# How to show spacecraft position, heading etc relative to an asteroid

Given all the cool asteroid and comet missions out there (OSIRIS-REx, Hayabusa2, etc) I want to track the approach of the spacecraft to the asteroid. My favorite tool, JPL Horizons, can provide highly accurate tabular output from the perspective of the Earth and other major bodies, but it doesn't seem to allow the "Observer Location" to be an asteroid, for some reason. They support "all major bodies in the solar system", but not the smaller ones I guess. (Am I right?)

Is there another tool to provide accurate tabular output for this sort of encounter? Or cool plots and simulations? I use Linux, so I want it to work on open platforms.

And can we get the planning documents with initial estimates for such maneuvers? Or for more precisely-lanned maneuvers like the first flyby of Venus by the Parker Solar Probe? E.g. currently Horizons says PSP will come within 14470 km of Venus at 2018-Oct-03 09:11 UTC, but I'm guessing that that is using the currently observed trajectory, not the planned trajectory.

• If this is so I would not be surprised. The orbits of the major bodies are calculated carefully (based on decades or centuries of data). While a few asteroids are as well, most aren't. This looks like more than one SE question, can you ask the last paragraph separately? – uhoh Aug 23 '18 at 17:19
• I don't really know what you mean by "show spacecraft relative to an asteroid". Can you just subtract the two positions? Bennu and Osiris REx are both there, – uhoh Aug 23 '18 at 17:38
• @uhoh See my revised question. I appreciate the variety of output columns in Horizons. But as I clarify it, I think I know the answer. The major bodies have well-characterized rotational axes, coordinate grids etc to be used as observer locations, but not the asteroids.... – nealmcb Sep 16 '18 at 23:41
• I can't state or explain the exact methodology, but there does seem to be some documentation. Have a look at this answer for example. – uhoh Sep 17 '18 at 2:57

tl;dr:

Here's the asteroid Bennu observed from OSIRIS-REx spacecraft,

Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)    _DEC             APmag
2018-Sep-17 00:00     17 46 40.89            -08 41 51.7      11.64


But so far I can not do the reverse, and observe OSIRIS-REx from Bennu. Let's see what is possible, then as an absolute last resort, read the documentation.*

This is an interesting question. Going to horizons and playing around with some sites to observe Mars I found that in addition to locations on the surfaces of some solar system bodies such as the Apollo 11 landing site on the Moon (otherwise known as 500@301) and observatories in Jupiter Florida and the Luna Cafe in Flint, Michigan:

we can also observe from the solar system barycenter (a mathematical point in space and usually somewhere inside the Sun) as well as the body centers of the Earth, Moon, or Moons of other planets, which don't require surface coordinates.

or even from some NASA spacecraft:

and so some interesting combinations are possible

But I haven't been able to specify centers of major asteroids such as Ceres, Pallas, Vesta, which are physically huge and, have over 200 years of tracking observations. So there must be a method to this madness.

It looks like you can observer from anything that's in the JPL Development Ephemerides (so @705 is "moon #5 of planet #7") or from some spacecraft.

And so as a wise man once said: If all else fails, read the directions!

Which is in this case, the documentaiton which is something that NASA excels at almost always.

In the Horizons output there is a link to https://ssd.jpl.nasa.gov/?horizons_doc#site which says in part:

COORDINATE CENTER (OBSERVING SITE) SELECTION

Once a target is specified, the next step is to specify the origin of the coordinate system, or the "observing point", relative to which the ephemeris should be expressed.

While osculating element tables may be generated with respect to a major body center only, vector and observer tables may produce output with respect to an arbitrary observing site, defined with respect to a major body center.

EARTH SITES

For the Earth, a list with the locations of 1900+ sites is predefined. The list generally matches that of the Minor Planet Center while providing an expanded list on radar/radio sites (which have negative ID numbers). Station "500" is the geocenter.

NON-EARTH SITES

For non-Earth major bodies, station 500 also represents the body center. For those major bodies with IAU rotational models, additional topocentric sites may be defined. Spacecraft landing sites are typically predefined on non-Earth bodies.

SPECIFYING A PREDEFINED SITE

There are several equivalent ways of specifying an observing location. The most general form is ...

                            site @ body


... where "site" is a numeric code or name fragment to match, and "body" is a >numeric major body code or name fragment to match. A list of such major body codes follows later in this document, or type "MB" at the main Horizons prompt in the telnet interface, or send "COMMAND= MB" via e-mail interface.

Here are four equivalent ways of searching for the same Earth location:

 Code         Meaning
-----------  -------------------------------------------------------------
675@399      Site #675 on Earth (Palomar Mountain)
palomar@399         "
675@                "
Palomar             "           (observer table only)


OBSERVER & VECTOR TABLES:

If an observer or vector table has been requested, the "@" symbol may be dropped; the Earth will be assumed if an integer like "675" or a name fragment like "Palom" is input. However, if you are trying to specify an observing site not on Earth, you MUST use the "@" symbol for correct interpretation. For example, if an observer table as seen from the Sun is desired, it must be specified as "@10" or "@sun". Specifying "10" only will select the Caussols site.

After thinking about the many columns of output data that Horizons provides, I see that many wouldn't make sense for an asteroid. The major bodies have well-characterized rotational axes, coordinate grids etc to be used as observer locations, but not the asteroids....

• Well, my answer makes some sense in theory, but is wrong in practice. The answer by @uhoh digs in much deeper and notes that the spacecraft can be the observer, and the asteroid the target. Naming is odd though, I had to use "500@-96" for the Parker Solar Probe.... – nealmcb Sep 17 '18 at 16:25