May 22, 2008
Hitting the Moon
image from Science@NASA
Every day 33 tons of comet and asteroid fragments strike the Earth, mostly burning up in the atmosphere. A similar quantity of space debris hits the lunar surface at full cosmic velocities of tens of kilometers per second. Amateurs have recorded a number of such impact flashes, and since 2005 systematic and continuous observing has been been conducted by the Meteoroid Environment Office group at the Marshall Space Flight Center in northern Alabama. They now have images from two or more telescopes of 103 lunar impact events. The first thing that sticks out is how non-random the distribution of impact locations is. No impacts near the poles? And nearly none in the central-eastern face of the Moon? This seems like an observational selection effect, and MEO's description of their observing procedure suggests explanations. They only observe the dark part of the Moon to maximize the detectability of the impact flash against the surrounding terrain. And observations are only made between New and 1st Quarter and between Last Quarter and New Moon, when at least half of the visible surface is un-illuminated. This limits observing to only 10-12 nights per month, and the center area is infreqently imaged. The much greater number of impacts on the western half of the Moon, compared to the eastern half first suggests that its more pleasant to observe in the early evening than in early morning, but that doesn't make sense because this is, I think, an automated system. Perhaps clouds are more common in the early AM than in the early evening. Or perhaps there is some orbital mechanics reason for the western half to record more impacts (e.g. approach direction of meteor streams?). Why no impacts at the poles? I bet its because MEO maximizes coverage of dark lunar surfaces by centering their videocams near the equator. But a wide enough field of view would get the entire un-illuminated half. Perhaps someone from the MEO team will confirm or correct these guessed explanations for non-random distribution. In any case its remarkable work, inspired by earlier amateur successes.
Here is a list of observed impacts - I have not yet discovered why the impact numbers have different colors.
Amateur observations of lunar impact flashes
R. Suggs, et al., 2008, The NASA Lunar Impact Monitoring Program, Earth, Moon Planets 102:293-298.
Yesterday's LPOD: Apennine Snows
Tomorrow's LPOD: Virtually Perfect
1) Are these impacts big enough to see the crater they made with an amateur telescope? Just wondering. Aethrae, Andrew Martin SFO
(2) Chuck--Perhaps the colors are used to identify different years? I think they need a better system to ID impacts on an image of the Moon. The numbers placed on top of numbers that are on top of more numbers, etc. is difficult to read and confusing.
It seems there are different tonnage estimates for objects that impact the Moon on a daily basis. I've read an estimate of 10 metric tons of micrometeorites striking the Moon each day. You mention an estimate of 33 tons. Is this estimate larger because it includes objects larger than micrometeorites?
I imagine that measuring things on the Moon must be difficult. Every so often I notice a conflict between sources regarding height, depth, and diameter of craters, mountains, etc. However, I've also noticed the conflict in other published astronomical information--for example, one source states the solar system is 26,000 light years from the galaxy center; another source states it is 30,000 light years. (??)
I suppose in cases like this, the word "about" could come in very handy. I use it a lot when I describe things.
(3) Why the Poles? I think Chuck you may be right on the suggestion of the videocams being centered on the equator regions; as their seems to be a clear cut-off point above both these high latitudes. Or, I'm wondering, as wandering objects pass along these high lats.,, their likely occurance of striking the surface is less here because of the much smaller percentage of land-space available as opposed to the much larger land space open at the central regions??? John -- www.moonposter.ie
(4) Bill: Carefull, 26,000 light years and 30,000 light years can be equivalent in scientific terms, it depends on the error bar. One measurement could be 26,000 light years + 2,000 ly and the other 30,000 ly + 5,000 ly. Since there is overlap between the uncertainty of both measurements, both can be considered correct (unless the actual distance was 24,300 ly, then the first one would be correct and the second wrong...)
Rob Suggs, Space Environments Team Lead, provides these clarifications:
You have the bit about field of view correct. And it is orbital mechanics that accounts for the asymmetry, not observing preference. It is VERY hard to drag out of bed to make those morning observations but we actually have slightly more good quality hours of morning observations than evening, but they are nearly equal
The asymmetry is, we currently believe, because the apex sporadic source meteoroids (the ones coming at the Earth-Moon system head on in their orbit around the sun) are visible from the portion of the Moon we observe in the evening sky but are hitting the lunar farside during the morning observations. That leaves only the anti-helion and toroidal sources to account for the morning sky impacts. Although the apex source is the weakest, it is also the fastest with an average speed of 47 km/sec. The other sources average around 25 km/sec. Since the kinetic energy goes as the square of the velocity, and the luminous efficiency has some TBD velocity dependence, we preferentially see the fastest ones. We discussed this a bit in our recent paper in Earth, Moon, and Planets and are currently investigating the effect in detail for publication later this year.
The FOV is defined by a standard "1/2 inch" video chip (Watec H2 or StellaCam EX) operating with an effective focal length of about 1 m for all of our scopes. This gives a FOV with the long dimension around 20 arcminutes and we place this dimension parallel to the Moon's central meridian. We try to mostly fill the FOV with earthlit Moon but we need some sky visible so we can record stars which we use for photometric calibration. The left hand side of the map pretty clearly shows our FOV. We don't look at the poles or the region around the central meridian.
The yellow numbers are sporadics and the other colors are for different showers. The candidates table indicates that identification but we just didn't add it to the figure caption.
Rob mentioned that this is a problem that amateurs can contribute to - there are instructions on the website. It's possible for amateurs to monitor the polar regions to discover impacts that the MEO program doesn't look for.