# How will JWST manage solar pressure effects to maintain attitude and station keep it's unstable orbit?

The James Webb Space Telescope presents a huge reflective cross-section to the Sun's photons, and this can generate both torque and thrust.

How will JWST manage solar pressure effects to maintain attitude and station keep it's unstable orbit?

The thermally protective shield is about 21 x 14 meters, and will be in almost constant daylight in a halo orbit around Sun-Earth L2 in order to have constant electrical power for its thermal and attitude management systems as well as telescope and communications operations.

Source

• Wikipedia lists a station-keeping budget of 150m/s total and 3-4m/s required per year. However I did a quick calculation of the solar radiation pressure acceleration and assuming a perfect reflector I come to 12.8m/s/yr already. On top of this you need station-keeping just because of the inherent instability of the halo-orbit. 13m/s/yr is of course very manageable with the 150m/s budget but I would like to know how they get to 3-4 m/s/yr. Commented Apr 9, 2019 at 7:44
• @AlexanderVandenberghe That's a different question. Here, it's only about cancelling some of the torque Commented Apr 9, 2019 at 8:19
• With regard to linear momentum (the question asks about "maintain attitude and station-keep....") the obvious solution is to position the telescope slightly sunward of the unstable equilibrium point (or orbit). Then the light pressure balances the net gravitational force, at least to a first approximation. Commented Apr 9, 2019 at 8:59
• @SteveLinton Great idea! I don't know about JWST's halo orbit, but SOHO's orbit was designed such that station-keeping could always be radial (toward or away from the Sun). That was done partly to simply attitude control so that it could simply use the Sun to orient itself before a burn. Of course it spent pretty much all of its time looking at the Sun anyway. See Is this what station keeping maneuvers look like, or just glitches in data? (SOHO via Horizons), especially Roberts 2002
– uhoh
Commented Apr 9, 2019 at 9:52
• @Antzi actually I left both things open: "...to maintain attitude and station keep it's unstable orbit?" That doesn't mean that both are done, I just didn't want to overly restrict the question.
– uhoh
Commented Apr 9, 2019 at 9:54

Brilliant question. I'm surprised nobody referred to the amazing JSWT User Documentation available at STScl-JWST. (This question was active again, somehow, so adding an answer.)

Antzi's answer was only regarding momentum management, stationkeeping is about orbit maintenance. But let's address both here.

From the JWST user documentation, it is clear that the solar torque although balanced out by reaction wheels is enough for momentum management, but for orbit maintenance around L2, frequent station-keeping maneuvers are necessary.

While orbits about the L2 point are inherently unstable, the orbit size is large and the orbital velocity is low (~1 km/s), so the orbit "decays" slowly. However, JWST's large sun shield, roughly the size of a tennis court, is subject to significant solar radiation pressure which results in both a force and a torque. The direction of solar force varies as the observatory's attitude changes from observation to observation. The solar torque is balanced by reaction wheels, but periodically, the accumulated momentum is dumped by firing thrusters. Because JWST operations are event-driven, the observatory attitude profile and momentum dumping cannot be accurately predicted months in advance. These two perturbations increase the acceleration of JWST from its orbit about L2, and necessitates more frequent orbit maintenance (station keeping) maneuvers than other Lagrange orbit missions (which are typically 3–4 times per year).

Thus, for station keeping JWST uses thrusters, while maintaining relative sun-pointing requirements, as follows:

Orbit perturbations along the Sun-L2 axis have the greatest impact on-orbit stability. Thrusters are mounted on the spacecraft bus on the side of the sun shield facing the Sun; those used for orbit correction are oriented as far away from the sun shield as possible, and the sun shield can support a larger sun-pitch angle1 for orbit correction than is allowed for science operations. This architecture allows thruster firing at angles up to 90° from the Sun consistent with Sun avoidance restrictions, which is sufficient to provide orbit correction in all cases.

For momentum management, JWST suffers from massive momentum buildup, as suggested in the question, described here.

During science observations, the observatory will be pointed at a target, in an orientation at which the sun shield center of pressure is not aligned with the observatory center of mass. As solar photons hit the large sun shield, they place a torque on the observatory as a whole. The attitude control subystem (ACS) counteracts this torque by appropriately changing the spin rate on the reaction wheels, with the consequence that angular momentum accumulates in the reaction wheels. Momentum accumulation depends on the solar pitch angle, the roll orientation of the telescope, and the visit duration at a particular pointing position. The angular momentum (spin rate) of the reaction wheels must be managed to be kept within operational limits.

Mission planners are creative in using a technique for momentum management that is passive:

Momentum changes can be managed at some level by the way a sequence of observations is planned; this is done by observing at an orientation that builds momentum in a particular reaction wheel, followed by an observation at an orientation that removes momentum from that wheel.

But, not always, as some need-based science require a quicker slew and orientation which overrides the above momentum management program, and hence "momentum dumping" is performed by unloading the wheels as required.

However, managing momentum is only one of a number of planning constraints. At some point, one or more wheels will need to be adjusted to stay within operational bounds. The planning and scheduling system inserts planned momentum unloads into the schedule as needed, based on the modeling of expected momentum buildup, currently expected to be 1–2 times per week. Each unload activity takes a few hours, in which the observatory slews to a particular orientation to minimize the impact on the orbit and then fires thrusters as needed to allow the spin rate of the reaction wheels to be adjusted.

P.S: What Steve Linton mentioned in the question comments, is also correct.

The orbit will be biased to compensate for mean outward forces associated with gravitation of the planets and radiation pressure on the sun shield.

Above quoted passages are all from the PDF JWST Cycle 1 Documentation for Telescope and Spacecraft found on this page

• Excellent answer, thank you for all of this! I'll give thorough read this morning. I'd thought that JWST sat slightly sunward of the ideal orbit so that radiation pressure tended to generally push it toward stability, and that orbit corrections were made mostly along the Sun-Earth line as well. So the sentence "This architecture allows thruster firing at angles up to 90° from the Sun consistent with Sun avoidance restrictions, which is sufficient to provide orbit correction in all cases." surprised me because it sounds like corrections could sometimes be made at very large angles to the line.
– uhoh
Commented Sep 2, 2019 at 22:04
• Glad to provide a quality answer. Ideally, the corrections should be made at 180 degrees, you're right but you can notice that the sun avoidance constraint means they cant flip the craft, also that's a lot of inertia to counter. I'm personally a fan of their momentum management scheme, minimalist! P.S: The PDF link you added is available as a HTML, the version I linked to. Let me edit that link. Commented Sep 3, 2019 at 8:11

Unfortunately I don't have much information, I hope we get a better answer but in the meantime:

According to Wikipedia

The sunshield segment also includes that trim flap at the end of a sunshield deployment boom. This is also called the momentum trim tab. The trim tab helps balance out solar pressure. The trim tab also manages the effects of the reaction wheels. The reaction wheels are located in the Spacecraft Bus (JWST)

The trim flap reduces the amount of fuel needed, because the spacecraft does not have to balance out the force from solar pressure.

Image shows the solar vane/trim tab. Taken from this pdf red arrow added

Which also contains the interesting paragraph:

The Sunshield size, shape and aft momentum trim flap minimize torque build-up due to solar pressure, thus reducing fuel consumption.

The trim flap is not articulated or adjustable in flight. According to this JWST page:

The momentum flap balances the solar pressure on the sunshield, like a trim flap in sailing. It's not adjustable on orbit, but it is while it's on the ground.

To add: I learned here somewhere on Space.SE: When the Mariner satellite goes off axis due to perturbation one side of the sail exposure to sun light increases (or in JWST case can be increased) and the radiation pressure pushes the satellite back on its axis. This is a spin off of the Mariner 4 that has solar vanes that work this way (but didn't do it well enough to be of use) while also being a solar shield.

I see that JWST was made with layers to distribute the light through semi transparent/reflective material while functioning like the Mariner 4 passive stabilization was intended to. Having an ideal separation allows it to operate at an ideal temperature and not to get the material too hot. https://en.wikipedia.org/wiki/Mariner_4

How does temperature affect a solar sail?