Moving Objects

by Tony Perreault last modified Mar 20, 2017 by Heidi Medlin

Introduction

The VLA is able to observe moving objects (solar system bodies) in standard continuum modes as part of general observing. It is not currently possible to observe spectral lines in planets or comets, except in unusual circumstances (background source occultations, for instance), or as part of the Resident Shared Risk Observing (RSRO) program. There is an observational limit on the rate at which objects can be tracked, but it is fast enough that observation of all natural solar system bodies is allowed, including Near Earth Asteroids (NEAs). As an example, the NEA 2005 YU55 was observed during its closest approach in 2011, when its motion was many arcseconds per second.

Generally, observing solar system bodies is no different than any other source in terms of the calibrations that are necessary (frequency setups for continuum observing, etc.). Observers should follow the recommended practices described elsewhere in the setup of the scans in their Scheduling Blocks (SBs), and the setup of the hardware (tuning and correlator). The main difference is in the setup of the source itself, of course, and there is a minor difference in how calibrators need to be selected. These will be described next.

Setting Up a Solar System Source

When starting from the Observation Preparation Tool (OPT) page, click on the Sources link. Create a new source catalog and/or group, or select an existing one (see the OPT documentation for instructions on how to do this). Click on File (located in the dark blue area at the top), then click Create NewSource. You are presented with a screen that looks like Figure 7.5.1.

There are now two choices for setting up a solar system source:

  1. Sources known internally to the VLA software system, or;
  2. Sources for which you can provide an ephemeris file.

Note that in the near future you will be able to specify the motion terms of a polynomial, but that is not implemented in the OPT yet.

Internal Sources

For the planets, the software system of the VLA uses an internal representation of the JPL DE410 ephemeris. The list of bodies supported in this way are:

  • Sun
  • Moon
  • Mercury
  • Venus
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

Please note that the positions for the objects specified in this way are barycentric, not bodycentric. The latter introduces small offsets to the actual on-the-sky positions for the VLA. If this is an important effect, you must use the other method of setting up your moving source. When in doubt whether this is affecting your observations, read on about the JPL Horizons page below or consult the NRAO Helpdesk.

To set up an internal ephemeris source, go to the SOURCE POSITIONS section of the New Source page (seen in Figure 7.5.1), and select Solar System Body with Internal Ephemeris in the POSITION TYPE pull-down menu. You will then see something similar to Figure 7.5.2 for the SOURCE POSITIONS section. You can now choose the object from the above list in the SOLAR SYSTEM BODY pull-down menu.

 

Ephemeris File Sources

If your body is not included in the list of sources known to the VLA, or if you care about bodycentric vs. barycentric positions, you may use an ephemeris file to specify the position of your source as a function of time. To set up an ephemeris file source, go to the SOURCE POSITIONS section of the New Source page (seen in Figure 7.5.1), and select Solar System Body with Uploaded Ephemeris in the POSITION TYPE pull-down menu. You will then see something similar to Figure 7.5.3 for the SOURCE POSITIONS section.

Click on the Browse… button to select the ephemeris file, and then click the Import button. The SOURCE POSITIONS section should now look similar to Figure 7.5.4, with the times and positions displayed.

 

Format of Ephemeris Files

Ephemeris files to be used in this way are created with the JPL Horizons system. Go to: http://ssd.jpl.nasa.gov/horizons.cgi, and specify:

To select your body, click on change next to Target Body, and use the lookup tool. JPL’s Horizons system knows about most solar system bodies, including comets, moons, and asteroids (NEAs included), and even spacecraft. To select your time range, click on change next to Time Span, and input the proper time range. Note that, for most bodies, using an ephemeris tabulated at 1 hour entries is sufficient. For some fast-moving near-Earth objects, a shorter interval between tabulated entries may be needed. To be sure that the Table Settings are correct, click on change, and then be sure that only options (1.) Astrometric RA & DEC and (20.) Observer range & range-rate are selected, then go to the Optional observer-table settings section (below Select observer quantities from table below), and be sure that the extra precision box is checked.

After everything is set up correctly, click on the Use Settings Above button and then Generate Ephemeris.After you are taken to that page, you will need to save the web page as a text file. Please note that currently Google Chrome does not allow for simple saving of web pages as text; Firefox, Safari, and IE do not suffer from this shortcoming. Once the file is saved to your computer, you can select it for use as described above.

Finding Calibrators

Finding calibrators for moving sources proceeds in much the same way as for other sources, but since the target object moves you must be a bit careful about it. For slow-moving sources in the outer solar system, using the same calibrators over periods of years is fine, since the motion is slow. For inner solar system bodies, however, this cannot be done—each new observation might require a new calibrator, and in extreme cases a single calibrator will not even suffice for a single Scheduling Block (for example, the case of 2005 YU55 mentioned above).

Fortunately, the SCT knows about moving sources, and their locations will be plotted properly in the bulls-eye source plots in that tool. See Figure 7.5.5 for an example. You can either use the normal search cone with radius method of finding a calibrator, or click on the bulls-eye icon for the moving source itself to identify good calibrators. The rules for choosing a calibrator for a moving source are no different than for other sources at the observing frequency.