JWST orbits around the Sun-Earth L2 Lagrange point, located about 1.5 million km from Earth.
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JWST was placed in an orbit about the Sun-Earth L2 Lagrange point, located about 1.5 million km from Earth, four times the distance between the Earth and the Moon. It is incorrect to say that JWST "is at L2." Rather, JWST orbits around L2.
The distance of JWST from the L2 point varies between 250,000 to 832,000 km, as shown in Figure 1. The period of the orbit is about 6 months. The maximum excursion above or below the ecliptic plane is 520,000 km. The maximum distance from the Earth is 1.8 million km, and the maximum Earth-Sun angle is <33°.
L2 is a saddle point in the gravitational potential of the Solar System. Because saddle points are not stable, JWST will periodically fire onboard thrusters to maintain its orbit around L2. Station-keeping maneuvers are scheduled every 21 days, and skipped if the computed correction is negligibly small. During the six months of JWST commissioning, four station-keeping burns were fired, with typical durations of tens of seconds, and three were skipped.
To maintain solar power, the orbit was designed such that JWST will never be in the shadow of the Earth or the Moon during the mission.
Rationale for the orbit dimensions
A larger orbit makes it easier to get the spacecraft to L2, as well as maintain its orbit. However, larger orbits can also permit stray light from the Earth or Moon to get past the sun shield and strike the primary or secondary mirrors. In addition, a larger orbit reduces communication contact opportunities.
Because JWST is solar powered, it cannot pass through the Earth's shadow during the mission. Launch times were selected to avoid shadow crossings.
JWST's orbit around L2 has an orbital period of 6 months. 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 2 perturbations increase the acceleration of JWST from its orbit about L2, and may necessitate more frequent orbit maintenance (station keeping) maneuvers than other Lagrange orbit missions (which are typically 3–4 times per year). Accurate orbit determination requires daily tracking measurements over a period of 19 days, so station keeping is performed approximately every 21 days.
Orbit perturbations along the Sun-L2 axis have the greatest impact on orbit stability. Thrusters are mounted on the spacecraft bus (located 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. The sun shield can support a larger sun-pitch angle† for orbit correction than that 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.
The orbit is biased to compensate for mean outward forces associated with gravitation of the planets and radiation pressure on the sun shield.
† The angle between the pointing direction and the satellite-Sun line. The "pointing direction" is the "boresight" of the telescope, also called the V1 axis of the observatory.