JWST 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

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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 0.030"/s (30 mas/s or 108"/hour). This will provide access to Mars and all of the more distant planets and their moons, asteroids, KBOs, and most comets over some part of their orbit (typically when a comet is more than two or three AU from the Sun; see Kelley et al., 2016). 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 or the MIRI spectrometer field of view. 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 30 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)

Target of opportunity (ToO) policy

Main article: JWST Target of Opportunity Observations

The proposer should be aware that only a small number of disruptive (activation <14 days prior to observation) target of opportunity activations will be allowed during any particular cycle due to the effect on the schedule. See JWST Target of Opportunity Observations for a more thorough description of the ToO policy for JWST.

Duplication policy

Main article: JWST Duplicate Observations Policy
See also: JWST 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 Duplicate Observations Policy for a general discussion on 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

Main article: JWST Observing Overheads Summary

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


Kelley, M.S.P., et al., 2016, Publications of the Astronomical Society of the Pacific, Vol. 128, 018009
Cometary science with the James Webb Space Telescope

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



Latest updates
    Fixed in-text links.

    Added wording about GTO and ERS to duplication policies section