Observation of Rosetta the night before its 3rd Earth swingby

J.M. Mondelo

Below are some exposures taken in the evening-night of Nov. 12, 2009, in an attempt to distinguish the movement of the Rosetta spacecraft against the background stars prior to its 3rd (and final) Earth swingby. Astrometry has been performed in all the pictures in order to detect aiming inaccuracies and obtain coordinates for Rosetta when found. Rosetta seems to have been found in the three first exposures.

The observation has been carried out from the observatory in the Les Planes de Son nature centre. I would like to thank all the people there for hosting me and kindly allowing me to use all the equipment there.

Hardware and software used

Ephemeris used

Aiming coordinates obtained via JPL Horizons, with these settings: 
   Ephemeris Type [change] : OBSERVER
   Target Body [change] : Rosetta (Spacecraft) [-226]
   Observer Location [change] : user defined ( 1°04'54.8''E, 42°37'06.7''N, 1500 km )
   Time Span [change] : Start=2009-11-12 16:00, Stop=2009-11-13 16:00, Step=5 m
   Table Settings [change] : QUANTITIES=1,4
   Display/Output [change] : plain text
The coordinates obtained with these settings will be called ephem01 coordinates from now on. In order to consider location inacuracies, a second set of coordinates will be considered, obtained also from JPL Horizons but with these settings: 
   Ephemeris Type [change] : OBSERVER
   Target Body [change] : Rosetta (Spacecraft) [-226]
   Observer Location [change] : user defined ( 1°04'43.9''E, 42°37'04.7''N, 1590 km )
   Time Span [change] : Start=2009-11-12 16:00, Stop=2009-11-13 16:00, Step=5 m
   Table Settings [change] : QUANTITIES=1,4
   Display/Output [change] : plain text
In this second set, longitude, latitude, and height have been deduced from Google Maps. The coordinates obtained with this settings will be called ephem02 coordinates. The full set of coordinates for ephem01 is here, and the one for ephem02 is here.

Pictures and astrometry results

  1. Exposure of 3 min, taken on 2009-11-12 18:55:36 UT (precision: 2 secs, due to vfat filesystem).
    File: rst1845001.00000796.FIT.

    Click on picture for full-sized version.

    Intended center of picture (when aiming the telescope): 02:03:55.61 -19:40:39.9, corresponding to 18:45 UT ephem01.

    GCX astrometric fit with WCS: 02:03:11.36 -19:39:03.9 2000 1.268620 1.268620 176.7528 (in this order: RA, dec of center of picture, equinox of coord, horizontal number of pixels per arcsecond, vertical one, rotation of FOV in degrees).

    Rosetta coordinates from astrometry (clicking on the middle): 2:3:21.87 -19:32:43.4

    Some lines from ephem01, ephem02: 

       Time (UT)         | RA ephem01  | dec ephem01 | RA ephem02  | dec ephem02
   2009-Nov-12 18:45 | 02 03 55.61 | -19 40 39.9 | 02 03 57.40 | -19 41 11.5
   2009-Nov-12 18:50 | 02 03 51.86 | -19 41 23.4 | 02 03 53.64 | -19 41 55.4
   2009-Nov-12 18:55 | 02 03 47.99 | -19 42 07.3 | 02 03 49.75 | -19 42 39.7
   2009-Nov-12 19:00 | 02 03 43.99 | -19 42 51.4 | 02 03 45.74 | -19 43 24.3
   2009-Nov-12 19:05 | 02 03 39.86 | -19 43 35.9 | 02 03 41.59 | -19 44 09.2
   2009-Nov-12 19:10 | 02 03 35.61 | -19 44 20.7 | 02 03 37.32 | -19 44 54.4
   2009-Nov-12 19:15 | 02 03 31.23 | -19 45 05.9 | 02 03 32.92 | -19 45 39.9

  2. Exposure of 3 min, taken on 2009-11-12 19:00:16 UT (precision: 2 sec).
    File: rst1845001.00000797.FIT

    Click on picture for full-sized version.

    Intended center of picture at the same coordinates as before.

    GCX astrometric fit with WCS: 02:03:10.36 -19:38:50.5 2000 1.268564 1.268564 176.7357
    (11 pairs, error in ra: 0.28", error in dec: 0.60")

    Rosetta coordinates from astrometry: 02:03:15.92 -19:33:38.5

  3. Exposure of 3 min, taken on 2009-11-12 19:10:44 UT (prec: 2 sec).
    File: rst1845001.00000798.FIT.

    Click on picture for full-sized version.

    Intended center of picture at the same coordinates as before.

    GCX fit with WCS: 02:03:11.21 -19:38:41.5 2000 1.269875 1.269875 176.7362
    (15 pairs, error in RA 0.41", error in dec 0.92")

    Rosetta coordinates from astrometry (clicking on the middle): 02:03:08.07 -19:34:53.9

    Animated GIF with the three previous pictures:

  4. Exposure of 3 min, taken on 2009-11-12 22:11:54 UT (prec: 2 sec).
    File: rst2205.00000799.FIT.

    Click on picture for full-sized version.

    Intended center of picture at 01:59:35.71, -20:13:39.3, corresponding to 22:05 UT ephem01.

    GCX astrometric fit with WCS: 01:59:46.18 -20:14:28.8 2000 1.273249 1.273249 169.9497

    According to ephem01, Rosetta was at 01:59:25.95, -20:14:35.0 at 22:10 UT.

  5. Exposure of 3 min, taken on 2009-11-12 22:37:52 UT (prec: 2 sec).
    File: rst2230.00000800.FIT.

    Click on picture for full-sized version.

    Intended center of picture at 1:58:45.13, -20:18:21.2, corresponding to 22:30 UT ephem01.

    GCX astrometric fit with WCS: 01:58:31.43 -20:21:15.6 2000 1.272694 1.272694 169.9543

    Some lines of ephem01: 

       Time (UT)         | RA          | dec         | Az       | El
   2009-Nov-12 22:30 | 01 58 45.13 | -20 18 21.2 | 181.0034 | 27.1195
   2009-Nov-12 22:35 | 01 58 34.46 | -20 19 18.7 | 182.3703 | 27.0755
   2009-Nov-12 22:40 | 01 58 23.60 | -20 20 16.5 | 183.7361 | 27.0086

  6. Exposure of 3 min, taken on 2009-11-12 22:48:30 UT (prec: 2 sec).
    File: rst2230.00000801.FIT.

    Click on picture for full-sized version.

    Intended center of picture at the same point as before: 1:58:45.13, -20:18:21.2,
    corresponding to 22:30 UT ephem01.

    GCX astrometric fit with WCS: 01:58:31.05 -20:21:29.6 2000 1.272755 1.272755 169.9486

    Some lines of ephem01: 

       Time (UT)         | RA          | dec         | Az       | El
   2009-Nov-12 22:30 | 01 58 45.13 | -20 18 21.2 | 181.0034 | 27.1195
   2009-Nov-12 22:35 | 01 58 34.46 | -20 19 18.7 | 182.3703 | 27.0755
   2009-Nov-12 22:40 | 01 58 23.60 | -20 20 16.5 | 183.7361 | 27.0086

  7. Exposure of 3 min, taken on 2009-11-12 23:29:08 UT (prec: 2 sec).
    File: rst2320.00000802.FIT.

    Click on picture for full-sized version.

    Intended center of picture at 01:56:49.67 -20:28:13.3, corresponding to 23:20 UT ephem01.

    GCX astrometric fit with WCS: 01:56:33.28 -20:31:31.1 2000 1.270967 1.270967 169.8294

    Some lines ephem01: 

       Time (UT)         | RA          | dec         | Az       | El
   2009-Nov-12 23:20 | 01 56 49.67 | -20 28 13.3 | 194.5334 | 25.6581
   2009-Nov-12 23:25 | 01 56 37.01 | -20 29 14.9 | 195.8569 | 25.3895
   2009-Nov-12 23:30 | 01 56 24.13 | -20 30 17.0 | 197.1725 | 25.0993

  8. Exposure of 6 min, taken on 2009-11-12 23:42:54 UT (prec: 2 sec).
    File: rst2330.00000803.FIT.

    Click on picture for full-sized version.

    Intended center of picture at 01:56:24.13 -20:30:17.0, corresponding to 23:30 UT ephem01.

    GCX astrometric fit with WCS: 01:56:06.52 -20:33:18.2 2000 1.272559 1.272559 169.9793

    Some lines of ephem01: 

       Time (UT)         | RA          | dec         | Az       | El
   2009-Nov-12 23:30 | 01 56 24.13 | -20 30 17.0 | 197.1725 | 25.0993
   2009-Nov-12 23:35 | 01 56 11.04 | -20 31 19.5 | 198.4797 | 24.7878
   2009-Nov-12 23:40 | 01 55 57.73 | -20 32 22.6 | 199.7781 | 24.4552
   2009-Nov-12 23:45 | 01 55 44.20 | -20 33 26.2 | 201.0671 | 24.1019

  9. Exposure of 6 min, taken on 2009-11-12 23:54:44 (prec: 2 sec).
    File: rst2345O.00000804.FIT.

    Click on picture for full-sized version.

    Intended center of picture at DON'T REMEMBER!! (it was a desperate try :-) eye-gessing coordinates from the finder charts in the Rosetta blog).

    GCX astrometric fit with WCS: 01:56:57.08 -20:19:13.1 2000 1.275535 1.275535 170.009

    MISTERY: the star in the center is missing!!

    Some lines of ephem01: 

       Time (UT)         | RA          | dec         | Az       | El
   2009-Nov-12 23:45 | 01 55 44.20 | -20 33 26.2 | 201.0671 | 24.1019
   2009-Nov-12 23:50 | 01 55 30.44 | -20 34 30.3 | 202.3464 | 23.7281
   2009-Nov-12 23:55 | 01 55 16.45 | -20 35 35.0 | 203.6156 | 23.3342

Conclusions

Wrong conclusions: see update2 below.

I seems that Rosetta was a little higher than predicted by Horizons, even having into account errors in the position of the observing site. See the following figure (click to enlarge). Actually, Rosetta seems to have been caught in the first picture thanks to a combination of aiming errors plus atmospheric refraction.

In the rest of pictures, Rosetta seems not to be in a circle of 10.5 arcseconds (half the FOV of all the CCD pictures shown) centered in the coordinates given by Horizons. Perhaps the Horizons system did not have the last trajectory data?

Update: Kristin Wirth, Rosetta Science Operations Manager, has reported in the Rosetta blog that, in far more serious observations from ESA's Optical Ground Station on Tenerife, they did not find any disagreement between Horizons' predicted position of Rosetta against the background stars and the one found. If anyone is thrilled by the mistery and finds out where did I go wrong, please let me know :-)

Update2: mistery solved! I used km as units for the height of the observatory location (see Ephemeris used above), and therefor I got wrong coordinates. Thanks to A. Jorba for pointing out that. By using the following settings in Horizons, 

Ephemeris Type [change] :  	OBSERVER
Target Body [change] :  	Rosetta (Spacecraft) [-226]
Observer Location [change] :  	user defined ( 1°04'43.9''E, 42°37'04.7''N, 1590 m )
Time Span [change] :  	discrete time(s)=2009-11-12 18:55:36
Table Settings [change] :  	QUANTITIES=1,4
Display/Output [change] :  	plain text
the RA and dec obtained are 02:03:18.35 -19:33:11.1, which are a few seconds away from the bottom right end of the trace of Rosetta in the first picture.
Last update: Mon Nov 23 18:21:08 CET 2009