The Importance of Lunar Landing Ejecta and Emissions
There are many exciting opportunities to be found as gradual progress is made to return humans to the Moon to stay in the coming years. With these opportunities there are also various challenges to overcome related to an increasing frequency of landed missions. When spacecraft land they emit large quantities of gases and displace significant amounts of regolith. Because of the Moon’s low gravity and thin atmosphere relative to the Earth, both of these actions have magnified effects.
In the case of lunar regolith blasted away from launch and landing sites due to exhaust forces, the thin atmosphere and lunar gravity environment leads to long distance and high velocity debris displacement. In a 2011 NASA recommendation document, an exclusion radius of 2 km (around 1.2 miles) was recommended around heritage Apollo lander sites. This distance is distinctly similar to the roughly 1.8 km (around 1.1 miles) distance to the lunar horizon from any point on the surface.
Because of the Moon’s low gravity and thin atmosphere, the small particles traveling at high velocity will fly over the horizon and above an exclusion zone. Larger sized ejecta travel at lower velocity and impact before the horizon. Mid sized particles fall in a range between this, with some impacting beyond the horizon after being gradually brought near to the surface with lunar gravity. Despite this, these medium range particles make up the smallest fraction of the lunar regolith so their impact contributes minimal damage.
There are multiple considerations to be made about how the gases emitted during the launch and landing of spacecraft on the Moon effect the Moon’s thin near-vacuum atmosphere. This effect is called vacuum pollution and it occurs when the Moon’s natural rate of atmospheric gas loss is exceeded by the injection rate of gas from non-natural sources.
In the example of Apollo, each mission released ~10,000 kg (around 22,000 pounds) of rocket exhaust into the lunar atmosphere. The natural rate of atmospheric loss on the Moon has been measured as between 1-10 g/s, or around 7 g/s (about 0.015 pounds per second). With this rate of loss, the gases released from a single Apollo-sized mission take about two weeks to dissipate.
The danger in this is that the Moon’s atmosphere can switch from being photoionization loss dominated to thermal escape dominated, with a sufficient injection of gas. The source injection rate above which this effect could be seen is about 60 kg/s (about 132 pounds per second). The reason this switch is important is that in a thermal escape dominated atmosphere, pollutant gases take hundreds of years to dissipate, whereas in the current photoionization loss dominated atmosphere those gases are lost in a matter of days.
Importantly, the atmospheric changes by these gas emissions would also remove some of the thin atmosphere conditions that currently make the lunar surface excellent for astronomical observation. The changes necessary for that only require gas injection exceeding 1 kg/s (about 2.2 pounds per second).
With spacecraft landings on the Moon promising to increase in frequency in the near future, these concerns must be addressed. With multiple countries and companies planning missions, coordination and cooperation will also be important. There is also a big challenge ahead in both preventing and managing damages.
For dealing with lunar ejecta, we must employ both exclusion zones and durable solutions meant to deal with the issue. For the issue of gas injection we must quantify the potential dangers and take measures to avoid unintended harm, both by limiting emission and employing methods for gas removal where practical and necessary.
Interestingly, all of the problems outlined with lunar regolith ejecta and lunar atmospheric gas emission also provide opportunities for space resources. The creation of landing pads that limit ejecta during landing and takeoff using in-situ resources, and the management of ejecta in other ways to prevent damage on the Moon is a viable future service. Likewise, there is room to develop methods to reduce gas emissions, or capture gas emissions, either for usage or as part of a potentially needed atmospheric management strategy in the long term.
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