Before undocking from Skylab, Gerald Carr had fired Apollo's attitude-control thrusters for three minutes, nudging the cluster 11 kilometers higher, into an orbit 433 x 455 km. After the crew had returned to earth and the end-of-mission engineering tests were finished, flight controllers vented the atmosphere from the workshop, oriented the cluster in a gravity-gradient-stabilized attitude with the docking adapter pointed away from the earth, and shut down most of its systems. Skylab could still respond to telemetry signals whenever its solar panels were in sunlight. A suited astronaut could enter it-assuming he could reach the hatch and had some reason to go inside.) 1 But no plans contemplated such a visit or any other reuse of the huge hulk. With one control moment gyro inoperative and another ailing, with two coolant loops behaving erratically and several of the power-supply modules approaching the end of their expected life spans, the $2.5-billion orbiting laboratory was junk.
It was, in fact, inexorably headed for a flaming death in the earth's atmosphere. Calculations made during the mission, based on current values for solar activity and expected atmospheric density, gave the workshop just over nine years in orbit. Slowly at first-dropping 30 kilometers by 1980-and then faster-another 100 kilometers by the end of 1982-Skylab would come down, and some time around March 1983 it would burn up in the dense atmosphere.2 If, as planners hoped, Shuttle development went smoothly, one of the new craft's early missions would attach a propulsion module to the workshop to boost it into a higher orbit. If not, the 75 OOO-kilogram cluster would probably attract more public attention than NASA wanted when it returned to earth. Flight controllers could do little to change the course of its reentry.
The nine-year lifetime of the orbiting laboratory seemed ample in 1974, and in any case NASA had more pressing problems to worry about.  During the next three years the agency's annual budgets shrank to record low levels, delaying the development of Shuttle. Meanwhile the Russian manned program showed every sign of vitality. Soviet cosmonauts surpassed Skylab's endurance records, and Soviet space officials spoke of establishing permanent stations in earth orbit.
By early 1977 the first Shuttle orbiter Enterprise was being prepared for landing tests, and planners could begin to think about payloads and missions. Early in the year Headquarters directed Johnson Space Center and Marshall Space Flight Center to outline schedules and funding requirements for a Shuttle mission to boost Skylab into a higher orbit. Houston was not optimistic. Problems of rendezvous and docking with the inert workshop had not been thoroughly studied; and JSC's studies showed that a visit to Skylab could not be carried aloft earlier than the fifth test flight of the Shuttle orbiter, expected to be launched in late 1979. As the next solar maximum approached (1980-1981), it was becoming clear that the sun was considerably more active than anyone had predicted three years before-bad news for Skylab, because solar activity heated the earth's upper atmosphere, increasing its density at orbital altitude and dragging the workshop down faster than anticipated.3
Marshall's experts told Headquarters in March 1977 that a study contract to define the booster stage for the Skylab mission should be awarded not later than midyear. Headquarters then set the fifth test flight as the target mission and 1 September as the latest date for decision. This would allow just over two years for hardware development. Meanwhile the centers continued to compile the data necessary to make that decision.4
In September the word was GO, and in November Marshall awarded a $1.75 million letter contract to Martin Marietta Corporation to conduct analysis and design studies for a teleoperator retrieval system to be carried in Shuttle's cargo bay and used to attach a propulsion module-also still to be designed-to Skylab's docking port. Since time was critical, developed and qualified hardware was to be used to the extent possible- very much in the Skylab tradition. A preliminary design review was set for March 1978.5
Within a month, however, this schedule seemed inadequate. A meeting of the American Geophysical Union heard in December from Howard Sargent, chief forecaster for the National Oceanic and Atmospheric Administration (NOAA), that the current sunspot cycle was the second most intense in a century. Sargent's forecast was based on a model different from that used by NASA; he (and others) criticized the space agency for using what he considered to be an inaccurate model. Asked by journalists whether he thought the Skylab reboost mission would succeed, Sargent offered the opinion that NASA was "in a pile of trouble" if it was counting on the cluster to stay in orbit long enough for Shuttle to reach it on the current schedule.6
 Critics of manned spaceflight tried to make capital of the discrepancy between NASA's predictions and those of NOAA, but in fact no single method of predicting sunspot activity was universally accepted by solar scientists. (Ironically, Skylab's own results-unavailable in 1974- would eventually contribute to refining those methods.) All were based on analysis of historical data. NASA's scientists used more observations and predicted less sunspot activity than their counterparts at NOAA. Sargent and his colleagues insisted that some of the very early (17th century) observations that NASA used were unreliable and reduced the accuracy of the predictions. The space agency had ignored the forecasts NOAA published in 1976, leading some cynics to attribute self-serving motives to the forecasters at Marshall: since Huntsville still had thoughts of using Skylab somehow, it was not in their center's interest to acknowledge that the space station might fall to earth before it could be rescued.7 Since no such proposals were ever formalized, the simpler explanation-that Skylab was simply forgotten in the press of more urgent business-is equally credible.
Early in 1978 Skylab was rudely thrust into the glare of publicity- like earlier NASA activities, by the Soviet space program. The unmanned Cosmos 954, apparently as a result of systems failure, flamed into the atmosphere over northern Canada, scattering pieces of its nuclear-fueled electrical power module over a wide area. The module contained 45 kilograms of uranium highly enriched in the fissionable uranium-235 isotope, and an intensive search for the pieces was started immediately.8
Coming so soon after the arguments of the previous month, the Cosmos reentry produced immediate concern for what might happen when Skylab came down. NASA's public affairs office assured the world that the cluster contained no radioactive material and that it would not drop below 278 kilometers before October 1979.9 That was hardly reassuring, since it cut nearly four years from previous estimates of Skylab's life expectancy.
As far as NASA was concerned the most stimulating reaction was a query from the State Department. In view of worldwide interest in Cosmos, State wanted to know, what did NASA propose to do about Skylab ?10 Diplomatic repercussions were possible almost anywhere in the world if a piece of Skylab fell on a citizen somewhere, since the laboratory's orbital path took it over the heads of 90% of the world's population. Although NASA's studies had shown that the risk to humans was small, it was not zero-a fact that was important to any agency sensitive to public opinion in the late 1970s.
NASA immediately got to work to determine the condition of Skylab's  systems. If the derelict were to be reboosted for later use or brought out of orbit at a site of NASA's choosing, it was necessary to determine how much control could be exercised from the ground. In the most favorable circumstances this was limited to controlling the cluster's attitude, thereby decreasing or increasing atmospheric drag; it was impossible to increase its altitude. If everything worked well Skylab's orbital lifetime might be extended by as much as five months, which might-just might- give Shuttle engineers enough time to get the reboost mission aloft. Toward the end of February, an eight-man team-four from Marshall and four from JSC-went to Kindley Naval Air Station, Bermuda, the only tracking station that could still transmit the UHF signals that operated the obsolete telemetry equipment aboard Skylab. 11
Meanwhile, during NASA's budget hearings, Administrator Robert Frosch explained to the Senate space committee what the agency was trying to do and the difficulties it was encountering. He was still hopeful that the teleoperator retrieval system could be built in time for launch in October 1979, but by his own estimates the odds were only 50-50 that Skylab would still be in orbit by then. Frosch pointed out that the projections were based on forecasts of sunspot activity and were therefore much less accurate than he would prefer. William C. Schneider explained to the senators what the reboost mission involved. The 4540-kilogram teleoperator unit, mostly fuel tanks and engines, would be guided by an astronaut in the Shuttle orbiter to dock at the multiple docking adapter, whereupon its thirty-two 100-newton thrusters would push the workshop into a higher orbit. Design studies were already under way. Fabrication and assembly were scheduled to begin in six months, and the completed module was to be delivered to the Cape in early September 1979 for an October launch on the third orbiter test flight.12 It was an ambitious schedule, considering that the first orbiter had not yet been launched.
The engineers in Bermuda made their first contact with Skylab the following month. Working with the North American Air Defense Command (NORAD), they located the workshop by radar, aimed a radio signal at it, and received a response. For two minutes Skylab reported on the condition of its systems, then fell silent. Apparently it was rotating at about 10 revolutions per hour, and when its solar panels turned out of the sunlight the radio transmissions ceased. The first thing the engineers needed to do was to charge the batteries, and since they could transmit commands only briefly once during each orbital pass, this would take time. Within a week, however, they had charged two batteries, determined the workshop's attitude, and ascertained that the onboard computer could be used to help control the spacecraft.13
The next goal was to gain control of the workshop systems, principally the control moment gyros, the thruster attitude control system,  and the attitude-sensing rate gyros. Once these were in hand, flight controllers could keep the workshop in a minimum-drag attitude, conserving altitude until the fate of the Shuttle mission was clear. After that they could either maintain the low-drag profile or increase the drag, which would give them some control over the point of impact when the workshop finally reached the end of the road. Since all these operations required power, the solar panels had to be kept in sunlight as much as possible. Balancing these requirements was a complex job that could not be handled by a skeleton crew at a remote site, so in June a control center was jury-rigged at Johnson Space Center and manned by two teams of flight controllers. Shortly thereafter the station at Madrid was brought into the tracking network; later, Goldstone in California and a station near Santiago, Chile, would be added.14
By early June the JSC team had turned on the two functioning control moment gyros and used them to stabilize the cluster in a low-drag attitude that allowed them to keep the batteries charged. This was not accomplished with great ease, for the gyro that had given Houston so much worry during the last days of the third mission again showed signs of stress-decreased wheel speed and increased motor currents. Besides that, the refrigerating systems that cooled the batteries in the airlock module were ailing; one had lost nearly all its cooling fluid, and the other was not completely reliable. Juggling the demands of power production and minimum drag with these complications thrown in took a great deal of planning, and crews worked 10-hour shifts through the summer. In July they almost had to start all over again when a spurious telemetry signal caused the computer to switch the control moment gyros off and the gas thrusters on; a significant fraction of the remaining propellant was used before the Houston team could regain control.15
Meanwhile Headquarters was setting up an organization to deal with the problems that would arise when Skylab came back to earth. On 25 July a Skylab Contingency Working Group was established to coordinate interagency planning. Under the direction of William G. Bastedo, this group was responsible for a host of activities, from keeping track of Skylab's condition to informing foreign governments of the current state of affairs. Besides NASA participants, the group included members from the departments of State, Justice, and Defense.16
The effort to save Skylab was becoming costly. Not counting expenditures for hardware development, NASA had spent $750 000 on the dying workshop by 1 June 1978 and expected to lay out at least $3 million more by the end of the year. At least one official thought this money was largely wasted. Chris Kraft, director of JSC, publicly expressed his opinion that the effort was futile. He did not expect the Skylab systems to continue functioning long enough for its reentry to be controlled (tacitly implying that there was no hope for the reboost mission). He conceded  that his engineers were obliged to do everything they could, but thought that NASA would not have gone to such lengths if the Cosmos accident had not focused so much attention on falling spacecraft fragments- attention that Kraft evidently felt Skylab did not deserve. In his opinion the money would have been far better spent on the Shuttle program, which was falling behind schedule for lack of adequate funding. In Washington, however, where the White House and the State Department could look over his shoulder, Robert Frosch reiterated the agency's determination to continue the effort in spite of the very small chance that Skylab would hit anyone.17
As summer turned to fall the Houston operation, directed by Charles Harlan of JSC's Flight Control Division, began working around the clock. Addition of the tracking station in Chile gave complete coverage throughout each of Skylab's revolutions, and by October 1978 Harlan had enough people to set up five flight control teams that worked three shifts a day. A few had sat behind control consoles during the Skylab manned missions, but most were new.18
The Skylab working group had a rehearsal of sorts in September, when the unmanned satellite Pegasus 1 came out of orbit.i The exercise served mainly to evaluate impact prediction models, using orbital data from NORAD, as well as to establish interagency procedures. Having checked out its communications and models, the group monitored Pegasus's uneventful reentry over the southwestern African coast on 17 September. From this exercise, goals were set for the eventual demise of Skylab. 19
Having started with little confidence in the aging systems on board the orbital cluster, but having discovered that those systems were better built than they expected, flight controllers developed real enthusiasm for their task. The problem was important enough to be worthwhile and difficult enough to be challenging. Early in the summer they had determined that they could use the onboard computer, and Marshall control system engineers devised new programs to control the spacecraft's attitude without using the gas thrusters. The remaining fuel for these had to be kept in reserve, for they would be needed if the reboost module should reach the workshop. The batteries had to be watched constantly. As those in use heated up, others were put on line to replace them; occasionally they all warmed up and the cooling system had to be switched on long enough to return them to normal temperature. As the relation of Skylab's orbital plane to the sun changed, all the variables changed.
 Many hours were spent devising and testing new combinations. Then in November the sick control moment gyro slowed down even more. The workshop was turned around, to expose the gyro to sunlight and warm up its bearings so that lubricant might flow more freely. The maneuver had been used during the third mission with ambiguous results, but since the gyro was now operating far outside the limits reached during the manned mission it seemed worth trying; loss of this gyro would seriously complicate the problem. In the event, both gyros lasted until reentry.20
The year-long effort to keep Skylab aloft ended in December 1978. Although the teleoperator propulsion module was approaching final assembly, problems with Shuttle's main engines had delayed critical tests, and program officials clearly saw that the reboost mission could not be launched by October. Frosch advised the President on 15 December that Skylab could not be saved but that NASA would do all it could to control reentry to minimize the risk to populated areas. John Yardley, associate administrator for the Office of Space Transportation Systems (successor to the Office of Manned Space Flight), provided details of the decision to the press on the 1 9th. Shuttle schedules had slipped so far that the reboost mission could not be launched before March 1980, and the workshop's rapidly decaying orbit, plus the increasing difficulty of controlling its attitude, made rescue impossible.21
The decision would simplify the work of Houston's flight control teams, though not immediately. For six more weeks they worked three shifts a day, holding Skylab in its low-drag attitude until policymakers could decide exactly how to manage the reentry. Choices were severely limited. As soon as the decision was reached, Bastedo sent a detailed reentry plan to the departments of State, Defense, and Justice and to the Federal Preparedness Agency for comment. A meeting with NORAD on 9 January 1979 established radar tracking requirements and set up formal technical liaison. Reentry information from NORAD would be transmitted to NASA field centers and to a coordination center to be set up by Yardley's office to direct the reentry.22 The operation was only slightly less elaborate than preparations for the return of an Apollo flight.
One of the Skylab group's chief functions was to ensure that NASA spoke with a single voice during the months remaining before reentry. Since NASA and NORAD used different models to predict reentry times, it was important that public statements about the date and place of reentry be consistent. This precaution was wiser than it seemed at the time. Three months later, when a nuclear reactor accident in Pennsylvania almost required evacuation of several thousand people, much confusion resulted when different experts made conflicting public statements as to the level of danger.
 Now that Skylab was certain to come down, television and the press looked forward to an event that might prove spectacular and in any case would be newsworthy. Much as they had done with Comet Kohoutek (app. F), reporters and headline writers began to play up the coming reentry. Some bizarre by-products of the event provided an occasional flash of weird humor. In Washington, two computer specialists established a firm called Chicken Little Associates, offering to provide up-to the-minute estimates of the danger to any specific person, for a fee. With the implication that NASA's predictions were unreliable, Chicken Little drew publicity-especially abroad. Then, just a month before reentry, a group from the Brookline (Mass.) Psychoenergetics Institute attempted to increase Skylab's altitude by telekinesis. They staged a "coordinated meditation" session in several eastern states, but produced no effect detectable on NORAD's radars.23
In Washington and Houston, more serious preparations continued. Bastedo's staff finished the NASA reentry plan and sent it to the White House on 30 January. March offered a second opportunity to check out refined procedures when HEAO 1, a NASA astronomical satellite, returned to earth. Data links between NORAD, Washington, and Huntsville were checked out. As a final rehearsal, the Skylab group, NORAD, MSFC, and JSC followed the reentry of a Soviet rocket body 27-29 April. This target of opportunity was used to determine the state of readiness of all participants in the Skylab reentry. In June, a paper simulation was run as a last check.24
Work at the control center at JSC had slacked off somewhat in early February, following a decision to return the workshop to solar-inertial attitude. Since power management was much easier in this attitude, round-the-clock monitoring of systems was suspended for several weeks and many of the flight control people were sent back to their regular jobs. Attitude control too was comparatively easy in solar inertial, in spite of the increased drag, but it was expected to become more difficult as the workshop lost altitude. From February through May, however, the control center simply kept an eye on Skylab while plans were made for its last few orbits.25
Toward the end of April, Headquarters issued its first forecast of a reentry date calculated from NORAD's model. On the 25th, when the workshop had fallen to about 320 kilometers, NORAD estimated a probability of 50% that Skylab would come down by 19 June; there was a 90% chance that it would reenter between 13 June and 1 July. This format was used consistently for the rest of the waiting period, because it was impossible to give a more precise estimate until reentry had virtually begun. Marshall's engineers used a slightly different forecasting model; they estimated reentry between 15 and 22 June, but their estimates were never publicized. NORAD was in the business of tracking satellites and NASA used NORAD's forecasts for public utterances.26
 Since flight controllers were not vitally interested in NORAD's predictions, the discrepancy was not particularly bothersome. The two groups did exchange information, however, and determined the different ways the two computer models treated data. NORAD made a fairly straightforward extrapolation based on recent observations, while NASA continuously took account of changing atmospheric density and the spacecraft's drag profile as it came down. Harlan reasoned that the two predictions would converge rapidly as reentry approached, which turned out to be the case.27
By the end of May, engineers and managers had agreed on a method of controlling the reentry. Skylab would be placed in a high-drag "torque equilibrium" attitude, in which aerodynamic forces were balanced by the control moment gyros as long as they had the capability. This would subject the workshop to a known retarding force from which impact predictions could be made. Flight controllers could then reduce drag if necessary to shift the reentry point. When the cluster fell to 140 kilometers, it would be set to tumbling end-over-end, reducing the drag to a known level and allowing a reasonably accurate prediction of impact. Theoretically impact could be shifted by as much as five orbits by changing the tumbling altitude, but that would tax the systems to their limit. A shift of one to three orbits was a more realistic expectation. The torque-equilibrium attitude made power management more complex, so the Houston center went back to 24-hour surveillance and control.28
Meanwhile each ground track covered by Skylab was assigned a "hazard index," ranging from 0 to 100, depending on the population exposed on that track compared with the least dangerous track. On the basis of these numbers Harlan might have to shift the impact point to an orbit of lower risk in the last hours of flight. It was a statistical game- sensible, but offering no assurance of safety. As Harlan commented later, "Clearly you could come in on an orbit with a lot of people and not hit a soul, or you could come in on an orbit with a few people and hit a schoolhouse and kill a bunch of kids." Administrator Frosch's testimony before a House subcommittee in June pointed up NASA's predicament. He reiterated the small risk of human injury (1 chance in 152), and emphasized that the fragments would be widely scattered. Although Frosch could not give absolute assurance that no one would be injured, he tried his best to convince his audience that there was really not much to worry about.29
Statistical arguments, however, are inherently unconvincing, at least to the general public; and Frosch's assurances were the less comforting because a few of the fragments might weigh several hundred kilograms when they reached the surface. It was clear that the decision made in 1970 was definitely embarrassing nine years later. The space agency was feeling the effects of a change in public attitude toward technology generally and space technology in particular. A large fraction  of the public was unwilling to accept any risk from a high-technology program, especially when the average citizen could do nothing to protect himself from that risk. Congressmen and editors demanded to know why Skylab had been launched without the means of controlling its reentry, and Frosch could only answer that it had seemed too expensive at the time.
The workshop was down to 261 kilometers on the 20th of June, having fallen 60 kilometers in the previous four weeks. During June NORAD issued predictions periodically; the median date (by which time there was an even chance that the workshop would have come to earth) moved from 16 July to 12 July, while the spread narrowed: 7-25 July predicted on 14 June, 10-18 July on the 28th. 30
As reentry approached, the difference between NORAD's predictions and NASA's caused some small problems. Television networks, needing time to prepare for coverage of the event, called Houston to ask when they should send reporting teams. Harlan and the JSC Public Affairs Office felt obliged to give them a date in which they themselves had some confidence, so they told the media officials to come a day or two before the official predictions called for reentry. This could have caused some embarrassment for Headquarters, but nobody publicized the point.31
Early in July the end was approaching rapidly. The workshop became harder to control as it dropped into the denser atmosphere, and power supplies were increasingly difficult to manage. On 9 July 1979 the Skylab Coordination Center opened in NASA Headquarters. With direct telephone lines to NORAD, NASA field centers, the State and Defense departments, and the FAA, the center was capable of relaying information and orders almost instantly. A closed-circuit TV display from Houston pictured Skylab's ground track for several orbits, as well as the current position. Newsmen and other nonessential personnel were kept out of the operations room itself, but the closed-circuit TV, tracking charts, and periodic briefings kept the crowd in the larger newsroom informed. On opening day the center issued the prediction that Skylab would come down on 11 July between 2:10 a.m. and 10:10 p.m. EDT, most probably on its 34 981st orbit. It was then at an altitude of 190 kilometers. The following day it dropped 17 km and the reentry time was bracketed between 7:02 a.m. and 5:02 p.m. EDT on the 11th.32
In Houston, Charles Harlan and his team stood by to make their last decision. For some hours before reentry the computers gave the same prediction: the workshop was coming down on 11 July. The only question that remained was the timing of the final tumbling maneuver. During the last hours of 10 July it appeared that Skylab would reenter on the best possible orbit of the day on the 11th, an orbit passing across southern Canada and the east coast of the United States and then over a long stretch of open ocean to Australia, the next landfall. But early calculations of the  debris pattern showed that if tumbling were initiated at 140 kilometers as planned, the western end of the 7400 X 185 km "footprint" would slightly overlap North America. JSC officials then recommended, and Headquarters concurred, that the cluster be tumbled sooner, to move the predicted impact area downrange. Harlan picked an area about 1300 kilometers south-southeast of Cape Town, South Africa, halfway between North America and Australia and south of the shipping lanes, which would require tumbling at 148 kilometers. The command was executed at 3:45 a.m. EDT, and the workshop went into an end-over-end spin.33
Skylab had one more trick up its sleeve, however-one that gave flight controllers some anxious moments on the last orbit. They expected the cluster to come apart before it passed over the east coast of the U.S., but radar operators at Bermuda reported only a single image. Over Ascension Island the workshop still had not broken up; a NORAD imaging radar clearly showed that even the fragile solar arrays were still intact. But the telemetry was faltering and stopped entirely as the craft passed south of Africa. Its unexpected tenacity had shifted the impact ellipse considerably to the east, however, and there was a possibility that Australia would catch some of the heavy fragments, which would fall at the eastern end of the ellipse.34
NORAD computed that impact occurred at 12:37 p.m. EDT. Shortly before 1 p.m., the Washington control center received word that the area southeast of Perth, Australia, had indeed been showered with pieces. Spectacular visual effects were reported and many residents heard sonic booms and whirring noises as the chunks passed overhead in the early morning darkness. Officials waited anxiously for news of injury or property damage, but none came. Skylab was finally down and NASA had managed it without hurting anyone.35
One Australian, in fact, profited handsomely from the overshoot. A San Francisco newspaper had offered $10 000 for the first authenticated piece of Skylab brought to its office within 48 hours of reentry, and on the morning of 13 July a claimant appeared. Stan Thornton, a 17-year-old beer-truck driver from the small coastal community of Esperance, had found some charred objects in his back yard, bagged them up, and caught the first plane for California. He arrived without passport and with only a shaving kit for luggage, but the pieces were identified as remains of plastic or wood insulation from Skylab, and Thornton got his prize.36
Examining their data after reentry, Harlan and his team decided that they had miscalculated drag during tumbling. It was a small relative error-only 4%-but it had shifted the impact zone hundreds of kilometers farther east than they had wanted. Fortunately the reentry orbit passed over the sparsely settled ranch country of Western Australia, but it was a slightly inelegant end to an otherwise well managed reentry.37
 Little remained to be done. The makeshift control centers at Headquarters, JSC, and MSFC were dismantled; Harlan and his co-workers went back to their jobs grappling with Shuttle's problems. Five Marshall engineers went to Australia to test the fragments that had been recovered, search for others, and try to establish the actual pattern. Some indignation had been expressed by Australian newspapers just after the reentry, but the NASA team was greeted warmly and given all possible assistance in their mission. Some pieces of the workshop had been put on display in Coolgardie and other nearby towns, but a cursory search found no others. Doubtless many remained scattered across the dusty ranches of the outback, to be stumbled upon some day by a herder or fence rider.38
Meanwhile, just three days after Skylab's reentry, two Soviet cosmonauts aboard Salyut 6 established a new record for endurance in earth orbit. The record they broke was not Skylab's but one that had been set only the year before by another Soviet crew.39
i Pegasus 1 was launched 16 February 1965 as part of the payload on the test mission AS-9, which also carried a boilerplate Apollo spacecraft into orbit for tests. Two more Pegasus satellites were flown on similar missions; they carried equipment to measure and report the number and velocity of micrometeoroids at orbital altitude.