SP-424 The Voyage of Mariner 10


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painting of Mariner flying over a planet with the Sun rising

Chapter 5

Venus Bound - Success and Near Failure

 

[45] FOR BEST SCIENCE RETURN, the spacecraft had to be launched during a short, 1.5 hour "window" on November 2, 1973. All had to be ready: people, electronics, a worldwide operation-men and women at the Jet Propulsion Laboratory in Pasadena, at tracking stations around the world, at the launch center at Cape Kennedy where the gleaming spacecraft protected by polished thermal blankets rested securely within the shroud atop the Atlas/Centaur on Launch Complex 36B.

All was ready for the epic mission to explore Mercury, closest planet to the Sun, mothlike orbiter in the solar glare; the final countdown had proceeded without a hitch. Then, at 12:45 a.m. Eastern Time, within a few thousandths of a second of the scheduled launch time, Atlas/ Centaur No. 34 blossomed into life as its triple engines turned night into day at the launch complex and pounded the eardrums of observers outside the blockhouse. Mankind's first explorer of the planet Mercury was on its way (Fig. 5-1).

For about 15 seconds Atlas/Centaur 34 rose vertically, then began its programmed pitch along its path toward space. Exactly as scheduled, the outer engines of the Atlas lost their fiery exhaust trails at just over two minutes after liftoff and....

 


Fig. 5-1. Within a few thousandths of a second of the opening of the launch window, Mariner was sent on its way to Mercury: November 2, 1973, 9:45 p.m. PST.

Fig. 5-1. Within a few thousandths of a second of the opening of the launch window, Mariner was sent on its way to Mercury: November 2, 1973, 9:45 p.m. PST.

 

[46] ....were jettisoned. Almost two minutes later the fire also died in the main engine. The Centaur upper stage flew free of the Atlas bulk, as an explosive charge sliced through the interstage adapter and retro rockets slowed the spent booster preparatory to its tumbling back into the Atlantic Ocean. Within 12 seconds of Atlas engine cutoff, the bright nucleus of the Centaur's twin engines blossomed in the night sky, to burn fiercely for 5.1 min to push the spacecraft into Earth parking orbit at an altitude of 188 km (117 mi) and a speed of 28,046 km/hr (17,428 mi/hr).

Silently the Centaur and the spacecraft moved weightless nearly a third of the way around the Earth. Again the Centaur's engines erupted into flame, expanding exhaust jets into the vacuum of space. The Centaur and its payload bounded forward in orbit, breaking free of Earth's gravity within 2.25 min at a speed of 40,969 km/hr (25,458 mi/hr) headed backwards along Earth's orbit around the Sun.

Robbed of some of Earth's orbital motion, the spacecraft and the Centaur could no longer balance orbital action against the pull of the Sun's gravity. They began to fall toward the center of the Solar System, following a long orbit around the Sun that would take them ultimately to the orbit of Venus.

About a minute and a half after the Centaur engines shut down, the spacecraft separated from the Centaur. Then, 8 1/2 min later, the spent rocket turned and blew out its remaining propellant through the rocket nozzles to thrust it away from a trajectory that might cause it to tangle later with the spacecraft or crash onto the surface of Venus. Now Mariner Venus/Mercury was on its own: a true spacecraft in its natural environment. Now its name was Mariner 10. Pyrotechnic squibs were fired aboard the spacecraft; its various movable elements unfurled and extended. Mariner 10 had reached maturity as a spacecraft in its cruise configuration.

Very soon after launch, the planet-viewing experiments were turned on, a first time for planetary missions. The aim was to calibrate the instruments in the well-known environment of the Earth-Moon system. The charged particle telescope was turned on within 3 hours of liftoff, the ultraviolet experiment within 7 hours, and the TV cameras shortly thereafter. First TV pictures of Earth were obtained 16 hours and 15 minutes after liftoff.

There were some problems. The two thermal strap-heaters surrounding the aluminum lens barrels of the cameras were designed to hold the camera system at a temperature of 4 to 15°C (40 to 60°F). But they failed to operate as programmed following launch. Mission controllers, watching the engineering data coming back to the Mission Operations Center, saw that the heaters were not activated. Quickly a command was sent to the spacecraft to deactivate the heaters and then to activate them by triggering the relay switch, which seemed to have stuck. Nothing happened. The telescopes continued to cool down.

There was concern that without the heaters operating the television cameras would cool down too much and affect sensitive optics so as to distort pictures of the planets and cause a degradation of camera focus. Part of the problem was caused by the screening of the spacecraft against solar heating. It was so protected by a sunshade and by surface coatings and thermal blankets that when the camera heaters failed to come on, the cameras began to cool. Engineers from JPL and Boeing studied the problem to determine how heat might pass from the rest of the spacecraft in place of that missing from the heaters. They found that the thermal insulation of the spacecraft was so good that there was no way to heat the cameras from the spacecraft itself. The fall in temperature had to be lived with. They also checked the backup spacecraft poised at Cape Kennedy in an attempt to determine what might have caused the relay to stick. Had this problem degraded the spacecraft capability to an unaccepted degree, it would have been necessary to launch the backup.

Fortunately, the cooling stabilized at an acceptable level, and the cameras did maintain their sharp focus. The lens elements and the optical tube elements were self-compensating to changes in temperature. But an ever-present danger was that the Invar rods might contract, fracturing the vidicon potting compound if the temperature fell below -40°C ( - 40°F). Project scientists halted this temperature drop by keeping the vidicons switched on to maintain some heat within the cameras. Normally the vidicons would have been rested in the cruise between the planets, but it was considered prudent to change this mode of operation and take the chance that the lifetime of the vidicons might be shortened somewhat rather than risk the cameras' becoming too cold. This [47] being done, the temperature of the cameras stabilized, at low but livable values-the vidicons were about -10°C ( + 14°F), the backs of the optics were -20°C ( - 4°F ), and the telescope fronts were about -30°C ( - 22°F).

Mariner's cameras transmitted good pictures of the Earth and the Moon despite the temperature problem. The pictures of Earth (Fig. 5-2) provided stereo photographs of clouds with revealing depth and structure. They appeared to be the clearest pictures yet received from a television camera in space. If the spacecraft returned similar-quality pictures from Venus, the project could obtain a completely unprecedented look at the brilliant clouds of that mysterious planet.

In all, Mariner 10's cameras provided a series of five Earth mosaics (Fig. 5-3) within the first....

 


Fig. 5-2. A few hours later it was testing its cameras and sending photographs back of its home planet.

Fig. 5-2. A few hours later it was testing its cameras and sending photographs back of its home planet.

 

 

....few days of flight. These mosaics revealed intricate cloud patterns at about the same resolution expected during the Venus flyby. The Earth pictures could provide valuable comparisons with the Venus clouds. Earth observations also provided in-flight verification of the cameras' "veiling glare" performance, thus confirming that the preflight calculations of settings of camera exposures for Venus were correct. This was important, since Venus encounter geometry did not allow an incoming far-encounter sequence to check the exposures.

Another problem arose almost at the beginning of the flight when, on November 5, the plasma science experiment was turned on. Scientists were surprised to find that no solar wind particles were being observed. There appeared to be a good vacuum in the detectors, and the device was scanning back and forth as it should. Engineers performed a series of tests and sequences of switching commands without positive results. One possibility was that the instrument door had failed to open so that plasma could not enter the detector. Another was that the high-voltage sweep was stuck at the high end, thus permitting only a few high-energy particles to register. The operation of this experiment was, unfortunately, restricted throughout the mission, and it was concluded that the protective door had failed to open fully. However, plasma data were obtained by the scanning electron spectrometer part of the instrument, which was unaffected by the failure of the door.

As the spacecraft left Earth, the ultraviolet air glow instrument looked back at the home planet, observing the same emission regions that it expected to check later at Venus and Mercury. Lyman-alpha hydrogen emission was recorded, together with helium emission at 584 angstroms.

All subsystems of the spacecraft were performing exactly as expected. The trajectory was also very good; less than 8 m/see (27 ft/sec) of the spacecraft's total maneuvering capacity of 120 m/sec (396 ft/sec) was expected to be needed to move the Venus aiming point of the spacecraft and change the arrival time about 3 hours to bring Mariner 10 to its later pass within 1000 km (600 mi) of Mercury's surface.

Mariner 10's series of five Earth mosaics was intermixed with six mosaics of the Moon (Fig. 5-4) within the first week of flight as calibration tests for the Mercury encounter. The path of....

 


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Fig. 5-3. In the next few days mosaics of Earth and Moon were built up as imaging team members tried out the cameras to see if the failing heaters had degraded the optical system. The Earth is revealed here in great detail, auguring well for the capability of Mariner to produce pictures of details in the clouds of Venus.

Fig. 5-3. In the next few days mosaics of Earth and Moon were built up as imaging team members tried out the cameras to see if the failing heaters had degraded the optical system. The Earth is revealed here in great detail, auguring well for the capability of Mariner to produce pictures of details in the clouds of Venus.


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Fig. 5-4. The good quality of the Moon mosaics made promising the prospect of later photographing the surface of Mercury, which was believed to be a Moonlike body.

Fig. 5-4. The good quality of the Moon mosaics made promising the prospect of later photographing the surface of Mercury, which was believed to be a Moonlike body.


[
50] Fig. 5-5. As it passed the Moon, Mariner provided views of the north polar regions needed to update lunar maps.

Fig. 5-5. As it passed the Moon, Mariner provided views of the north polar regions needed to update lunar maps. One of these pictures is shown in (a) [left]; (b) shows how the new photographs can be applied to fill in the poor details so far obtained of the lunar north pole.

 

Fig. 5-6. Mariner's cameras also checked their sharp focus on stars to show that they had not degraded from the heater failure. A stellar field is reproduced in (a) [left]; (b) identifies the stars photographed.

Fig. 5-6. Mariner's cameras also checked their sharp focus on stars to show that they had not degraded from the heater failure. A stellar field is reproduced in (a) [left]; (b) identifies the stars photographed.

 

[51] ....Mariner allowed images to be obtained of the north polar region of the Moon (Fig. 5-5), which, because of constraints on paths of other space vehicles, had previously been covered only obliquely. The Mariner 10 photographs provided a basis for cartographers to improve the lunar control net, the relationship of points on the lunar surface one to another in precise definitions of lunar latitude and longitude of craters and other features. The exercise in lunar cartography provided a useful prelude to applying the same techniques to map Mercury using the images to be obtained during the flyby.

Diagnostic tests were conducted on November 6, including photography of stars (Fig. 5-6) and additional tests on the Moon (Fig. 5-7). The Moon tests, as well as providing better information about how the TV system was performing,....

 


Fig. 5-7. Gradually the Moon was left behind and Mariner 10 was heading for Venus.

Fig. 5-7. Gradually the Moon was left behind and Mariner 10 was heading for Venus.

 

[52] ....allowed scientists to evaluate the practicality of proposed measurements of the diameter of Mercury. At this stage of the mission, optical performance of the television system continued to be good even though the TV optics had not yet stabilized in temperature. As of November 7, Mariner 10 had returned almost 900 pictures to Earth. Experimenters were enthusiastic about the excellent quality. The Moon pictures recorded objects a mere 3 km (2 mi) across (Figs 5-8 and 5-9 ). Since the pictures to be returned from Mercury were expected to be of three times higher resolution than those of the Moon, there was good reason for excitement. At last, it seemed, mankind would have a chance to resolve those dusky markings on the innermost planet, those indistinct features that earlier astronomers had interpreted as Marslike, even erroneously with linear "canal" type features. Another test conducted was photographing the Pleiades cluster in the constellation of Taurus: a galactic cluster in the Milky Way which is visible to the unaided human eye as seven faint stars and is often called the "Seven Sisters". These stars are about 20,000 light years from the Sun and are immersed in nebulosity. A total of 84 pictures were taken, verifying the focus of the television system.

On November 8, commands were executed in the spacecraft to calibrate the charged particle telescope. Scientists were pleased to see good data. Also, the scanning electron spectrometer of the plasma science experiment produced excellent data. These data were routed, as they arrived, to the NASA-Goddard Space Flight Center in Maryland so that members of the science team at Massachusetts Institute of Technology and at Los Alamos Scientific Laboratory were able to follow the test in real-time by telephone links with....

 


Fig. 5-8. The imaging team was enthusiastic about the quality of the lunar images. The close-ups obtained were excellent.

Fig. 5-8. The imaging team was enthusiastic about the quality of the lunar images. The close-ups obtained were excellent.


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Fig. 5-9. The details revealed on the Moon's surface showed the capabilities of this new television system for planetary photography.

Fig. 5-9. The details revealed on the Moon's surface showed the capabilities of this new television system for planetary photography. The computer-enhanced terminator regions showed that the system would also provide great detail in the terminator regions of Mercury. The small picture is a mosaic; the computer-enhanced terminator regions are shown alongside.

 

....NASA-Goddard. The team was able to compute approximate plasma density, electron temperatures, and the flux of charged particles by using this real-time data. The Principal Investigator, Herbert S. Bridge, stated that although the experiment was "painfully" degraded with apparent loss of the data from the scanning electrostatic analyzer, valuable information concerning the solar wind was being obtained, and this experiment was still expected to produce new information about the interaction of the solar wind with Venus and Mercury later in the mission.

By November 11, over 2000 commands had been successfully sent from the Mission Operations Center to the spacecraft. Of these, 1019 were to update the central computer and sequencer preparatory to making the first trajectory correction maneuver for the spacecraft. By this time the navigation teams had determined the trajectory of the spacecraft and knew that, if uncorrected, [54] Mariner 10 would fly by Venus on the wrong side of the planet, some 55,000 km (34,000 mi) off the aiming point and 3 hours later than desired, and would miss Mercury. The spacecraft had now to be turned and reoriented in space so that its rocket engine could be fired for a short period and apply a change in velocity to the spacecraft in the right direction to ensure that it would arrive at Venus at the right time and place to permit the later encounter with Mercury.

The velocity change required was some 7.8 m/sec ( about 25.5 ft/sec or just less than 18 mph), which required the rocket engine to burn for about 20 sec and consume 1.8 kg (about 4 lb) of propellant. On Sunday November 11, project personnel gathered at the Mission Operations Center for a maneuver conference at which the maneuver scheduled for November 13 was given the go-ahead. At 1:45 p.m. PST on that date the maneuvering sequence started aboard Mariner 10 with the command that maneuver events would start clocking at the next hour pulse within the spacecraft. This pulse occurred at 2:38 p.m., and at three sec after 3:00 p.m. the gyros began to whirl within Mariner. Just over an hour later the cold jets at the tips of the spacecraft solar panels spurted nitrogen gas into space and the spacecraft began its roll turn, taking about 4.5 min to roll through 49 deg. Then, equally as abruptly, opposing nitrogen jets stopped the roll. A few minutes later jets of nitrogen spurted from other thrusters mounted on the outriggers that support the high-gain antenna and the magnetometer. The spacecraft started to slowly pitch over, taking another 12 min to pitch through 127 deg before opposing jets stopped the pitch. Now it was ready for the hydrazine rocket engine burn. A valve in the propellant system opened. Nitrogen gas pressing against a rubber diaphragm in the propellant tank forced hydrazine into the rocket thrust chamber, where it was decomposed by a catalyst to produce a hot jet. The thrust lasted for the required 19.9 see; then the valves closed and the engine shut off.

Four minutes later the central computer and sequencer started the pitch jets operating, followed by the roll jets, to return the spacecraft to its correct orientation with respect to the Sun and the stars. Then the gyros were switched off. At 5:08 p. m. the first trajectory correction maneuver had been completed.

Meanwhile the tracking data were being examined by the navigation staff at JPL to check the effects of the maneuver. There was momentary anxiety at the Operations Center when telemetry signals from the spacecraft indicated that the Canopus tracker had lost the star. It seemed that a bright particle had moved past the spacecraft-perhaps a meteor or a particle from the spacecraft itself-and attracted the star sensor. But soon Canopus was reacquired and the spacecraft returned to normal by 6:40 p.m. PST.

As doppler data were analyzed, the performance of the maneuver looked good. The navigation team had been able to monitor both the roll and the pitch turns and to ascertain that the velocity change of the main engine thrust caused a doppler shift of 71 Hz, while 72 Hz was required. This corresponded to an error of 1.5%. However, analysis of tracking data for 15 days after the trajectory correction maneuver was needed before the exact trajectory could be determined.

By November 28, it was known that the spacecraft was headed much closer to its required rendezvous with Venus, but there was still a relatively small error of 1380 km (860 mi) too far from the planet and an arrival time 2 min early over that required (see Fig. 5-10). A scheduled....

 


Fig. 5-10. By this time navigators had checked the aiming point at Venus and could prepare for a first trajectory correction maneuver to change the post-injection orbit to the desired flyby point.

Fig. 5-10. By this time navigators had checked the aiming point at Venus and could prepare for a first trajectory correction maneuver to change the post-injection orbit to the desired flyby point. This was done by TCM-1 a few days after the Earth-Moon system had been left.

 

[55] ....further trajectory correction maneuver would have to be made later in the mission to refine the position and time of encounter. Two maneuvers before reaching Venus had always been a part of the mission plan.

However, other troubles had shocked operations personnel. On November 21, the gyros were commanded on to put the spacecraft through a roll calibration maneuver. Immediately, the flight data system reset itself automatically to zero, but it was not known if this uncommanded reset was a problem in the spacecraft power or in the grounding system or was a sensing error of the flight data system itself. The roll calibration maneuver was postponed.

It was not until two weeks later, on Friday, December 7, 1973, that Mariner 10 performed a successful roll calibration maneuver and a calibration of the high-gain antenna. Again during the turn-on of the gyros for this maneuver, the flight data system automatically reset itself to zero as it had done previously. But the most significant and ominous power-related problem did not occur until nearly a month later, January 8, 1974, when the spacecraft automatically switched from its main to its standby power chain. This automatic switchover was irreversible: it was of concern primarily because of the possibility of a fault common to both power circuits causing the backup power circuit to fail also and thus raising the possibility of the mission's being ended right there. So, following this power problem, extreme caution was exercised for some time in changing the power status of the spacecraft and in maneuvering relative to the Sun, the latter to avoid an automatic switchover from solar panel to battery power.

There was another problem connected with the high-gain antenna which seemed to stem from its low temperature. On Christmas Day 1973, shortly before 1:00 p.m. PST, a part of the feed system of the high-gain antenna failed and caused a drop in signal power emitted by the antenna. Mission controllers tested the system, issuing diagnostic commands to the spacecraft. They deduced that a joint in one of the feed system's two probes may have cracked or fractured due to temperature changes during the flight. The problem was regarded as severe because it would prevent realtime TV sequences from being transmitted to Earth at Mercury encounter so that less area of the planet's surface could be covered by the photomosaics.

On December 29, the feed system healed itself. The high-gain antenna performed normally again. But the joy of the engineers was short-lived. Within four hours the fault developed again. Analysis indicated that the problem might have been caused by the low temperature of the feed system, and it was hoped that by the time the spacecraft reached Mercury the antenna temperature would be high enough to clear the fault and permit full operation of this antenna so that the full complement of mosaics would be obtained.

However, the antenna problem caused cancellation of some planned ultraviolet spectrometer airglow experiments, together with a roll calibration maneuver and other tests of the spacecraft. Engineers devised tests using duplicate hardware on Earth to simulate possible causes of the high-gain antenna problem. But as the orientation relative to the Sun changed and there was some heating of the antenna feed, the problem cleared up again by itself on January 3. A predetermined contingency plan was immediately put into effect by Mission Operations to position the high-gain antenna so that the Sun would continue to warm the feed. By positioning the antenna to gather some solar heat, the temperature was maintained well above the temperature at which the high-gain antenna problem had originally developed. In addition to thermal considerations, the new position of the high-gain antenna was selected to direct a side lobe of the antenna pattern toward Earth, since the side lobes carried more radiated power than the low-gain antenna.

After recovery on January 3, the high-gain antenna again failed on January 6, and the antenna was pointed back to Earth and use of the side lobe discontinued. Meanwhile, other events had occurred. On December 14, 1973, the solar panels were tilted 25 deg off the Sun to reduce the surface temperature of the panels by approximately 10°C (18°F). On December 18, the scan platform was also tilted to its maximum so that the ultraviolet airglow spectrometer could make new measurements of emissions from interstellar helium gas in a direction opposite from the Sun.

On December 19, the gyros were turned on and another roll calibration maneuver made. This time there was no power-on reset in the flight data system as had occurred during the previous maneuvers. The spacecraft seemed to be behaving....

 


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Fig. 5-11. On the way to Venus an opportunity arose to observe the Comet Kohoutek from Mariner 10.

Fig. 5-11. On the way to Venus an opportunity arose to observe the Comet Kohoutek from Mariner 10. Although images were not obtained because the comet was much fainter than originally expected, ultraviolet scans provided new information on the comet that could not be obtained from Earth. (Photo: Table Mountain Observatory)

 

 

....quite neurotically and confounding its designers and controllers.

Early in January the scan platform aboard the spacecraft was slewed so that the ultraviolet airglow spectrometer could be ready for observations of the Comet Kohoutek ( Fig. 5-11). The prime objective was to obtain unique observations of Kohoutek in the ultraviolet region of the spectrum which could not be obtained from Earth or from orbiting vehicles due to the Earth's hydrogen corona. Mariner 10 was well outside the hydrogen corona, thus being in a superior position to Skylab, which was also being used to observe the comet. Observations began with passive ultraviolet measurements of the tail of Kohoutek starting January 9 and concluded with the passage of the comet's nucleus through the field of view by January 17. Active ultraviolet scanning and TV imaging of the comet took place toward the end of the month. Neutral hydrogen emission intensities were measured by Mariner as far as 17 deg from the comet's nucleus compared with only 2 deg for the Skylab-based observations from within the Earth's hydrogen corona.

The attempt to photograph comet Kohoutek was not, however, successful, mainly because the comet disappointed everyone by being such a faint object-nearly 50 times less bright than anticipated. The comet was too faint to reveal any useful information in the TV pictures from Mariner. But Mariner's ultraviolet spectrometer did obtain some very good Lyman alpha (neutral hydrogen) radiation measurements through the comet's tail and into the nucleus. Preliminary results of this ultraviolet scanning showed a very large hydrogen corona to the comet, having a diameter of about 20 million km (12.5 million mi).

The next major event in the Mariner 10 mission was the second trajectory correction maneuver required to refine the flyby of Venus to a greater precision, making it possible to reach Mercury after the Venus encounter. On January 16, some of the preliminary commands for the maneuver were sent to the spacecraft and stored in its memory within the central computer and sequencer. The objective was to make sure that Mariner 10 would fly through a 400-km (248-mi) diameter "hole in the sky" which lay about 16,000 km (10,000 mi) to the right and in front of Venus as seen from the approaching spacecraft. The gravity of Venus would bend Mariner's path from that aiming point to pass within 5784 km [57] (3594 mi) of Venus's surface about 1O:00 a.m. PDT on February 5.

The navigation team redetermined the orbit of the spacecraft following the trajectory correction maneuver performed shortly after launch by processing over 60 days of tracking data consisting of 2600 measurements of the distance of the spacecraft from Earth. If the error at Venus were left uncorrected, the spacecraft would miss Mercury by 1.5 million km (nearly 1 million mi).

On January 21, at 11:50 a.m. PDT in response to stored commands, Mariner rolled itself about 46 deg, pitched over nearly 35 deg, and then, 24 min later, fired its rocket engine for 3.8 sec to change the spacecraft velocity by about 1.3 m/sec (4 ft/sec). At Mission Operations, project personnel were jubilant when the doppler frequency was measured as having shifted 17.41 Hz, which was within 0.04 Hz of the required amount. Following another 10 days of tracking, the navigation team confirmed that the flyby point was within 27 km (17 mi) of the aim point. All science equipment was working well, ready for Venus encounter; the cameras were stabilized in temperature; the only science problem was the still-closed door of the plasma experiment. But other troubles beset the spacecraft.

On January 28, Mariner started a series of eight calibration rolls that were to be completed in 79 min. At the end of each roll the scan platform was moved to obtain records of the diffuse ultraviolet emissions observed over wide regions of the sky. Suddenly, an oscillation occurred in the roll channel of the attitude control system, causing expulsion of attitude control nitrogen gas at a disastrous rate. As the gas pressure telemetry data dropped inexorably, mission controllers knew they were watching a spacecraft die. In the hour that it took to recognize, analyze, and respond to the problem, some 16% of the spacecraft's attitude control gas had been ejected into space. W. I. Purdy, the Guidance and Control Analyst, hastily called from a meeting, quickly determined that the gas loss was a result of a gyro-induced instability. He commanded gyros off, and the gas loss stopped. The nitrogen gas supply had dropped from 2.7 to 2.1 kg (6.0 to 4.7 lb).

Later analysis showed that the gas loss resulted from a mechanical oscillation of the spacecraft induced by impulses from the jets mounted on the extreme ends of the solar panels. Following extensive analysis, mission controllers issued commands for the movable solar panels and scan platform to be positioned in such a way as to prevent the oscillation and thus avoid further loss of gas in the future. It was hoped that spacecraft attitude maneuvers and trajectory corrections might be conducted under certain conditions without inducing further gas-consuming oscillations.

But the cause of the problem was not known during the final preparations for the Venus encounter. A gyro malfunction was at that time a viable explanation, and a disastrous, uncontrollable spacecraft spinup was thought to be a possible result if the gyros were turned on again, Thus, the flyby of Venus was now planned to take place under Sun and star reference instead of inertially by gyro control. This presented a hazard to the spacecraft in that Mariner 10 might suddenly swing around to lock onto the bright planet instead of the star Canopus. Project engineers analyzed the characteristics of the Canopus tracker and decided that the design of the baffles to protect the sensor from stray light made the probability of losing lock on Canopus acceptably low. The risk was therefore taken, and Mariner 10 bore down on Venus oriented to the celestial references of the Sun and Canopus. The three-axis gyro system remained idle.

On January 17, during the time that heaters for other Mariner 10 instruments were being turned off in preparation for the second trajectory correction maneuver, the heaters for the TV cameras, which had mysteriously been off since launch, equally as mysteriously came back on by themselves. Actually, the explanation for the failure was that there had been a short in another heater which resulted in biasing the TV heater to its switched-off mode. The "healing" of the camera system was most welcome, since the science investigators had been concerned that the cameras might not operate properly during Venus encounter because their temperature had dropped below freezing. After the trajectory correction maneuver had been completed, the original plan was to turn the heaters on again. But to avoid any risk of affecting the camera heaters, heaters in the same circuit as those for the cameras were left turned off. Mariner had by now warmed up sufficiently in its approach to the Sun so that some of the heaters were no longer needed. On January 23, the movable scan platform on which the TV cameras were mounted was given its final [58] pointing calibration by taking three sequences of test pictures of star clusters. Then the cameras were idled for a week.

By February 4, Mariner 10 was 640,000 km (about 400,000 mi) from Venus and approaching the planet at a speed of over 29,600 km/hr (18,400 mi/hr). On this day, as the high-gain antenna was being moved during a calibration sequence, the feed system problem suddenly righted itself; then a little later it returned, but not as badly as before. Despite all of the spacecraft problems, it appeared that Mariner 10 was capable of conducting the Venus encounter as conceived long before launch. Much credit was due the project personnel who had nursed the neurotic spacecraft through its troubles and had devised ways to continue the mission by operating around the various problems. Everything was now ready for the encounter (Fig. 5-12).

 


Fig. 5-12. A further trajectory correction maneuver and Mariner 10 was all set for its encounter with Venus.

Fig. 5-12. A further trajectory correction maneuver and Mariner 10 was all set for its encounter with Venus.


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