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 . 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.