Cannot find file /Users/jewitt/Dropbox/CYCLE30/C30_Bennu_GO/Bennu.pdf0Assigned ID: 2866
----- Attempting Submission 1 (Wed Mar 23 09:19:32 GMT 2022) -----
HST Phase I Proposal 2866 successfully submitted.
Receipt: # 2866-1
----- Attempting Submission 2 (Thu Mar 24 20:36:40 GMT 2022) -----
HST Phase I Proposal 2866 successfully submitted.
Receipt: # 2866-2
----- Attempting Submission 3 (Fri Mar 25 18:03:58 GMT 2022) -----In this resubmission, orbital elements for Bennu are included now that APT can access JPL Horizons. -- Max Mutchler-206673543----- Attempting Submission 1 (Thu Jun 30 14:50:20 GMT 2022) -----
----- Attempting Submission 2 (Tue Jul 05 14:59:30 GMT 2022) -----Asteroid Bennu Large Particle TrailThe sporadic ejection of centimeter sized particles from Bennu was one of the major surprises from NASA's OSIRIS-REx mission and revealed this object as one of the Active Asteroids. The cause of particle ejection is not yet well established. Archival data obtained for another purpose in 2012 show evidence for an ultra-faint, narrow particle trail that likely corresponds to the summed emission from Bennu over months. We propose to image the ultra-faint particle trail from asteroid Bennu at a more favorable geometry in order to better assess the properties of the trail. Through models, we will interpret the trail parameters to determine the average particle size, ejection speed, mass loss profile and mass, in order to better constrain the mass loss process.We request 8 HST orbits to begin an investigation of these questions. The need for 8 orbits is based on analysis of the archival data, showing the trail to be very faint (in 6 orbits) and the assumption that the trail production rate in 2023 is the same as in 2012. In 8 orbits we estimate that the trail signal-to-noise ratio will be <1 per pixel but, with a measurable trail length of 200 to 300 pixels, the integrated SNR is 14 to 17, and there will be enough SNR to measure resolved perpendicular segments and parallel profiles of the trail. The orbits should be as close together as possible but do not suffer from modest gaps, for example as may be needed for gyro-resets. In each orbit we will take five long exposures using WFC3 with the wide bandpass filter F350LP for maximum sensitivity - this time request and set up is similar to that used in the earlier program GO 13118. The F350LP filter provides a 1.6x higher count rate than more standard (e.g. R, V) filters for a target with a solar-type spectrum, which matters because we are interested in ultrafaint, diffuse emission near Bennu. We will dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. The timing of the visit is determined by optimizing the geometry. We would like to observe Bennu from a vantage point in the orbital plane of the asteroid. In this geometry, all low velocity particles (which are confined to move close to the orbit plane) are lined up along the line of sight, so maximizing the surface brightness and detectability. (Depending on the thickness perpendicular to the orbit plane and the exact value of the angle of the Earth from the plane, we estimate an increase in the surface brightness by factors from 100 to 1000, offering a considerable observational advantage). In addition, we would like to minimize the geocentric distance, so as to better resolve the particle trail. The thickness of the trail provides a projection-free measure of the out-of-plane extent of the dust and of the perpendicular ejection speed. And we would like to minimize the heliocentric distance because spacecraft data suggest that particle emission is slightly stronger at small rH than at large rH (Hergenrother et al. 2020).
These conditions are best met on UT 2023 September 25. The Earth crosses the orbit plane on two occasions in 2023: on UT March 23 and September 25. The second of these crossings is the better of the two, in view of the much smaller geocentric distance (0.55 AU on September 25 compared to 1.37 AU on March 23). The heliocentric distance is also better (0.92 AU on September 25 against 1.35 AU on March 23). By the inverse square law, the geometry gives a 2.8 magnitude advantage to observations on September 25. This is slightly decreased by the larger phase angle on the latter date. However, Hergenrother et al. report that the phase function of Bennu particles is small, reducing the photometric advantage of the September 25 crossing to a still respectable 2.3 magnitudes (factor of 8 in flux density). We also note that the phase function of the particles is smaller than that of Bennu itself, acting to reduce the relative brightness of Bennu compared to a dust trail by a further V = 1.04 magnitudes relative to the March 23 date. For these reasons, we request observations at the plane-crossing on UT 2023 September 25. Bennu has been unobservable as a result of being in the HST Sun exclusion zone since 2019 September 15. It will emerge on UT 2023 February 9 and remain accessible throughout Cycle 30. The apparent rates of motion in late September are 180 arcsec/hr N and -15 arcsec/hr S, which are easily within Hubble's tracking capabilities. The ephemeris uncertainty is <1 arcsec, which is negligible compared to the WFC3 field-of-view. Ephemeris issues are of no concern to this observation. We understand that we may have essentially no control over the spacecraft roll angle, which means we will not be able to optimize the orientation of the dust tail on the CCD (i.e., to orient the tail along the longest dimension of the detector). However, the field-of-view of the camera is large enough that we should obtain excellent data, no matter what spacecraft roll angle is used. As described above, we request that the observations be obtained as close as possible to UT 2023 September 25. The actual moment of plane crossing is UT 2023 September 25.7. Dates within +/- 1 day of plane crossing are sufficient to negate the effects of projection. Therefore we request 8 orbits in the window UT 2023 September 25.7 +/- 1 day.David Jewitt is a long-time user of HST and ground-based telescopes to explore the nature of the solar system's primitive body population. Yoonyoung Kim is experienced with Monte Carlo modeling of dust and the production of simulated comet images used to infer dust ejection properties. Max Mutchler is an expert in optimization of the use of HSt for solar system science. Jing Li is experienced with handling large datasets and coordinating measurements. Jessica Agarwal is an expert on the near-nucleus dynamics of cometary debris and on subsequent motions under the action of radiation pressure and solar gravitySolar System Astronomy6falsefalsefalsefalsefalsefalsefalsefalseSUPPORTEDSOLAR SYSTEMNOASTEROIDActive asteroid visited by the OSIRIS-REx spacecraftTYPE=ASTEROID,A=1.1263910258948120,E=0.2037450762416414,I=6.0349437702479403
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