Some of the most productive orbiting telescopes operate at the sun-Earth Lagrange points L1 and L2. Currently, those positions afford us some very incredible science. What they can't afford is easy access for repairs and servicing. That limits the expected lifetime of facilities such as JWST to about 10 to 15 years.

In the future, more missions will be deployed a Lagrange points. These include the Nancy Grace Roman Telescope, ESA's PLATO and ARIEL missions, and the Large Ultraviolet Optical Infrared Surveyor (LUVOIR).

These observatories need propellants for attitude thrusters to help them stay "in place" during their observations. There's only so much "gas" you can send along with these observatories. In addition, components wear out, as they did with HST.

So, people are looking at ways to extend their lifetimes through servicing missions. If failing components can be replaced and propellant delivered, the lifetimes of these observatories should be extended quite a bit, giving astronomers more bang for the observational buck.

Planning future spacecraft servicing missions

Researchers at the Satellite Servicing Capability Office (SSCO) at the Goddard Space Flight Center (GSFC) investigated the possibilities for servicing missions to distant space telescopes. In a recently published paper in Acta Astronautica, they focus on the feasibility of on-orbit refueling missions for space telescopes orbiting at sun-Earth Lagrange 2 (SEL2).

There are many challenges. For one thing, present-day launch technologies are (at this writing) inadequate to do that kind of mission at such distances. Clearly, the technology has to advance for servicing visits to take place. In addition, it's important to remember that current telescopes, such as Gaia and JWST, weren't designed for such access.

However, future telescopes can be fitted with servicing ports, etc. to enable servicing. Finally, there are the challenges of actually getting the servicing missions to the observatories.

The Goddard team focused on this final issue by computing models of various launch and orbital solutions for such missions. Not only did they take into account the launch trajectories themselves, but also sun-Earth-Lagrange point dynamics, plus the relative positions of observatories at SEL2.

In addition, the team considered the stability of the observatories during and after rendezvous and attachment. All of these factors count when planning whether or not a servicing vehicle can be launched at a reasonable cost to extend the lifetime of the observatory enough to make the effort worth the time and expense.

Getting a spacecraft refueling mission underway

The team created models for a theoretical mission for on-orbit fueling at SEL2. That's where JWST and Gaia are sitting, for example, along with WMAP, Planck, and others. The paper examines robotic refueling missions out to SEL2 for modeling purposes.

To do that, however, there must be an optimal trajectory for the robotic spacecraft to take out to SEL2. They need to be able to perform autonomous navigation to the correct point in space. Once at the target observatory, the refueling robot would then need to make a careful approach for its docking maneuvers.

That requires on-orbit assessment of the target's motion in space with respect to the sun as well as its position in its SEL2 orbit. Docking itself can affect the observatory's position and motion and the robot needs to take that into account, as well. The idea is to keep the observatory in the same position after docking.

However, the big question is: How do we get it out there inexpensively, fast and safely?

The Goddard team primarily investigated the best and most efficient trajectories to get to SEL2. In particular, they looked at the best approaches to get to the Gaia spacecraft, which will run out of its propellant sometime in the next year. They also examined JWST as a possible target for such a mission. If such a mission was possible today, those observatories would gain years of "point and shoot" access to the universe.

How to get there

In their paper, the team looks at two approaches to the SEL2 refueling mission. One is a direct launch trajectory from Earth and the other is a spacecraft leaving from a geostationary transfer orbit (GTO). They assumed that the point of the mission was the fastest possible restoration of telescope operation. That dictates the shortest and safest possible trajectory along which the spacecraft can maintain constant thrust.

The Goddard team created a "forward design" approach for computing low-energy and low-thrust transfers from an Earth departure orbit to a space telescope orbiting the SEL2 point. Then they did the same for a servicing spacecraft leaving from a point in geostationary space.

Essentially, either an Earth-departure or GTO-centric departure will work. Once the robotic servicing mission leaves Earth orbit, it travels at low thrust during a spiraling transit to SEL2. Once there, it does a rendezvous with the target, matches its motion in space, and then "locks on" to perform its delivery mission.

It's important to remember that a launch from Earth or GTO is part of several solutions to SEL2 servicing missions. The team's analysis resulted in a simplified process of generating possible orbits and trajectories for such activities.

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