When satellite communications at NASA’s Super Pressure Balloon Imaging Telescope, SuperBIT, failed, the team knew they were in danger of losing their astronomical imaging data, which the telescope had painstakingly collected from more than 99 .5 percent of the atmosphere. But mission planners had a backup plan.
His idea was to drop the entire telescope, with all its data, to the ground by parachute so that it could be used again. Having circumnavigated the southern hemisphere five times, mostly over the ocean, the team understood that their opportunities were limited. Their best chance for recovery was to make landfall in Argentina on May 25, 2023. By simulating wind speed and weather patterns, they were able to predict, more or less, where the instrument would land.
In the end, a crucial part of this plan went wrong. After the telescope landed in a remote area of the Argentine province of Santa Cruz, the parachute did not release as planned. Then, in the hours it took the search and rescue team to find the instrument, the wind blew it for 3 kilometers through the hills of the Argentine wilderness, destroying the telescope and leaving a trail of debris.
But the team had a backup plan; one that had never been attempted. Now Ellen Sirks and Richard Massey of Durham University in the United Kingdom, along with the rest of the SuperBIT team, tell the story of this mission and suggest that other balloon missions could mitigate the risk of failure with a similar approach.
The SuperBIT mission began on April 16 with a launch from Wanaka, New Zealand. The telescope was raised to an altitude of 40 kilometers and for the next 40 days it observed distant galaxy clusters in the hope of recording gravitational lensing events that could indicate the existence of dark matter in these parts of the universe.
A feature of the mission was the relatively high rate of data produced by the imaging system. The telescope was therefore equipped with two downlinks, one via the commercial Starlink satellite communication system and the other via NASA’s Tracking and Data Relay Satellite System (TDRSS). He also kept onboard copies of all data as a backup.
Then, on May 1, the Starlink connection was lost for reasons that are still unknown. And then, on May 24, the TDRSS system began to fail, raising the possibility of the entire mission being lost.
The team immediately made the decision to return the instrument to Earth in the maneuver that culminated in its total loss.
But the team had planned for such a failure. Part of the design included four capsules equipped 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 full backup of mission data. The capsule also contained a Global Navigation Satellite System receiver, a battery, and an Iridium satellite communication system transmitter to transmit its whereabouts. It also had a servo-powered clamp mechanism that would release it from the telescope.
During the mission, a capsule failed, possibly because its data cable was disconnected from the telescope during launch. That left three pods operational as backup.
Then, 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 that the terminal velocity of the parachuting capsules was 4 m/s. They also knew the local wind patterns. This allowed them to calculate where the parachutes would land and time the drop so that these locations were isolated to avoid injuries on the ground, but not so remote that recovery would be impractical.
“Therefore, we targeted a large region of open, uninhabited land west of Highway 12,” say Sirks, Massey and company. Then they crossed their fingers and waited.
Things quickly began to go wrong. During the descent, the capsules had to transmit their position in order to control their trajectory and review the position of the landing zone.
But during the mission, the capsule’s batteries froze and the cold did not supply enough power to the GNSS receiver and transmitter. “Unfortunately, neither (the capsule) reported or recorded its location during descent, as they are supposed to,” say Sirks, Massey and company.
But the batteries warmed up after landing and the capsules began transmitting their position, one landing 2 kilometers further than expected and the other 2 kilometers earlier. With bright red parachutes easily visible, the search and rescue team found a capsule about 15 miles off Highway 12.
They found the other capsule in a patch of snow surrounded by mountain lion tracks. “We guess foam and parachute-type nylon are intriguing but not tasty,” reflect Sirks, Massey and company.
It turned out that both capsules contained complete copies of the entire mission data set. The team was 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 didn’t need it because the data from the released capsules had already been recovered, but having the original copy allowed us to verify that no data on the SD cards was corrupted,” say Sirks, Massey and company. “The first use of the data recovery system capsules during a live scientific mission was a great success.”
This is a remarkable story with an important takeaway message. “For a relatively small cost, we insure the scientific performances of SUPERBIT against a loss event. that came true: High-bandwidth communication links failed and the telescope was destroyed upon landing,” the team says. “We recommend that future balloon missions consider the use of this or similar systems.”
Final 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 American spy satellites used photographic film to record images of Soviet military installations. The film was then parachuted to Earth over the Pacific Ocean and caught in the air by a specially equipped aircraft.
These missions were the first in the Keyhole series of spy satellites that the United States continues to operate today. The current incarnation is believed to be the size of the Hubble Space Telescope but pointed toward Earth. They no longer parachute their images to the ground.
Ref: Data downloaded by parachute from a NASA superpressure balloon: arxiv.org/abs/2311.08602