Reconstruction of Bennu Particle Events From Sparse Data

Abstract

OSIRIS-REx began observing particle ejection events shortly after entering orbit around near-Earth asteroid (101955) Bennu in January 2019. For some of these events, the only observations of the ejected particles come from the first two images taken immediately after the event by OSIRIS-REx's NavCam 1 imager. Without three or more observations of each particle, traditional orbit determination is not possible. However, by assuming that the particles all ejected at the same time and location for a given event, and approximating that their velocities remained constant after ejection (a reasonable approximation for fast-moving particles, i.e., with velocities on the order of 10 cm/s or greater, given Bennu's weak gravity), we show that it is possible to estimate the particles' states from only two observations each. We applied this newly developed technique to reconstruct the particle ejection events observed by the OSIRIS-REx spacecraft during orbit about Bennu. Particles were estimated to have ejected with inertial velocities ranging from 7 cm/s to 3.3 m/s, leading to a variety of trajectory types. Most (>80%) of the analyzed events were estimated to have originated from midlatitude regions and to have occurred after noon (local solar time), between 12:44 and 18:52. Comparison with higher-fidelity orbit determination solutions for the events with sufficient observations demonstrates the validity of our approach and also sheds light on its biases. Our technique offers the capacity to meaningfully constrain the properties of particle ejection events from limited data.

Figure 1

Track identification and radiant point estimation for the 6 January event. Bennu is saturated in the long‐exposure NavCam images used to create this figure, so no surface detail is visible (see the image processing description in Lauretta, Hergenrother, et al., 2019). (a) The NavCam images of the event are registered and differenced. Objects from the first image appear white, and objects from the second image appear black. (b) Particles detected in both images allow us to find a repeated pattern, (c) which is used to make associations that show the particles' apparent motion. (d) When the apparent motion is traced backward, the lines intersect at a common radiant point (red cross) on Bennu.

Figure 2

The linear geometry that results from assuming that the particles' velocities remain constant. A particle following a linear trajectory connecting the 3‐D positions ${\overline{r}}_{1,2,3}$ appears within the image at the locations l _1,2,3 when observed at the times t _1,2,3. The x and y components of each 3‐D position with respect to the radiant point are also shown where the x direction is parallel to the line connecting the observations within the image plane, and the y direction is parallel to the camera boresight. The z direction completes the right‐hand rule, such that the z component of each position is zero.

Figure 3

The 6 January ejection event reconstruction shown from (a) Bennu's −Y side and (b) Bennu's south (−Z) pole on a shape model of the asteroid. Vectors originate from the particles' estimated positions at the time of the first image of the event (20:56:13 UTC) and point in the velocity vector direction. Vector lengths show distance traveled over roughly 7 min for particles detected in two images (blue) or 1 min for the higher‐velocity particles that appeared as streaks in a single image (red). Also shown is the estimated ejection location (orange dot).

Figure 4

(a) Ejection location solutions for all events shown with 3‐sigma uncertainties on a global grid. The three largest observed events (Table 1) are shown in color as specified in the legend. Eight smaller events (Table 2) are shown in gray. (b–d) Locations of the three events with the most observed particles: (b) 6 January, (c) 19 January, and (d) 11 February. Also shown are the independent solutions for the 19 January and 11 February events from Leonard et al. (2020).

Figure 5

Estimated local solar times for all analyzed ejection events with error bars indicating 1‐sigma uncertainties. Both near and far solutions are shown for all events except those for which one solution could be ruled out either through independent solutions from Leonard et al. (2020) or through observations of particles in front of Bennu indicating that the far solution was not plausible.

Figure 6

Ejection velocity magnitudes of the particles analyzed for the three largest ejection events on (a) 6 January, (b) 19 January, and (c) 11 February. Far solution velocities are shown for all three events.

Figure 7

The 11 February event radiant point estimation using the registered and differenced NavCam images. The slower‐moving particles have begun to curve back toward Bennu under the influence of its gravity. If we assume that they travelled in straight lines and trace their paths backward (yellow vectors), the estimated radiant point (red cross) is off‐body, showing the biasing effect of the approximations that we made with respect to the independent OD solution of Leonard et al. (2020) (blue cross, on body).