Two and a half months after Starfish Space’s first orbital mission teetered on the edge of failure because its Otter Pup satellite docking system took a wild tumble, the Kent, Wash.-based startup says that it has stopped the spin and is moving ahead with preparations to rendezvous with another satellite.
Mission controllers still have to make sure that Otter Pup is in working order, and they still have to identify a satellite they can link up with. But Starfish co-founder Austin Link said the team has gotten over the highest hurdle: “de-tumbling” a spacecraft that had been rotating at a rate of roughly one revolution per second.
“This is the first time that we as a company have gone and done something really unique and really extraordinary in space,” Link told GeekWire. “It wasn’t the thing that we set out to do with this mission. We still have that ahead of us. But to do that is, to me, another proof point for how excited I am to get to work with all the incredible folks we have at Starfish.”
The challenge began shortly after Otter Pup and dozens of other spacecraft were sent into low Earth orbit by a SpaceX Falcon 9 rocket on June 12. Starfish’s spacecraft, which is about the size of a dorm-room fridge, was designed to be deployed from an orbital space tug.
That space tug that would then serve as the target for a months-long demonstration showing how Otter Pup — and eventually, Starfish’s full-size Otter — can rendezvous and dock with other spacecraft for servicing.
Unfortunately, the space tug went into a rapid spin after separating from the Falcon 9 upper stage, with Otter Pup still attached. Mission controllers made an emergency decision to release Otter Pup immediately, but Starfish’s spacecraft continued to tumble. Atmospheric drag slowed down that tumble somewhat; nevertheless, the Starfish team worried about the solar-powered spacecraft’s ability to recharge itself.
“It’s disheartening when you put so much effort into a mission and it ends up at risk,” Link said. “I mean, the Otter Pup died twice during the de-tumbling process, and it just happened to come back to life.”
To stop the spin and get the solar arrays pointing at the sun again, Starfish’s engineers devised an algorithm that made use of the interaction between Earth’s magnetic field and three magnetized torque rods on Otter Pup. The rods are basically electromagnets that can be turned on and off to slow down (or speed up) the rotation of a spacecraft.
“Earth’s magnetic field is pointing a certain direction in space, and when your satellite rotates through it, the way that you need to turn on your torque rods changes,” Link said. “If you have a one-second delay in your system, well, if you’re rotating 5 degrees per second, that’s OK. But if you’re rotating 300 degrees per second, you’ll be way off from where you thought the magnetic field was aligned.”
Starfish’s engineers devised software code that checked the alignment on the order of 10 times per second, and they spent weeks testing the code in simulated space conditions. “We were making super-quick-turn engineering decisions … and then you’d say, OK, this worked, or this didn’t work,” Link said. “It was pretty fast-paced and exciting, really.”
At the end of July, the team uploaded the code for a 30-minute test run and waited for the results. Link said he and Starfish’s other co-founder, Trevor Bennett, made a bet on how much improvement they’d see after the test. “It ended up right between our bets,” Link recalled.
As soon as they could, engineers uploaded another batch of code for a three-hour run. That brought the rate of spin down to zero.
“We managed to get the Otter Pup stable and pointing toward the sun and not spinning along the way,” Link said. “I’ll confess, it happened much faster than I expected we’d be able to do it.”
Starfish’s team was supported by partners including Astro Digital, which built Otter Pup’s frame to fit Starfish’s requirements; Advanced Solutions Inc., a subsidiary of Rocket Lab that contributed some of Otter Pup’s onboard flight software; and Vast as well as its Launcher team, which provided the Orbiter SN3 space tug.
Chris Biddy, co-founder and CEO of Astro Digital, told GeekWire that rescuing Otter Pup “was a big recovery effort and the most difficult we have been involved with.”
“Things can get very weird and scary when a satellite is not under three-axis control and spinning at rates that are nearly 100 times what is normal,” Biddy said in an email. “Off-nominal temperatures, power generation and communications are big threats to the survival of the satellite under these conditions. To have recovered fully from that and to have the satellite now under three-axis control is a huge relief, and a testament to the teams involved and the design of the hardware.”
The Starfish team still needs to determine whether Otter Pup can do the things it needs to do.
“It was never designed to spin this fast,” Link said. “It hit conditions that may have stretched the satellite in multiple ways. We need to go test and turn on a bunch of different parts, and that’s a process that we’ve been doing and will continue to do. We need to see if everything works in the way that it needs to for the satellite to do proximity operations, to maybe go try to dock with another satellite.”
Starfish is also looking for another satellite to fill in as the mission’s target. The original target, Orbiter SN3, is out of the picture because of the anomaly it suffered.
“Even if we do both of those things well, then that just means that we get to do the initial mission, which was already a really challenging mission,” Link said. “So there are definitely hurdles in front of us here. But in a weird way, it’s exciting. … The engineer in you just loves solving crazy-hard problems.”
As the Otter Pup saga continues, Link is keeping the bigger picture in mind.
“It’s a great moment for the team to go de-tumble the satellites, and what that means as a technical accomplishment,” he said. “But it’s also a great moment in that we get a chance to continue with our Otter Pup mission. If we’re able to dance around another satellite, or even dock with another satellite, then that’s paving the way for the Otter in the future — and paving the way for it becoming a more regular and common thing for humans to go interact with satellites on orbit.”
