New Astrometric Observations of Deimos with the SRC on Mars Express

New Astrometric Observations of Deimos with the SRC on Mars Express

Overview Deimos Mars Express Mission HRSC / SRC Camera Astrometric Observations Verification of Pointing Limb Measurements Results Slide 2

Deimos Size and shape parameters (Source: WGCCRE2009): Mean radius: 6.2 ± 0.18 km Subplanetary equatorial radius: 7.8 km Along orbit equatorial radius: 6.0 km Polar radius: 5.1 km RMS dev. From ellipsoid: 0.2 km Mean orbital elements referred to local Laplace plane (Source: MAR080): Semi-major axis a = 23458 km Eccentricity e = 0.0002 Argument of periapsis ω = 260.729 Inclination i = 1.788 Longitude of ascending node Ω = 24.525 Orbital period P = 1.2624 days Slide 3

Mars Express ESA s first mission to Mars Scientific payload: 7 instruments HRSC, SRC Orbit insertion: 25 December 2003 Highly elliptical orbit: Periapsis: 258 km Apoapsis: 11,560 km Polar orbit: Inclination 86.3 Orbital Period: 6 h 43 m Due to pertubation forces the orbit drifts slightly around Mars Credits: ESA, Medialab Credits: ESA Slide 4

HRSC/SRC Camera Framing camera To show details within HRSC scenes Optics Maksutov-Cassegrain design Focal length f = 988.6 mm F-number f/11 CCD detector 1,024 x 1,024 CCD sensor Read-out-electronics Credits: DLR Credits: Oberst et al. 2008 Slide 5

Relative Astrometric Observations Given: Spacecraft position Spacecraft pointing Measured: Deimos center-of-figure Background stars positions Desired: Deimos center-of-mass Assumption: Deviations between COF and COM should be small. Credits: cf. Kolyuka et al. 1990 Slide 6

Coverage of Deimos orbit with SRC images Credits: H. Hoffmann, DLR Slide 7

Verification of Pointing Given: Nominal pointing (CK kernels) Verification and improvement of pointing by measuring positions of background stars Pointing of camera ω rotation about x-axis φ rotation about y-axis κ rotation about z-axis Required: Sample and line positions of at least two stars Generally: Measurement of the position of one background star Correct orientation of Deimos in the image => rotation about z-axis could be neglected Credits: Klaus-Dieter Matz, DLR Slide 8

Comparison between the limbfit method based on an triaxial ellipsoid and a plate model Credits: T. Roatsch, K.-D. Matz, DLR Slide 9

Limbfit workflow Credits: T. Roatsch, K.-D. Matz, DLR Slide 10

Results and Outlook Position of Deimos SRC image coordinates (sample, line) SRC camera coordinates (X C, Y C, Z C ) S/C centered, intertial J2000, Equatorial coordinates Position of MEX spacecraft Mars-centered, inertial J2000, Cartesian coordinates Outlook Determine accuracies Comparison with existing orbit models 2 a Credits: Oberst et al. 2006 2 b Credits: ESA / NAIF c arctan r 2 Slide 11

References Willner, K.; Oberst, J.; Wählisch, M.; Matz, K.-D.; Hoffmann, H.; Roatsch, T.; Jaumann, R.; Mertens, V., 2010: New astrometric observations of Phobos with the SRC on Mars Express. In: Astronomy and Astrophysics, Volume 488, Issue 1, 2008, pp.361-364. Oberst, J.; Matz, K. D.; Roatsch, T.; Giese, B.; Hoffmann, H.; Duxbury, T.; Neukum, G., 2006: Astrometric observations of Phobos and Deimos with the SRC on Mars Express. In: Astronomy and Astrophysics, Volume 447, Issue 3, March I 2006, pp.1145-1151. Kolyuka, Yu.; Tikhonov, V.; Ivanov, N.; Polyakov, V.; Avanesov, G.; Heifetz, V.; Zhukov, B.; Akim, E.; Stepanyants, V.; Papkov, O.; Duxbury, T., 1991: Phobos and Deimos astrometric observations from the PHOBOS mission. In: Astronomy and Astrophysics (ISSN 0004-6361), vol. 244, no. 1, April 1991, p. 236-241. NASA-supported research. Duxbury, T. C.; Callahan, J. D., 1989: PHOBOS and Deimos astrometric observations from Mariner 9. In: Astronomy and Astrophysics (ISSN 0004-6361), vol. 216, no. 1-2, June 1989, p. 284-293. Duxbury, T. C.; Callahan, J. D., 1988: PHOBOS and Deimos astrometric observations from Viking. In: Astronomy and Astrophysics (ISSN 0004-6361), vol. 201, no. 1, July 1988, p. 169-176. Slide 12

Credits: ESA / DLR / FU Berlin (G. Neukum) Thank you for your attention. Slide 13