Rescuing Intelsat

Several motors were considered for this mission. As Intelsat 603 had used a substantial fraction of its propellant to stave off orbital decay (which would reduce its operating life from 15 years to 12 years if it attained its assigned station) an IUS was an attractive option because a two-stage vehicle would be able to make the circularisation manoeuvre. However, bolting an HS-393 onto an IUS would not be easy. The second option was the TOS that had been developed specifically to boost heavy Shuttle payloads into geosynchronous transfer orbit. This had the advantage that it, too, was cleared for flight on the Shuttle, but had yet to be used. The third option was to fit the same type of motor as that used originally. In fact, the TOS and the first stage of the IUS used variants of this same motor. It was eventually decided to carry a new Orbus in a modified TOS cradle and mate it with the satellite. The challenge was to find a means of capturing the satellite, which was much larger than any that astronauts had worked on previously. A stinger was impractical because the support ring at the base of the satellite was 3 metres wide. The only option was to develop a capture bar incorporating a trunnion pin to enable the remote manipulator to manoeuvre the captive satellite into position directly above the motor. If the motor had a suitable adapter, the satellite could be connected to the motor using the bar, the arm could be withdrawn, the trunnion pin removed, and the payload released. The tricky part would be capturing the satellite. The astronaut would position the bar diametrically across the base of the satellite and activate the mechanism that would engage both ends simultaneously. One complication was that the target would be rotating, but as the rate was only half a revolution per minute underwater trials indicated that this would not pose a problem.

On its debut mission in May 1992 as STS-49, Endeavour was launched to rescue

The launch of a Commercial Titan on 23 June 1990 with Intelsat 6F4.

Intelsat 603. Bruce Melnick swung Pierre Thuot on the arm up to the slowly rotating satellite while Rick Hieb remained in the payload bay. After Thuot had carefully aligned the bar across the ring on the bottom of the enormous cylinder, he eased it forward to establish contact. An automatic mechanism was supposed to trigger latches to secure both ends simultaneously, but Thuot had not contacted the target with sufficient force to trigger the mechanism. On a second attempt, he made a firmer contact and the latches fired. Unfortunately, when Thuot tried to use the bar to cancel the residual spin, it slipped off and the satellite resumed its spin. Furthermore, Thuot's actions had caused it to precess, which made positioning the bar for a third capture much more difficult. Not only did his attempts to attach the bar fail, but each time he touched the satellite its precessional motion became more pronounced. After three hours he admitted defeat and Endeavour withdrew, leaving Intelsat 603 in a slow 50-degree nutation. It was belatedly realised that the simulations in the water tank had not accurately replicated the degree to which the satellite would be disturbed by a light contact. The clue was the fact that it had reacquired its spin after the bar had slipped off. This meant that the liquid propellant had retained sufficient momentum to spin up the satellite again, and the precession was due to the liquid sloshing around. If he ever managed to capture the satellite, Thuot would need to hold it steady until the liquid had shed its kinetic energy, and then take care each time he moved it. Despite having trained in a water tank, they had been caught unawares by the physical properties of fluids in weightlessness. Overnight, the satellite's handlers eliminated both the precession and the residual spin. When Endeavour returned the next day Thuot ventured out again, confident that the satellite would be captured. This time he eased the bar against the ring and activated the latches manually. However, they failed to engage. After five hours he retreated, this time leaving the satellite in a 'flat' spin. When Endeavour had withdrawn, the controllers once again set out to stabilise their charge, but it was beginning to look as if it was beyond rescue.100

While the crew slept, their colleagues on the ground tried to find an alternative way of capturing the rogue satellite. It was all too evident that the capture bar would have to be abandoned. How could an astronaut retrieve the 4-metre-diameter, 6-metre-tall drum without using the bar? And, in any case, the bar would have to be in place to enable the arm to fit the satellite on the motor. Could Thuot capture the satellite by hand? Could he hold it in place while Hieb added the bar? If Thuot stood on the arm to hold the satellite, Hieb would not be able to reach it unless the orbiter manoeuvred really close in. If Thuot lost his grip, the satellite might strike and damage the payload bay doors or the vertical stabiliser. Meanwhile, the crew of Endeavour had also reached the conclusion that the ideal tool for grappling a satellite was the human hand, and they added a twist of their own. The Shuttle carried a spare suit, and if Tom Akers, who was scheduled for a later excursion, was to lend assistance, Thuot could then grab the satellite and slowly swing it down into the bay, where Hieb and Akers would be waiting for it. They could then hold it steady while Thuot attached the bar, free of any concern about disturbing the satellite's motion. As soon as the bar was in place, they could revert to the original plan. When this idea was put to Houston, a test was ordered to verify that the airlock could accommodate three spacesuited astronauts. It was a tight fit, but was feasible, so in the absence of a better idea the plan was approved.

Endeavour had propellant for just one more rendezvous. If Intelsat 603 eluded them this time it would have to be written off, even though it was perfectly healthy. If they failed, it would not be for lack of experience. Dan Brandenstein, chief of the Astronaut Office, manoeuvred the orbiter to position the satellite a mere 2 metres above the bay, with its base facing down. Hieb had taken up station on a foot restraint on the starboard sill; Akers was on a truss that spanned the mid-bay, and the satellite was just above their heads. Melnick, further demonstrating his prowess with the arm, stationed Thuot around the far side. For 10 minutes they simply observed the satellite's motion. It had a slight

Pierre Thuot, riding the Shuttle's arm attempts to engage the ring fixture on the base of Intelsat 603 using the 'capture bar'.

Pierre Thuot (on the arm), Richard Hieb (on the sill) and Tom Akers the payload bay) manually manipulate Intelsat 603.

(in the centre of nutation with a 3-minute rate. They waited until it was vertical with respect to the bay, and then six hands simultaneously grasped the base ring and held the satellite still for several minutes to allow its fluids to settle. Although they had captured the drum in a vertical attitude, it was not conveniently oriented for fitting the capture bar, which Hieb had stowed on the side wall below his station. Working in concert, they rotated the massive satellite, a little at a time, through 120 degrees. When it was restabilised, Hieb let go, retrieved the bar, and positioned it just beneath the ring. From his position on the sill he could not reach the controller in the centre of the bar to engage the latches, so he held it in position with one hand and the ring with the other. Akers then acted as an anchor while Thuot manoeuvred beneath the satellite, indicating his instructions to Melnick by way of hand movements. With the latches of the bar engaged, Thuot tightened a number of bolts to complete the attachment, then dismounted the arm. Once the foot restraint had been deleted from the arm, Melnick grasped the satellite via the trunnion pin on the bar, and Akers, who had held the satellite in a vise-like grip for 90 minutes, was finally able to let go. After a clamp had been attached to each end of the bar, the satellite was mated with the motor. There was a moment of concern when the command to eject the stack from its cradle was issued and nothing happened, but the backup circuit functioned and Intelsat 603 finally set off on the first stage of its journey to geostationary orbit. NASA set up a review of the rescue by a panel chaired by Eugene Covert of the Massachusetts Institute of Technology, and this reported that the operation had not been cost-effective, and was not worth the risk of the astronauts' lives.101

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