Simulated LSST Observations of Real Metre-scale Impactors

Abstract

Using real metre-sized asteroid Earth impactors from the last decade, we ask the question: ``If the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) had been operating over the last 10 years, how many imminent impactors would it have observed and discovered pre-impact?'' We feed 216 large impacts detected by global coverage orbital sensors through the LSST Solar System Survey Simulator Sorcha, and find that it would have made 99 observations of 28 unique objects, and discovered one object four days pre-impact. Recently proposed pipeline modifications would increase this discovery number to seven. Scaling our results to take into account the biases of our reference population, we estimate that LSST will discover 12 +/- 3 imminent impactors over its nominal 10 year survey, with an average warning time of 3.5 days. While this is at the low end of previous estimates of 1 - 10 discoveries per year, the significant increase in warning time compared to the current average (9 hours across 11 impactors) will bring significant opportunities for follow-up telescopic observations, deployment of specialised equipment for fireball observations, and planetary defence operations. We also show that the LSST will provide substantial precovery data for impactors discovered by other surveys, instantly lengthening observation arcs and thereby reducing the orbital and impact location uncertainties. In some cases, these observations may also enable the linkage of telescopic observations with observed fireballs post-impact, providing valuable pre-impact astrometric and photometric data. This has significant implications for both asteroid research and planetary defence.

Figures (6)

• Figure 1: Map of the impact locations for the 216 USG events. Grey dots are not observed pre-impact, blue $\times$ markers are observed pre-impact, orange triangles are objects observed 9+ times. The red square (USG_2015-05-10T07-45) would have been discovered as an imminent impactor, according to this work (note its location on the western edge of the map). The pink diamond (USG_2024-09-04T16-39) corresponds with the known imminent impactor 2024 RW$_1$. 17 of the observed bodies impacted in the Southern Hemisphere.
• Figure 2: Comparison of all impactors (observed and not observed; grey) and the total number of observations (blue). Orange shows the regions where the objects with 9+ observations sit. (a) Impact energy: all observed objects have E $>$$\sim$ 0.08 kT, distributed towards higher energies. (b) Speed: all observed objects have relatively low impact speeds (v $<$ 25 km s$^{-1}$). The four bodies observed 9+ times all have 11.2 km s$^{-1}$$<$ v $<$ 11.6 km s$^{-1}$. (c) H$_r$: observations tend to be of objects with H$_r$$>$ 31.5 (D $>$ 1.5 m). The four objects observed 9+ times all have H$_r$$>$ 30.5 (D $>$ 2.5 m). This shows that, unsurprisingly, objects that have a chance of being discovered with the USG dataset are large and slow.
• Figure 3: Calculated impact energy vs altitude for all 317 objects with reported altitudes, including objects without reported speeds (red $+$), objects with speeds that were not observed by LSST (grey dots) and objects with speeds that are observed (blue $\times$). Speeds are rarely reported for events $>$ 50 km. (Almost) all events with E $>$ 2 kT TNT have reported speeds.
• Figure 4: Semimajor axis compared to eccentricity and inclination. Grey dots are not observed pre-impact, blue $\times$ markers are observed pre-impact, orange triangles are objects observed 9+ times. The red square (USG_2015-05-10T07-45) would have been discovered as an imminent impactor, according to this work (note its location on the western edge of the map). The pink diamond (USG_2024-09-04T16-39) corresponds with the known imminent impactor 2024 RW$_1$. The four objects with 9+ observations are all on evolved orbits with low $a$, $e$ and $i$.
• Figure 5: Days before impact of the first observations for the observed bodies (blue), and of the bodies with 3+ observations on the same night which could be detected with modifications to the SSP (orange). Most objects are observed 0 - 6 days pre-impact. Three of the 28 events have first observations earlier than 22 days (at 36, 70 and 1262 days before impact; see Table \ref{['tab:obs_full_list']}). Median value is 3.5 days pre-impact for the first observations of all objects, and 4.3 days pre-impact for the 3+ tracklets to be created.