# With independent control of attitude and thrust direction, could JWST be kept pointing in a “safe” direction during "towing"?

Context: JWST is considered un-repairable in its current L2 location for a number of reasons:

• Diagnosis is difficult since JWSR has no “selfie” capability. This makes planning for any repair mission (crewed or robotic) problematic.
• Distance and radiation exposure are vastly beyond the experience of crewed missions
• JWST was not designed to be serviceable so repair would likely be complex and require the flexible talents of a crewed mission, perhaps multiple missions.
• Rocket exhaust could damage optical surfaces.
• JWST cannot transfer itself closer to Earth for repair since this would mean pointing its optics towards the sun for the burn.

Proposal: Built a robotic “tugboat” with thrusters which are positioned to be safe for the optics. Have the tugboat dock with JWST using the launch mount or docking ring. The tugboat would then transfer JWST to an Earth/Moon L1 orbit. The Gateway/Artemis resources would be available for diagnosis and repair. This would avoid the expense and radiation exposure of a dedicated crewed repair mission to L2. Once repaired, the “tugboat” could reposition JWST at Sun/Earth L2, and remain with JWST in case a repeat mission is needed. The tugboat could also be used for end-of-life decommissioning, extending JWST service life.

Transfer of JWST from SEL2 to EM halo orbits or “Gateway” orbit using low-energy trajectory is within the delta-v budget of typical earth-SEL2 missions. Is the inter-manifold transfer of JWST between Sun-Earth L2 and Earth-Moon L1 within the capability of existing propulsion systems?

All components of JWST can tolerate room temperature, but the optics are capable of focusing Sunlight, Earthlight and Moonlight to destructive intensity if the image of the radiating body falls on the secondary mirror. A purpose-built “tugboat” could give independent control of attitude and thrust direction.

Question: With independent control of attitude and thrust direction, could JWST be kept pointing in a “safe” direction (for instance at the celestial pole) during the proposed transfer and repair?

• At this point of complexity, for a uncertain outcome, it’s just going to be better to send a new space telescope Feb 22 at 4:24
• The Shuttles used something called "low z mode" to steer away from sensitive stuff (like hubble or MIR). They did not ose the thrusters aimed towards the thing that had to be protected but several thrusters aimed at angles away from it. the resulting vector is the vectorial sum of the thrust. This is inefficient (two thrusters each 45° away from the target would "waste" about 41% of the fuel) but helps keeping the target save. Feb 22 at 6:47

Partial answer.. perhaps more of a comment... downvote expected...

In theory the answer could be yes.

It relates to the concept behind the JWST successor of soughts, the LUVOIR set for launch in 2039.

Large Ultraviolet Optical Infrared Surveyor is effectively composed of two separate spacecraft flying in close formation.

This brings two main benefits:

1. This minimises any transmission of structural heat and vibration from the propulsion supporting spacecraft (bus) to the delicate and highly sensitive payload spacecraft (telescope). Their only connection is in the cabling.

2. The payload spacecraft (telescope) is able to slew in any direction (within reason due to the sunshield) independent of the supporting spacecraft bus.

If the retrieval spacecraft had a tether, and secondary bus that kept station somewhere in between JWST and the retrieval spacecraft, you could sort of have a controlled-attitude tow in effect.

This is OTTOMH

Related Q and A from here:

How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation?

How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation? What compensates?

It could be, but the main challenge is going to be executing burns with thrusters that aren't aligned with the center of mass of the tug-JWST pair. Assuming the goal is to keep the sun behind the sun shield at all times, the tug will need to be able to perform translation maneuvers in a few directions without JWST changing its attitude (except for the yaw axis). For example, if you needed to burn in a direction that would normally place the optics in sunlight, the tug would instead have to provide a force from its current attitude, without rotating the JWST towards the sun. Doing this would create an unbalanced torque/moment about the spacecraft's center of mass which would need to be balanced. The tug would have to be specially designed for this, with thrusters along the sides rather than inline.

I think the torque-balancing could be managed with a few boom-mounted rcs pods, but it'd be awfully inefficient, as countering the moment will also reduce your effective thrust.

If you're instead proposing a cable link between the tug and JWST, then in theory it could maintain its attitude, but only effectively if the cable is in line with the center of mass, else it would waste all of its onboard RCS propellant. That would be impossible to maintain for different maneuvers as the tug ends up on different sides of JWST, no matter where the cable is mounted. Also I suspect that whole system would be much too unstable for such a fragile spacecraft.

• +1 but some readers (including myself) are 3D spatially challenged; if you can add a drawing or diagram, perhaps with some arrows added by hand, it would be easier for folks like me to visualize the problem. Thanks!
– uhoh
Jan 21 at 22:10
• @wyattg --- The goal is not to “to keep the sun behind the sunshield”, but to prevent damage to JWST. Jan 22 at 5:23
• @wyattg --- You make valid points but the question concerns safe orientation of JWST during transfer and repair, not design challenges of the "tug". I did not propose a tethered tow. The question specifically says "docking ring" Jan 22 at 6:27
• In that case, I’m not sure what you mean by preventing damage or by “be kept pointing in a ‘safe’ direction”. If there are attitude constraints during burns, then my answer applies (minus the tethers, as you say). If there aren’t, then it’d be the same as any other docked spacecraft pair, much like Apollo’s CSM-LEM pair. JWST doesn’t even have to be operational to do any of this, given the tug is designed for the job (and assuming it is already docked). That’s why I mentioned the tethers, because JWST’s independent controls aren’t independent once it’s docked. Jan 23 at 0:15