Lunar and Planetary XLVIII (2017) 1577.pdf

THE FEMTOSPACECRAFT IMPACT MISSION (FAIM): A LOW COST MISSION TO MONITOR THE DART IMPACT ON THE DIDYMOON T. Marshall Eubanks1, T. Cash1, B. Blair2, M.E. Eubanks1, 1Asteroid Initiatives LLC, Clifton, VA 20124 USA; 2NewSpace Analytics, Canterbury, NH 03224; [email protected];

Introduction: 63803 Didymos is a binary near- In order to meet the more demanding requirements Earth asteroid which will make a close approach to of operation in deep space and to create true space- Earth in early October, 2022. Observations show that craft swarms to meet various operational goals Asteroid the Didymos primary is roughly spherical with a diame- Initiatives LLC is developing the Pixie femtospacecraft ter of 800 m [1, 2, 3], while the secondary, the “Didy- (see Figure 1). A Pixie is 80 x 40 x 9 mm with a mass ”,∼ is roughly 150 m in diameter with an orbital < 50 grams, includes a battery and a variety of instru- period of 11.9 hours and an orbital radius of 1.18 km. ments, and is intended to operated in a with other The ESA∼ Asteroid Impact Monitoring (AIM) mission nodes (Pixies or otherwise) within communi- [4] was intended to rendezvous with the Didymos sys- cation range. tem as one component of the Asteroid Impact & Deflec- The assumption for FAIM is that the requirements of tion Assessment (AIDA), mission, a planetary defense AIDA would require swarms of femtospacecraft, ac- test [5]. The other AIDA component, the NASA Dou- cepting the risk that a substantial fraction may be lost ble Asteroid Redirection Test (DART) [6], is intended at deployment (due to failure to land on the Didymoon to impact the Didymoon in October, 2012, demonstrat- surface) or during the DART impact phase (when nodes ing the effectiveness of kinetic impactors for asteroid might be buried by debris or even ejected from Didy- deflection. The DART impact is expected to change the moon surface). Didymoon velocity by 0.3 β mm s−1, where β is a scale In order to operate as a single unit spacecraft swarms factor between the momentum of DART and the Didy- must be able to perform data fusion and ideally dis- moon post-impact momentum change. β is expected to tributed information processing, and in many cases may be > 1 due to the effects of impact ejecta; determining need to perform internal swarm positioning and external the impact β is a primary goal of the AIDA mission. communication fusion as well. To act as a Swarm, the On December 2, 2016, ESA announced that funding Pixies must collectively: was not approved for the AIM mission; shortly there- Aggregate : Determine which swarm members after NASA announced that work was continuing on the ◦ DART impact mission. While the combined AIDA mis- are within reach and establish an ad-hoc commu- sion was designed so that the DART planetary defense nications network encompassing them. test could go forward even without AIM, there is no Observe : Collect and share data from the aggre- question that the goals of AIDA would be enhanced by ◦ gated swarm. having in situ observation of the results of the impact. Here, we show that the core programmatic goals of Distill : Convert the analyzed data into a higher- AIM can be met with a relatively low cost Femtospace- ◦ level summary. craft Asteroid Impact Mission (FAIM), a 6-U “Brad- Communicate : Report the high level data sum- bury” CubeSat deploying a swarm of dozens of 50-gm ◦ “Pixie” femtospacecraft. FAIM would also provide sci- maries to Earth. entific information about the Didymos system, and an The FAIM femtospacecraft deployment would advance to expand the technological readiness of the technology of such sensor networks for asteroid small spacecraft for asteroid exploration and mining. expiration, would test innovative means of networked The Pixie Femtospacecraft: The exploration of the communication over the surface of an asteroid, would Solar System can be enhanced by the use of femtospace- provide fine-grained information about the surface prop- craft, small autonomous units which can be used to pro- erties of the secondary, and would provide important in- vide multiple observation points or in hazardous envi- sight into the reaction of the secondary to the DART ronments (such as near the DART impact) where there impact. is a strong risk of loss of individual nodes. The first fem- Scientific and Planetary Defense Objectives: The tospacecraft sent into space (in 2013) were the “Kick- Didymos primary appears to have been spun up by Sats” developed at Cornell University [7]. These Kick- Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) ra- sats were stateless, having no battery, long-term mem- diative torques [8], with the primary currently rotating ory or ability to receive commands, and reviving in a near the predicted rotational disruption spin rate, and default configuration every time sufficient power the secondary presumably resulting from material dis- was available. rupted from the primary. The Didymoon is thus highly Lunar and XLVIII (2017) 1577.pdf

unlikely to be monolithic, but instead can expected to a poorly consolidated “rubble-pile” [9], which will com- plicate its response to the DART impact. The small explored by spacecraft and to date largely have surfaces with very few impact craters, presumably smoothed out by seismic shaking after a large impact [10]. Recent work [11] indicates that seismic shaking from impacts can be more severe than previously indicated; characterizing this shaking, and material transport associated with it, are important goals of FAIM. In the proposed mission, the Bradbury CubeSat would be a femtospacecraft carrier with propulsion by either a Figure 1: The Asteroid Initiatives Pixie Asteroid Instru- solar or an electric sail. After rendezvousing with Didy- mentation set (without insulation). mos, this CubeSat would deploy its Pixie femtospace- craft near the Didymoon L1 or L2 points, where there is a high probability of low velocity deploy- Seismology: The seismic shaking after ments impacting the Didymoon surface [12]. The Brad- the DART impact might last for 1 or 2 hours bury would then proceed to leave the Didymos system [10]. A scientific goal of FAIM is to measure this and observe the DART impact remotely. shaking at multiple locations with accelerometers −3 −2 Scientific and planetary defense goals for FAIM in- with an accuracy of 10 ms / √Hz or better. clude Pixie Astrometry: At the time of the DART impact Approach: The Bradbury would take pre-deployment Pixie transmissions could be used as a source images both for optical navigation and to provide for differential Very Long Baseline Interferome- pre-impact geological context of both Didymos try (VLBI) observations from Earth. The scien- and the Didymoon. tific goal would be to provide astrometry good to 10 m in transverse position, sufficient to de- Descent: Deployment by ballistic descent would take termine the velocity β at the 10% level within 3 at least an hour, providing an opportunity to es- days of the DART impact. timate the mass of the secondary through mutual tracking of the femtospacecraft swarm. The sci- References: [1] P. Pravec, et al. (2006) Icarus 181:63 entific goal of this phase is the measurement of doi. [2] P. Scheirich, et al. (2009) Icarus 200:531 doi. the mass of the secondary to 1% or better, lim- [3] L. A. M. Benner, et al. (2010) in AAS/Division for ited by measurement accuracy∼ and solar radiation Planetary Sciences Meeting Abstracts #42 vol. 42 of Bulletin pressure on the femtospacecraft. of the American Astronomical Society 1056. [4] P. Michel, et al. (2016) Advances in Space Research 57:2529 doi. Surface Anchoring: The experience with the bounc- [5] A. Cheng, et al. (2015) Acta Astronautica 115:262 ISSN ing of the Philae shows the difficulty with 0094-5765 doi. [6] A. F. Cheng, et al. (2016) in Lunar and landing spacecraft in a microgravity environment. Planetary Science Conference vol. 47 of Lunar and Planetary We propose the use of a Microgravity Electro- Science Conference 2032. [7] Z. Manchester, et al. (2013) in Magnetic Anchoring System (MEMAS) system Proceedings of the 27th Annual AIAA/USU Conference on Small Satellites, Standards and Education 111.[link]. [13] to attach the Pixie femtospacecraft to the sur- [8] W. F. Bottke, Jr., et al. (2006) Annual Review of Earth and face of the Didymoon, and to validate this anchor- Planetary Sciences 34:157 doi. [9] N. Murdoch, et al. ing technology for further use in microgravity. (2015) ArXiv e-prints.arXiv:1503.01931. [10] E. Pre- and Post-Impact Rotational Dynamics: If the Asphaug (2007) in Lunar and Planetary Science Conference vol. 38 of Lunar and Planetary Science Conference 2432. Didymoon is an synchronously rotating ellipsoid [11] R. F. Garcia, et al. (2015) ArXiv e-prints. then it may undergo forced librations due to arXiv:1503.01893. [12] F. Ferrari, et al. (2015) in orbital perturbations [14] and the DART impact Proceedings, 23rd Conference of the Italian Association of is likely to excite free librations in either latitude Aeronautics and Astronautics (Edited by E. Carrera, et al.). or longitude, depending on the location of the [13] M. A. Graule, et al. (2016) Science 352(6288):978 ISSN impact and the angular momentum carried by 0036-8075 doi. [14] B. G. Bills, et al. (2009) in Europa impact ejecta. A scientific goal of FAIM is the (Edited by R. T. Pappalardo, et al.) The University of Arizona measurement of rotation rate with a precision space science series 119–134 University of Arizona Press. better than 1 deg/hour.