Engineers and scientists involved with the Galileo mission to Jupiter have devised several creative techniques to enable the spacecraft to achieve the majority of its scientific objectives despite the failure of its main communications antenna to open as commanded.
Upgrades to Galileo's onboard computer software along with modifications of its groundbased communications hardware have been developed and tested by the Jet Propulsion Laboratory (JPL) in response to the high-gain antenna problem, which could have caused a profound loss of data during the orbiter portion of the mission. (The spacecraft's Jupiter atmospheric probe mission can be accomplished without the new techniques, but the upgrades will enhance the orbiter's ability to reliably record and retransmit the data collected by the probe as it descends on a parachute.) The new telecommunications strategy hinges on more effective use of the low-gain antenna, which is limited to a very low data rate compared to the main, high-gain antenna.
The switch to the low-gain antenna and its lower data rate means that far fewer data bits will be returned from Jupiter. However, new software on the spacecraft will increase Galileo's ability to edit and compress the large quantity of data collected by the spacecraft and then transmit it to Earth in a shorthand form. On Earth, new technology is being used to greatly sharpen the hearing of the telecommunications equipment that will be receiving Galileo's whisper of a signal from Jupiter. Together, these efforts should enable Galileo to fulfill at least 70% of its original scientific objectives.
Galileo's original mission plan called for the high-gain antenna to open shortly after launch. For the Venus-Earth-Earth Gravity-Assist (VEEGA) trajectory mission, however, the heat-sensitive high-gain antenna had to be left closed and stowed behind a large Sun shade to protect it during the spacecraft's passage through the inner solar system. During this portion of the journey, two small, heat-tolerant low- gain antennas provided the spacecraft's link to Earth. One of these S-band antennas, mounted on a boom, was added to the spacecraft expressly to bolster Galileo's telecommunications during the flight to Venus. The other primary low-gain antenna mounted to the top of the high-gain was destined to become the only means through which Galileo will be able to accomplish its mission.
Before being launched from the shuttle, the Galileo orbiter was tested in a space simulation chamber. The test chamber, located at JPL, is designed to subject spacecraft to approximately the same environmental conditions that they will encounter in space. The high-gain antenna can be seen fully extended.
In a crash effort over the next several weeks, a team of more than 100 technical experts from JPL and industry analyzed Galileo's telemetry and conducted ground tests with an identical spare antenna. They deduced that the problem was most likely due to the sticking of a few antenna ribs, caused by friction between their standoff pins and sockets.
The excessive friction between the pins and sockets has been attributed to etching of the surfaces that occurred after the loss of a dry lubricant that had been bonded to the standoff pins during the antenna's manufacture in Florida. The antenna was originally shipped to JPL in Pasadena by truck in its own special shipping container. In December 1985, the antenna, again in its own shipping container, was sent by truck to Kennedy Space Center (KSC) in Florida to await launch. After the Challenger accident, Galileo and its antenna had to be shipped back to JPL in late 1986. Finally, they were reshipped to KSC for integration and launch in 1989. The loss of lubricant is believed to have occurred due to vibration the antenna experienced during those cross-country truck trips.
Extensive analysis has shown that, in any case, the problem existed at launch and went undetected; it is not believed to be a result of sending the spacecraft on the VEEGA trajectory, delaying antenna deployment.
Engineers now conclude there is no significant prospect of the antenna being fully deployed and used during the mission. Laboratory tests verified that holding ribs 9, 10, and 11 in the stowed position most nearly modeled the spacecraft telemetry. Nevertheless, one last attempt will be made in March 1996, after the orbiter's main engine is fired to raise Galileo's orbit around Jupiter. This "perijove raise maneuver" will deliver the largest acceleration the spacecraft will have experienced since launch, and it follows three other mildly jarring events: the release of the atmospheric probe, the orbiter deflection maneuver that follows probe release, and the Jupiter orbit insertion engine firing. It is possible, but extremely unlikely, that these shocks could jar the stuck ribs enough to free the antenna. This will be the last attempt to open the antenna before radioing the new software to the spacecraft to inaugurate the advanced data compression techniques designed specifically for use with the low-gain antenna.
After arriving in orbit on December 7, 1995, Galileo will perform its tour of Jupiter as originally planned, including 10 close encounters with the giant gas planet's major moons over the 23-month orbital mission. The spacecraft will also return nearly uninterrupted streams of data about the complex charged particles and powerful magnetic field surrounding Jupiter. Galileo will return more than 1500 images of Jupiter and its moons. The technology innovations developed for the low-gain antenna operations will also expedite the return of 100% of the atmospheric probe data and provide a backup technique to guarantee the capture of the most important subset of data from the probe.
This Phase 2 software will provide programs to shrink the voluminous science data the Galileo orbiter will collect during its two-year mission, while retaining the scientifically important information, and to return that data at the lower data rate. The first use of the Phase 2 software will be to return data collected during the orbiter's final approach and arrival at Jupiter, including the close encounters with Europa and Io, observations of the atmospheric probe's Jupiter entry site prior to entry, and detailed profiles of the planet's inner magnetosphere.
Without enhancements, the low-gain antenna's data transmission rate at Jupiter would be limited to only 8-16 bits per second (bps), compared to the high-gain's 134,400 bps. However, the innovative Phase 2 software changes, when coupled with hardware and software adaptations at Earth-based receiving stations, will increase the data rate from Jupiter by as much as 10 times, to 160 bps. The data compression methods will allow retention of the most interesting and scientifically valuable information, while minimizing or eliminating less valuable data (such as the dark background of space) before transmission. Two different methods of data compression will be used. In both methods, the data are compressed onboard the spacecraft before being transmitted to Earth.
The first method, called "lossless" compression, allows the data to be reformatted back to their original state once on the ground. This technique is routinely used in personal computer modems to increase their effective transmission rates. The second compression method is called "lossy," a term used to describe the dissipation of electrical energy, but which in this case refers to the loss of some original data through mathematical approximations used to abbreviate the total amount of data to be sent to the ground. Lossy compression will be used to shrink imaging and plasma wave data down to as little as 1/80th of its original volume.
Another critical, ongoing DSN upgrade will be the addition of so-called Block V receivers at the tracking stations. These receivers, which are being installed for multimission use, will allow all Galileo's signal power to be dedicated to the data stream by suppressing the traditional carrier signal, thus allowing use of higher data rates.
Finally, starting early in the orbital tour, the 70-meter and two 34-meter DSN antennas at Canberra will be arrayed to receive Galileo's signal concurrently, with the received signals electronically combined. The arraying technique allows more of the spacecraft's weak signal to be captured, allowing a higher data rate to capture more data. Additional arrays are planned as well: the 64-meter Parkes Radio Telescope in Australia will be arrayed with the Canberra antennas, as will the 70-meter DSN antenna in Goldstone, California, when its view of Galileo overlaps with Canberra's.
Onboard data processing made possible by the Phase 2 software will allow the spacecraft to store and transmit nearly continuous observations of the jovian magnetosphere and extensive spectral measurements of the planet and its satellites in the infrared, visible, and ultraviolet, including more than 1500 high-resolution images.
While tens of thousands of images would be required for large-scale movies of Jupiter's atmospheric dynamics, the hundreds of images allocated to atmos-pheric imaging will allow in-depth study of several individual features in the clouds of Jupiter. Cooperative observations with Hubble Space Telescope investigators and groundbased observers have long been planned as part of the Galileo mission to provide information on the global state of Jupiter's atmosphere.
Like a tourist allotted one roll of film per city, the Galileo team will select its observations carefully at each encounter to ensure the maximum amount of new and interesting scientific information is returned. The imaging campaign will focus on the planet and the four large Galilean moons, but it will also cover the four inner minor satellites and Jupiter's rings. Eleven close satellite encounters will be conducted: one Io flyby (on approach), three of Europa, three of Callisto, and four of Ganymede. Five additional midrange encounters (from closer than 80,000 kilometers, or about 50,000 miles) with these moons will also occur.
The Galileo orbiter mission, with its sophisticated instruments, close satellite flybys, and long duration in jovian orbit, was specifically designed to answer many of the questions that the Pioneer and Voyager spacecraft were unable to answer. Scientists expect that Galileo can still achieve this goal despite the fact that the total volume of data has been reduced. Thus, when Galileo examines a class of phenomena, fewer samples of that class can be studied, and the spectral or temporal resolution will often be reduced to lessen the total volume of data. The resulting information, however, should nevertheless provide unique insight into the Jupiter system.
Some specific impacts from the loss of the high-gain antenna include elimination of color global imaging of Jupiter once per orbit; elimination of global studies of Jupiter's atmospheric dynamics such as storms, clouds, and latitudinal bands (efforts to image atmospheric features, including the Great Red Spot, are still planned, however); a reduction in the spectral and spatial coverage of the moons, which would provide context for study of high-resolution observations of their key features; and reduction of much of the so-called fields and particles microphysics (requiring high temporal- and spectral-frequency sampling of the environment by all instruments) during the cruise portion of each orbit. Most of the fields and particles microphysics, however, will be retained during the satellite encounters.
Galileo has already returned a wealth of surprising new information from the targets of opportunity it has observed on the way to Jupiter. Two first-ever asteroid encounters yielded close-up images of the asteroids Gaspra and Ida, and the extraordinary discovery of a moon, Dactyl, orbiting Ida.
The spacecraft was in a unique position to observe the remarkable impact of Comet Shoemaker-Levy 9 as its fragments slammed into Jupiter in July 1994. The observations provided key information on the initial impact, the duration, size, and temperature of the fireball, and also of the splash-back phase, when the material flung into space by the impact fell back into the planet's atmosphere.
Important scientific observations were also made of Venus, Earth, and the Moon during those flybys. These events served as demonstrations of the orbiter's ability to make excellent scientific observations with all its instruments. The Ida/Dactyl and SL-9 observations were returned to Earth at data rates substantially lower than those that will be available with the new Phase 2 capabilities.
In the face of adversity, the Galileo project team has succeeded in developing an innovative strategy to maximize the science return from the mission. Beginning in May 1996, Galileo's newly streamlined data pipeline should be providing a constant flow of new information, including an average of two to three images per day, through the end of the mission. Galileo's scientific instruments represent the most capable payload of experiments ever sent to another planet. The data they will return promises to revolutionize our understanding of the jovian system and reveal important clues that it holds regarding the formation and evolution of our solar system.
For more information: http://www.jpl.nasa.gov/galileo/