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There is evidence from previous missions for enhanced hydrogen deposits at the poles of the Moon. Due to limitations in the data sets that measured this hydrogen, we still do not know the form of the hydrogen, that is, is it in the form of water (H2O) or some other hydrogen-bearing compound, such has hydrated minerals or hydrocarbons. One possibility is that the hydrogen is indeed in the form of water (as ice, ice-coated dust grains, and/or hydrated minerals) and it is accumulating in craters whose floors are in permanent shadow, and so very cold: only 60 °C (110 °F) or so above absolute zero. LCROSS will make the first definitive measurements for water within a permanently shadowed crater. Using a suite of instruments, including infrared and visible spectrometers and cameras, LCROSS will be able to identify water (ice, ice-coated dust grains, or vapor) in the impact ejecta cloud and, hopefully, shed some light (pun intended) on this question. [Top]
Why we care if water exists at the poles of the Moon can be organized into two categories: Exploration and Science. With respect to exploration, an in-situ source of lunar water would prove greatly useful for establishing significant local infrastructure. If one wants to “live off the land”, water is a key ingredient, but even if one simply wanted to make a semi-permanent establishment more versatile and efficient, locally derived water might prove invaluable. There are a variety of reasons for establishing infrastructure at either lunar pole, including a more benign thermal and illumination environment: you can find spots where it’s light nearly all the time. Having a possible water resource may be the coup de grâce!
Looking into the future, when missions to other parts of the solar system and beyond might depart directly from the Moon – whose 1/6th gravity allows use of much smaller rockets to go the same distance as a mission from Earth – a means to produce rocket fuel on the Moon could make a more ambitious space exploration program feasible at lower cost. While the Moon’s surface is full of oxygen in various mineral forms, hydrogen is the other key element that could make rocket fuel production practical on the Moon.
A byproduct of this prospecting for possible exploration resources is effective and exciting science. Understanding the source of the hydrogen concentration will help us understand not only the lunar environment, but also the environment of the inner solar system. Depending on the form of the hydrogen and its concentration, we will be able to say something about the flux of comets and asteroids, and possibly even the position of the lunar poles through time. Even the impact of LCROSS itself, and the formation and evolution of the ejecta cloud and a possible lunar “exosphere,” (an atmosphere that has no air) will tell us more about impact cratering, one of the most dominant physical processes at work in shaping the planets and moons of the entire solar system and galaxies beyond. [Top]
It is very important for us to have a reasonable expectation of what the impact will look like, how it will evolve in time, and how much material it will excavate. These expectations help us design the mission and select instruments. The impact itself has been studied numerically, for example, with sophisticated hydrodynamic computer models, and empirically, with high-velocity projectile guns fired into a simulated lunar surface. Similar efforts were used to understand the Deep Impact impact (in fact, a number of our Co-I’s were involved with planning for and observing Deep Impact). These efforts are not only critical to planning the mission, but also to analyzing the data. Indeed, what was predicted for Deep Impact and what was observed to happen were quite different. But that is exploration and science! We learned, and are still learning about, comets from the Deep Impact mission. It will be similar for LCROSS. While we have a bit of an advantage over Deep Impact in that we have a better understanding of the Moon and its physical properties compared to those of a comet, that fact that we are purposely targeting a part of the Moon we can’t see (it being permanently shadowed) does keep things interesting!
In addition to the observations made from the LCROSS spacecraft, we are organizing efforts for both ground- and space-based assets (telescopes, cameras, spectrometers) to observe the impact. As the impact date nears, LCROSS will provide the exact impact location and time to the science community and public via the web. The goal is to muster as many eyes and instruments as possible to observe, record and analyze the impact event and the following ejecta and vapor cloud. [Top]
Countless objects have hit the Moon since its formation (in fact, the Moon’s formation was quite possibly the product of a very large impact to the Earth). Most of the large craters one sees on the Moon resulted from large asteroid or comet impacts early in the history of the solar system; however, numerous impacts by much smaller objects continue even today. These smaller objects range in size from smaller than a grain of sand to a basketball. Most of the shooting stars one sees at night are indeed small grains to rock-sized fragments entering the Earth’s atmosphere. If they are hitting the Earth’s atmosphere, you can bet some are also hitting the Moon! (which has no atmosphere to burn them up or slow them before they reach the surface). While these objects are small, due to their high velocity (~40 km per sec), even these relatively small objects pack a considerable punch! The energy associated with the LCROSS impact is about 6 billion Joules (1 Watt = 1 Joule per sec, so the energy of LCROSS is what you’d get from 100 million 60 Watt light bulbs in a second). A 10 kg (about 22 lbs) meteorite would impact with about 8 billion Joules of energy. There are probably several of these size objects striking the Moon every few months (some have recently been imaged by small ground-based telescopes using high-speed-film cameras). So, in short, the Moon has been, and continues to be, pummeled by objects of all sizes over the last 3.9 billion years, many of them having energies many, many times greater than that of the LCROSS impact.
The LCROSS impact will not be noticed by the Moon and only noticed by those on Earth with telescopes trained on the impact site. What makes the LCROSS impact special, compared to the ongoing, natural barrage, is that we control the LCROSS impact to occur at a precise place and time. allowing us to sample a specific piece of lunar real estate and be in position to monitor it. [Top]
The LCROSS impact will have the same effect on the water (if it is indeed there) as any other object that might naturally impact it. Most (>90%) of any water that is excavated by LCROSS will most likely return to nearby “cold traps”. The LCROSS impact is actually a slow impact and, thus, most of the material is not thrown very high upward, rather outward, adjacent to the impact site. Of the water that does get thrown upward, much of it will actually return to the Moon and eventually find its way back to the dark, cold craters. This is actually one possible way that the water was supplied in the first place: it was deposited following the impacts of comets and asteroids.
There is about 12,500 square km of permanently shadowed terrain on the Moon. If the top 1 meter of this area were to hold 1% (by mass) water, that would be equivalent to about 4.1 x 1011 liters of water! This is approximately 2% the volume of the Great Salt Lake in Utah. The LCROSS impact will excavate a crater approximately 20 meters in diameter, or about one-trillionth the total permanently shadowed area. It is safe to say the LCROSS impact will not have a lasting effect on lunar water, if it does indeed exist. [Top]
The LCROSS impacts will have a similar effect on the lunar exosphere as a “natural” impact might have. The ejecta cloud from the LCROSS impacts will quickly (in about 5 minutes) settle back to the surface. Very little to none of the ejecta has the potential to go into lunar orbit as the initial ejecta velocities are ballistic (what goes up must come down!) and without a sufficient lateral component to match orbit velocities (approximately 1.8 km per second). To enter orbit, the impact ejecta would need to have its ballistic flight path altered and still maintain sufficient velocity to remain in orbit. The only way this can occur in an impact the size of LCROSS is in the case of a mid-flight collision, but as the amount of material traveling with sufficient speeds is so small (90% of the material is traveling slower than 200 meters per second) the chances of this occurring, and occurring in just the right way as to create a stable orbit, are exceedingly small. As the LCROSS impact velocity is relatively slow, compared to natural impacts, impact temperatures will be to low to vaporize much of any material (rock or rocket). Any volatiles that are sublimed during impact or when the eject reaches sunlight and warms, will also return to the lunar surface in a few days, or by lost to space through usual escape processes. [Top]
The Centaur upper stage and LCROSS spacecraft will not vaporize or disintegrate during impact, but will mostly crumple and break apart. Most of the material will be warmed to several hundred degrees just through the compaction process during impact. Any unspent rocket fuel (primarily hydrazine) will most likely “flash”, or burn, at impact. With regards to the Centaur we are being very careful to purge as much of the rocket fuel (hydrazine, liquid oxygen and hydrogen) as possible so as not to contaminate our measurements. The bulk of the rocket material will stay within a crater diameter of the impact site (about 20 meters) so mess we make will be fairly confined to a very small area. It should be noted that there have been at least 20 impacts of terrestrial spacecraft into the moon. Some on purpose, others not. Some of these impacts were of the very large Saturn IV upper stage (about a fact or 8 times larger than LCROSS!) or the comparable Lunar Module. In the cases when we've known where they where planned to impact, we have been able to identify a few of the impact craters. In the cases when we had only approximate knowledge of where to go looking for the fresh crater, we could not find it. The point being, the mark we make on the moon will be very hard to notice – even when looking for it. Lastly, constantly bathed in cosmic rays and solar particle radiation, the LCROSS impact sites are be no less hospitable a place following the impacts. [Top]
The impact crater that will be made by the LCROSS Centaur impact will be around 20 meters in diameter. Impacts that generate crates this size have occurred though out the history of the moon. Depending on the exact age of the crater floor, one can find 30-100 craters of a similar size to the LCROSS crater per square kilometer. LCROSS is just one more for the record book. The only thing special about the LCROSS impact, relative to all the other natural lunar impacts, is that we know precisely where and when the impact occurs. In terms of size, shape and modification of the terrain, there is little difference (except perhaps the LCROSS crater being a bit more shallow, due to its lower density and slower velocity compared to natural impacts).
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