When the satellite communications failed on NASA’s Super Pressure Balloon-borne Imaging Telescope, SuperBIT, the team knew they were in danger of losing their astronomical imaging data, which the telescope had painstakingly gathered from above 99.5 per cent of the atmosphere. But the mission planners had a backup plan.
Their idea was to drop the entire telescope, with all its data, to the ground by parachute so it could be used again. Having circumnavigated the southern hemisphere five times, mostly over the ocean, the team understood their opportunities were limited. Their best chance for recovery was the upcoming landfall over Argentina on 25 May 2023. By simulating windspeeds and weather patterns, they were able to predict, more or less, where the instrument would land.
In the event, a crucial part of this plan went wrong. After the telescope touched down in a remote part of Santa Cruz province Argentina, the parachute failed to disengage as planned. Then, in the hours it took the search and rescue team to find the instrument, the wind dragged it for 3 kilometers across the rolling hills of Argentinian wilderness, destroying the telescope in a trail of debris.
But the team had a back up to the backup plan; one that had never been attempted. Now Ellen Sirks and Richard Massey at Durham University in the UK, along with the rest of the SuperBIT team, tell the story of this mission and suggest that other balloon-borne missions could mitigate the risk of failure with a similar approach.
Data Dive
The SuperBIT mission began on 16 April with a launch from Wanaka, New Zealand. The telescope rose to an altitude of 40 kilometers and during the next 40 days, it observed distant galaxy clusters in the hope of recording gravitational lensing events that might signal the existence of dark matter in these parts of the universe.
One feature of the mission was the relatively high rate of data produced by the imaging system. So the telescope was equipped with two downlinks, one via the commercial satellite communication system, Starlink, and the other via NASA’s Tracking and Data Relay Satellite System (TDRSS). It also kept on-board copies of all the data as a backup.
Then on 1 May, the Starlink connection was lost for reasons that are still unknown. And then on 24 May, the TDRSS system began to fail, raising the possibility that the entire mission could be lost.
The team immediately made the decision to bring the instrument back down to Earth in the maneuver that culminated in its complete loss.
But the team had planned for such a failure. Part of the design included four capsules fitted with small parachutes. Each capsule contained a Raspberry Pi circuit board connected to five 1TB microSD cards of solid-state memory that can hold a complete back up of the mission data. The capsule also contained a Global Navigation Satellite System receiver, a battery and an Iridium satellite communication system transmitter to broadcast its whereabouts. It also had a servo-operated pincer mechanism that would release it from the telescope.
During the mission, one capsule malfunctioned, possibly because its data cable became disconnected from the telescope during launch. That left three working capsules as backups.
So with the telescope dangling from a helium-filled balloon at an altitude of 33 kilometers, they dropped two of the remaining capsules somewhere over Argentina.
The team knew the terminal velocity of the parachuting capsules was 4 m/s. They also knew the local wind patterns. This allowed them to work out where the parachutes would land and to time the drop so that these locations would be isolated to prevent injuries on the ground but not so remote to make retrieval impractical.
“We therefore aimed for a large region of open, uninhabited land west of main road 12,” say Sirks, Massey and co. Then they crossed their fingers and waited.
Drop Zone
Things quickly started to go wrong. During the descent, the capsules were supposed to broadcast their position so that their trajectory could be monitored and the position of the landing zone revised.
But during the mission, the capsule batteries had become frozen and cold not supply enough power to the GNSS receiver or to the transmitter. “Unfortunately, neither [capsule] reported or recorded its location during descent, as they are supposed to,” say Sirks, Massey and co.
But the batteries warmed after landing and the capsules began to broadcast their position, one landing 2 kilometers further than predicted and the other 2 kilometers earlier. With the bright red parachutes easily visible, the search and rescue team found one capsule some 24 km from main road 12.
They found the other capsule in a patch of snow surrounded by cougar prints. “We surmise that foam and parachute nylon are intriguing but not tasty,” muse Sirks, Massey and co.
Both capsules turned out to contain complete copies of the mission’s entire data set. The team were able to verify that this data was an exact copy of the onboard data because they eventually found the telescope’s intact solid-state drive in the debris field left after its destruction.
“We did not need it, because data had already been retrieved from the released capsules, but having the original copy enabled us to verify that no data on the SD cards were corrupted,” say Sirks, Massey and co. “The first use of the Data Recovery System capsules during a live science mission was a huge success.”
That’s a remarkable story with an important take-away message. “For a relatively small cost, we insured the scientific returns of SUPERBIT against a loss event that came true: high bandwidth communication links failed, then the telescope was destroyed upon landing,” say the team. “We recommend that future balloon missions consider using this or similar systems.”
End note: SuperBIT may have been the first balloon-borne telescope to use parachutes for data collection, but this approach has a long history. In the early 1960s, the first US spy satellites used photographic film to record images of Soviet Military facilities. The film was then parachuted back to Earth over the Pacific Ocean and caught in mid-air by specially equipped aircraft.
These missions were the first of the Keyhole series of spy satellites that the US continues to operate today. The current incarnation is believed to be the size of the Hubble Space Telescope but pointing Earthwards. They no longer parachute their images back to the ground.
Ref: Data Downloaded Via Parachute From A NASA Super-Pressure Balloon : arxiv.org/abs/2311.08602
