Solar System Targets

The Solar System Targets form is used to specify moving targets in some JWST observations. The Astronomer’s Proposal Tool (APT) is used to enter the targets into the proposal.


Purple text indicates the parameter is Limited Access.

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning)Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

Brown text indicates notes for the developers.

Green text indicates the name of the parameter used by Commanding.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

Ephemerides are generated using fundamental ephemeris information from NASA’s Jet Propulsion Laboratory (JPL). Ephemerides can be generated for all known types of solar system targets, including planets, satellites, comets, asteroids, surface features on planets and satellites, and offset positions with respect to the centers of all the above bodies. The following instructions demonstrate how to define solar system targets in a way that allows accurate ephemeris generation.

The body-axes definitions, body dimensions, directions of rotation poles, rotation rates, and the definitions of cartographic coordinates used by STScI are normally identical to the values adopted in the report of the “IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 1982” (Davies, M.E., et al., Celestial Mechanics, 29, 309-321, 1983). In a few instances, the latter data have been updated due to new results obtained from the flyby spacecraft. Also, some new bodies have been added which were unknown at the time of the IAU report. For Jupiter and Saturn, the lambda(III) coordinate system is assumed, but lambda(I) or lambda(II) can be used. For Uranus and Neptune, coordinates follow the “Report of the IAU/IAG/COSPAR Working Group on Cartographic Coordinates and Rotation Rates of the Planets and Satellites” (Celestial Mechanics and Dynamic Astronomy, 46, 197, 1989). If you need further information on these, please contact the JWST Help Desk at

One exception exists to the requirements outlined above. Observers for solar system Targets of Opportunity (e.g. a “new” comet or asteroid, a solar-wind disturbance reaching the Jovian magnetosphere, etc.), should complete a Generic Target specification  and the Observation Specifications (to the extent possible) in time for the proposal deadline. If and when a suitable target appears, the proposer must complete a Solar System Target specification and update the Observation Specifications. No target can be observed until the complete proposal information is provided.

Target Identification 

The following information is required to identify and classify each target.


Each target in your proposal will be assigned its own unique number (which can be changed by the user) by APT (warning)The numbers go from 1 to 999. Target numbers must be positive, monotonically increasing integers. You should define a different target whenever a different target position or timing description is required.  For example, separate targets should be defined if you plan to take spectra of several different surface features on a planet.

Name in the Proposal 

The name is used to identify a target; all target names within a proposal must be unique. The target name can be selected from the STScI list of standard targets ( explanations of “Level 1  and “Level 2  are given below), or a name can be defined by the GO. The use of standard names is encouraged whenever possible.

(lightbulb) Since solar system targets in the Archive cannot be found by a positional search, target names should contain the full name of the primary object (e.g. Jupiter-Red-Spot, Io-Volcano, or Saturn-Aurora). Construct target names so they make sense for your observing proposal. For example, if your proposal consisted of consecutive observations of three surface features on Mars, then three appropriate target names might be: MARS-FEATURE1, MARS-FEATURE2, and MARS-FEATURE3.

(warning) The following restrictions should be noted for the target name in the proposal:

  • The length of a target name can be anywhere from 2 to 31 characters.
  • Only upper and lower-case letters, numerals, '.', '+', '_' and '-' are allowed.  Spaces are not allowed.
  • Two targets may not have the same name with different cases.


(red star) Do not use just a standard name for a target when a specific portion of a body is being observed. For example, do not use “Saturn” as the target name for a feature or specific location that is defined relative to the position of Saturn as a standard body because this is confusing for the software that computes the positions of moving targets.

Name for the Archive 

When observing extended objects, annotating the NAME IN THE PROPOSAL is often necessary to make the proposal readable (e.g. EUROPA-EAST, EUROPA-WEST). But these names are not as helpful for people searching the Archive. So, there is a second target name - the NAME FOR THE ARCHIVE. Proposers and archival researchers use these names to determine whether JWST has observed a particular object. APT can infer this name if the target is a standard body, or if the comet or asteroid name was resolved with Horizons. But if a comet or asteroid name has not been resolved with Horizons please supply a name. (warning)This name is not required, but if supplied, must be 2 to 31 characters in length.

Target Description 

The target's Descriptions will be one of the fields used by archival researchers in searching through the JWST data archive. Select up to five Descriptions from Table 1. 

Table 1. Target Description Keywords

Descriptive Keywords for Solar System



This parameter is used to indicate if the target is extended to support data pipeline processing, and is recommended for spectroscopic observations. When left as UNKNOWN, the pipeline will use default values based on the instrument configuration (e.g. template, wavelength, etc.).

Note for developer: Add the line "Recommended for spectroscopy (for advice to data reduction pipeline)" after the parameter.

Target Pointing 

The following information is required to properly point JWST at your target.

Target Position 

Target Pointing Specification (TPS) and "Levels" 

Three fields are used to describe the target’s position, referred to here as the Target Pointing Specification (TPS). The TPS has been defined using a hierarchical structure.

  • Level 1 refers to a target in orbit about the Sun. Examples of Level 1 targets include planets, asteroids, and comets. When a Level 1 object is the desired target for observation, complete the Level 1 field and leave the other two target position fields blank.
  • Level 2 refers to a target whose motion is normally described with respect to a Level 1 object. Examples of Level 2 targets include planetary satellites, surface features on planets or asteroids, and non-nuclear positions in the coma of a comet. When a Level 2 object is the desired target for observations, the Level 1 field contains information on the parent body, and the Level 2 field gives positions relative to this body. In this case, leave the Level 3 field blank.
  • Level 3 refers to a target whose motion is normally described with respect to a Level 2 object. Examples are a surface feature on a planetary satellite or a pointing which is offset from the center of a planetary satellite. When a Level 3 object is the desired target for observation, then all three fields must be completed, with Level 1 giving the parent of the body described in Level 2, and Level 3 giving the position of the observed target with respect to the body in Level 2.

(warning)No more than three levels are allowed. If you believe that your target cannot be described in this form, contact the JWST Help Desk at

Describing Targets 

The targets specified in the target position fields can be described in up to four ways:

PIs are responsible for obtaining up-to-date orbital elements for bodies not in this table. Objects must be denoted by their IAU-adopted name. A good reference for object names can be found in the Astronomical Almanac, and in the Marsden comet catalog (Marsden, B. G., Catalog of Cometary Orbits, Enslow Publishers, Hillside, NJ, 1983). If you are uncertain whether or not your target can be referenced by name, contact the JWST Help Desk at

In those cases where the target’s position is given with respect to one of the standard objects, the latest available data from JPL on the bodies’ physical dimensions, orientation, and rotation rates are used in calculating the target’s position. In those cases where all or part of the TPS for your target can be described using standard names, we strongly recommend that you do so. Generally, this will result in the most accurate ephemeris generation for your target.

Minor Body Name Resolver and Ephemeris Retriever 

When specifying an asteroid or a comet as the target, APT provides an option to resolve the target name and download the orbital elements directly from the JPL Horizons system. Using this APT capability to communicate directly with Horizons avoids possible copy and paste errors.

Alternatively, the user may manually enter the target name and the orbital elements. In all cases, the observer retains the responsibility for the correctness of the orbital elements regardless of the source from which they are obtained.

Specifying Time 

Wherever there is an entry involving time, the suggested format for that entry is (although there are alternate formats (future link)) :


where YYYY are the full four digits of the Gregorian calendar year, MMM is the first three letters of the month name, DD is the day of the month, hh is the hour, mm is the minute, and ss are the seconds. Only the necessary precision need be specified.

Systems of Time (TDB, TDT, UTC) 

Three different systems of time are used in APT:

  • Barycentric Dynamical Time (TDB), which is synonymous with ET (Ephemeris Time)
  • Terrestrial Dynamic Time (TDT)
  • Coordinated Universal Time (UTC)


2011-AUG-19:13:04:31 TDB

2012-JAN-19 UTC

Observing Windows 

The observability of solar system targets is often constrained by various geometrical conditions (e.g. satellites observed at greatest elongation from their parent planet), or the desirability of coordinated observations (e.g. the observation of a planetary system at the same time as a spacecraft encounter with the system). These types of scheduling windows are controlled by special requirements  on the observation.


This (optional) field should include in words what you are trying to define by coordinates.

Examples of Moving Target Specifications 

The sample targets defined in this section are provided as examples of completed forms using the syntax described in these instructions. This collection does not provide an example for every type of keyword but does give a good overall representation of the types of target selections that can be accommodated. Numerical data in these examples is fictional.

Example 1: In this example the proposer wants to perform spectroscopy of a volcano on Io. The position of the target is given in planetographic coordinates.  The proposer also wants to observe the target when it lies close to the central meridian and, thus, uses the special requirement CENTRAL MERIDIAN LONGITUDE to specify the allowable range of the central meridian longitude.

Target Number:


Name in the Proposal:








Level_2:STD = IO








Observe IO volcano Loki when it is near the central meridian.

Example 2: In this example the proposer wants to perform spectroscopy of the western ansa of the Io torus when Io is near greatest eastern elongation. The elongation condition is specified using the ORBITAL LONGITUDE special requirement.

Target Number:


Name in the Proposal:















RADIUS = 4.3E5


West ansa of IO Torus when IO is at greatest eastern elongation.

Example 3: In this example the proposer wants to perform spectroscopy in the tail of comet Halley near the time of a spacecraft encounter. The latest orbital elements for the comet have been supplied by the proposer and these will be used for the ephemeris generation. The POSITION ANGLE target reference system is used to specify the tailward pointing.

Target Number3
Name in the ProposalCOMET-HALLEY-TAIL
Level_1:TYPE = COMET

Q = 0.5871167

E = 0.9672815

I = 162.2397156

O = 58.144397

W = 111.8489075

T = 2012-FEB-06:11:01:04

EPOCH = 1986-MAR-06




Comments:30 arcsec into tail of Halley during TBD spacecraft encounter. New orbital elements based on recent observations are provided.

Change log

February 28, 2023

  1. PROPINSTJWST-91555 Updated text entry constraints in the NAME IN THE PROPOSAL section.

May 24, 2022

  1. PROPINSTJWST-91520 Updated target tracking in Planetographic, Planetocentric, and Magneto sections.

Version 12 (April 27, 2021)

  1. PROPINSTJWST-89165 - Updated Table 1 to reflect that descriptions are now chosen from a list instead
    of free text.

Version 11 (May 27, 2020)

  1. PROPINST-91372 - Added new moons of Jupiter (Pandia and Ersa).

Version 10 (February 21, 2020)

  1. PROPINSTJWST-91445 - Updated Table 1 (added Non-gravitational parameters for  Comet)
  2. PROPINSTJWST-91446 - Updated Table 1 (added Orbital Element parameters for  Comet)
  3. PROPINSTJWST-89781 - Updated Table 2 (added Non-gravitational parameters for Asteroid)

Version 9 (November 18, 2019)

  1. PROPINSTJWST-91397 - Added Valetude (Jupiter) and Hippocamp (Neptune)

Version 8 (September 10, 2019)

  1. PROPINSTJWST-91402 - added Source parameter to Comet and Asteroid tables.

Version 7 (April 16, 2019)

  1. Editorial change (moved change log to main article)

Version 6 (November 20, 2017)

  1. PR 83610 - added Extended parameter
  2.  PR 88303 - added Torus diagrams to complete formal definition

Version 5 (July 5, 2017)

  1. PR 87996 - revised text on Planetographic Longitude
  2. PR 88303 - revised text on Torus Coordinate System

Version 4 (May 8, 2017)

PR 87694 - added Name for the Archive

Version 3 (October 6, 2016)

  1. PR 85913 - deleted UAT text and added developer note for Target Description
  2. PR 85608 - remove text on target flux and ETC Run number

Version 2 (July 16, 2016)

  1. PR 84886 - for Planetographic Coord - changed Longitude, Latitude, and Altitude Rate Changes to Rate of Change
  2. PR 84886 - for Planetocentric Coord - changed Longitude, Latitude, and Altitude Rate Changes to Rate of Change
  3. PR 84886 - for Angle Coord - changed Radius and Angle Rate Change to Rate of Change

Version 1 (May 26, 2016)

This is the converted Word File from Chapter 4.