Does anyone have solid information regarding the scheduled/anticipated station keeping burns for JWST at L2? In an online video I've heard mention of 'every 21 days' which at first look seems excessive but on second look may not be nuts if very brief.

I would think station keeping burns would be on an an as-needed basis. Perhaps the original calculated useful 10 year life was based on frequent burns.

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    $\begingroup$ The original ten year life time was based on conservative estimates of how much propellant would be left after the transfer and orbit insertion burns. Those initial estimates were indeed overly conservative. $\endgroup$ Commented Feb 25, 2022 at 1:16

2 Answers 2


How frequent are (or will be) JWST station keeping burns at L2?

21 or 42 days

I've heard mention of 'every 21 days' which at first look seems excessive but on second look may not be nuts if very brief.

"How brief?" would be an excellent follow-up question!

It's exactly this high cadence that helps keep the station-keeping delta-v budget so low.

Halo orbits are exponentially unstable so making the cadence substantially longer would dramatically increase the budget.

Also with all the variation in torque due to solar photon pressure on the giant reflective sunshield, these scheduled propulsive maneuvers include momentum unloading as well as trajectory station-keeping.

From this answer to What happens to JWST after it runs out of propellant?:

This Northrop Grumman video (starting at 09:31) illustrates JWST's orbit in a non-rotating (normal) frame. It's really in an orbit around the Sun about 1% farther than Earth's, but the weak tug of Earth pulls it along a bit faster so that it remains in 1:1 resonance with the Earth. The orbit is called a "Halo orbit" because in a rotating frame it looks like it's a ring around the L2 point.


Regular, but very small station keeping propulsive maneuvers keep it in this otherwise unstable configurations. The more frequent the adjustments, the lower overall fuel consumption per year. According to these and James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism and Station Keeping Monte Carlo Simulation for the James Webb Space Telescope there will be a small propulsive station-keeping event every 21 days.

From the "Monte Carlo" paper:

JWST will fly in a Libration Point Orbit (LPO) around the Sun-Earth/Moon (SEM) L2 point, with a planned mission lifetime of 10.5 years after a six-month transfer to the mission orbit. Stationkeeping (SK) maneuvers will be performed every 21 days to keep JWST in an LPO around the unstable SEM L2 point. The LPO orbit period is about six months. SK maneuvers are needed to correct for orbit determination errors, maneuver execution errors, uncertainty in Solar Radiation Pressure, and other force modeling errors, as well as momentum unloads (MUs).


As an additional challenge, the JWST observation schedule in the next 21-day period will not be known at the time of SK maneuver planning. A planned observation schedule one week ahead will be available, but the actual observation schedule will be event-driven. If a ‘target of opportunity’ arises then the schedule can be changed within 48 hours to point at the new target. Also if JWST’s Fine Guidance Sensor (FGS) is unable to lock onto a guide star for a scheduled observation, then the observation will be skipped [2]. Thus there can be significant variation in SRP between SK maneuvers, and the future variation in SRP is unknown.

Basically, the 21 day cadence is planned to be fixed, and the observation schedule will be dynamic and adjust itself around them.


In our simulation we did include the condition, mentioned at the end of Section 3, that a planned maneuver that would be smaller than 12 cm/sec would be skipped for efficiency. We found that we can fly the mission successfully, without an impact on the SK budget. In fact, the simulation results indicate that we could skip 48% of the planned SK maneuver, so in most cases an SK maneuver would be performed every 42 days, not every 21 days. We allowed at most one maneuver to be skipped, even if the next SK maneuver would also be smaller than 12 cm/sec. We made this choice for mission safety. If we skipped one maneuver, and then for some reason we could not perform the next maneuver 21 days later, we could end up waiting 63 days between SK maneuvers, presenting a potential risk for an LPO mission.

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    $\begingroup$ It would be interesting to see why they chose 21d & 12cm/s as the threshold and not something smaller. Is the limiting factor % of time spent not doing science? Accuracy of the rocket engine? Fixed costs of starting of the rocket engine? Accuracy of measurement? Other? $\endgroup$
    – TLW
    Commented Feb 26, 2022 at 2:47
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    $\begingroup$ Has JWST done its first station keeping yet? The last burn I know about was the MCC2 burn on Jan24; more than 21 days ago; and that was a pretty long burn with a $\Delta_V$ of 1.6 m/sec or 160 cm/sec, which is much larger than the 12cm/sec so I would assume they would want another burn at 21 days just because JWST must always stay on the Earth side of the halo orbit, and there had to be some reasonable margin in the MCC2 burn. $\endgroup$
    – Sheldon
    Commented Feb 26, 2022 at 17:51
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    $\begingroup$ @Sheldon I don't know and that's a great new question! My feeling is that the 21/42 day cadence is for the steady state situation after it's settled down and there's sufficient tracking history to accurately project its orbit in 3D. The delay-doppler and vlbi measurements by the DSN don't necessarily give the position and velocity to sub-meter and sub mm/sec accuracy from a single measurement... $\endgroup$
    – uhoh
    Commented Feb 27, 2022 at 0:58
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    $\begingroup$ @Sheldon ...they need to measure several times and fit those measurement to orbit propagation calculations to get the full picture. But I'm no expert in this, I think that "Has JWST done its first station keeping yet?" is a great new question to post, that way folks who do know or at least know how to find out will see it and be able to see it. $\endgroup$
    – uhoh
    Commented Feb 27, 2022 at 0:59
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    $\begingroup$ @TLW "Why 21 days & 12cm/s thresholds for station keeping decisions?" would also be an excellent new question! Again I'm no expert, but my hunch is that "diminishing returns" and "risk minimization" would be parts of the answer. Shutting down the observatory and turning on rocket engines, then turning them off and starting up the observatory again every three weeks steals observing time and costs time and money on the ground, and each one is an opportunity for something to go wrong; miscalculation, stuck valve, cosmic ray event in the navigation computer... $\endgroup$
    – uhoh
    Commented Feb 27, 2022 at 1:07

They will be as frequent as needed.

The original 10 year lifetime was based on fuel that was reserved for orbital correction that wasn't needed for that post-rocket boost. Usually they have a 95% chance required for a 10 year mission, which means there is a very good chance there will be margin for any such mission.


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