Estimating the mass of asteroid 253 Mathilde from tracking data during the NEAR flyby

Science

Volumn 278, Issue 5346, 1997, Pages 2106-2109

  Yeomans, D K ^a   Barriot, J P ^b   Dunham, D W ^c   Farquhar, R W ^c   Giorgini, J D ^a   Helfrich, C E ^a   Konopliv, A S ^a   McAdams, J V ^c   Miller, J K ^a   Owen Jr , W M ^a   Scheeres, D J ^a,d   Synnott, S P ^a   Williams, B G ^a  

^a CALIFORNIA INSTITUTE OF TECHNOLOGY   (United States)

^b CNES   (France)

^c JOHNS HOPKINS UNIVERSITY   (United States)

^d Iowa State University of Science and Technology   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; ASTRONOMY; DENSITY; IMAGING; MASS; PRIORITY JOURNAL; SPACE FLIGHT;

EID: 0031470607     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.278.5346.2106     Document Type: Article

References (22)

1. For an overview of the NEAR mission, see J. Astronaut. Sci. 43, 345 (1995). For comprehensive accounts of the Gaspra and Ida encounters by the Galileo spacecraft, see M. J. S. Belton et al., Science 257, 1647 (1992); M. J. S. Belton et al., Icarus 120, 1 (1996).
2. For an overview of the NEAR mission, see J. Astronaut. Sci. 43, 345 (1995). For comprehensive accounts of the Gaspra and Ida encounters by the Galileo spacecraft, see M. J. S. Belton et al., Science 257, 1647 (1992); M. J. S. Belton et al., Icarus 120, 1 (1996).
3. For an overview of the NEAR mission, see J. Astronaut. Sci. 43, 345 (1995). For comprehensive accounts of the Gaspra and Ida encounters by the Galileo spacecraft, see M. J. S. Belton et al., Science 257, 1647 (1992); M. J. S. Belton et al., Icarus 120, 1 (1996).
4. B. G. Williams et al., paper presented at the AAS/AIAA Space Flight Mechanics Meeting, Huntsville, AL, 10 to 12 February 1997.
5. The early Mathilde orbits included only 60 observations over the interval 3 May 1927 to 6 January 1994. In January 1995, E. Goffin made available a data set wherein he had re-reduced many of the existing observations with respect to more modern reference star catalogs and extended the observational interval back to the time of this asteroid's discovery in mid-November 1885. Extensive sets of observations were received from a number of observatories, including McDonald (Texas), Klet (Czech Republic), Modra (Slovakia), Siding Spring (Australia), Oak Ridge (Massachusetts), the Carlsberg Automatic Meridian Circle (Canary Islands), and the U.S. Naval Observatory Flagstaff Station (Arizona). Some of the observations from Flagstaff were reduced with respect to extragalactic radio sources; these observations were given increased weight in the orbit determination process because they were relatively unaffected by the systematic errors present in most reference star catalogs. See R. C. Stone et al., Astron. J. 111, 1721 (1996). Observations taken during the 3 months before the flyby were instrumental in providing an accurate Mathilde ephemeris to the flight project. Beginning on 7 April 1997, observations became available from the observatories of Garradd (Australia), Mauna Kea (Hawaii), and Table Mountain (California). Many of tne observations from Table Mountain were reduced with respect to reference stars from the Hipparcos and Tycho star catalogs and were thus upweighted in the orbital solutions because these positions were considered an order of magnitude more accurate than observations reduced with respect to traditional reference star catalogs. Twenty-four of these observations became available covering the dates 21 May, 30 May, 31 May, 17 June, 20 June, 22 June, and 24 June 1997. If these Hipparcos-based positions of Mathilde had not been available before the encounter, and if we had not weighted them strongly in our final orbital solution, the ephemeris errors at encounter would have been at least an order of magnitude larger.
6. Positions for the three bright reference stars in the field were taken from the highly accurate Hipparcos catalog [M. A. C. Perryman, The Hipparcos Catalogue (European Space Agency, SP-1200, Noordwijk, Netherlands, 1997)]. Although Hipparcos reference star positions had been made available in advance of publication for improving the orbit of asteroid Ida before the Galileo spacecraft flyby in August 1993 [W. M. Owen Jr. and D. K. Yeomans, Astron. J. 107, 2295 (1994)], the Mathilde encounter marks the first operational use of the Hipparcos catalog by JPL navigation.
7. Positions for the three bright reference stars in the field were taken from the highly accurate Hipparcos catalog [M. A. C. Perryman, The Hipparcos Catalogue (European Space Agency, SP-1200, Noordwijk, Netherlands, 1997)]. Although Hipparcos reference star positions had been made available in advance of publication for improving the orbit of asteroid Ida before the Galileo spacecraft flyby in August 1993 [W. M. Owen Jr. and D. K. Yeomans, Astron. J. 107, 2295 (1994)], the Mathilde encounter marks the first operational use of the Hipparcos catalog by JPL navigation.
8. Our centroiding algorithm modeled the camera's point-spread function as an elliptical Gaussian with 1σ values of 1.7 pixels in the sample direction and 1.0 pixel in the line direction. The DN value in each pixel was modeled by the integrated flux from the point source plus a constant background, with the integration extending over only the light-sensitive part of the pixel. An iterative linearized least-squares procedure then solved for the (x,y) coordinates of the centroid, the brightness of the point source, and the background. This procedure failed at the low signal-to-noise ratio typical of co-added Mathilde images, and we estimated Mathilde's center by eye and lowered the weight appropriately.
9. J. E. Riedel et al., paper presented at the AIAA/AAS Astrodynamics Conference, Portland, OR (20 to 22 August 1990).
10. Besides the spacecraft position and velocity at a 2 June epoch, these parameters include the solved-for magnitude and direction of the maneuver performed on 18 June, nongravitational accelerations, and the Mathilde ephemeris and mass. Error sources treated as considered covariance parameters were the assumed uncertainties in the troposphere and ionosphere refraction and delays, the station location and Earth-moon ephemeris uncertainties, and the optical navigation center-finding and focal length errors. Station range and camera pointing biases were modeled as uncorrelated nondynamic stochastic parameters with a process noise of 0.7 m (range) and 0.01° to 0.03° (camera pointing).
11. J. Veverka et al., Science 278, 2109 (1997).
12. E. M. Standish, personal communication.
13. A. S. Rivkin et al., Icarus 127, 255 (1997).
14. J. Schubart and D. L. Matson, in Asteroids, T. Gehrels, Ed. (Univ. of Arizona Press, Tucson, 1979), pp. 84-97.
15. W. Landgraf, Astron. Astrophys. 191, 161 (1988).
16. E. M. Standish and R. W. Hellings, Icarus 80, 326 (1989).
17. E. Goffin, Astron. Astrophys. 249, 563 (1991).
18. G. V. Williams, Asteroids, Comets, Meteors 1991 (Lunar and Planetary Institute, Houston, TX, 1992), pp. 641-643.
19. B. Viateau and M. Rapaport, Astron. Astrophys. Suppl. Ser. 111, 305 (1995).
20. _, Astron. Astrophys. 320, 652 (1997).
21. M. J. S. Belton et al., Nature 374, 785 (1995).
22. A portion of this research was carried out by the JPL, California Institute of Technology, under contract to NASA. We thank P. W. Chodas, A. B. Chambertin, M. S. Keesey, E. M. Standish, and R. N. Wimberly for technical assistance and helpful discussions. Important ground-based astrometric observations of Mathilde were provided by observers including M. Buontempo, D. Clowe, A. Galad, G. J. Garradd, E. Goffin, O. Hainaut, A. K. B. Monet, A. Mrkos, R. McNaught, P. J. Shelus, R. Stone, J. Surace, D. Tholen, J. Ticha, A. Whipple, and R. J. Whiteley. M. J. Perryman of the Hipparcos project kindly provided Hipparcos reference star positions in advance of publication. Prepublication estimates of Mathilde's size and volume were kindly provided by the NEAR MSI team, in particular by P. Thomas and the team chief, J. Veverka. A large part of the success of the very difficult encounter with Mathilde was due to the tireless efforts of the NEAR Flight Team (both at Applied Physics Laboratory and JPL). The outstanding leadership of NEAR's systems engineer, A. Santo, deserves special recognition.