David W. Dunham

Combining asteroid models derived by lightcurve inversion with asteroidal occultation silhouettes

Abstract Asteroid sizes can be directly measured by observing occultations of stars by asteroids. When there are enough observations across the path of the shadow, the asteroid’s projected silhouette can be reconstructed. Asteroid shape models derived from photometry by the lightcurve inversion method enable us to predict the orientation of an asteroid for the time of occultation. By scaling the shape model to fit the occultation chords, we can determine the asteroid size with a relative accuracy of typically ∼10%. We combine shape and spin state models of 44 asteroids (14 of them are new or updated models) with the available occultation data to derive asteroid effective diameters. In many cases, occultations allow us to reject one of two possible pole solutions that were derived from photometry. We show that by combining results obtained from lightcurve inversion with occultation timings, we can obtain unique physical models of asteroids.

Libration Point Missions, 1978 - 2002

This paper summarizes the six missions to the vicinity of libration points that have been flown up to the time of this conference in June 2002. The first libration-point mission, the third International Sun-Earth Explorer (ISEE-3), is emphasized because it laid the groundwork for so many later missions, most of which are covered more thoroughly in other papers given at this conference. First, the authors present some basic properties of libration-point orbits, and some history of their development for early missions. Only brief information is given here; details can be found in the references.

Volumes and bulk densities of forty asteroids from ADAM shape modeling

Context. Disk-integrated photometric data of asteroids do not contain accurate information on shape details or size scale. Additional data such as disk-resolved images or stellar occultation measurements further constrain asteroid shapes and allow size estimates.Aims. We aim to use all the available disk-resolved images of approximately forty asteroids obtained by the Near-InfraRed Camera (Nirc2) mounted on the W.M. Keck II telescope together with the disk-integrated photometry and stellar occultation measurements to determine their volumes. We can then use the volume, in combination with the known mass, to derive the bulk density.Methods. We downloaded and processed all the asteroid disk-resolved images obtained by the Nirc2 that are available in the Keck Observatory Archive (KOA). We combined optical disk-integrated data and stellar occultation profiles with the disk-resolved images and use the All-Data Asteroid Modeling (ADAM) algorithm for the shape and size modeling. Our approach provides constraints on the expected uncertainty in the volume and size as well. Results. We present shape models and volume for 41 asteroids. For 35 of these asteroids, the knowledge of their mass estimates from the literature allowed us to derive their bulk densities. We see a clear trend of lower bulk densities for primitive objects (C-complex) and higher bulk densities for S-complex asteroids. The range of densities in the X-complex is large, suggesting various compositions. We also identified a few objects with rather peculiar bulk densities, which is likely a hint of their poor mass estimates. Asteroid masses determined from the Gaia astrometric observations should further refine most of the density estimates.

Contingency Plans for MESSENGER's Mercury Orbit Insertion Maneuver

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will be the first spacecraft to orbit the planet Mercury when it begins its one-year Mercury orbital mission phase next year. On 18 March 2011 MESSENGER will perform the critical 862 m/s Mercury orbit insertion (MOI) maneuver. This paper summarizes strategies for recovering MESSENGER’s science mission in the event of an aborted or anomalous MOI maneuver. If 70% or more of the MOI burn is completed, MESSENGER will be captured into a high Mercury orbit. One or two maneuvers would then be required to achieve the planned 82.5inclination, 12.0-hour orbit, and all science objectives can be met for most of these cases. If less than 70% of the MOI burn is completed, MESSENGER would remain in a heliocentric orbit, and a recovery maneuver must occur either 10 to 14 days after the 18 March 2011 MOI attempt, or approximately one Mercury year (87.97 days) later in June 2011. For these heliocentric trajectories, solutions were found by which the spacecraft returns to Mercury after either one Mercury year or multiple (Earth) years (subsequent to completing one more or one less revolution of the Sun than Mercury). None of the successful return solutions exceeds the 7-year maximum preferred return time to Mercury.

A Voyager-style tour of comets and asteroids 1994-2005

A low cost program that links a dual-comet flyby sample-return mission with a multicomet/asteroid tour is proposed. Two spacecraft are used to carry out this program: a three-axis stabilized Observer-class spacecraft and a smaller spin-stabilized sample-return probe. The Observer spacecraft uses earth-swingby and propulsive maneuvers to accomplish the small-body tour, which includes flybys of three comets (Tempel-1, Tempel-2, and Encke) and two asteroids (46-Hestia and 433-Eros) over a 12-year period. Two of these comets (Tempel-1 and Tempel-2) are also the shared targets, the Observer serves as a navigational aid for the probe, which scoops up dust particles as it flies through the cometary atmosphere. After collecting the cometary dust samples, the probe returns to a low earth orbit where it is recovered by the Space Shuttle.