If an engineer could rescue a still working telescope in the fall, which is going down, and never is able to carry it up by itself, how does the engineer do that? This is the issue with NASA’s Neil Gehrels Swift Observatory, a gamma-ray-burst hunter tasked to orbiting in low earth orbit since its launch in 2004. Although it is scientifically valuable, the air is constantly dragging Swift down. The first task is not to fix the damaged equipment. It must get to the spacecraft, precisely match its velocity, and boost its orbit before it falls into a much tougher environment due to drag. From afar, that seems easy to do.
One of the hardest things to do in space when it came to not building the target for servicing was orbital rendezvous. The amount of machinery involved in such a rendezvous, proximity operations and docking exercise can be seen in the work that NASA itself has done on these systems and subsystems: guidance, navigation, control, sensor calibration, collision avoidance, contact dynamics, and real-time simulation. There’s an additional wrinkle to Swift’s situation. The path it will follow in the future is not exactly certain since the drag on the telescope varies from week to week due to variations in solar activity in the upper atmosphere. That is why flight dynamics teams have been constantly revising their predictions, rather than assuming a fixed orbit. These forecasts are subject to changes over time, depending on space weather predictions and other considerations, such as the orientation and elevation of Swift, said Michael Shoemaker, deputy flight dynamics lead for Space Science Missions Operations at NASA Goddard.
Link is a rescue vehicle being designed by Katalyst Space Technologies (KST) under a NASA contract. The success of the mission would be a rare example of a government science mission involving commercial satellite servicing, whose technical history is longer than most people aware. In 1984, NASA serviced the SolarMax satellite, and in the subsequent shuttle missions 1989, 1993, and 1997 extended Hubble’s life. Later work focused on robotics techniques, such as spacecraft rendezvous and refueling of previously untouched satellites.
Swift falls into that tougher end of the spectrum. Designed to watch the violent aftermath of cosmic explosions, not a visitor. This requires Link to be able to approach a non-cooperative spacecraft, determine its precise relative motion, and complete the encounter without putting it in jeopardy of damaging or destroying the spacecraft or itself, but also with sufficient margin to execute an orbit raising maneuver. The team is forecasting weekly to determine when to suspend science observations and reconfigure the observatory to minimize drag, NASA officials report. That operating discipline has already extended the time by slowing orbital decay.
This is not just a telescope story. If the reboost is successful, its success would help bolster the case for satellite inspection and in-space logistics as practical tools, as opposed to futuristic gadgets. It would prove that aging spacecraft can be useful even when their orbital margins are slowly eroding, and that traffic-management prowess can be as important to mission life as instrument health. “The project to re-boost Swift has generated intense interest across the flight dynamics community”, said Russell Carpenter, Space Science Missions Operations’ deputy project manager. The task is barren and unsentimental for now: Get the telescope in position accurately enough, then reach it cleanly enough and push it up high enough so that one of NASA’s most long-lived observatories can continue to peer at the tempestuous cosmos for a bit longer.