Prepare for Impact!

Impacts of meteoroids on the Moon provide important clues for better understanding the microphysical properties of its regolith (Berezhnoy et al. 2019), as well as of the projectile itself (Yanagisawa et al. 2021). Unfortunately, collisions with meteoroids having a high kinetic energy occur quite seldom. However, an impact happening on the dark side of the Moon, not far from the terminator could be possible to detect through remote observations of light scattering from the ejecta cloud. Clearly, it is impossible to predict such events in advance and, hence, their observations are rarely reported in the literature.

An impact of space debris has a better chance to be predicted, although the overall population of space debris in the vicinity of the Moon is quite low. An upcoming impact of such an object, tagged as WE0913A, is being discussed on world-wide mass media. This object, first identified as the Falcon 9 upper stage and now thought to be the Long March 3C third stage, the Chang'e 5-T1 booster, is expected to strike the Moon on 2022 March 4. ⁶ Using the Horizon System of JPL NASA, we have retrieved the most recent orbital elements of the WE0913A object (as of 2022 February 19) and, using our time-domain dynamical model (e.g., Nozdrachev et al. 2020), we simulate motion of the WE0913A object starting from 2022 February 10. We examined four values of time increment, 0.1, 0.01, 0.001, 0.0001 s, all yielding virtually the same orbit of WE0913A. Our modeling does suggest its impact on the Moon on 2022 March 4. However, according to our simulation, the impact is expected to happen on 12:34 pm (UTC) that is somewhat later compared to what is circulated in the media, 12:25 pm (UTC)¹. It is important to stress, however, when we implement older orbital elements of WE0913A (as of 2022 January 20), our modeling also suggests impact on 12:25 pm (UTC), so this latter time is a result solely from the latest observations and not our modeling.

Figure 1 shows the cross section of the Moon at the expected impact point, which is parallel to the lunar equatorial plane, but shifted by 12.7% of the lunar radius toward the north pole. The orbit of the WE0913A object is shown with the red line; its plane nearly coincides with the plane of the cross section. As one can see in Figure 1, the impact will take place on the far side of the Moon, making ground-based observations impossible. Nevertheless, due to the close proximity to edge of the apparent lunar disk, there is a chance to observe the ejecta cloud, appearing above the lunar limb shortly after the impact. The radial distance from the point of impact to the altitudes directly visible from Earth is equal to 119.5 km. It could be reached by particles whose vertical ejection velocity exceeds ∼610 m s⁻¹.

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We constrain the mass ejected from the Moon at velocity faster than V_ej, using the equation given in Table 1 of Housen & Holsapple (2011). The parameters of that equation are set as follows: C₁ = 0.55, k = 0.3, μ = 0.41, and ν = 0.4, the best matching case of impact onto regolith surface. The bulk material density of the lunar regolith was set to 1500 kg m⁻³; whereas, for the projectile, we set 3500 kg m⁻³. The mass of the Long March 3C third stage is poorly known. Therefore, for this projectile, we adapt the dry mass of the third stage of the Long March 3A, m = 2800 kg. ⁷ This is somewhat lower compared to the Falcon 9 upper stage, 4000 kg. The impact velocity of WE0913A we infer by means of our dynamical modeling, 2600 m s⁻¹. These input parameters yield 677 kg of mass ejected from the Moon at velocity V_ej ≥ 610 m s⁻¹. Such an ejecta cloud could become apparent in ground-based observations of the Moon shortly after impact on 2022 March 4. Note, for the case of lunar escape velocity, V_ej ≥ 1680 m s⁻¹, the ejected mass is reduced to 195 kg. It is worth noting that a lower bulk material density of the projectile somewhat decreases the ejected mass. For instance, at 1500 kg m⁻³, the ejected mass is about 16% lower compared to the case of 3500 kg m⁻³.

Finally, we estimate the apparent magnitude of the ejecta cloud. We emphasize that the submicron and micron-sized dust particles appear to be the most efficient at light scattering and, hence, their contribution to the light-scattering response is crucial (Zubko 2020). We set 10% of the ejected mass to particles with diameter of 1 μm and compute their apparent magnitude. It also is worth noting that the impact is expected to happen shortly after the new Moon. On this epoch, dust particles appear in ground-based observations at a phase angle of nearly 180°. In other words, the ejecta particles will be observed in the forward scattering. In this regime, the apparent brightness of the micron-sized particles is 5–6 mag stronger compared to side scattering or backscattering and, also, it is hardly affected by the material absorption (Zubko 2020). For instance, in the V filter of the Johnson photometric system, the absolute magnitude of an agglomerated debris particle having size of 1 μm is ∼58.35 mag (Zubko 2020). At V_ej ≥ 610 m s⁻¹, their population is 1.57 × 10¹⁷ particles; whereas, at V_ej ≥ 1680 m s⁻¹, 4.51 × 10¹⁶ particles. In ground-based observations on 2022 March 4, these particles should be visible as a cloud with brightness of ∼3.8 mag and 2.4 mag, respectively. Furthermore, removing the V filter could increase the total brightness of the cloud by ∼1 mag, making its detection possible with the naked eye or small binoculars, even in the evening dawn.