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U P D A T E # 3 2 Part 1: Testing
tape recorder strategies March 8, 1996 The Galileo spacecraft is performing normally in orbit around Jupiter, and has resumed playing back the atmospheric probe data. The probe scientists are planning to update their preliminary conclusions later this month at a scientific meeting in Houston, TX, and are preparing them for journal publication. Playback operations paused last week so the Galileo team could test operating strategies for using the tape recorder during the Ganymede encounter in June and the rest of the orbital science mission. Engineers are still studying the test results, which suggest possible modification to the operating strategies. Playback will be interrupted again next week for a planned major spacecraft maneuver, in which Galileo's Jupiter orbit will be reshaped, increasing the future close approach distance to Jupiter from 185,000 kilometers (115,000 miles) to 786,000 kilometers (488,000 miles) and setting up for the Ganymede encounter on June 27, 1996. Probe science playback will resume late in March. Operations in the Galileo testbed are continuing at a fast pace, preparing the spacecraft's new operating system to be transmitted in May. This laboratory collection of spacecraft spare and engineering-model hardware, including scientific instruments, is an imitation spacecraft used to test new software, commands, and sequences before they are tried out on the actual spacecraft. Most of the new software, almost three times as long as the one presently operating the Galileo spacecraft, has been exercised in the testbed once since the testing began in January. A complete second round is scheduled before the software is ready to be installed in the spacecraft in May. Nine scientific instruments and the two main spacecraft computers will be reprogrammed to construct the new Galileo system in orbit around Jupiter, roughly 700 million kilometers (400 million miles) away. Galileo is 19.3 million kilometers (12 million miles) from Jupiter, with an orbital speed near 450 meters per second or 1,000 mph. It is 18.5 million kilometers (11.5 million miles) from Ganymede. Radio signals take 47 minutes to reach Earth from the spacecraft. March 14, 1996 Shortly after noon Pacific Time today NASA's Galileo spacecraft fired its German-made main rocket engine for the fourth and last time to complete the set-up for its 11-orbit tour of Jupiter's system. Operating on a sequence of computer commands sent to the spacecraft a week ago, Galileo turned yesterday morning to the correct orientation for the rocket firing and then increased its spin rate to 10.5 rpm yesterday afternoon to stabilize the spacecraft during today's burn. The rocket firing started at 12:01 p.m. PST today, and stopped at 12:25 p.m. Tomorrow the craft will spin down and return to its normal orientation, with its antenna pointed toward Earth. The rocket engine, part of a propulsion system built by Messerschmitt-Bolkow-Blohm and furnished by the German government as a partner in the Galileo project, delivers 400 newtons or about 88 pounds of thrust. Acting on the 1-1/2-ton spacecraft for about 24 minutes, this force increased Galileo's speed in its orbit around Jupiter by about 377 meters per second or 842 miles per hour, nearly doubling its speed at the outer end of the orbit. Now Galileo is aimed for an 844-kilometer (524-mile) encounter with the satellite Ganymede on June 27. Equally important, Galileo will not pass so close to Jupiter as it would have otherwise, resulting in much less radiation exposure from the planet's trapped radiation belts. It passed through the intense inner radiation zone last December of necessity to accomplish the probe mission, and had no problems then, but the spacecraft was not designed for multiple passes through that hazardous region. Today the spacecraft is 19.3 million kilometers (12 million miles) from Jupiter, just past the farthest point in its orbit. Starting Saturday Galileo will begin to fall, and pick up speed, back toward Jupiter. Earth and the spacecraft are now less than 840 million kilometers (about 520 million miles) apart, as Jupiter and Earth approach each other in their solar orbits. As a result, Galileo's radio messages now take only 46 minutes, 19 seconds to get here. March 18, 1996 Scientists continuing to analyze information returned by the Galileo atmospheric probe that plunged into Jupiter last December report more surprises about the giant gas planet. Most significantly, the ratio of the elements that make up 99 percent of the Jovian atmosphere -- helium and hydrogen -- now closely matches that found in the Sun, suggesting that Jupiter's bulk composition has not changed since the planet formed several billion years ago. Estimated amounts of key heavy elements such as carbon and sulfur have increased, but minimal organic compounds were detected, and estimates for Jupiter's wind speeds have climbed still higher. Probe scientists are reporting these refined results today at the Lunar and Planetary Science Conference in Houston, TX. The ratio of helium to hydrogen by mass is key to developing theories of planetary evolution. In the Sun, this value is about 25 percent. During a January 1996 press conference, Galileo probe scientists estimated that this number for Jupiter was 14 percent. More comprehensive analysis of results from the probe's helium abundance detector has raised this estimate for Jupiter to 24 percent. "This increase implies that the amount of helium in the Jovian atmosphere is close to the original amount that Jupiter gathered as it formed from the primitive solar nebula that spawned the planets," according to Galileo probe project scientist Dr. Richard Young of NASA's Ames Research Center, Mountain View, CA. "The revised helium abundance also indicates that gravitational settling of helium toward the interior of Jupiter has not occurred nearly as fast as it apparently has on Saturn, where the approximate helium-to-hydrogen ratio is just six percent," said Young. "This then confirms that Jupiter is much hotter in its interior than its neighbor Saturn, the next largest planet in the Solar System. It also may force scientists to revise their projections for the size of the rocky core believed to exist deep in the center of Jupiter," he said. The new estimate of the helium-to-hydrogen ratio on Jupiter is supported by analysis of complementary data from the Galileo probe's neutral mass spectrometer. These new helium results are raising related estimates for the abundances of other key compounds, such as methane. Several heavy elements, including carbon, nitrogen and sulfur, are significantly greater in abundance on Jupiter than in the Sun. "This implies that the influx of meteorites and other small bodies into Jupiter over the eons since its formation has played an important role in how Jupiter has evolved," said Young. However, minimal organic compounds were detected, indicating that such complex combinations of carbon and hydrogen are rare on Jupiter and that the chances of finding biological activity on Jupiter similar to that found on Earth are extremely remote. The strong Jovian atmospheric winds continue to exceed expectations. Wind speed estimates announced in January of up to 330 mph have grown to more than 400 mph. The winds persisted far below the one cloud layer detected, strongly suggesting that heat escaping from deep in the planet's interior drives the winds, rather than solar heating. Since all the outer giant planets exhibit strong winds, scientists hope that understanding Jupiter's winds will lead to important new insights into their unusual meteorology, Young said. The scientists continue to report that the probe apparently entered Jupiter's atmosphere near the southern edge of a so- called infrared hot spot, which is believed to be a region of reduced clouds. "The probe's nephelometer observed only one distinct cloud layer, and it is tenuous by Earth standards. It is likely to be an ammonium hydrosulfide cloud," said Young. Three distinct cloud layers (an upper layer of ammonia crystals, a middle layer of ammonium hydrosulfide, and a thick bottom layer of water and ice crystals) were expected. Further analysis of probe data has confirmed the preliminary report that the Jovian atmosphere appears to be relatively dry, with much less water than anticipated on the basis of solar composition and predictions from data sent by the Voyager spacecraft that flew by Jupiter in 1979. These studies predicted a water abundance for the planet of twice the solar level (based on the Sun's oxygen content.) Actual probe measurements now suggest an amount of water less than that of the Sun. Scientists confirmed that the probe's instruments found much less lightning activity on Jupiter per unit area than on Earth. Lightning on Jupiter was found to be about 1/10th of that found on Earth in an area of the same size. "Although we found much less lightning activity, the individual lightning events are about ten times more energetic than similar events on Earth," Young said. "This is the sort of unique and exciting information that could not have been obtained in any way other than an atmospheric entry probe," Young said. Complete detailed results of the Galileo probe data analysis will be reported in the May 10 issue of Science magazine. The cone-shaped Galileo probe entered the atmosphere of Jupiter on Dec. 7, 1995, at a speed of over 106,000 mph and survived deceleration forces of 228 times Earth's gravity. After deploying a parachute, it relayed data to the Galileo orbiter spacecraft overhead for 57 minutes. The Galileo orbiter is beginning a two-year, 11-orbit tour of Jupiter and will have its first major encounter with a Jovian moon on June 27 when it flies closely by Ganymede. The orbiter successfully conducted a key engine burn on March 14 to prepare for this encounter. The Galileo probe project is managed by Ames. Hughes Space and Communications Co., El Segundo, CA, designed and built the probe. Lockheed Martin Hypersonic Systems (formerly General Electric), Philadelphia, built the probe's heat shield. NASA's Jet Propulsion Laboratory, Pasadena, CA, built the Galileo orbiter spacecraft and manages the overall mission. April 1, 1996 The Galileo spacecraft has resumed tape-recorder playback of atmospheric probe data, scheduled to conclude April 15. After the final main-engine maneuver on March 14, the spacecraft performed a series of engineering activities including checkout of the camera and the scan platform, followed by a final repetition of the of the "hammering" procedure, an attempt to free the stuck high-gain antenna. The camera and scan platform are operating nominally, but, as expected, the antenna remains stuck. The project team has analyzed an unexpected tank pressure situation in Galileo's propulsion system that was observed after the March 14 maneuver. The problem of possible internal leakage is in the helium pressurization system and is somewhat similar to a problem observed last July. Tank pressures can be controlled by maintaining appropriate temperatures with electric heaters in the system, which requires careful management of all spacecraft electric power loads. This strategy has been applied since July. Telemetry now suggests that at least one check valve is closed. All tank pressures are within acceptable limits. The Galileo engineers are continuing to develop and check out the new spacecraft operating system, which includes new programming for many of the science instruments as well as the attitude control and command and data computers. This mass of computer code is scheduled to be sent up to the spacecraft in May and June. The spacecraft is performing normally, spinning at about 3 rpm, collecting interplanetary dust and magnetic-field measurements and transmitting them to Earth, together with the tape playbacks and engineering telemetry, at 16 bits per second. Galileo is now just 19 million kilometers (11.9 million miles) from Jupiter, falling back toward the planet at about 892 meters per second (2,000 mph). It is 792 million kilometers (492 million miles) from Earth. Ron Baalke reports: Doppler plots of Galileo's Perijove Raise Maneuver on March 14 are now available on the Galileo home page: http://www.jpl.nasa.gov/galileo/ Ron Baalke baalke@kelvin.jpl.nasa.gov If this is your first message from the updates-jup list, welcome! We are presently in a down mode where an update will be sent about once per month. We hope to reactivate the project more fully after a variety of science data begins streaming in. The likely timeframe for any such reactivation is early 1997. To catch up on back issues, please visit the following Internet URL: gopher://quest.arc.nasa.gov:70/11/interactive-projects/jupiter/journals
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