Satellite on a String: The “Weird Science” of STS-46

Twenty years ago, International Space Year 1992 saw representatives of nine discrete nations visit low-Earth orbit. Among them was the first Italian national, Franco Malerba, who rode STS-46 to participate in the deployment of his country’s ambitious Tethered Satellite System (TSS). In this view, the satellite inches its way into space from its deployed mast. Photo Credit: NASA

One day in 1987, European Space Agency (ESA) astronaut Claude Nicollier came to Jeff Hoffman with an invitation to an interesting meeting. It was about an Italian project called the Tethered Satellite System (TSS).

“Jeff,” Nicollier said, “you’re a physicist. This is something that might intrigue you.”

A year had passed since the loss of Challenger and several scientist-astronauts were taking sabbaticals or pursuing advanced degrees. Hoffman was enrolled at Rice University on a master’s course in materials science, focusing on crystal growth, when Nicollier invited him to the TSS meeting. Nicollier was about to be sent to the Empire Test Pilots’ School in Boscombe Down, Hampshire, in the United Kingdom, to work on the development of ESA’s proposed Hermes spaceplane, and when the two men attended the meeting Hoffman was quickly hooked. So began a nine-year involvement with one of the strangest Shuttle experiments of all time – an experiment which astronaut Marsha Ivins once described as “weird science”.

Weird is certainly an apt choice of descriptor for the TSS, which encompassed nothing less than a satellite trawled ‘on a string’ through the electrically-charged ionosphere, as part of efforts to demonstrate the electrodynamics of a conducting tether to convert kinetic energy into electrical energy. Originally proposed to NASA in the early 1970s by Guiseppe Colombo of Padua University and Mario Grossi of the Smithsonian Astrophysical Observatory, it was hoped that the concept might ultimately lead to systems featuring electricity-generating tethers, using Earth’s magnetic field as a power source. Moreover, by ‘reversing’ the direction of current in the tether, the force created by its interaction with Earth’s magnetic field could potentially place objects into motion, boosting the velocity of a spacecraft without propellant, thereby counteracting the effects of atmospheric drag.

Following Colombo and Grossi’s proposal, the Facilities Requirements Definition Team (FRDT) met in 1979 to consider the applications of tethered satellites and its report, published the following year, strongly endorsed a Shuttle-based research mission. A memorandum of understanding was signed in 1984, which called for NASA to develop the deployment mechanism and Italy to build the satellite. A science advisory team provided guidance in preparation to a formal Announcement of Opportunity, in April of that same year, for experiments. A total of 12 experiments were selected, featuring participation from the Italian Space Agency, NASA and the Air Force’s Phillips Laboratory.

The TSS Deployer Core Equipment and Satellite Core Equipment (DCORE/SCORE), provided by Carlo Bonifazi of the Italian Space Agency, was responsible for controlling electrical current between the satellite and the orbiter. It featured an electron accelerator with two electron beam emitters, each capable of ejecting up to 500 milli-amperes – around half an amp – of current from the system. The Research on Electrodynamic Tether Effects (RETE), supplied by the Italian National Research Council in Rome, employed a pair of instrumented booms to measure the electrical potential in the plasma ‘sheath’ formed around the satellite during deployment. Franco Mariana of the Second University of Rome was principal investigator for the Magnetic Field Experiment for TSS Missions (TEMAG), which used two magnetometers to map fluctuations in magnetic fields around the satellite. Elsewhere, the University of Genoa sought to investigate the extent to which TSS could ‘broadcast’ from space, with magnetometer emplacements around the world and extreme-low-frequency receivers at the Arecibo Radio Telescope in Puerto Rico primed to track emissions and plasma wave directions. Finally, the University of Padua supplied a pair of experiments to analyse the satellite’s oscillations over a range of frequencies in real time.

The STS-46 crew included three nations amongst its seven-strong crew. Five US astronauts were joined by Italian Franco Malerba (bottom right) and Swiss Claude Nicollier (far left). Photo Credit: NASA

The United States’ side of the experiment payload included the Research on Orbital Plasma Electrodynamics (ROPE), provided by NASA’s Marshall Space Flight Center of Huntsville, Alabama, which studied the behaviour of ambient charged particles in the ionosphere and ionised neutral particles in the vicinity of the satellite itself. Peter Banks of the University of Michigan at Ann Arbor provided the Shuttle Electrodynamic Tether System (SETS) to evaluate the capacity to collect electrons by determining current and voltage and measuring the resistance to current flow in the tether. The Smithsonian Astrophysical Observatory conducted research into electromagnetic emissions and dynamic noise from TSS, whilst the Tether Optical Phenomena (TOP) experiment featured hand-held, low-light-level television cameras for visual data. Other experiments provided theoretical electrodynamic assistance and measured the levels of the Shuttle’s own electrical potential in comparison to the ambient space plasma.

In the late 1980s and early 1990s, the TSS was a particularly appealing concept for the designers of Space Station Freedom and its successor, today’s International Space Station, as a means of compensating for the effects of atmospheric influence on the gigantic outpost. Additionally, it was hoped that the electrodynamic tether concept might lead to the development of new devices to trail scientific platforms far below orbital altitudes in difficult-to-study zones, including the fragile ozone layer above the South Pole. Other possibilities included providing a foundation for extremely low-frequency antennas, capable of penetrating land and seawater, and perhaps generating artificial gravity or accelerating payloads into higher orbits.

At the time of the Challenger disaster, future Shuttle manifest projections ended in August 1988 and did not include TSS, suggesting that the mission would have been conducted at some point after that date. The satellite itself was a 5-foot-wide sphere, weighing 1,130 pounds, with an outermost skin of aluminium alloy, coated with an electrically-conducting layer of white paint. It was, however, far more than just an oversized metallic football. Piercing its shell were windows for Sun, Earth and charged-particle sensors, a connector for the umbilical tether and access doors for its on-board batteries. Extending from one side of the TSS was a long, fixed instrument boom, whilst a shorter antenna sprouted from the other side. To assist with thermal control, the interior of the spherical shell was painted black. If one were to open the shell, like an egg, two compartments would be revealed: a Payload Module for the scientific experiments and a Service Module for the subsystems. Additionally, in the centre of the shell was a tank of pressurised nitrogen, which provided propellant for the satellite’s 12 cold-gas manoeuvring thrusters. According to Italian prime contractor Aeritalia (today part of Alenia Aeronautica), the TSS required more than a million man-hours of work to prepare it for final integration.

Deployed at the end of a 0.08-inch-thick tether, the satellite was designed to crawl away from the TSS mast to a distance of 12.6 miles. Photo Credit: NASA

If the satellite itself represented a technological marvel, then the 0.08-inch-thick tether which connected it to a supporting mast on a Spacelab pallet in the Shuttle’s payload bay was an equally impressive creation. Surrounding its Nomex core was electrically-conducting copper wire, insulated with Teflon and coated with ultra-strong braided Kevlar-29 and an outer ‘jacket’ of brained Nomex to protect it from abrasion and the corrosive effects of atomic oxygen in low-Earth orbit. During deployment operations, the tether was unreeled from a mechanism affixed to the Spacelab pallet and a Mission-Peculiar Equipment Support Structure (MPESS). This regulated the length, tension and rate of deployment of the tether and was capable of unreeling at a maximum speed of about 4 mph.Essentially, the structure took the form of a four-sided erectable ‘tower’, not dissimilar in appearance to a small broadcasting pylon, which unfolded slowly from a storage canister using a series of rollers. As the canister rotated, fibreglass batons popped out of their stowed, bent-in-half positions to form cross-members – ‘longerons’ – which supported the vertical segments. The tower was deployed to a height of 39 feet above the payload bay, so that when the satellite was unreeled to its full length of 12 miles the risk of impacting the Shuttle was minimised. “The complexity of the experiment,” said astronaut Andy Allen, who served as pilot on the first TSS mission and later commanded the second, “is extreme.” Aside from the risks, the sheer audacity of the mission is amply illustrated by the numbers: when fully deployed, the TSS/orbiter combination would be a hundred times longer than any other spacecraft in human history, its electrical potential was anticipated in the region of 5,000 volts and its maximum current output was expected to be one ampere.

That a European astronaut was one of the first to draw attention to the TSS concept within the astronaut corps offers a nod toward its international flavour, for Claude Nicollier was the first non-US citizen to fly aboard the Shuttle as a fully-fledged mission specialist. In July 1977, he had been selected – alongside West German physicist Ulf Merbold and Dutch physicist Wubbo Ockels – as one of the first European astronauts. Initially assigned to prepare for a payload specialist position on the first Spacelab mission, in May 1980 Nicollier began training as a mission specialist candidate. At the time of the Challenger disaster, he was scheduled to fly on STS-61K in August 1986.

As circumstances transpired, Nicollier and Hoffman ended up flying together on the first TSS flight. They were named as mission specialists for STS-46 in September 1989, working towards a scheduled launch date in May 1991. Also named was Franklin Chang-Díaz, the first Hispanic-American astronaut. Little could they have foreseen at the time, but Hoffman, Nicollier and Chang-Díaz would fly together on both missions of the TSS.

Years later, Hoffman remembered the excitement of the TSS effort. “It was something that nobody had ever done before,” he told the NASA oral historian. “It was like learning how to go to the Moon. How do you do it? Nobody knew how to control a tethered satellite. In fact, the way that it had originally been designed, they had thought that this was going to be an easy thing to do. It was going to be completely automatic and all you would do is push a button. It would go ‘up’ and then you’d push a button and it would come back.” The satellite did have an attitude-control mechanism, but there was no control over pitch or roll controllability…and this raised concerns in Hoffman’s mind. What if the there went ‘slack’, he asked, or if a malfunction required them to halt it at mid-deployment? Would there be a chance of loss of control?

“The problem is when the thing really gets going, it’s coming out at several metres per second,” he recalled. “That’s pretty fast. If you just slam the brakes on, it’s going to go wildly unstable. They basically had never designed for all these contingencies. As we did more and more simulations, and we learned more and more about the system, we came up with more and more scenarios where you need these manual capabilities.” Hoffman worked closely with TSS project management, which proved very accommodating in organising the necessary financial resources to install extra capabilities on the satellite.

In addition to the three mission specialists, NASA also assigned veteran astronaut Robert ‘Hoot’ Gibson as commander of STS-46, indicative, perhaps, of the need for a pilot’s perspective from the outset. It would mark Gibson’s fourth Shuttle mission…but fate would dictate otherwise. In the summer of 1990, he was abruptly removed from the mission and dropped from T-38 flight status for a year, apparently as a punitive measure following “violation of a policy which restricts high-risk recreational activities for astronauts named to flight crews”. On Saturday 7 July, he participated in an air race at the civilian airshow in central Texas, when his aircraft collided with another. Gibson nursed his machine to the ground, but the other pilot was killed. Two days later, Don Puddy, the director of Flight Crew Operations, formally grounded him from flight status. “Our high-risk activity policy defines plain and simple guidelines for astronauts assigned to flight crews,” Puddy explained in a NASA press release. “They are intended to preserve our crews as assigned and apply regardless of the time prior to launch.”

At the time of the incident, the launch of STS-46 had slipped slightly in the manifest and was tentatively scheduled for the late summer of of the following year. Five months after Gibson’s grounding, in December 1990, fellow astronaut Loren Shriver was assigned to fill his shoes. Shriver shared many of his crewmates’ concerns about the controllability of the TSS during deployment. Like Hoffman, he knew that the danger of the tether slackening was a serious worry; although gravity gradient forces would keep it tight during part of its deployment, the risk was very real in the early stages. “The dynamics,” said Shriver, “could do most anything and it could go frontwards and then backwards and side to side.” The obvious implication was that the Shuttle itself could be endangered. “The trouble was finding any computer system that NASA had that could model that,” he continued. “Within the last couple of months of going to fly, [we] had some stand-alone trainers that began to show some of that in a reasonable manner, but we never did have really good training set-ups in the Shuttle mission simulator. There were some approximations, but they were never that complete or that good.”

As for his crewmates themselves – and this was his first Shuttle mission to feature an international team of astronauts – Shriver had nothing but the most extreme praise. With an Italian-built primary payload, NASA always intended to fly an Italian payload specialist and in September 1991 physicist Franco Malerba was selected for the position. Together with Claude Nicollier and five Americans, this made STS-46 only the fourth Shuttle mission to feature representatives of three discrete nations. (They even informally added Jeff Hoffman’s English-born wife, Barbara, to the crew roster.) “It never ceases to amaze me,” said Shriver, “how quickly crews coalesce into a highly-functional unit. Once you get worked out who is going to be doing what…then the plans start to fall in place and you go off and start training and everybody knows what they need to do.”

Franco Malerba (left) and Claude Nicollier, clad in their Shuttle partial-pressure suits, are pictured during STS-46 training. Nicollier became the first non-US astronaut to serve as a Shuttle mission specialist, whilst Malerba became the first Italian spacefarer. Photo Credit: NASA

The demands of the TSS deployment and science-gathering operation demanded that the crew worked in two teams – nicknamed ‘red’ and ‘blue’ – for around-the-clock activities. On the red team were Hoffman, Chang-Díaz and Marsha Ivins, whilst the blue team consisted of Claude Nicollier, Franco Malerba and pilot Andy Allen.

STS-46’s seven days in space were to be tightly packed with activities; the activation and checkout of the TSS was scheduled to begin only hours after orbital insertion, followed by experiments with the TOP investigation and the actual deployment on the fourth day. Within hours of deployment, the satellite was expected to reach its maximum distance of 12 miles from the Shuttle, whereupon it would be reeled back in to about 1.5 miles for further experiments, then finally retrieved and docked back onto the top of the mast. As if these operations were not complex enough, TSS formed one of two primary payloads on STS-46. The other was ESA’s European Retrievable Carrier (EURECA), a unique, box-like satellite, loaded with 15 experiments in protein crystallisation, space biology, fluid mechanics, solar physics, aeronomy and climatology and electric propulsion. At the time of the Challenger disaster, it was scheduled for its first launch in March 1988, but was extensively delayed. After deployment from Atlantis’ RMS mechanical arm at an altitude of 260 miles on the second day of the STS-46 mission, EURECA was to employ its own thrusters to ascend to around 325 miles, spending the next nine to ten months untended, ahead of retrieval by a subsequent Shuttle crew in the early summer of 1993. Europe paid NASA $14.1 million to launch EURECA and an additional $3.9 million to recover it from orbit.

With international participants including Belgium, Germany, Denmark, France, Italy, the United Kingdom and The Netherlands, EURECA measured 8.2 feet long and 14.7 feet in diameter and weighed almost 10,000 pounds. It was constructed from high-strength carbon-fibre struts, connected by titanium nodes, to form a framework of cubic elements which provided relatively few thermal distortions, enabled higher alignment accuracy and was easy to assemble and maintain. It was powered by a pair of deployable solar arrays and was designed (as its name implies) to be ‘reusable’, with at least one other mission scheduled for the mid-1990s. This was subsequently cancelled. As a result, EURECA flew only once and is today housed at the Swiss Transport Museum in Lucerne.

During its mission, the satellite was managed by ESA’s Space Operations Centre in Darmstadt, Germany, which exercised control through a pair of ground stations in Kourou, French Guiana, and Maspalomas in southern Spain. It would actually be in direct contact for a relatively short period during each day and was thus designed to operate highly autonomously, detecting and responding to systems problems and isolating and recovering from faults. Its scientific payload was complex and varied: protein and semiconductor crystallisation; analysis of the impact of space radiation on spores, seeds and eggs; Italian and German materials science furnaces, capable of reaching temperatures of up to 1,400 degrees Celsius; a high-precision thermostat; a pair of solar physics investigations; a study of aerosol and trace gas densities in the mesosphere and stratosphere; a wide-angle gamma ray and X-ray telescope; an inter-orbit communication device to relay data through ESA’s Olympus satellite; observations of meteoroids and cometary dust; an advanced gallium arsenide solar panel; and an experimental electric propulsion thruster. Magnetic torquers and EURECA’s own attitude and orbit-control system was designed to ensure that accelerations would be kept to a minimum, thereby avoiding disturbance to the experiments.

Deftly manipulated by Swiss astronaut Claude Nicollier, the RMS mechanical arm manoeuvres EURECA above the payload bay, prior to deployment. The spherical TSS, atop its mast, can be seen in the foreground. Photo Credit: NASA

With two ambitious satellites thus tucked inside her cavernous payload bay, Atlantis rocketed into orbit at 9:56:48 am EDT on 31 July 1992. The countdown clock had been held at T-5 minutes for 48 additional seconds, when the ground launch sequencer verified that the fuel isolation valve position for one of the Shuttle’s Auxiliary Power Units (APUs) was closed. The valve was subsequently opened by Andy Allen and the countdown continued towards a perfect launch. Upon achieving orbit, the astronauts began preparations to deploy EURECA. Six hours into the mission, Nicollier completed the checkout of the RMS, with no anomalies, and positioned it above the payload bay in an ‘overnight park’ position, preparatory to deployment on 1 August.

However, mission events began to slip when a series of intermittent data problems were encountered with EURECA whilst attached to the RMS. At the ‘lower hover’ position above the payload bay, the orbiter’s payload data interleaver lost its communications link with the satellite, although the solar arrays and antenna were successfully unfurled. It was decided to postpone the deployment by 24 hours, whilst ground controllers commenced troubleshooting, and it was not until the late evening of 1 August that Nicollier once again returned EURECA to its pre-deployment position. At length, at 3:07 am EDT on the 2nd, Nicollier released the satellite and Shriver and Allen began the procedure to withdraw Atlantis to a safe distance. The pilots maintained a station-keeping position at a distance of 1,000 feet until the checkout of EURECA had been satisfactorily concluded.

For a time, during this station-keeping, Allen was alone on the flight deck, and when Atlantis slipped into orbital darkness it was almost impossible to see the satellite. As a result, he flew off the radar, making sure that he kept an appropriate distance from EURECA. “On a night pass, we have about ten minutes where we lose radio contact with the ground,” Allen told an interviewer for the Smithsonian, years later. “We had just entered one of those periods, so there was no contact with Mission Control and no view of the satellite.” All at once, the radar started picking up ‘range-rate’ data, indicative of a changing position of EURECA relative to Atlantis. “At first I thought it was a problem with the radar, but the rate kept increasing,” he continued. “It finally got to the point where I figured if it was really true, it was either going to hit us in a few seconds or I had to do something about it.”

Allen started firing off a number of “pretty aggressive” bursts from the Shuttle’s thrusters, whose cannon-like reports quickly drew a concerned Loren Shriver up to the flight deck. Neither man could see EURECA in the darkness and, even with flashlights aimed through the windows, their view was poor. Eventually, contact was re-established with Mission Control. “What had happened,” said Allen, “was that the control centre in Germany had tried to rotate the satellite, but had sent the wrong command, which fired all the satellite’s jets. Unbeknownst to Mission Control and to the crew, they fired all the jets in such a fashion as to send EURECA towards us at four to five feet per second. They couldn’t have planned it better if they had wanted to hit the orbiter!” Moreover, flight rules forbade them from using the Shuttle’s thrusters within a distance of about 700 feet of the satellite, for fear of damaging its delicate solar arrays. Allen had gotten well within that range, but thankfully no damage to either craft was sustained.

In the meantime, the RMS was kept in a poised-for-recapture position, to hedge against the possibility of a contingency retrieval of the satellite. Eventually, it was decided to leave the mechanical arm uncradled for a further day, in order for its wrist camera to acquire imagery of one of the TSS scientific experiments. The RMS was finally cradled early on 3 August. However, the gremlins were not yet finished with EURECA. Seven and a half hours after deployment, its thrusters were ignited for what should have been a 24-minute burn to position the satellite into its operational orbit. Unfortunately, this firing was cut to just seven minutes, due to the occurrence of unexpected attitude data. Ground controllers worked the issue over the following couple of days, uploading corrected values into the satellite’s Low-Attitude Conical Earth Sensor (LACES), until, on 6 August, it was successfully boosted into its correct orbit.

Marsha Ivins and Jeff Hoffman at work on Atlantis’ flight deck during Tether Optical Phenomena (TOP) experiments. Photo Credit: NASA

The delayed departure of EURECA had already prompted the STS-46 Mission Management Team to approve an additional (eighth) day to the flight and pre-deployment activities associated with the TSS had been correspondingly pushed back. Original planning called for the deployment to occur with Atlantis’ payload bay facing away from Earth – tail slanted ‘upward’ and nose pitched slightly ‘down’ – such that the satellite would initially unreel at an angle of about 40 degrees behind the Shuttle’s flight path. Departing at a relatively slowpoke pace, it would be halted temporarily at a distance of just under a mile to reduce the deployment angle from 40 degrees to five degrees, putting the satellite in the same plane, directly overhead. Deployment would then resume and upon reaching a distance of 3.4 miles, a quarter-revolution-per-minute spin would be imparted via TSS’s attitude thrusters, thus kicking off a lengthy series of scientific investigations.

The next stage would see the satellite extended to a peak velocity of around 8 km/h, reaching a distance of 9 miles from the Shuttle, after which the rate of tether unreeling would be slowed. Five and a half hours after first motion from the top of the TSS mast, at a maximum distance of 12 miles, the spin would be briefly stopped in order for measurements of tether dynamics to be taken. Of specific interest to the physicists were exploring the validity of theoretical concepts that tethers became increasingly more stable with increasingly length, together with analysing the effects of induced disturbances, such as ‘bobbing’, pendulum-like vibrations (known as ‘librations’) or backwards-and-forwards skipping-rope-type motions. One particular test involved adjusting the Shuttle’s autopilot to cause a slight ‘wobble’ of up to ten degrees in any direction – five times more than normal – before executing a thruster firing to damp them out. This ‘Ten-Degree Deadband’ test would be employed to judge disturbances caused through looser attitude control on the part of Atlantis herself.

Working on the theoretical prediction that payload instability reached its most acute whilst the tether was at its shortest deployed extent, the retrieval speed of TSS was planned to decrease at a distance of about 4 miles. At length, it would be halted again, preparatory to several hours of final science operations, followed by the final retrieval and reberthing back onto the deployment mast in the payload bay. Since many of the dynamic features of the tether/satellite combination were unknown, a guillotine feature provided for the cutting of the payload at any stage.

Many of these plans were thrown into some disarray, when problems were encountered with TSS from the outset. Four days into the mission, at 12:12 pm EDT on 4 August, the mast was deployed without incident and – despite a troublesome umbilical which initially refused to separate – the first attempt to release the satellite started a little over five hours later. It was quickly aborted by the crew, due to excessive side-to-side motion in the tether. Following a lengthy check of the reel mechanism and vernier motors, a second attempt at 6:51 pm was successful and the tether deployed smoothly to a distance of 590 feet. It was stopped at 7:47 pm, because of possible buried winding on the reel, and was reeled in a few metres, then reeled back out at a slightly higher rate to a total extent of 840 feet.

From the back of Atlantis’ cabin, the view was spectacular. “When the Sun set,” recalled Jeff Hoffman to the NASA oral historian, “and everything turned red, it was just glorious.” Ninety minutes later, deployment resumed, but stalled again after two minutes and was powered down to survival levels to maintain the satellite’s battery lifetime during a crew sleep period. Next day, at precisely 9:00 am EDT on 5 August, the tether was retracted from 840 to 730 feet, whereupon it refused to move in either direction.

“I remember clearly looking through the camera” at the tether, Hoffman said. “All of a sudden, it started to get all these wiggles in it.” Wiggles were worrisome, implying a sudden slackness in the tether and suggestive of either a break or a jam. “The tether hadn’t broken,” he continued. “We could see that. The tether had jammed. The satellite had a jet of nitrogen gas to pull it away, and that was still on, but now it had bounced back.” The nitrogen burst was pushing the satellite in a sideways direction and Hoffman and Nicollier struggled to regain control. Loren Shriver knew that its current extent “was right in the middle of the so-called unstable zone”; moreover, mission rules prohibited the tether from exceeding a 45-degree ‘red-line’ angle, beyond which it would have to be cut loose for safety reasons. As the mission commander, Shriver was determined to prevent that from happening. “Every once in a while, I listen to the audio from that,” Hoffman reminisced. “It was certainly the wildest time that I’ve ever been in space. We were really up against the wall. We got very close to the red line, where we would’ve had to cut the tether.”

Jeff Hoffman and Franklin Chang-Diaz during pre-breathing exercises in support of a possible contingency EVA. Photo Credit: NASA

A contingency EVA was another option and the cabin pressure was duly reduced to enable Hoffman and Chang-Díaz to begin the necessary ‘pre-breathing’ period. The plan called for Hoffman to climb the TSS mast “and basically pull it in, hand over hand, and Franklin was going to wrap up the tether”. One hundred and fifty metres above the payload bay, at length, the satellite’s motions were calmed. The location of the jam was believed to reside in the upper or lower tether control mechanism and, mindful of the problems associated with the actual deployment, a consensus was reached to clear the glitch and bring the satellite back into the payload bay. “The motor that extends the boom was actually more powerful than the motor which reels in the tether,” said Hoffman, “so maybe if there was a kink in there, by extending the boom, that would be able to pull the kink free.”

At 3:52 pm EDT on 5 August, the procedure got underway. Firstly, the mast was retracted by one panel, in order to allow the crew to visually check for any tether slackness, but nothing was detected. This indicated that the problem existed in the upper tether control mechanism. The astronauts then re-extended the mast with its reel brakes engaged, which successfully cleared the jam and enabled them to retrieve and reberth the satellite at 6:54 pm. A little over an hour later, the mast was finally retracted and the satellite secured.

The maximum deployed distance had been 840 feet and although the predicted 45-volt electromagnetic flux was developed, the levels of induced voltage proved insufficient to excite the physical process for many of the mission’s primary (Category I and II) scientific objectives. Some Category III tasks – including studies of electron beam propagation, beam-gas cloud interactions and Shuttle surface glow – were achieved and the basic concept of the tethered satellite was successfully demonstrated.

After eight intensive days in orbit, Atlantis swept onto KSC’s Runway 33 at 9:11 am EDT on 8 August, completing the final Shuttle mission to land without the benefit of the drag chute. In the months after STS-46, she would be extensively upgraded and her next landing, in November 1994, would do so with a chute. The limited data return from the TSS proved sufficient to warrant calls for a follow-on mission, but in the days after landing a Board of Investigation – chaired by Darrell Branscome, the Chief Engineer at NASA’s Langley Research Center in Hampton, Virginia – convened to assess the problems. Later in August, the board reported back to Jeremiah Pearson, the Associate Administrator for Space Flight, concluding that the tether had snagged on a bolt in the deployment mechanism and corrective action should not impair the prospect of a repeat flight. When the Tethered Satellite flew next, in February 1996, it would be accompanied by most of the original STS-46 crew…although its outcome would be markedly different.


Tomorrow’s article will focus on STS-47, another international science mission flown 20 years ago, during International Space Year 1992.

One Comment

  1. Ben, I’d like to ask you if you have compiled all the magnificent space history articles that you write here, into a book, and if so, where can I find it?



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