Observing Lunar Impact Flashes
Every astronomer is familiar with shooting stars in our night sky, and knows that these are the result of sand grain, or larger mass, space debris ploughing into our atmosphere at tens of kilometres per second, to be consumed by the heat from friction against air molecules. However, have you ever wondered what happens when the Moon, or indeed any solid surface planetary body, intersects the path of the meteoroid? Obviously a small impact crater will be formed. The bulk of the kinetic energy (K.E. = ½mv2) is converted into mechanical and heat energy by excavating a crater, generating a minor moon-quake from the impact thud, and chucking ejecta over typically a distance of up to ten times the crater diameter away. However just under 1% of the K.E. from gram sized, or larger, impactors gets converted into a flash of light which may be seen by Earth-based telescopes. This effect was first confirmed by several amateur astronomers videoing the dark lunar earthshine during the Leonid meteor shower in 1999. Over 400 impact flashes have since been confirmed, mostly by a lunar impact flash observing programme at NASA’s Meteoroid Environment Office at the Marshall Space Flight Center, though an ESA funded observatory team is also now conducting searches from Greece.
If you choose to take part in the impact flash project, then observations that you can make for the BAA Lunar Section, will help to refine our knowledge of the modern era impact rate in the vicinity of the Moon. This is scientifically useful to know about for several reasons: Firstly it can be used to compare against impact rates detected by past and future lunar seismometers. Secondly lunar space mission planners need to have an improved understanding of the risks from meteoroids, and potential damage from associated shrapnel-like ejecta. Lastly it can tell us about whether the present-day era impact rate varies, and how this might have affected crater dating estimates from the past.
Given that there is already an extensive NASA programme, and another one in Europe, how can we contribute? Firstly, we are not constrained by professional astronomer research grants, and as volunteers could in theory bring many more telescopes and combined square metre mirror aperture on-line, than professionals can. Secondly, amateur astronomers can do this work in conjunction with the lunar occultation programme, killing two birds with one stone. Finally, we can experiment with different observational techniques, to glean new information about impact flashes that NASA/ESA does not have time for.
So what are the requirements? Ideally a 20 cm, or larger, aperture telescope, with focal ratios of f/6 or smaller – this optimizes the number of detections by recording as large an area of the Moon as possible, and gives an improved signal to noise ratio, so fainter magnitude flashes can be recorded. However, we never say never to slightly smaller scopes, or higher f/numbers, so long as you can record earthshine. Focal reducers have been shown to be useful in this respect. Tracking telescopes are ideal, but impact flash observing using a Dobsonian is also feasible too. As for cameras, the Watec 902H, or equivalent, seems to be the preferred choice as they can video 10th magnitude stars, or fainter, in real time at 25 frames per second (or 30 frames per second on an American NTSC TV system) and it has a wide spectral response from the blue up into the near Infra-Red. However, any camera that is capable of these specifications, or even at slower rates of 10 frames per sec, is suitable. Video should be recorded as ‘Raw’ monochrome, if possible; otherwise Video 8 camcorder style (each frame is effectively a JPEG image) AVI can be used, but certainly not MPEG as this destroys too much information.
Software exists for detecting impact flashes, to avoid the tedious need to watch your video in slow motion looking for brief < 0.1 sec duration flashes of light. You can either use Lunarscan by Peter Gural, and hosted on the following download site: https://www.nasa.gov/centers/marshall/news/lunar/observing_schedule.html , or a prototype version of a Europlanet program called ALFI (European Union’s Horizon 2020 research and innovation programme, under grant agreement No 654208), available by emailing the coordinator email@example.com or eventually from this website: http://users.aber.ac.uk/atc/alfi.htm . Both software have their advantages and disadvantages, and what one might miss as a flash, the other one might detect – so it is sensible to run any earthshine recordings through both. Impact flashes observed from Earth are typically many hours apart, so detection rates are low. This is why it is important to combine this with lunar occultation work, or other aspects of the lunar section observing programmes.
In terms of what we can do differently to the NASA program:
1) We would really like to bring together white light impact flash light curves, from three or preferably more simultaneous observations, as this can tell us which aspects of light curves published are real,or due to scintillation and seeing. It will also improve the signal to noise ratio of the images, and allow us to improve on time resolution.
2) Making simultaneous recordings in different waveband filters, can allow us to determine the blackbody temperature of each flash.
3) Simultaneous higher angular resolution videos of impact flashes can tell us the spatial extent of these flashes, as there are four instances where elongations or shapes have been attributed to impact flashes.
A white light image of an impact flash near Wolf crater, recorded at 17:47UT on 2017 Jan 01 by Aberystwyth University. Images are 1/50th second apart. This was also observed, and discovered, by Stefano Sposetti (GLR) in Switzerland.
To date, only one confirmed lunar impact flash has been captured from the UK, let us start adding more to this total and do some real science.