Space & Astronomy

home *** CD-ROM | disk | FTP | other *** search

/ Space & Astronomy / Space and Astronomy (October 1993).iso / mac / TEXT_ZIP / jplnews / 0529.ZIP / 0529.PR

Wrap

Text File | 1993-04-21 | 33KB | 659 lines

OFFICE OF PUBLIC INFORMATION JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION PASADENA, CALIFORNIA. TELEPHONE 354-5011 ________________________________ The Mariner 6 and Mariner 7 spacecraft will fly past Mars on the nights of July 30 and August 4, 1969, respectively. Time of closest approach is now estimated at 10:18 p.m. PDT, July 30, for Mariner 6 and 10:00 p.m. PDT, August 4, for Mariner 7. Altitude at encounter will be about 2000 miles for each spacecraft. Mariner 6, launched from Cape Kennedy on February 24, will fly a total of 241 million miles in 156 days. Communications distance from Mars at encounter will be 59.5 million miles (about 5 1/2 light minutes). Total Earth-to-Mars distance to be travelled by Mariner 7, launched March 27, is 197 million miles in 130 days. Communi- cations distance at encounter will be 61.8 million miles. Both spacecraft were boosted into space by Atlas-Centaur launch vehicles. The Mariners were developed and their missions are conducted for the National Aeronautics and Space Administration by the Jet Propulsion Laboratory in Pasadena, California. Mariner 6 will examine the equatorial regions of Mars. Mariner 7 will cover some of the same area, but will concentrate on the southern hemisphere and a portion of the south polar cap. -2- Together, they are expected to furnish data as different as possible from the standpoint of geography and climate. The mission follows the 1964-65 flight to Mars by Mariner 4 and is a precursor to the 1971 and 1973 Mars missions. Mariner 4 was the only other spacecraft to have photographed another planet. In 1971 two Mariner-class vehicles will orbit Mars for three months, and in the 1973 mission, Project Viking, two spacecraft will orbit Mars and detach landing craft to descend to and operate on the surface. Mariner 1969 mission objectives are to study the surface and atmosphere of Mars to establish the basis for future experiments in the search for extra-terrestrial life and to develop technology for future Mars missions. The 1969 flights will not determine the presence of life on Mars but will help establish whether or not the Martian environment is suitable for life. Television cameras aboard each spacecraft will photograph the full disc of Mars during the approach to the planet and selected surface areas at high resolution during the close Mars passage. Thermal mapping of the areas photographed will be provided by an infrared radiometer (IRR) to correlate temperatures with surface visual appearance. A principal goal of the experi- ment is to determine whether the Martian polar caps are frozen carbon dioxide or frozen water. -3- The chemical constituents of Mars' upper atmosphere will be measured by an ultraviolet spectrometer (UVS). The experiment will identify and measure the distribution of a number of gases in the atmosphere--principally oxygen, nitrogen, and perhaps hydrogen. Composition of the lower atmosphere and possibly the surface of Mars will be determined from measurements by an infrared spectrometer (IRS). The instrument may be able to detect the presence of some organic molecules in atmospheric concentrations as small as two parts in one-million. An occultation experiment, in which the Mariners disappear from Earth behind Mars and their radio signals pass through the Martian atmosphere, will yield information on atmospheric pressures and densities. Radio tracking data during encounter, as well as throughout the entire flight, contributes to still another experiment--celestial mechanics--which will provide information to refine astronomical data. The Mariner encounter can be divided into three phases-- far encounter or approach to Mars; near encounter or close passage by Mars; and playback of recorded near encounter science data after the fly by. _________________________________ As the two Mariners approach Mars, they will take a series of TV pictures while the planet revolves through several -4- Martian days (a Martian day is 24 hours, 37 minutes). The pictures will reveal general surface features not visible from Earth and the planet will be photographed at all longitudes. Only the north pole area will not be covered in the pictures. Some information may be obtained on the formation and motion of clouds and other Mars meteorological phonomena. Mariner 6 will begin taking full planet pictures two days before it reaches Mars, Mariner 7 about three days before encounter. (SEE PAGES 12 and 13). A new high-rate telemetry system--16,200 bits per second--on the Mariners and the use of the 210-foot antenna at the Goldstone Space Communications Station in the Mojave Desert allows the two spacecraft to record and play back an enormous amount of picture data during the approach to Mars. In the standard mission, programmed into the on-board computer prior to launch, Mariner 6 will take 50 approach pictures beginning 48 hours and 770,000 miles from Mars and ending 7 hours and 112,000 miles from Mars. Mariner 7 will take 93 approach pictures beginning 72 hours and 1,140,000 miles from Mars and ending 4 hours and 65,000 miles from Mars. Only TV camera B, the high resolution camera, will be used for taking far encounter pictures. Each spacecraft must receive and act upon certain ground commands to initiate the standard mission sequence. These commands must be transmitted to Mariner 6 about 52 hours prior to closest approach and to Mariner 7 about 76 hours before its closest approach. -5- (As a backup to the standard mission in the event that certain problems occur between now and encounter, a conservative mission has been designed and programmed into each spacecraft to operate on an automatic basis or by specific command. It consists of eight approach pictures taken by each spacecraft between 22 and 11 hours before closest approach. The pictures would be stored on tape and played back at the normal science playback data rate--270 bits per second--after the spacecraft passes Mars. The near-encounter sequence would remain the same as in the standard mission. Neither high-rate telemetry nor ground command capability is required to conduct the conservative mission.) If both Mariner 6 and Mariner 7 conduct the "standard mission," it is possible to acquire as many as 143 far encounter TV pictures. The high-rate telemetry system, the 210-foot antenna at Goldstone and a microwave link between Goldstone and the Jet Propulsion Laboratory in Pasadena, permits the real time display of the pictures as they are played back from each spacecraft. Approximately 12 hours of real time TV may be available in five playback sessions. Every five minutes, a new picture-- each containing more than half-a-million photo elements--is seen on monitors at JPL. The disc of Mars gets larger with successive pictures until the planet fills, then spills over, the edges of the frame. (SEE PAGES 12 and 13). The five playbacks occur as follows: -6- Mariner 6, 33 pictures, 7/29 6:35 p.m. - 9:27 p.m. PDT Mariner 6, 17 pictures, 7/30 6:00 p.m. - 7:27 p.m. Mariner 7, 34 pictures, 8/2 6:05 p.m. - 9:00 p.m. Mariner 7, 34 pictures, 8/3 7:24 p.m. - 10:19 p.m. Mariner 7, 25 pictures, 8/4 6:08 p.m. - 8:19 p.m. Each of the five real-time TV playbacks occurs during the evening hours California time due to the 210-foot antenna view period. ______________ Mariner near encounter can be defined as a one-hour period beginning 35 minutes before closest approach to Mars and ending when the spacecraft re-appears from behind the planet. Duration of near encounter including occultation is 68 minutes zfor Mariner 6 and 74 minutes for Mariner 7. At about 15 minutes before closest approach, the two TV cammeras--shuttering alternately every 42 seconds--the IR radio- meter, IR spectrometer and UV spectrometer will begin taking planetary data, some of which is transmitted directly to Earth and all of which is recorded on board the spacecraft. During near encounter, real-time transmission of data to Earth will be at the high-rate 16,200 bits per second. It will include every seventh TV picture element for photometric measurements. î Receipt of the entire picture on Earth will occur during the post-encounter tape recorder playback. Near encounter TV totals 24 pictures--12 high resolution and 12 medium resolution-- during a period of about 17 minutes. The Mariners reach their -7- nearest proximity to Mars during the last few minutes of the close-up TV sequence. When the TV swath of overlapping pictures crosses the day/night terminator, picture recording ceases. The other instruments continue taking and recording dark-side data out to and beyond the limb of Mars about 10 minutes after closest approach. Occultation--that period when Mars is between the spacecraft and Earth--begins several minutes after the end of science recording and lasts about 20 minutes for Mariner 6 and 29 minutes for Mariner 7. The occultation data, from which can be determined the density of the Martian atmosphere, is obtained at Earth tracking stations at both entry and emergence from behind the planet. Tracking data obtained throughout encounter as well as during the entire flights, contributes to the celestial mechanics experiment. _______________________ Following occultation, the near-encounter science data recorded on two tape recorders--one analog, one digital--aboard each Mariner is played back. The digital recorder, which stores all near encounter data, including TV, is played back at the normal science playback rate, 270 bits per second. At about 19 hours after closest approach for both Mariners, the digital playback is interrupted for a five-hour playback of the analog recorder (TV near encounter only) at the high rate, 16,200 bits -8- per second. Mariner 6 playback is interrupted also for the Mariner 7 far encounter sequence. After both spacecraft have completed the playback several times--about August 17--they continue to provide additional tracking and spacecraft performance information until the mission is terminated. -0- 529-7-28-69 -9- ________6______________________________ ___________________________ EVENT _____________________ 6:19 p.m. Ground command, transmitted from Goldstone, turns on Mariner 6 science power and starts shuttering TV camera. 8:49 p.m. Mariner 6 scan platform is pointed at Mars so that Far Encounter Planet Sensor (FEPS) sees planet and begins tracking Mars' center of brightness to keep TV camera pointed accurately at Mars. 10:26 p.m. Mariner 6 takes first of 33 far encounter pictures from a distance of 771,500 miles. Entire sequence consumes 19 hours, 44 minutes, with one picture taken each 37 minutes. ________________ 6:10 p.m. Mariner 6 takes picture #33. 6:35 p.m. Mariner 6's first picture (M6-1) is received at JPL following high-rate transmission from the spacecraft. All 33 pictures are displayed on TV monitors at JPL as they are received about five minutes apart. Playback duration for 33 pictures is 2 hours, 52 minutes. 9:27 p.m. Receipt of M6-33 is completed. __________________ 12:23 a.m. Mariner 6 takes picture #34 (M6-34), the first of a series of 17 pictures. One picture is taken each 56 minutes during a duration of 15 hours, 56 minutes. 3:19 p.m. Mariner 6 takes picture #50 at altitude of 111,950 miles. 6:00 p.m. Beginning of real-time receipt and display of M6-34. Playback duration for 17 pictures is 1 hour, 27 minutes. -10- (PDT) EVENT ________________________________ 7:27 p.m. Receipt of M6-50 is completed. 9:43 p.m. Start Mariner 6 near encounter sequence with cooldown of Infrared Spectrometer. 10:03 p.m. Mariner 6 begins recording data from science instruments--Infrared Spectro- meter, Ultraviolet Spectrometer, Infrared Radiometer. 10:04 p.m. High and medium resolution TV cameras each take 12 pictures with the medium resolution pictures overlapping and high resolution covering small areas within the overlaps. Twenty-four pictures are recorded in 17 minutes. 10:18 p.m. Mariner 6 makes its nearest approach to Mars. Estimated altitude is about 2000 statute miles. 10:21 p.m. Mariner 6 takes last near encounter TV picture (M6-74). Other science instru- ments continue taking and recording data into the martian night. 10:28 p.m. End recording Mars science. 10:34 p.m. Start playback to Earth of science data recorded on spacecraft's digital recorder during near encounter. 10:36 p.m. Enter occultation. Ground station at Goldstone loses Mariner 6 radio signal as spacecraft disappears behind Mars. 10:56 p.m. Exit occultation. Goldstone regains Mariner 6 radio signal as spacecraft emerges from behind Mars. Digital science playback continues. -11- (PDT) EVENT _________________ 5:36 p.m. Start high-rate playback of Mariner 6 (approx.) near encounter pictures (M6-51 to M6-74) from spacecraft's analog tape recorder. (Near encounter pictures will be played back twice during this session. They will not be displayed on TV monitors.) 11:17 p.m. End high-rate playback M6-74. Resume (approx.) digital playback until interrupted for Mariner 7 far encounter TV. ______________________ 5:53 p.m. Transmit ground command to Mariner 7 to turn on power for science instruments and start shuttering TV cameras. 8:23 p.m. Mariner 7 science scan platform slews to far encounter position. 9:59 p.m. Mariner 7 takes M7-1, first of a total of 93 far encounter pictures of Mars, 34 of which are taken during this first of three sequences. Duration of 34-picture sequence is 19 hours, 48 minutes. A picture is taken each 36 minutes. M7-1 is taken from a Mars altitude of about 1,140,000 miles. __________________ 5:47 p.m. Mariner 7 takes M7-34. 6:05 p.m. Start playback M7-1 and continue real- time display of 34 pictures, one each five minutes. Playback sequence lasts 2\hours, 55 minutes. 9:00 p.m. End playback M7-34. 10:59 p.m. Mariner 7 starts second series of 34 far encounter pictures, recording one frame each 36 minutes for 19 hours, 48 minutes. -12- (PDT) EVENT ________________ 6:47 p.m. Mariner 7 takes picture #68. 7:24 p.m. Start playback M7-35. Duration of playback sequence is 2 hours, 55 minutes. 10:19 p.m. End playback M7-68. ________________ 1:01 a.m. Mariner 7 takes frame M7-69. This final far encounter series numbers 25 pictures taken at 42-minute intervals. The series consumes 17 hours, 48 minutes. The last picture, M7-93, is taken from a Mars distance of 65,550 miles. 5:49 p.m. Take M7-93. At this point, Mariner 7 is 4 hours and 11 minutes from its closest approach to Mars. 6:08 p.m. Mariner 7 plays back final series of far encounter TV pictures, M7-69 to M7-93. Receipt of all 25 pictures takes 2 hours, 11 minutes. 8:19 p.m. End playback M7-39. 9:25 p.m. Begin Mariner 7 near encounter with IRS cooldown. 9:45 p.m. Start recording Mars science, including 24 near encounter TV pictures (frames M7-94 to M7-117). 10:00 p.m. Closest approach to Mars (about 2000 miles). 10:02 p.m. Take TV frame M7-117, concluding TV recording. Continue recording other science data. 10:10 p.m. End recording Mars science. 10:15 p.m. Enter occultation. Goldstone loses Mariner 7 radio signal. -13- (PDT) EVENT _________________________ 10:16 p.m. Near encounter science data begins playing back, although it begins while Mariner 7 is behind Mars. 10:44 p.m. Exit occultation. Goldstone regains Mariner 7 radio signal and receipt of Laboratory in P Two-huf Mars were taken by the two Mariners, including 57 high and medium resolution views of selected Martian surface areas from an altitude of only a few thousand miles. The bulk of photos was taken as Mariners 6 an Mariner 7 mad 4, tone station in on the evening of Julyplatform was pointed at Mars so that a light sensitive sensor would track the planet's center of brightness to keep the TV camera pointed accurately at Mars. Mariner 6 took the first of 50 approach pictures at 10:26 p.m. PDT on July 28 from a distance of 771,500 miles. Although the camera took a picture each 84 seconds, only one picture each 37 minutes was recorded on the spacecraft's analog -4- tape recorder. (A second TV camera also was operatting-- alternately with the first--but its pictures were recorded only during the close passage.) Nearly 20 hours later, a full tape load of 33 pictures had beecture elements wseen on the monitors. Seven potal of more than 17 hours of real-time TV display. In addition to the 200 pictures stored on the analog recorder and played back, Mariner 6 and 7 transmitted to Earth 1177 digital pictures as they were taken every 42 seconds by the cameras. The digital pictures contained every seventh picture element of each picture line and are used for photometric measurements. On July 30, just seven hours before Mariner 6 was to cross the orbit of Mars, the radio signal from Mariner 7 fell silent. The Deep Space Station at Johannesburg reported loss of signal at 3:11 p.m. PDT. It was believed, and the cause is still uncertain, that Mariner 7 may have been struck by a meteoroid. One of the Deep Space network's two Madrid stations was tracking Mariner 6. The other broke off tracking Pioneer 8 and joined the search for Mariner 7. As the Earth turned under the spacecraft and Mariner 6 neared Mars, three Goldstone stations came into view. The Pioneer Station at Goldstone picked up a very faint signal about 10:30 p.m. PDT on July 30, just a few minutes after Mariner 6 whipped by Mars. Commands were trans- mitted to Mariner 7 to switch antennas and 11 minutes later--round -6- trip light time between Earth and Mariner 7--a healthy signal was detected by stations at Goldstone and in Australia. It was learned that Mariner 7 had lost lock with its celestial reference, the star Canopus. Hence, the high-gain antenna no longer pointed at Earth. Canopus lock was re-estab- lished and further tracking indicated Mariner 7 had been damaged, including the loss of some 20 of 90 telemetry channels. Mariner 7 also had changed velocity. It apparently was receiving a small amount of thrust, possibly from an outgassing pressure vessel. The measured continuing acceleration of a few millimeters per second changed the spacecraft trajectory slightly, causing it to arrive 10 seconds later than predicted but very close to the predicted altitude. The Mariners began their near encounter sequences 35 minutes before closest approach with the cryogenic cooldown of one of the science instruments--the infrared spectrometer. At encounter minus about 15 minutes, the two TV cameras--shuttering alternately every 42 seconds--the infrared spectrometer, infrared radiometer and ultraviolet spectrometer began taking planetary data, some of which was transmitted directly to Earth and all of which was recorded on board the spacecraft. Mariner 6 took 24 near-encounter pictures--12 high resolution and 12 medium resolution--during a 17-minute period. It reached its nearest proximity to Mars during the last few minutes of the close-up TV sequence. -7- Mariner 7 took 33 near-encounter pictures--16 high resolution and 17 medium resolution. The scan platforms on both spacecraft were slewed four times during the Mars passage, resulting in a multi-segment trace across the planet. The Mariner 7 scan program had been revised on the basis of the Mariner 6 data to provide maximum coverage of the south polar cap. When the TV swath of overlapping and nested pictures crossed the day/night terminator, picture recording ceased. The other instruments continued taking and recording dark-side data out to and beyond the limb of Mars about 10 minutes after closest approach. All instruments aboard Mariner 7 functioned perfectly. On Mariner 6, one of the infrared spectrometer's two channels did not get cold enough to operate. Following occultation, the near-encounter science data recorded on two tape recorders--one analog, one digital--aboard each Mariner was played back. The digital recorder, which stored all near encounter data, including TV, was played back at the normal science rate, 270 bits per second. At about 19 hours after closest approach for both Mariners, the digital playback was interrupted for a six-hour playback of the analog recorder (TV near encounter only) at the high rate, 16,200 bits per second. By mid-August both spacecraft had played back all the science data several times. The two Mariners remain in their solar orbits and are tracked periodically by stations of the Deep Space Network. -8- Two experiments which required no special instrumenta- tion were conducted by Mariner 6 and 7. One was occultation which provided atmospheric pressure measurements through the analysis of changes in the spacecraft radio signal as the spacecraft disappeared behind Mars relative to Earth. Tracking data obtained throughout encounter, as well as during the entire flights, contributed to the celestial mechanics experiment. The Mariner Mars 1969 Project provided the United Statared Spectrometeon of the lower Martias Dr. George C. Pimentel of the University of California at Berkeley. Co-investigator is Dr. Kenneth Herr, also of U.C.B. The Infrared Radiometer made Mars surface temperature measurements. Principal investigator is Dr. Gerry Neugebauer of Caltech. Co-investigators are Dr. Guido Munch, Caltech, and Stillman C. Chase of the Santa Barbara Research Center. -10- The S-Band Occultation Experiment determined the pressure and density of the Martian atmosphere. Principal investigator is Dr. Arvydas J. Kliore of JPL. Co-investigators are Dr. S. I. Rasool, Goddard Institute of Space Studies; Gunnar Fjeldbo, Stanford University; and Boris Seidel, JPL. The Celestial Mechanics Experiment provided data for the continuing effort to refine astronomical values. Principal investigator is Dr. John D. Anderson of JPL. Co-investigator is Warren L. Martin, also of JPL. -11- PRELIMINARY SCIENCE RESULTS MARINER MARS 1969 TELEVISION The principal results from preliminary study of the tian atmosphere. anti- ates this conclusion, a very key chemical compound is missing from the Martian environment. If this is true, any life chemistry on Mars will have to be very much different than we know on Earth. The Mariner 7 UV spectrometer measured the composition of the upper atmosphere on two passes over the limb of the planet and confirmed the results of Mariner 6 in finding no molecular nitrogen in the upper atmosphere. The new result associated with Mariner 7 comes from the pass over the polar cap. The intensity of ultraviolet light from the planet increased abruptly as the spectrometer view passes from the desert onto the polar cap showing that ultraviolet radiation at very short wavelengths penetrates to the surface of the planet. This result shows that the planet is bathed in this energetic solar radiation. INFRARED SPECTROMETER The Mariner 6 IR spectrometer successfully recorded data in the spectral region 2 to 6 microns. With 60 by 60 mile spatial resolution, significant thermal variations were detected, revealing temperatures up to 75'F. The data also reveal the local reflectivity of solar light and it is clear that the darker -13- spots on Mars are warmer than the bright areas. The carbon dioxide intensity reveals topographical detail that remains to be analyzed. Neither ammonia nor nitric oxide was present in the Martian atmosphere. Carbon monoxide was detected. Perhaps the most exciting result is that the spectrum of ice was recorded. It seems unlikely that this ice could be in the spectrometer--the only possible misinterpretation--and we tentatively attribute it to a very thin ice fog. Finally, there are two or three uncertain spectral features that remain to be verified and identified. The Mariner 7 Infrared Spectrometry experiment produced data from both channels. These data were first believed to have contained evidence of gaseous methane and ammonia, and the suggestion that a portion of the south polar cap is composed of water ice. Further analysis of the data is required before a firm conclusion can be reached. INFRARED RADIOMETER The heat radiated by areas approximately equal to thbserved. A smaller ledge of 63 miles. Mariner 7: A pressure of 3.5 millibars and a tempera- ture of 205'K (-90'F) was obtained at a latitude of 59'S and Longitude 28'E. The low value of surface pressure may indicate that this region (near Hellespontica Depressio) is substantially higher than the average (about 4 miles). An ionosphere was also observed, with a mean peak at an altitude of about 130 kilometers (81 miles). -15- CELESTIAL MECHANICS Function of the celestial mechanics experiment is to investigate the effects of gravity on the spacecraft range and Doppler tracking data. These data are affected directly by the predictions of general relativity and also indirectly by gravita- tion effects on the spacecraft trajectory. By analyzing three months of data from Mariners 6 and 7, it has been possible to determine a ratio of the mass of the Earth to that of the Moon of 81.3000 with an uncertainty of 0.0015. Although we think of the Moon as revolving about the Earth, in fact both the Earth and Moon are revolving about their common center of mass which is at a distance of about 2900 miles from the center of the Earth. The Earth's motion about this point at a speed of about 27.8 miles per hour is reflected in the Mariner data and it is possible to deduce that the Earth is about 81.300 times more massive than the Moon. Another result of the early analysis of the data is that the mass of Mars is about one tenth the mass of the Earth; the exact figure is 0.1074469 _ 0.0000035. The only other precise determination of the mass was obtained from the Mariner 4 Doppler data. A recent re-examination of these data by George W. Null at JPL indicates a value of 0.1074464 with an uncertainty of _ 5 units in the last place; Mariners to Mars in 1964 and 1969 agree very well in their determinations of the mass. Tracking data from Mariner 6 indicate that it approached the planet at a speed of 16,113 miles per hour. This was increased to a speed of 17,953 mph at closest approach because of the gravity field of -16- the planet, a total increase of 1840 mph. In addition the closest approach to Mars was decreased from an altitude of 2619 miles to 2134 miles and the trajectory was deflected because of the gravity field. The precise measurement of these changes in the trajectory resulted in a determination of the mass of the planet. Further analysis should also yield information on the shape of the gravitational field and on the orbit of Mars itself. This will be important to future, more advanced missions to Mars and to an extended analysis of the data for the general relativistic effects.