Meteor Hazards for a Lunar Settlement
The threat represented by meteorites to a lunar settlement is real, but not insurmountable.
Direct Meteor Impact
Meteor strikes that represent a danger to a lunar settlement will be rare, but real. The size and danger of the strikes follow a power law such that there will be many small strikes for every major strike. Over the lifetime of a lunar settlement, small meteors are a greater danger than the very rare large ones. Also a large object on the surface of the Moon, like a solar collector, is more likely it is to be hit than a small object like a man.
Meteors arrive at the surface of the Moon with very high energies and there is no air to slow them down. The result is that virtually the entire mass of the meteor is vaporized on impact along with some lunar material. The white hot and expanding vapor then generates an explosion that sends material flying out from the crater, called ejecta. The material from the meteor itself cools as the vapor expands and condenses into very small grains and spheres that eventually get gardened into the regolith.
It is much more realistic to think of a meteor hitting the Moon as an explosion rather than a bullet. They will not shoot holes in things, they blow them apart.
A meteor can approach the Moon about equally from any point in the sky. Lunar terrain does provide some screening near the horizon so meteors are more likely to come from above. Only the few meteors that come in at very low angles can blow elliptical craters and such craters are rare.
The Earth and Moon periodically cross the orbits of burned out comets. These orbits contain ribbons of dust particles spread out over the entire ellipse. The individual particles are mostly of rocky minerals from the size of a grain of sand up to perhaps that of a pea. Some ribbons within the orbit contain much higher particle counts than others. The distribution is both uneven and lumpy.
When the Earth crosses one of these old comet orbits a great many small shooting stars and even fireballs, called bolides, light the night sky. This effect is called a meteor shower. The shooting star trails all appear to radiate from the same point in the sky and the shower is named after the constellation containing their point of radiation.
The major showers occur at specific times of the year and are at peak for a day or two. The best time to watch for meteors is between midnight and dawn. The events can easily run up to tens of visible effects a minute. You should watch the sky between the named constellation and the horizon. The effect can be awe inspiring and very romantic and will certainly give you a real appreciation for the dangers of meteors to a lunar settlement.
Some of the major showers as experienced on Earth are:
- Quadrantids in January
- Lyrids in April
- Eta Aquands in May
- Delta Aquands in July
- Perseids in August, a large shower
- Orionids in October
- Tourids in November
- Leonids in November, a large shower
- Geminids in December
A major shower occurs about every 30 years. Abraham Lincoln witnessed a major Leonid shower in 1833 from Illinois and described the experience in a speech many years later. The sky was lit up all night with thousands of shooting stars, including many bolides, often with sonic reports like distant cannon. It was thought by many, but not by Lincoln, to be of major religious significance. The end of the world did not come, but such a shower would be a major risk to a lunar settlement.
There was a major Leonids shower in 1998 that disrupted ISS operations. A Russian landing had to be delayed and the crew simply waited it out. Fortunately there was no major damage.
Different locations on the Moon will have different risks from the various showers. The place of the Moon in its orbit will also have an effect. The risk is greatest if your location is exposed to the leading direction in Earth’s orbit at the peak time. A lunar settlement will need to generate a computer model of all meteor showers as seen from its location and provide alerts of major events.
Fortunately the timing of these showers is well known. Known meteor showers may require curtailment of outside manned activities and the placing of equipment in a safe mode for several days a year.
Outside of the showers, there is a lower level constant background of meteors arriving at the Moon and Earth from all directions, throughout the year. These meteors are called "sporadic".
These cannot be predicted and only rarely will they be spotted far enough in advance to provide any warning at all. They represent a risk to large exposed structures like solar power equipment but only a small, acceptable risk to small targets like a person. Again, think in terms of small explosions and not bullets.
The 1178 Event
A major celestial event was reported by a number of monks on a warm summer’s evening in Canterbury, England in 1178. The event was taken by the monastery to be significant and sworn testimony of four witnesses was entered into the monastery’s official chronicle.
The Moon was only a few days past new and would have appeared as a narrow bright crescent with a darker area filling in most of the Moon’s disk. The reports describe a flash and bright streams, like golden hair, crossing the dark areas of the Moon.
The report is now taken to be credible, but the exact cause is under debate. The leading theory is that it was a major hit by a meteor on dark area of the Moon that threw ejecta up into the sunlight. This would be largest lunar meteor hit in human history and gives us a good idea of how often lunar settlers should expect a really big one.
The problem is that this event was not reported elsewhere; not in Europe, not in India, and not in China. All of these societies would have recorded any significant lunar event they witnessed. This puts a limitation of the size and time extent of the event. The visible effects must not have been observable from Earth to the unaided human eye for more than a few hours. More important, there was no report of a major meteor shower in the days that followed.
One candidate for the 1178 hit site has been put forward. It is Crater Giordano Bruno which is a very new crater in the right location. The problem is that Crater Bruno is too big, about 22 km (14 miles) in diameter. An event that large would have launched thousands of tons of lunar material on paths that would take it to Earth over the next few days. The resulting meteor showers would have been awe inspiring and lasted for days if not weeks. It could not have been missed anywhere on Earth.
Another possibility is that the monks saw a large bolide, or fireball, in Earth’s atmosphere exactly in front the Moon as seen from Canterbury. The visible effect would have appeared similar to that reported, but would have been visibly associated with the Moon from only a limited area. A sonic report would be common from such an event, but none was reported.
The event could still have been a meteor strike on the Moon, but the resulting crater must simply be very much smaller the Crater Bruno. Probably no more than a few hundred meters. Such an event would be short in time span, limited visually on Earth, and would not create a notable meteor shower. There are plenty of small candidate craters, but their age is very difficult to judge without a visit.
No matter what theory wins in the final analysis, a major meteor strike anywhere on the Moon would be a major problem for a lunar settlement. Most of the danger comes from the ejecta rather than from the initial hit.
Lunar meteor strikes large enough to throw dangerous ejecta over significant areas are very rare, but they are still a major danger to a lunar settlement. The ejecta will affect a much larger area than direct meteor hit and can do great damage.
The effects of lunar ejecta are completely different from similar explosive debris on Earth. First off, much of the energy in an Earth explosion is carried away by the air pressure wave which does not occur on the Moon. Second, at any significant distance from an Earth explosion, air resistance slows the flying debris so that the major danger becomes one of being crushed or buried.
On the Moon there is no air to slow the debris or to carry off energy as sound waves. The individual ejecta rocks will come at you hard and fast. They will not be energetic enough to vaporize on impact like a meteor, but they can act very much like shrapnel from a hand grenade, even at a great distance.
The high speed also means there will be little warning. The most reliable warning instrument for events over the horizon would be a seismograph that reads sound waves traveling through the rocky body of the Moon. The time between the first strong seismic waves arriving and the first flying rocks arriving could be as little as a few seconds. People outside may need to quickly dive for cover.
Ejecta will prefer low angles of arrival. It can simply cut you off at the knees. It will also be a major danger to fixed equipment.
Protecting Living Space
The meter or two of lunar regolith proposed for radiation shielding on lunar settlement buildings, (see Architecture as Mole Hills), will also provide substantial meteor protection for the living spaces. Nothing is likely to penetrate that deeply and the explosive force will not travel far in the lose regolith.
Emergency Surface Procedures
People operating of the exposed surface of the Moon will need to have procedures for defensive actions in the event of a meteor warning. These procedures will require periodic practice drills for instant response.
In the settlement and industrial areas, these procedures may require spider holes for people to run into. These might be slit trenches with a roof covered with sandbags. They would require the people to remain in their spacesuits but would provide spare air and power. They would also provide some radiation protection. Such spider holes might also be used in the event of rocket landing or launching emergencies. They could also provide protected places for electronics, communication nodes, power distribution nodes, and robot recharging stations.
In the open field, emergency procedures might involve hiding under a lunar rover or jumping into a small crater. Even a large rock might be better than nothing, if you knew which side to hide on.
Large equipment, such as Sandworms, cannot be brought inside in a meteor shower. The best we will be able to do is to place the equipment in a safe mode to minimize the damage from a hit.
For example, most heavy equipment will include thermal panels to get rid of waste heat. Large versions of these panels usually include a coolant fluid, such as ammonia or Freon, that is actively pumped though tubes. In normal operating mode, a single small hit to a thermal panel could result in the loss of all the coolant. Without the coolant the entire machine could soon overheat and be damaged. The unit could be out of service for a long time and require very expensive repairs.
In the safe mode, the coolant would be drained into a storage tank and the plumbing valved off. The storage tank could be protected with sandbags. The machine could not operate without cooling, but it could take a much bigger hit with minimal damage.
All lunar equipment will have to be designed with a safe mode. There are many possible emergencies where the safe mode will be needed.
The risk to a lunar settlement of damage from a large meteor strike is comparable to living in an Earthquake zone, like California. The risk is there and we must take it into account in the design of the settlement, but it is not one of our Show Stoppers. In fact, the risk of a major hit to a lunar settlement is much lower than the risk of the "Big one" to LA.
The risk of small meteor hits to people and equipment on the moon again is real and must be addressed in both infrastructure design and operational procedures. It is comparable to the risk of storm damage on Earth in that some of the danger is periodic and some is random. This risk is lower than that from hurricanes on the American Gulf Coast.
We can address these risks and live with them.