'The Tether Is Broken': The Second Flight of the Tethered Satellite (Part 2)

The Tethered Satellite System (TSS) begins its deployment from the payload bay mast on STS-75. Photo Credit: NASA

The Tethered Satellite System (TSS) begins its deployment from the payload bay mast on STS-75. Photo Credit: NASA

Eighteen years ago this week, Space Shuttle Columbia rocketed into orbit carrying one of the most unusual experiments on record: the reflight of the Tethered Satellite System (TSS-1R). Originally conceived by the late Professor Guiseppe Colombo of Padua University, it was intended to demonstrate the “electrodynamics” of a conducting tether in an electrically charged region of Earth’s atmosphere, known as the ionosphere. It was envisioned that Colombo’s idea might ultimately lead to systems that would use tethers to generate electricity for spacecraft, employing our planet’s magnetic field as a power source. Furthermore, by reversing the direction of the current in the tether, the force created by its interaction with the magnetic field could potentially put objects into motion, thus boosting a spacecraft’s velocity without the need for precious station-keeping fuel and counteracting the effects of air drag. In the late 1980s and early 1990s, this was particularly appealing for the designers of the International Space Station as a means of compensating for atmospheric drag on the colossal outpost. Additionally, it was hoped that the concept could lead to the development of devices to trail scientific platforms far below orbital altitudes in difficult-to-study zones, such as the fragile ozone layer over the South Pole. Other applications for tethers included serving as extremely low-frequency antennas, capable of penetrating land and seawater, and perhaps generating artificial gravity or delivering payloads into higher orbits.

As noted in yesterday’s history article, the mission began smoothly on 22 February 1996, and within days the seven-man STS-75 crew—Commander Andy Allen, Pilot Scott “Doc” Horowitz, Payload Commander Franklin Chang-Díaz, Mission Specialists Jeff Hoffman, Claude Nicollier, and Maurizio Cheli, and Payload Specialist Umberto Guidoni—was in position to deploy the satellite. The intention was that, as the tether neared its maximum length of 12.7 miles (20.5 km), the deployment rate would have been gradually reduced. “It was within 1 km of its final length,” Hoffman told the NASA oral historian, “at which point we were going to put on the brakes and just let it sit there and start all the experiments. I was recording this huge arc in the tether through the camera, when I started to see little ripples in the tether.” To Hoffman’s eyes, it reminded him of the tether jam on STS-46, and a horrible sense of déjà vu dawned on him. However, at 8:29:35 p.m. EST on 26 February, at a tantalisingly closet-to-target distance of 12.2 miles (19.6 km), it became clear that the tension was due to something else: the tether had not jammed … but snapped.

The shocked astronauts recorded video footage of the incident, and the breakage appeared to have taken place near the top of the mast. “The tether has broken at the boom!” Hoffman radioed urgently. “It is going away from us.” In fact, the tether and the satellite were accelerating away from the shuttle at a rate of about 415 miles (670 km) during each 90-minute orbit. By the morning of the 27th, it was trailing Columbia by 3,000 miles (4,830 km), flying some 30 miles (50 km) “above” the shuttle. After winding the remaining 30 feet (10 meters) of tether back into the mechanism, the astronauts retracted the mast to its original configuration. It was fortuitous that the breakage occurred close to the top of the mast, rather than further outward, close to the satellite. “If it breaks at the bottom, it will fly away from you and you’re not in any danger,” said Hoffman, “but if it breaks at the top you’ve got 20 km of tether coming snapping back at you. We had practiced for that eventuality in the simulator. You’ve got to then cut the tether at the bottom and fly away from it.”

Agonizingly, TSS-1R reached 12.2 miles (19.6 km) from the Shuttle when the tether snapped. Photo Credit: NASA

Agonizingly, TSS-1R reached 12.2 miles (19.6 km) from the shuttle when the tether snapped. Photo Credit: NASA

Although nearly 24 hours of electrodynamic measurements had been lost, the $154 million reflight was far from a total failure. Already, when the satellite was less than 3.7 miles (6 km) from the mast, the Deployable Core Experiment (DCORE) recorded its first data. This experiment was mounted in the payload bay on the MPESS, and its task was to control the flow of electric current in the tether using a pair of electron guns. Before the break, its first performance run had successfully generated a current of 480 milliamps from the electrical charge that had collected on the satellite’s surface. This was about 200 times greater than the levels obtained during the first TSS flight on STS-46 in July 1992. Other experiments in the payload bay continued to function in support of the satellite and tether until as late as 6 March.

“We did get a lot of good data during the deploy,” Hoffman told journalists during a space-to-ground news conference. Currents measured during the deployment were at least three times higher than predicted in analytical models. Voltages as high as 3,500 volts were developed across the tether, achieving current levels of 480 milliamps. It was also possible for researchers to study the interaction of gas from the satellite’s thrusters with the ionosphere. A first-ever direct observation of an ionized shockwave around the satellite—impossible to study or model in laboratories on Earth—was also accomplished. Moreover, as the satellite and its trailing tether sped through the ionosphere, it was possible to continue investigations in spite of the fact it was no longer physically linked to the shuttle. It did not detract from the disappointment, however. “If you don’t ever get your nose bloodied,” said STS-75 Lead Flight Director Chuck Shaw, paraphrasing Theodore Roosevelt’s famous comment, “you’re not in the game.” Capcom Dave Wolf told the astronauts that, whilst it was too early to speculate on the cause of the tether breakage, an investigative board was already getting established to explore the anomaly. The board was headed by Kenneth Szalai, the director of NASA’s Dryden Flight Research Center at Edwards Air Force Base in California.

On 27 February, as the satellite and tether flew above the Electronic Signal Test Lab at the Johnson Space Center (JSC) in Houston, Texas, ground controllers transmitted commands to successfully reactivate three of its on-board experiments: the Research on Orbital Plasma Electrodynamics (ROPE), the Magnetic Field Experiment for TSS Missions (TEMAG), and the Research on Electrodynamic Tether Effects (RETE). With a possible two additional days of data-gathering capability now re-established before the satellite’s batteries were predicted to expire on 1 March, teams scrambled to assemble a last-minute research timetable to squeeze as much as possible from their dying payload. The ROPE experiment sought to examine the behaviour of charged particles in the ionosphere, as well as those surrounding the satellite and tether, under a variety of conditions. Meanwhile, RETE measured the electrical potential in the plasma “sheath” around TSS-1R and identified waves excited by the tether.

Elsewhere, the Second University of Rome’s TEMAG experiment mapped fluctuations in magnetic fields around the satellite. It was hoped that, even though Columbia and TSS-1R were now physically separated, firing electron beams from guns mounted in the payload bay would still disturb the ionosphere and be detectable by the satellite’s instruments. On 28 February, scientists were able to observe a sunlight-induced electrical charge on the satellite’s surface as it moved through the daytime and nighttime portions of its orbit. They also succeeded in reactivating and acquiring valuable data from two other satellite-mounted experiments. It was even possible, according to Jeff Hoffman, for ground-based observers to see TSS-1R from the southern United States.

The frayed end of the tether is clearly visible, snaking away from the mast, in this view captured from STS-75. Photo Credit: NASA

The frayed end of the tether is clearly visible, snaking away from the mast, in this view captured from STS-75. Photo Credit: NASA

Since the tether breakage, orbital dynamicists had predicted that Columbia would approach to within retrieval distance of TSS-1R on 29 February, and such a scenario was briefly considered, but ultimately discarded due to insufficient propellant margins aboard the shuttle. Had it been approved, a retrieval would have consumed up to six days of crew time. In anticipation of its rendezvous, the satellite’s batteries were placed in a low-power mode from the late afternoon of 28 February until the morning of the 29th to keep it alive for long enough. Right on cue, at 12:15 p.m. EST on 1 March, Andy Allen spotted TSS-1R and its tether, from a distance of just 47 miles (75 km). “All we can really see are pinpoints of light, real close together,” he radioed to Mission Control. By this time, however, the satellite’s batteries were rapidly failing. Very weak signals had been detected through the Merritt Island and Bermuda tracking stations earlier that same day, and no further data was received after 1 March. Still, it endured for far longer than expected, prompting one manager to liken it to the Energizer bunny, for its capacity to keep going.

Despite the measure of success gained on STS-75, the mood aboard Columbia remained sombre. “Every time I turn around and look through the window and I see this empty bay,” said Maurizio Cheli, “it’s like a part of myself has left.” As STS-75’s commander, and a veteran of the previous TSS mission, Andy Allen derived an additional blow from the problems endured by the tethered satellite. “Scientists on the ground have lost a lot and we feel for them,” he said. “We were looking forward to demonstrating that we could actually retrieve a satellite from 20 km and we’ve put an amazing amount of work into it.” Jeff Hoffman added that the tether loss felt “like getting hit in the stomach.” Of course, as noted in scientific papers later presented at an American Geophysical Union (AGU) conference, the main scientific breakthrough was the discovery of tether currents three times higher than theoretically predicted. It was speculated that this might indicate some degree of ionisation around TSS-1R, even when its cold gas thrusters were switched off. In fact, when the thrusters were activated, the current climbed even higher, to 580 milliamps. Overall, its current-collection and power-generation capabilities proved to be several times higher than predicted.

With the completion of “direct” TSS-1R operations, late on 26 February, the astronauts returned to their dual-shift (“red” and “blue”) system of activities and focused on their other mission tasks. The most important of these was the third United States Microgravity Payload (USMP-3), which reflew several sensitive materials experiments. Although USMP had flown twice before, it had never been aboard a dual-shift shuttle mission, and this presented a number of obstacles. “Because they were growing crystals, it required an extremely quiet Shuttle,” recalled Jeff Hoffman. “When the scientists discovered that we were going to be a two-shift flight – so somebody would always be awake – they were pretty upset. Just because of the tight scheduling, it couldn’t be moved to another flight.”

Claude Nicollier is the sole clearly identifiable crewman in this view of Columbia's crowded flight deck during TSS-1R operations. Photo Credit: NASA

Claude Nicollier is the sole clearly identifiable crewman in this view of Columbia’s crowded flight deck during TSS-1R operations. Photo Credit: NASA

The crew promised they would be quiet at critical periods, in order to minimise disturbances through the shuttle’s structures and their potential impact on the sensitive USMP-3 experiments. “Very quickly, we learned what activities were causing disturbances and we would stop those,” Hoffman continued. “They told us after the flight that it was as quiet as they had ever seen it. They could see the vernier jets firing; they made more noise than we did! In order to accomplish this, they had to declare that these eight-hour periods, when the other shift was asleep, were the so-called “quiescent periods”. They weren’t allowed to give us any other experiments to do … so for the best part of a week, for eight hours a day, we just had to float and look out the windows. I felt as if I were a space tourist. It was really quite extraordinary!”

The USMP-3 operations ran so smoothly and generated such valuable data that on 4 March NASA decided to extend the STS-75 mission by 24 hours to almost 15 days. In terms of microgravity research, STS-75 had proven a superb success, hampered only by the tether breakage which lost almost a full day of electrodynamic measurements. Nevertheless, the reflight did demonstrate the concept of powering spacecraft using conducting tether systems. Columbia’s return to Earth, already extended until 8 March, was postponed by an additional 24 hours, due to a forecast of low clouds and the chance of rain and gusty winds in Florida. Although weather conditions were acceptable at Edwards Air Force Base in California, NASA managers decided to hold out for an improvement on the East Coast. A cold front passed through KSC on 7 March and was expected to become stationary by the 9th, perhaps leading to an upper-level low-pressure system which could produce clouds and showers. Fortunately, the weather on the 9th proved acceptable, and Allen and Horowitz guided Columbia to a smooth landing on Runway 33 at 8:58:21 a.m. EST.

“All right!” yelled the entire crew, in unison, at the instant of touchdown.

“We copy your elation,” came the reply from Mission Control.

With Allen and Nicollier both making their third shuttle missions, and Horowitz, Cheli, and Guidoni on their first flights, it was Hoffman and Chang-Díaz—both of whom were on their fifth space voyages—who were the most experienced members of the STS-75 crew. In fact, both men accumulated a total of 1,000 hours of experience aboard the shuttle during this mission; Hoffman was the first person to pass the milestone on 29 February 1996, with Chang-Díaz entering second place on 8 March, shortly before Columbia returned to Earth. “I was the first person to complete 1,000 hours on the Shuttle,” Hoffman told the NASA oral historian, “which I hadn’t really thought about.” However, test pilots Allen and Horowitz reminded him that in their military community, becoming the first person to accrue 1,000 hours was highly commendable. “Now it turned out that Franklin wasn’t going to get 1,000 hours if we had come back when we were supposed to land, but we had a weather delay, so we had one extra day in orbit,” explained Hoffman. “Then he was the second person to get 1,000 hours, so we have a nice picture of the two of us floating together holding a big sign saying 2,000. That was quite nice.”

Pilot Scott 'Doc' Horowitz is pictured during the re-entry phase of the STS-75 mission in his seat on Columbia's flight deck. Photo Credit: NASA

Pilot Scott “Doc” Horowitz is pictured during the re-entry phase of the STS-75 mission in his seat on Columbia’s flight deck. Photo Credit: NASA

Meanwhile, Kenneth Szalai’s review panel charged with investigating the TSS-1R tether breakage had been established on 26 February. “Given the public investment in the tethered satellite, it is important that we find out what went wrong,” explained Wil Trafton, NASA’s associate administrator for the Office of Space Flight. “To do any less would be a disservice to the American and Italian people.” By the time the board’s 358-page report was published in June 1996, it blamed “arcing and burning of the tether, leading to a tensile failure after a significant portion of the tether had burned away.” The arcing itself was caused by either penetration from a “foreign object” (though not orbital debris or micrometeoroids) or a tether defect breached its insulating material. (Certainly, it was stressed that “the degree of vulnerability of the tether insulation to damage was not fully appreciated.”) This had apparently triggered a local electrical discharge from the copper wire in the tether to a nearby electrical ground.

“The board found that the arcing burned away most of the tether material at that location,” Flight International noted, “leading to separation of the tether from tensile or pulling force.” In his concluding remarks, Szalai highlighted that the problem was “not indicative of any fundamental problem in using electrodynamic tethers,” adding that “constructing a tether that was strong, lightweight and electrically conducting took the project into technical and engineering areas where they had never been before.” To the STS-75 astronauts, a short circuit had been at the forefront of their minds from the outset. “I was able to hook up a very powerful train of optics [and] telephoto lenses and take a close look at the broken end of the tether,” said Jeff Hoffman. “I could see that it was brown and charred, so we knew before we ever came home that it almost certainly had been a short circuit that had melted the tether.”


This is part of a series of history articles which will appear each weekend, barring any major news stories. Next week’s article will focus on the 45th anniversary of Apollo 9, one of the most unsung missions in America’s bid to plant human bootprints on the Moon. Although it never left Earth orbit, Apollo 9 was critical in enabling Neil Armstrong’s “one small step” on the lunar surface.


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