The JPL Horizons service provides very accurate ephemerides for most solar system objects, including:

Comets and asteroids

Major planets, natural satellites and selected artificial satellites

It does not include:

Single opposition asteroids with observational data arcs less than 30 days, unless they are NEOs, PHAs or radar targets (which ARE included)

Current NEOCP objects (use Retrieve NEOCP ephemerides, Retrieve Scout ephemerides or   Retrieve NEOFixer ephemerides instead)

See the Horizons web application online documentation at https://ssd.jpl.nasa.gov/horizons/manual.html.

Horizons provides ephemerides from two internal database sources, the Small-body database and the Major-body database and it is important to understand how the source affects object selection.

Small bodies

The Small-body database contains over 1 million orbits for comets and asteroids.

Entering a minor planet or comet designation in Object and pressing the Retrieve Horizons ephemeris button will search the Small-bodies database for the object.

A designation can be the permanent number of a minor planet or comet, or a packed or unpacked provisional designation. It can also be the name of an object, e.g. Apophis. Provisional designations are case sensitive, names are not.

Major bodies

The Major-body database contains details of about 500 objects including the Sun, major planets, natural satellites, a selection of spacecraft, and several dynamical points such as Earth-Sun L1, L2, L4, L5, Earth-Moon L1, L2, L4, L5 and planetary system barycenters.

Hold the Ctrl key when clicking the Retrieve Horizons ephemeris button to search the Major bodies database. If an object is not found in the Major-bodies database then the Small-bodies database will also be searched. Searches in the major-bodies database are not case sensitive. To get a list all the objects in the Major-bodies database, search for object "mb".

Searching for part of a designation or name

Small-bodies database search

If you want to search for part of a name or designation, end it with an asterisk (*). Otherwise, it is assumed a complete name or designation is specified and the search must match exactly and completely. Searching for designations is case sensitive but searching for names is not. If a search results in more than one object being selected then the Select Horizons Target window will be displayed to allow the required object to be chosen.

Major-bodies database search

Entering an asterisk (*) is not required. The text entered will be searched for anywhere in the "Name" or "IAU/aliases/other" columns and as an exact match in the "Designation" column. Searches are not case sensitive. If the text entered is found in a single entry it will be selected and used to generate an ephemeris. If multiple entries are found the Select Horizons Target window will be displayed to allow the required object to be chosen. If the text does not match any record in the Major-bodies database the Small-bodies database will also be searched. To force a search for the text anywhere in any of the columns, including "Designation", end the text with an asterisk (*).

Note: The ‘*’ symbol used in searching either of the databases is NOT a positional wildcard match but only a switch that activates matching on the preceding text. The * must be at the end, for example, 19*3 is not a recognized search. Horizons does not support wildcard searches.

Select comet elements for a different epoch

Osculating elements may be available for more than one comet perihelion passage. By default the set of elements closest to the current date are used to generate an ephemeris. To select elements for a different epoch, force a search for the comet by adding an asterisk '*' after the comet designation. The available epochs will be listed in the Select Horizons Target window "Epoch-yr" column, allowing a suitable epoch to be chosen.

Comet fragments

Individual comet fragments are available from Horizons by entering their designation, e.g. 73P-BT. To select from a list of fragments, add an asterisk '*' after the comet designation, e.g. 73P*, then select the required fragment from the Select Horizons Target window. The list can be further filtered in the Select Horizons Target window.

Horizons ephemeris quantities

The information included in an ephemeris from the Horizons service can be configured using the HorizonsQuantities setting. This is a comma separated list of the quantities documented in the Definition of Observer Table Quantities at https://ssd.jpl.nasa.gov/horizons/manual.html#obsquan.

The default setting is "1,4,9,23,24,47,19,20,37", corresponding to the following quantities:

1. Astrometric RA & DEC

4. Apparent AZ & EL

9. Visual magnitude & surface Brightness

23. Sun-Observer-Targ ELONGATION angle

24. Sun-Target-Observer ~PHASE angle

47. Sky motion: angular rate & angles

19. Heliocentric range & range-rate

20. Observer range & range-rate

37. Plane-of-sky error ellipse

Note that the quantities do not have to be listed in numerical order and must be listed individually, do not enter ranges, i.e. enter 23,24, not 23-24.

The ephemeris format "JPLHrznAPI" is configured for the default  set of values and should not be changed. If the list of quantities in the HorizonsQuantities setting are changed then a new ephemeris format should be added to match the chosen columns. See Add new ephemeris format in the Ephemeris Formats Editor for more information.

Created with the Personal Edition of HelpNDoc: Maximize Your Reach: Convert Your Word Document to an ePub or Kindle eBook