Telescopes in space have a huge advantage over those on the ground: they can see the universe more clearly. The Earth’s atmosphere, weather conditions, and low-flying satellites don’t obscure their view. But space telescopes have a disadvantage too. They can’t be repaired, at least not since NASA’s Space Shuttle program ended in 2011.
But next-generation telescopes are being planned with robotic servicing missions in mind. And not just in low earth orbit, where the Hubble Space Telescope received repairs and upgrades five times during its lifespan from space shuttle crews. Today’s engineers are preparing for ways to repair telescopes in deep space, including at the Sun-Earth Lagrange point L2.
L2 is the current home of the James Webb Space Telescope (JWST) and ESA’s Gaia mission. In this position, the Earth is kept between the Sun and the telescopes, giving them pristine conditions for observing the universe.
“While neither Gaia nor JWST were explicitly designed to be serviceable, next-generation space telescopes now in development include serviceability in their baseline designs,” write the authors of a new paper from a team at the Grainger College of Engineering, University of Illinois Urbana-Champaign.
Service spacecraft could attach themselves to derelict telescopes, bringing extra fuel, working reaction wheels, or even repairing damaged mirrors and other key components.
But it isn’t an easy task.
The University of Illinois team, including Professor Siegfried Eggl and Ruthvik Bommena, used Gaia and JWST as test subjects to design a feasible service mission.
“Gaia is like a rotating cylinder with a solar panel. It is encapsulated, so it hasn’t been damaged, but after a decade out there it’s running low on fuel,” said Eggl in a press release. “Ruthvik Bommena designed a novel concept to add a sort of spider-looking attachment that can extend its life without impeding its data collection. Gaia will be decommissioned soon, so there isn’t enough time to reach it, but the James Webb might still be a possibility because it will be operating for several more years and they may decide to prolong its mission.”
JWST’s exposed mirrors have already been struck by micrometeorites multiple times, affecting the quality of its observations.
“We’re trying to stay a step ahead so there is a plan to replace broken mirrors, for example. If we don’t, it’s like buying an expensive sports car, then like throwing it away when it runs out of gas,” says Eggl.
One of the most significant barriers to long-distance servicing missions is designing a trajectory for rendezvous with the target.
“A spacecraft sent to repair or refuel a telescope needs to brake when it reaches it,” Bommena said. “Using the thrusters to slow down would be like pointing a blowtorch at the telescope. You don’t want to do that to a delicate structure like a telescopic mirror. How do we get there without torching the whole thing?”
In addition, the team is working to optimize both fuel efficiency and cost for such a mission.
As Professor Robyn Wollands, another author on the paper explains, “getting there is doable because of some hidden highways in our solar system. We have a trajectory that is optimal for the size of spacecraft needed to repair the JWST,” she said.
These ‘hidden highway’s are geometrically optimal paths that take advantage of orbital mechanics to make rendezvous safe and cost-efficient. The team have developed a new way to calculate and evaluate these optimal paths.
“After we create a map of initial solutions, we use optimal control theory to generate optimal end-to-end trajectories,” said PhD student Alex Pascarella. “Optimal control allows us to find trajectories that depart near Earth, and rendezvous with our space telescope in the least amount of time. The initial sampling of the solution space is fundamental—optimal control problems are notoriously difficult to solve, so we need a decent initial guess to work with.
“The novelty is in how we brought together two separate approaches to trajectory design: dynamical systems theory and optimal control theory,” Pascarella added.
With teams like this one laying the groundwork, the lifespan of space telescopes might be extended long past their original best-before date, and that’s good news for astrophysicists and space programs worldwide.
Learn More:
Alex Pascarella, Ruthvik Bommena, Siegfried Eggl, Robyn Woollands, “Mission design for space telescope servicing at Sun–Earth L2.” Acta Astronautica.
“A mission design for servicing telescopes in space.” EurekAlert.