ASTEROID SURFACE GRAVIMETRY OF 65803 DIDYMOS VIA A LANDER CARRIED BY AIDA’S AIM SPACECRAFT. K. A. Carroll1, H. Spencer2 and R. E. Zee2, 1Gedex Inc., 407 Matheson Blvd. East, Mississauga, Ontario, Canada L4Z 2H2,
[email protected], 2Space Flight Laboratory, University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, Toronto, Ontario, Canada M3H 5T6,
[email protected],
[email protected]. Introduction: The internal structure of the two bodies making up 65803 Didymos could be probed by making a series of gravity measurements at multiple locations on the surfaces of those bodies. A small (15 kg) spacecraft, GRASP (GRavitational Asteroid Surface Probe) has been designed that could accomplish that, if carried to that asteroid by a carrier “mothership,” which would also provide communicationsrelay support. The AIM rendezvous spacecraft component of the proposed AIDA mission could carry GRASP to Didymos as a secondary payload. Gravimetric surveying of the satellite body Didymos-B would enhance the ability of the AIDA mission to achieve its objectives, by providing knowledge about the mass of that body, and of its internal structure, which could help interpret the effects of the impact on that body of the DART spacecraft. Subsequent gravimetric surveying of the primary body Didymos-A could add information about how Didymos B formed. AIDA Mass-Determination Objectives: The main objective of the proposed Asteroid Impact & Deflection Assessment (AIDA) mission is to demonstrate asteroid deflection by spacecraft kinetic impact [1], by colliding the proposed Double Asteroid Redirection Test (DART) spacecraft with a target, which is the smaller of the two bodies that together comprise asteroid 65803 Didymos. Related objectives are to determine various characteristics of the target asteroid (which we will refer to as Didymos-B) before, during and after the impact event, using the Asteroid Impact Monitoring (AIM) spacecraft. These objectives include determining the mass of Didymos-A and Didymos-B, analyzing their geology, and deriving collision and impact properties. In particular, in order to properly assess the effectiveness of the momentum transfer from DART to Didymos-B, it is important to have a good estimate of the mass of Didymos-B; that is also needed to determine its density, which is a fundamental parameter to understand the asteroid’s internal structure and composition. Methods for Determining the Mass of DidymosB: While the mass of Didymos-A can be determined from the orbit period of Didymos-B, the mass of Didymos-B is much more difficult to determine from orbital observations and it is currently poorly known; it is assumed in [1] to be 3x109 kg. There are several possible methods for determining its mass using the AIM spacecraft as originally conceived --- i.e., a
spacecraft that would rendezvous with Didymos but not land on it, making observation using stand-off, remote sensing instruments only. However, all of these methods are quite challenging: A method is described in [2] whereby camera observations of both bodies of Didymos by AIM, presumably in conjunction with radio tracking of AIM from Earth and detailed multi-body orbital modeling, could be used to determine the semimajor axis of the Didymos system, and the distance from its barycentre to the mass centre of Didymos-A, from which the mass ratio between the two bodies of Didymos would be estimated. Given an expected mass ratio of 1:100 and a semimajor axis of about 1 km, in order to estimate the mass of Didymos-B to within (say) 10%, the distance between the barycentre and the mass centre of Didymos-A would have to be determined to within about 1m, which may be difficult. Should AIM fly, radio tracking of AIM from Earth would be done, and those data would be fit to a Didymos system model to estimate various parameters, including masses for both bodies. A confounding factor, discussed in [3], is the uncertainty in the amount of solar radiation pressure acting on AIM. AIM would need to loiter at an altitude of 1 km or less above Didymos-B in order for the solar radiation force to be small enough to enable a mass determination accuracy of 10%. Given a Didymos-B orbital semi-major axis of about 1 km, accomplishing that could be extremely challenging, without colliding with one or the other asteroid body. A similar challenge would face any attempt to use the same technique as Hayabusa at 25143 Itokawa, which determined the asteroid’s mass to within 5% by dropping from an altitude of 1 km, while measuring altitude by LIDAR. Since the mass of Didymos-B is expected to be only 10% that of Itokawa, this measurement would be difficult to make with useful accuracy even without the much more complex orbital dynamics issue. We propose an alternate concept, suggested first in [3], of emplacing a gravimeter on the surface of Didymos-B in order to “weigh” that body. While this would add complexity to the overall AIM mission de-
sign, it might be the only feasible means of determining the mass of Didymos-B to the level of accuracy needed for AIDA to accomplish its overall mission objectives; it could also allow additional science objectives to be achieved, enhancing the impact assessment effectiveness of the overall AIDA mission. Such a lander, which we refer to here as GRASP (for GRavitational Asteroid Surface Probe) could be added to the AIM component of AIDA as a secondary, deployable payload. If implemented as a “microspace” class spacecraft, its cost could be relatively low. GRASP Design Concept: Gedex and SFL have completed an initial preliminary design study of a GRASP asteroid gravity geophysics lander, whose design is suitable for measuring the mass of DidymosB. The GRASP spacecraft would be carried to Didymos as a secondary payload by AIM, and would be released in the near vicinity (within a few km) of Didymos. GRASP includes full 3-axis attitude control capabilities, as well as a propulsion subsystem capable of 13 m/s delta-V --- in order to avoid the failure mode suffered by Hayabusa’s MINERVA lander. GRASP would make its way to the surface of Didymos-B, where it would land. It would carry the Gedex VEGA (VEctor Gravimeter for Asteroids) instrument, to measure the surface gravity at the landing site with an absolute accuracy of 1-10 nanoG, with which the mass of Didymos-B could be determined with an accuracy much better than 10% from a single measurement, when compensated with asteroid rotation data determined from imagery provided by AIM. GRASP would be supported by a “mothership,” which could be the AIM spacecraft itself, or could be an additional microsatellite secondary payload, also released by AIM; using a separate microsat for the mothership would reduce the impact on the main AIM spacecraft of supporting GRASP to a very low level. The mothership would act as a communications relay between GRASP and its ground controllers (possibly via AIM), as well as providing navigation support to GRASP as it descends to the asteroid surface, manoeuvring within the Didymos system all the while. The GRASP spacecraft, shown in the accompanying figure, is a 30 cm cube, with 6 legs each about 30 cm long. It has an estimated mass of 15 kg, including a mass margin of 30%; an additional 4 kg of
GRASP interface equipment is expected to be left behind on AIM and/or the mothership microsat. Its power and thermal subsystems are designed to withstand thermal conditions on an asteroid surface, including eclipses of as long as 12 hours, while producing 6W of power in sunlight. It can receive and transmit data at 5 kBaud, and its propulsion system has an impulse capability of 200 N-s. Payloads include the VEGA instrument as well as several cameras, and a pair of magnetometers. GRASP is also designed to be able to rove about the asteroid surface, using a combination of tumbling (actuated by its reaction wheels), and hopping (using its propulsion system). Enhanced AIDA Science Potential: The main objective, of measuring the mass of Didymos-B to high accuracy, could be accomplished quickly by GRASP by making a single gravimetry measurements at its initial landing site. Using its roving capability, GRASP could move about the surface of Didymos-B, conducting a gravimetry survey of the entire body. Extrapolating from terrestrial geophysical surveying practice, inversion techniques could be applied to infer the body’s internal density distribution from these gravity measurements. This could help characterize the internal structure of the target asteroid, potentially identifying compositional or morphological inhomogeneities. If done in advance of the DART impact, this information could help guide the choice of the DART target impact point on the surface of Didymos-B. If left on the surface of Didymos-B during the DART impact, VEGA will function as a seismometer. Capable of measureing accelerations at up to 1 kHz, it could provide information about the details of how DART interacts with the asteroid during impact, and how Didymos-B responds. A second gravimetry survey could be conducted following the impact, looking for changes in the mass distribution of Didymos-B. GRASP’s propulsion capability is large enough to allow it to manoeuvre from Didymos-B to Didymos-A, following the impact event. Additional AIDA-relevant science could be done by surveying the surface of Didymos-A, gravimetrically and otherwise. The precise details of the first GRASP design study were not optimized for the needs of the AIDA mission. They could be improved upon. This design serves to illustrate the size and mass of a secondary payload that could add considerable value to the AIDA mission. References: [1] Cheng A. F. et al. (2013) 64th IAC, paper IAC13-A3.4.8. [2] Murdoch N. et al. (2012) AIDA Mission Rationale Interim Release, ESA. [3] Carroll K. A. (2014) LPS XLV, Abstract #2352.
