5.5. Solar System objects and their visualisation

5.5.1. Solar System objects in HSpot

As of mid-2007 when a major update of the HSpot database was made, there were already more than 360 000 solar system objects (SSOs) that had been catalogued. Since then this number has increased rapidly and is expected to increase more rapidly still in the next few years. HSpot does not have the capability to include this complete database, nor are the majority of these objects observable by Herschel for reasons of sensitivity and, in the case of many satellites, also because Herschel lacks the spatial resolution to resolve them from their primary. A total of approximately 500 SSOs are included in the database. These are listed in Appendix C, Solar System Objects that have ephemeris information in HSpot. HSpot uses a static ephemeris from the JPL Horizons system; that is, it does not update, even if the orbit file in Horizons is improved. This will potentially be a problem for planning observations with some comets and many NEAs [please note that our Mission Planning software does use an active ephemeris for scheduling purposes]. All the orbit files for SSOs were updated for launch on May 4th 2009 and will be updated on an occasional basis. New objects have been added to the database regularly since then on demand from users. Of the objects included in the database, 290 were actually requested at some point in submitted observations, but only around 200 distinct SSOs were finally observed by Herschel.

If you need to observe or to check an SSO that is not included in the HSpot database, for example, a newly discovered object, please contact Helpdesk (http://herschel.esac.esa.int/esupport/) at the Herschel Science Centre to have it included. A significant update in HSpot since the initial call for proposals has been an improvement in the way that ephemeris files are handled so that they are now no longer part of the HSpot build, so new objects can be included far more quickly and easily.

For close approach objects, the ephemeris in HSpot will usually be sufficiently accurate to plan visibility, but not to schedule observations. For campaigns involving close approach objects such as 103P/Hartley 2 the difference betweeen the position three weeks before observation, when the observation planning files are sent to the Mission Operations Centre in Darmstadt and the position at the time of observation was as much as 30 arcseconds: far too large to be acceptable - all the 103P observations had to be re-planned as close to the date of execution as possible with the latest ephemeris.

For 45P/Honda-Mrkos-Pajdusakova, which was observed in August 2011, a special late update to the ephemeris was required too, made four days before the date of observation. In this case the ephemeris was still found to be out by around 7 arcseconds at the epoch of observation, mainly due to the small amount of new astrometry available in the final updates and the lack of radar observations.

Experience shows that for these close approach objects, only when radar observations are available is the ephemeris accurate enough for Mission Planning purposes at HSC.