James Webb Space Telescope - Orbit

Orbit

The orbit of the JWST will be an elliptical orbit (with a radius of 800,000 kilometres (500,000 mi)) around the semi-stable second Lagrange point, or L₂. The Earth–Sun L₂ point, about which the Webb telescope will orbit, is 1,500,000 kilometres (930,000 mi) from the Earth, nearly 4 times farther than the distance between the Earth to the Moon. At such a great distance, the Webb telescope would be more difficult to service after launch than the Hubble telescope. Nevertheless, a docking ring was added to the design in 2007 to facilitate this possibility, either by a robot or future crewed spacecraft such as the Orion MPCV.

Normally, an object circling the Sun further out than the Earth would take more than one year to complete its orbit. However, the balance of gravitational pull at the L₂ point (in particular, the extra pull from Earth as well as the Sun) means that JWST will keep up with the Earth as it goes around the Sun. The combined gravitational forces of the Sun and the Earth can hold a spacecraft at this point, so that in theory it takes no rocket thrust to keep a spacecraft in orbit around L₂. In reality, the stable point is comparable to that of a ball balanced upon a saddle shape. Along one direction any perturbation will drive the ball toward the stable point, while in the crossing direction the ball, if disturbed, will fall away from the stable point. Thus some station-keeping is required, but with little energy expended (only 2–4 m/s per year, from the total budget of 150 m/s).

Since the JWST must be kept very cold to make accurate observations of distant astronomical objects, it has been designed with a large sunshield that blocks light and heat from the Sun. In order for such a shield to work properly, the Sun's rays must be constantly coming from the same direction. To achieve this outcome, JWST will be put into a relatively large "halo orbit" around L₂. From the L₂ point itself, the Earth constantly shades one third of the sun's light as it periodically wobbles about the Earth-Moon barycenter; occasionally lunar eclipses will partially obscure more of the solar disk. However, the radius of the Webb telescope's orbit around L₂ will be so large that neither the Earth nor Moon will eclipse the Sun, allowing the shield to deal with a relatively constant sunlight environment. This was considered to be more important than attempting to utilize the Earth's shadow to block some of the sunlight, in an orbit nearer the exact L2 point.