Moving Target Policies

Science requirements for moving targets, as well as brief descriptions and links to appropriate pages about policies that affect moving target observing programs, are presented in this article.

On this page

Science requirements for moving targets

JWST must be able to observe solar system targets when they move relative to the guide star at rates up to 75 mas/s. This will provide access to a large number of near-Earth asteroids (NEAs), Mars and all of the more distant planets and their moons, asteroids, KBOs, and most comets. Precise trajectories are needed at the angular resolution of the observatory to avoid loss of angular resolution and to enable placing the moving targets on specific NIRSpec slits, MIRI MRS, and NIRSpec IFU. It is recognized that the image quality requirements will have to be relaxed slightly while tracking moving objects, and that exposure times (or the track paths) will be limited by the path of the guide star. There are no requirements to track accelerating objects (curved trajectories), to track objects continuously as the guide stars cross sensor chip boundaries, to have special guide star availability, or to observe in any special orientations (Science Requirements Document, pg. 7-8).

"When commanded, the Observatory shall compensate for the apparent motion of a moving target which exhibits an angular velocity between 0 and 75 mas/s with respect to a guide star that remains within a single Fine Guidance Sensor field of view." (SR-31, Science Requirements Document, pg. 8-7, amended to a higher apparent rate as a result of commissioning)

Target of opportunity (ToO) policy

See also: JWST Target of Opportunity Observations

The proposer should be aware that only a small number of disruptive target of opportunity activations will be allowed during any particular cycle due to their effect on the schedule. See Target of Opportunity Observations for a more thorough description of the ToO policy for JWST.

Duplication policy

See also: JWST Duplicate Observations Policy, JWST Observation Duplication Checking

Observations of the same target with the same instrument and mode are considered duplicate observations. Duplicating an observation is typically discouraged unless the target shows intrinsic variability or a large-time baseline is needed to achieve the science goals.

Example of an encouraged duplication: Annual NIRSpec IFU observations of Jupiter's Great Red Spot.

Example of a discouraged duplication: Duplicate observations of a small asteroid with the NIRSpec IFU to increase the signal-to-noise ratio of a spectrum obtained with JWST in a previous cycle.

See JWST Observation Duplication Checking for how to check for duplicate observations. Keep in mind that target/mode combinations in Guaranteed Time Observation (GTO) and Early Release Science (ERS) programs are subject to these policies in Cycle 1.

Moving target-specific overheads

See also: JWST Observing Overheads Summary

See the Moving Target Overheads article for a description of general overheads that apply to moving target observations.


Kelley, M.S.P., et al. 2016, PASP, 128, 018009 (arXiv)
Cometary science with the James Webb Space Telescope

James Webb Space Telescope Project, Science Requirements Document (July 10, 2012)

Notable updates
    Updated for Cycle 2.

    Updated links.

  • Fixed in-text links.

    Added wording about GTO and ERS to duplication policies section
Originally published