Title: The Apogee Podcast – Lunar Flash during Lunar Eclipse!
Original Paper: https://arxiv.org/pdf/1905.04487.pdf
Description: Cosmic discusses the MIDAS system for monitoring flashes and meteor impact on the Moon from small Earth-based telescopes, and observations and measurements of such an event during the lunar eclipse that took place on 21 January 2019.
Bio: Cosmic (aka Matt Cheselka) is an independent research astronomer and space musician.
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Hello! This is Cosmic, and welcome to the Apogee podcast! The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal. Sometimes topics within these articles send me off on temporary tangents, but they’re always relevant to the overall topic. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.
If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.
The music you hear in the background are my own compositions, which are available upon email request. I hope you enjoy listening to space music as much as I do.
The apogee for this podcast will take place tomorrow, 26 May 2019, at 13:28 UTC. The lunar distance at that time will be 404133 km, which is 443 km closer than last apogee on 28 April, and 415 km closer than the next apogee on 23 June. This is the 2nd closest apogee of 2019, with the closest one of this year having occurred on 9 January.
Ok, on with the paper!
Speaking of the moon, there was an incredible lunar eclipse on 21 January 2019 from 03:33 to 06:50 UTC. For those of us in North America, this was the evening of 20 January. I hope you were able to witness it! A lunar eclipse happens when the Earth is directly between the Sun and the Moon, so that the Moon moves through the Earth’s shadow over a period of a few hours. This particular lunar eclipse was viewed by 100’s of thousands, if not millions of people from around the world live on the internet. At 04:41:38 UTC — about 21 seconds after the totality phase of the eclipse began — a sudden small bright flash was seen on the surface of the moon. Many people saw it right away, and many more saw it later when it was pointed out to them.
Although this has yet to be independently confirmed via an observation of the surface of the moon with the Lunar Reconnosance Orbiter (LRO), it is assumed that this bright flash was a meteor impact event.
For about the past ten years, a team of astronomers from Spain have been operating MIDAS – the Moon Impacts Detection and Analysis System. This system of small telescopes and sensitive multi-wavelength CCD video cameras observes the dark portion (the night side) of the Moon looking for flashes, which are these likely impact events. Because these flashes are usually faint, they can only be seen on portions of the moon that aren’t lit by the sun. There’s only a limited amount of time during the moon’s waxing and waning period when these flashes can be observered from Earth. Weather permitting, the MIDAS team observes the moon when it is between 5% and 60% illuminated (95% – 40% dark). This corresponds to about 10 days per month. So when an fairly rare opportunity comes up to allow MIDAS to observe an entire dark hemisphere of the moon — even for only a couple hours — it’s something worth doing because maybe a flash event will be recorded.
This is exactly what happened during the 21 Janaury lunar eclipse!!! The MIDAS team was at the right place at the right time, and was almost totally prepared to take full advantage of the situation. According to their paper (published in the Monthly Notices of the Royal Astronomical Society, 29 March 2019), the MIDAS team declares “This is the first time ever that an impact flash is unambiguously recorded during a lunar eclipse and discussed in the scientific literature, and the first time that lunar impact flash observations in more than two wavelengths are reported.”
Of the three MIDAS sites, only one of them (Sevilla) had clear skies at that
time. At this location, seven telescopes were used to observe the event: three 0.36m Schmidt-Cassegrain telescopes, two 0.28m Schmidt-Cassegrain telescopes, and two 0.1m refractor telescopes. One of the 0.28m telescopes used a Johnson-Cousins ‘I’ filter, while the two refractors used color cameras (R, G, and B). All other telescopes were unfiltered and looking at various portions of the Moon (the refractors could see the entire moon). The flash itself was observed with one of the 0.36m telescopes, and one of the 0.1m refractors. It’s the RGB color camera data from the 0.1m refractor that allows the authors to claim that the flash was observed in more than two wavelengths.
From this data, the authors were able to determine (or at least make some
decent guesses about) the likely source of the impactor, its kinetic energy and mass, temperature of the impact plume, and crater size.
The source of the impactor can be estimated by carefully measuring the location and time of the flash / impact, which the MIDAS team determined to be 29.2 degrees south, 67.5 degrees west, and 04:41:38 UTC. In this case, the impactor didn’t correspond to any known meteor shower. It was decided, therefore, that this was a sporatic meteor. The flash was unusually bright. Most of the lunar impacts observed sofar have been around 8th magnitude. This one was was measured at magnitude 4.2.
The kinetic energy and mass can be calculated knowing the brightness of the flash, and the total time the flash was visible. Since they’re able to measure the total amount of energy emitted as light, the total impact energy can be calculated taking into account the ‘luminous efficiency’ value (the fraction of the kinetic energy which is emitted as visible light as a consequence of the collision). Based on this result, the mass and impact velocity can be determined (45 kg, 17 km / s, respectively). Given a range of densities, reasonable guesses for the diameter of the impactor can be inferred (29 – 66 cm).
Color information taken from the 0.1m refractor telescope was used to compute the Johnson-Cousins B, V, and R magnitudes. From these, fluxes can be calculated for each wavelength band, which can then be translated to flux densities. Assuming that the flash behaves like a blackbody, a temperature of 5700 degrees kelvin can be inferred from these flux densities. That’s as hot as the surface of the sun!
An estimation of the size of the resulting impact crater can also be made. In this case, the authors employed two different models and compared their results. In one model, the densities of the impactor and target, the kinetic energy, and angle of impact are used. The second model is slightly more complex in that it takes into account surface gravity, and computes kinetic energy using the same parameters as the first model, but in a different way. Crater diameters from 10.4 meters to 16.8 meters are reported. This is not an insignificant or trivial impact event, and an impact crater should be easily visible to the Lunar Reconnosanse Orbiter.
I look forward to the LRO imagery confirming the impact location and crater size. Knowing the crater size will help improve the models. MIDAS is one of at least a couple of groups in the world, like NASA and ESA, monitoring the moon for flashes or impacts. I actually talked about the NASA system back at my first podcast for 365 days of astronomy on 8 May 2014 – so go check it out!
Determining the impact frequency, impactor mass and velocity, temperature, and other characteristics of lunar impacts will allow us to better understand the Earth-Moon environment.
Thanks for listening! Until next apogee, I bid you Peace.
End of podcast:
365 Days of Astronomy
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