Lucy Observations of the DART Impact Event

Abstract

The Lucy LOng Range Reconnaissance Imager (L’LORRI) took 1549 images of the Didymos–Dimorphos binary system, starting 12 hr before the Double Asteroid Redirection Test (DART) impact event on 2022 September 26 and ending 24 hr after it. The Lucy imaging campaign provided pre-impact monitoring of the baseline brightness of the Didymos system, as well as intensive 1 s cadence imaging starting 3 minutes prior to impact and extending until 4 minutes after impact, and then continued monitoring at increasing cadences and image exposure times to measure the Didymos system brightness changes produced by ejecta released during the impact. One of the L’LORRI images encompassed the exact time when the DART spacecraft impacted Dimorphos, but there is no evidence of a thermally generated optical flash in the image. L’LORRI observed a shell of fast-moving ejecta, and we derive a range of projected speeds (∼0.26–3.6 km s−1) for that material. The much slower moving ejecta (≲1 m s−1) stayed within a single L’LORRI pixel for the entire duration of the Lucy-DART program. We find that the slow ejecta were responsible for 57.4% ± 2.2% of the total post-impact brightness increase measured by L’LORRI, while the fast ejecta were responsible for 42.6% ± 2.3% of the post-impact brightness increase. The initial brightness increase relative to the pre-impact Didymos system brightness was significantly smaller for L’LORRI compared to that measured by some ground-based observers, which is plausibly explained by differences in phase angles and different responses to sodium emission depending on the camera spectral bandwidths.

Achievement of the Planetary Defense Investigations of the Double Asteroid Redirection Test (DART) Mission

“…The brightness of the fast plume was seen to correlate with the filter bandpass, such that Earth-based observations taken through filters that encompassed Na or K emissions observed a brighter fast plume (Fitzsimmons et al 2023). A lower brightness observed by Lucy in comparison to groundbased observations has also been attributed to the wider bandpass of that instrument though, different phase angle viewing conditions may have contributed as well (Weaver et al 2024). While the fast ejecta plume contributed substantially to the overall initial brightening of the Didymos system, the upper limit estimates of the mass associated with the fast ejecta plume are just a few hundreds to a few thousands of kilograms of material (Fitzsimmons et al 2023;Graykowski et al 2023;Weaver et al 2024), a small fraction of the total ejecta mass estimated, as discussed in the next paragraphs.…”

“…The moment of DARTʼs impact was also captured by a number of Earth-based telescopes (Fitzsimmons et al 2023;Graykowski et al 2023;Shestakova et al 2023) as well as the Lucy mission in space (Weaver et al 2024). An immediate fast plume of material was observed, as shown in Figure 6(a), composed of material with speeds reported as ranging from roughly 1 km s −1 up to 3.6 km s −1 (Fitzsimmons et al 2023;Graykowski et al 2023;Shestakova et al 2023;Weaver et al 2024).…”

“…The moment of DARTʼs impact was also captured by a number of Earth-based telescopes (Fitzsimmons et al 2023;Graykowski et al 2023;Shestakova et al 2023) as well as the Lucy mission in space (Weaver et al 2024). An immediate fast plume of material was observed, as shown in Figure 6(a), composed of material with speeds reported as ranging from roughly 1 km s −1 up to 3.6 km s −1 (Fitzsimmons et al 2023;Graykowski et al 2023;Shestakova et al 2023;Weaver et al 2024). In particular, alkali metal neutral emission lines were observed associated with DARTʼs impact event that suggested relative amounts close to solar system abundances, evidence that is consistent with the fast plume being composed of material that originated from Dimorphos (Shestakova et al 2023).…”

On the Fate of Slow Boulders Ejected after DART Impact on Dimorphos

“…From the measurements by J+23, we see that the abundance of such relatively high-speed boulders is presumably small (see Table 4), so that the actual frequency of early impacting boulders on Didymos might be greatly reduced. In fact, photometric measurements during those first minutes after DART impact do not reveal any significant brightness enhancement (see, e.g., Graykowski et al 2023;Weaver et al 2024) that could be attributed to reimpacting boulders. On the contrary, at later times, all the remaining impact events on Didymos and Dimorphos are associated with ejection speeds smaller than ∼0.22 m s −1 (see Figure 10) and are therefore much more likely events from the measurements by J+23.…”