"Although the next generation of large space telescopes are being designed with serviceability in mind, there are enormous challenges with implementation," said Siegfried Eggl, professor of aerospace engineering at the Grainger College of Engineering, University of Illinois Urbana-Champaign.

A primary challenge is the distance. Many modern telescopes, such as JWST and Gaia, are stationed at the Sun-Earth Lagrange Point L2, roughly a million miles from Earth. While this location is advantageous for minimizing interference, it also makes servicing missions time-intensive and costly. Nevertheless, Eggl emphasized that the scientific benefits justify the effort.

Eggl explained, "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. Ruthvik Bommena designed a novel concept to add a spider-like attachment that can extend its life without affecting its data collection. Although Gaia will soon be decommissioned, the James Webb Telescope might still benefit from such a mission since it is expected to operate for several more years."

JWST presents its own challenges, including unshielded, segmented mirrors that have already suffered micrometeorite damage. Eggl noted, "We're trying to stay a step ahead so there is a plan to replace broken mirrors. If we don't, it's like buying an expensive sports car and throwing it away when it runs out of gas."

Safe proximity operations are another focus of Bommena's work. "A spacecraft sent to repair or refuel a telescope needs to brake upon arrival," Bommena said. "Using thrusters to slow down is akin to pointing a blowtorch at the telescope. You don't want to do that to a delicate structure like a telescopic mirror."

To tackle these challenges, researchers are identifying efficient and cost-effective routes to L2. Robyn Woollands, professor of aerospace engineering at the University of Illinois, highlighted a key breakthrough: "We have a trajectory that is optimal for the size of spacecraft needed to repair the JWST."

Ph.D. student Alex Pascarella developed an innovative trajectory design technique that combines dynamical systems theory and optimal control theory. This hybrid approach accelerates computation, enabling researchers to quickly identify viable solutions.

"The traditional method of computing invariant manifolds in multibody systems works well for reaching target orbits but becomes challenging when trying to rendezvous with a spacecraft at a specific location," Pascarella said. "Our technique starts by propagating sample solutions and mapping how close they pass to the desired destination. Then, we use optimal control theory to fine-tune the trajectory for an efficient rendezvous."

Eggl confirmed that the plan to refuel or repair Gaia is ready for implementation, while additional engineering is needed for JWST servicing missions.

Research Report:Mission design for space telescope servicing at Sun - Earth L2

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University of Illinois Grainger College of Engineering
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