OpGrav - Precise Small Body Mass Measurements during High-Speed Flybys

We propose to mature an instrument that enables precise mass determination of small asteroids and comets during high-speed spacecraft flybys. Science Objectives: Mass is a fundamental property of an asteroid and is central to determining its density, composition, and porosity. These properties relate to the body's bulk composition, indicating whether the observed surface characteristics are homogeneous throughout. They also have implications for the asteroid's formation, extending to the early solar system's accretional and collisional environment. In planetary defense, mass and porosity define the threat that a hazardous object represents and restrict which mitigation methods are feasible. Despite this importance, asteroid mass is difficult to measure remotely. The only practical way to measure a small asteroid's mass is to send a dedicated rendezvous spacecraft that orbits the asteroid (or co-orbits with the asteroid). Alternative approaches have low accuracy, are limited to bodies with certain characteristics (e.g., very large, have distinctive spectra, or have a binary companion), or require many years of radar observations. This instrument development enables mass measurements from high-speed flyby missions. With flyby missions, a single spacecraft can pass and observe many asteroids, giving a distribution of characteristics over a large sample. With current practice, we can only obtain mass estimates from spacecraft flybys when the asteroids are exceptionally large or the flyby speed is exceptionally low. And in those cases, gravity science often has to compete with other important observations. Instrument: Our technique called Optical Gravimetry (shortened to OpGrav, to highlight its similarities to optical navigation) significantly increases the sensitivity of mass estimates from high-speed spacecraft flybys. The associated instrument, called the Small Body In-Situ Multi-Probe Mass Estimation Experiment (SIMMEE), precisely dispenses a set of test-masses, which the spacecraft tracks prior to- and following the encounter using an on-board camera. The test-masses are dispensed so that they pass much closer to the body than a spacecraft would typically consider, benefiting from an r-squared increase in acceleration. Our past efforts have demonstrated that, broadly speaking, SIMMEE obtains a mass measurement for bodies less than 1/10th of the size achievable with existing capabilities (>1000x mass sensitivity improvement). This opens access to a significant new portion of the asteroid population. This instrument is also pertinent to rendezvous encounters for measurements of high-order and degree gravitational terms. However, a flyby introduces stricter instrument requirements, which we are designing and testing to. Relevance to SMD: The 2023 Planetary Science Decadal Survey highlights the need to determine the distribution of asteroid properties. It points to spacecraft-based in-situ measurements as a means to achieve this. By enabling mass measurements in small body flyby tour missions, this instrument will substantially increase the number of characterized small bodies. Likewise, OpGrav is also relevant to SMD's Planetary Defense objectives by potentially increasing the sample size of near-Earth asteroid properties and enhancing rapid reconnaissance missions in threat scenarios. Instrument Description: Through previous NASA and internal funding, we have developed and tested a SIMMEE prototype (TRL 4). The test-mass is a 15 cm sphere, which collapses to a thin disk for low stowed volume. The dispenser uses a compression spring and release mechanism. The release time and rate are measured with an LED curtain. The design includes features to ensure operation after long storage durations. If funded, we will fabricate an engineering model to validate the driving requirements (especially deployment accuracy and repeatability) and conduct functional and environmental tests to reach TRL 6.

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