The detectability of lunar impacts in the near infrared

Abstract

Impact crater scaling laws are used to predict the diameter and amount of melt produced in impacts on quartz sand of projectiles with mass from 100 gm to 100 kg and velocity from 20-40 km/sec. A one dimensional cooling model incorporating conduction, change of phase, and radiation is used to predict the cooling history of the crater. Several possible initial distributions (exposed to surface, shallow or moderate burial by cooler material) of the impact melt are considered. Infrared spectra are calculated for the modeled surface temperature distribution at several times during the cooling. The impact IR signature is prominent in the wavelength range 1.5-6 μ against the lunar nightside background. The optimum wavelength for detecting the smallest accessible impact is between 3 and 4 μ. It is found that the maximum signal strength is dependent on the initial distribution of melt as well as impact energy. The duration of the signal above a minimum detectability threshold is proportional to impact energy with only modest dependence on the initial melt distribution. Basic design requirements and capabilities for sensors to detect the impact signature from lunar orbit and earth orbit are considered. The lunar orbiting sensor can detect impacts as small as ∼50 gm. With a field of view covering ∼640000 km² a rate of approximately 2 events per week might be expected. An earth orbiting sensor could detect impacts of ∼100 gm at the sub earth point. Larger impacts could be detected closer to the lunar limb. Monitoring a large fraction of the nighttime hemisphere visible from earth orbit the observable event rate is similar to that from the lunar orbiter. Ground based observation at wavelengths between 2 and 2.4 μ could detect ∼2 kg impacts with an event rate estimated at 1 per 400 hours observing time.