Saturday 16 March 2019

Investigating a meteoroid impact on the Moon during the 21 January 2019 Lunar eclipse.

On 21 January 2019 the only total Lunar eclipse of 2019 was watched by millions of observers across the Americas, North Africa and Europe. A few minutes after totality was reached (i.e. after the Moon was completely covered by the Earth’s shadow), witnesses across this area began to report a flash on the east side of the Moon, presumably caused by a meteoroid impact. Such impacts are thought to be extremely common on the Moon, which lacks an atmosphere to protect its surface from impacts as the Earth does (indeed a second impact, on the brighter western side of the Moon, was reported a few minutes later by the Royal Observatory in the UK, though this has not been confirmed by other observers), but few are witnessed by quite as many observers, both amateur and professional, as this event, opening the potential for it to b studied in detail in ways that are not possible for most Lunar impacts.

In a paper published on the arXiv database at Cornell University on 28 January 2019, and submitted to the journal Icarus, Jorge Zuluaga and Pablo Cuartas-Restrepo of the Solar, Earth and Planetary Physics Group at the University of Antioquia, and the Sociedad Antioqueña de Astronomía, Jonathan Ospina, also of the Sociedad Antioqueña de Astronomía, Fritz Pichardo of the Sociedad Astronómica Dominicana, Sergio López, again of the Sociedad Antioqueña de Astronomía, Karls Peña, also of the Sociedad Astronómica Dominicana, and Mauricio Gaviria-Posada of the Observatorio LaLoma, present an examination of the 21 January 2019 Lunar meteoroid impact, based upon observations by amateur and professional astronomers in Morocco, the Dominican Republic and Colombia.

The first set of observations come from the Mobile Observatory of the Time and Date Astronomy Portal, which at the time of the eclipse was located at Ouarzazate in Morocco, and which broadcast the entire eclipse on the Time and Date website. Zuluaga et al. extracted six frames from this video, and used them to estimate the duration of the flash.

From left to right, frames of the video taken at Ouarzazate, Morocco by Time and Date mobile observatory. The 21 January 2019 Lunar impact flash, which is visible close to the lower dark limb, appeared in four of the six frames. Zuluaga et al. (2019).

The second observation used was made by Fritz Pichardo in Santo Domingo, the Dominican Republic, who used a Canon T3i DSLR camera mounted on a 8 inch Celestron CPC 800 SchmidtCassegrain telescope to take an image of the eclipsed Moon with a 20 minute exposure, between 4.41 and 5.01 am GMT.

20 minute exposure of the total lunar eclipse at the time of the impact flash taken in Santo Domingo, the Dominican Republic. Impact site is labelled as L1-21-J. Fritz Pichardo in Zuluaga et al. (2019).

The third observation used came from the 25 inch telescope of the LaLoma Observatory at San Vicente Ferrer in Antioquia; the largest telescope in Colombia, with a focal length of 2700 m. This much larger aperture telescope was able to take images of the Moon with a much lower exposure time, enabling Zuluaga et al. to extract a single image taken at 37 seconds past 4.41 am GMT, the exact time recorded for the flash by the Moon Impacts Detection and Analysis System, a Spanish project which has constantly monitored the Moon for such impacts for the past two decades.

Picture of the total lunar eclipse at the time of the impact flash taken the observatory LaLoma, Colombia. Jonathan Ospina, Mauricio Gaviria and Sergio López in Zuluaga et al. (2019).

Zuluaga et al. were able to use the high resolution image from the LaLoma telescope in combination with maps produced by the Lunar Reconnaissance Orbiter Camera to determine the precise location of the impact, determining it to have happened within an ellipse measuring 18 km along its east-west axis and 15 km along its north-south axis, centred on a point at a latitude of -29.428816° and a longitude of -68.167435°, close to the Byrgius crater to the southeast of Mare Humoris. The Lunar Reconnaissance Orbiter has been surveying the Moon for six years, at a resolution of one metre per pixel, during which time it has covered 70% of the Moon’s surface, with 3% having been covered moor than once. The calculated area within which the impact occurred should be small enough that this, or other Lunar surveying satellites, should potentially be able to detect the impact site directly in the future.

Impact estimated location. (Upper panel) Original picture taken at LaLoma Observatory in Colombia. (Middle panel) Superposition of the original picture and a Lunar Reconnaissance Orbiter Camera ortographic map. (Bottom panel) Flash image and location on top of a high-resolution Lunar Reconnaissance Orbiter Camera cylindrical map of the impact area. Zuluaga et al. (2019). 

Determining the original orbit of an asteroid based upon a simple observation of a flash is impossible, but Zuluaga et al. were able to use Gravitational Ray Tracing, a technique developed by Jorge Zuluaga and Mario Sucerquia, also of the Solar, Earth and Planetary Physics Group at the University of Antioquia, to determine the most likely origin of the body. Gravitational Ray Tracing works by using a computer model to create a large number of random incoming trajectories, each separated by at least 5°, and then tracing them backwards for a calendar year at regularly spaced speeds, through the gravitational fields of the Solar System, to determine which paths are possible and how likely these are. Zuluaga et al. generated 997 random incoming trajectories, and ran each of these at 100 different speeds.

82% of the hypothetical objects generated by this process survived a year into the past, and impactors travelling at shallow angles relative to the surface of the Moon were found to be more probable, with the object most likely to have hit the Lunar surface at an angle of less than 35°, and almost zero chance that it was on a trajectory close to the vertical. The object was also found to be travelling quite slowly relative to the Moon, probably hitting the surface at about 13.8 km per second (most such impactors are calculated to be in the 16-22 km per second range). The object was probably on an orbital path consistent with it being an Aten Group Asteroid (a body that spends most of its time closer to the Sun than the Earth, but which does cross the Earth’s orbit), though it does not appear to have been on a trajectory close to that of any known asteroid or meteor shower.

The data produced by the Time and Date telescope suggests that the flash was 0.30 seconds in duration and did not vary significantly in duration during this time. This means that all of the light that contributed to the visible point of light in the image taken by Fritz Pichardo in Santo Domingo must have been emitted during that time, while the images of stars in the background of that photograph must reflect the light emitted from those stars over the full 20 minutes of the exposure. Therefore if the brightness of the flash can be compared to the brightness of any stars can be found in the image, then the light reaching the Earth from those over 20 minutes must have a direct ratio to the light reaching Earth from the impact flash over 0.30 seconds, and since the distance to the Moon and that to all nearby stars is known, as well as the brightness of those stars, then this can be used to calculate the actual energy released during the impact.

Zuluaga et al. (2019). used nine stars to calibrate this method, HD 67564, BD+20 2009, BD+20 2007, BD+20 2005, BD+21 1766, BD+21 1779, BD+21 1777, TYC 1385-899-1, and TYC 1385-939-1. This led to the calculation that the impact released energy equivalent to that released by 0.3-0.5 tons of TNT, which in turn suggests an object 10-27 cm in diameter, with a mass of 7-40 kg, which would leave an impact crater 5-10 m in diameter, well within the detection range of lunar prospecting satellites.

Picture of the total lunar eclipse at the time of the impact  ash taken in Santo Domingo, the Dominican Republic with the moon removed, highlighting the background stars. 9 stars were identi ed and used for the photometry. Fritz Pichardo in Zuluaga et al. (2019).

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