation; this is not one of the 12 advanced technologies whose testing was the focus of DS1's primary mission, but it is a new and sophisticated device.
The protective software turned the power off and then on again, but that did not help. After waiting and repeating its unsuccessful attempts to fix the star tracker, the software placed DS1 in one of its predefined safe configurations known as Sun standby SSA.
In this state, nonessential devices are turned off, the star tracker is not used, and the craft is pointed at the Sun, the only easily recognizable target from its vantage point in the solar system.
The event was discovered by controllers during Friday's scheduled communications session with the Deep Space Network, and now engineers are collecting data from the spacecraft to determine its exact condition.
The star tracker has displayed many unexplained intermittent failures to report its orientation to the spacecraft computer properly since shortly after launch. Indeed, just over two weeks after launch the star tracker's inability to provide data led to software taking nearly the same action (the programmed response has been slightly altered since then).
In all previous cases however, the device resumed normal operation within less than an hour, and usually in less than a minute. This time, the star tracker has not yet resumed functioning correctly.
Since the problems began over a year ago, Deep Space 1 has been working with the manufacturer of the star tracker and with other missions planning to use the same apparatus to try to understand its problems. Various experiments have been conducted with similar units in laboratories, and special data channels were activated on DS1 to gain greater insight into the device's operation.
To date, none of these investigations has revealed the source of the problems. For now, DS1 will be left in Sun standby SSA until all the data on the spacecraft health can be returned and analyzed and controllers can design the next steps.
Deep Space 1 is now 60% farther away from Earth than the Sun is and over 625 times as far as the moon. At this distance of 240 million kilometers, or 149 million miles, radio signals, traveling at the universal limit of the speed of light, take nearly 27 minutes to make the round trip.
DEEP SPACE ONE
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