LAUREL, Md. — The projected increase in the launch of small satellites, particularly cubesats, has raised new concerns about the growth of orbital debris, despite statements by ventures developing such satellites that they will be responsible citizens in low Earth orbit.
“There’s been a lot of concern the last couple of years about small satellites and their proliferation. There are those in the industry who derisively refer to cubesats as ‘debris sats,’” said Ted Muelhaupt, one of several Aerospace Corp. officials who discussed the issue during a session of the Small Payload Rideshare Symposium at the Johns Hopkins University Applied Physics Laboratory here June 10. “There’s some justification for that.”
One problem with cubesats is that their small size makes them difficult to track, and thus difficult to predict when they might come close to another satellite. “Because of tracking uncertainties, cubesats have about as big an uncertainty volume as a normal satellite, so you have to avoid them just as much,” said Brian Hansen, who leads Aerospace’s Debris Analysis Response Team.
Planned constellations of dozens or hundreds of smallsats exacerbate that problem. One simulation presented at the symposium compared the collision risk for a group of satellites posed by a single large spacecraft and also by 100 smaller ones that, combined, have the same cross-sectional area as the large one. The cluster of smallsats posted 30 times the collision risk as the single large satellite.
“The proliferation of cubesats can pose a significant collision and conjunction risk,” said Andrew Abraham of the Aerospace Corp., who developed the simulation. “The issue of long-term disposal of cubesats should not be ignored.”
The challenge in disposing of cubesats, or carrying out other maneuvers, is that most of them do not have propulsion systems. Instead, satellite developers say they plan to place satellites in orbits low enough that atmospheric drag will cause them to re-enter within 25 years, consistent with U.S. government orbital debris guidelines.
“We have to respect the 25-year deorbit rule, and we’re not really aiming to do 24.9 years,” Mike Safyan, director of launch and regulatory affairs for San Francisco-based Planet Labs, said at the conference June 9. The company has already launched dozens of three-unit (3U) cubesats, many from the International Space Station, and has plans to ultimately deploy a constellation of 150 Earth imaging satellites.
“To be responsible space actors, we have to match the orbital lifetime to the operational lifetime as closely as possible,” he said, which for Planet Labs’ satellites ranges from several months to a few years. “About 350 to 600 kilometers is the range we think is the responsible place to be putting our satellites.”
Spire, another San Francisco company with plans to start deploying a constellation of 3U cubesats later this year, takes a similar view. “We’re really limited by orbital debris and the 25-year deorbit rule,” said Jenna Barna, launch manager for Spire, discussing the orbits the company was considering. “So 650 kilometers is where things start to get a little bit hairy with that.”
Another debris concern regarding cubesats involves their deployment. Such spacecraft are often launched in groups as secondary payloads on other launches, or in batches from the ISS, which can complicate tracking of them.
“The space station, for instance, will release a lot of cubesats at the same time. Those cubesats come off in this cloud,” said Lauri Newman, who manages efforts at NASA’s Goddard Space Flight Center to identify potential collision risks for the agency’s robotic satellites. “Unfortunately, when they come off as this cloud, it takes a while for them to separate.”
That cloud, she said, delays cataloging the individual satellites. “Other spacecraft can’t take action against them because their position isn’t known,” she said. “So there’s this time lag after the deployment of a cubesat when other objects can’t be protected.”
Newman recommended that organizations doing cubesat deployments perform assessments of potential collisions with other satellites prior to releasing the cubesats, since it can take up to a week for the cubesats orbits to be cataloged.
However, conference speakers took pains to emphasize that despite the growth in the numbers of cubesats, and greater public attention to the orbital debris problem, there was little danger of a major debris catastrophe in Earth orbit for the foreseeable future.
“Debris is a serious concern and we need to stop it now, but the alarmists are wrong, at least in the short term,” Muelhaupt said. “Don’t believe the movies.”