New High-End Search for Lunar Impacts

A professional observatory in Greece has begun recording flashes created when bits of interplanetary debris strike the Moon.

The Moon's battered face bears witness to the countless times something has slammed into the lunar surface, and new craters (albeit very small ones) form all the time. Even these mini-collisions occur at 20 km (12 miles) per second, while the very fastest are 70 km/s. If the chunk of debris has a mass of at least a few tens of grams, it creates a momentary white-hot flash — and if that occurs somewhere on the Moon's night side, it's an observable event.

Lunar flashes seen by NELIOTA project

So far the NELIOTA project has captured 22 impacts on the Moon. Colors indicate the estimated temperature of each strike, from 1,770 K (dull red) to 3,730 K (white).
ESA / Chrysa Avdellidou

We have front-row seats for these crash landings, but they're rarely seen. Over the past 20 years only a handful of lucky telescopic observers on Earth have spotted one inadvertently.

In 2005, a team from NASA's Marshall Space Flight Center started routine monitoring of the lunar disk using a network of 14-inch telescopes, particularly during annual meteor showers such as the Perseids and Geminids, and it's captured hundreds of flashes to date. Other monitoring efforts are MIDAS, operating in Spain, and ILIAD in Morocco.

Recently a new player has upped the scientific stakes. Since February, European astronomers have been staring at the lunar night using the 1.2-meter Kryoneri telescope on Peloponnese in Greece.­ This project, led by the National Observatory of Athens under a contract with the European Space Agency, has developed a special observing system called NELIOTA (short for NEO Lunar Impacts and Optical Transients). The 22-month observing effort records strikes down to 12th magnitude — far fainter than other programs can achieve.

Lunar impact flash

The NELIOTA project records about a quarter of the lunar disk. Here an impact flash (right edge below center) punctuates the lunar night.
NELIOTA project

At this week's meeting of the AAS's Division for Planetary Sciences in Provo, Utah, researcher Chrysa Avdellidou (European Space Agency) reported that to date the system has captured 22 flashes. That's one impact per 1.8 hours of observing, compared to one per 2.8 hours for the NASA system.

The power of NELIOTA, apart from the telescope's large aperture, lies in using a beam-splitter to feed a 17-by-14-arcminute field of view to two high-frame-rate video cameras simultaneously. One camera records the lunar night in red light (R band, 641 nm) and the other in the near infrared (I band, 798 nm).

This combination captures longer events, lasting from 43 to 182 milliseconds, because the collision sites remain hot after the visible-light flash has faded from view. "You can watch the cooling of each impact plume," Avdellidou says.

Moreover, the two wavelengths provide a way to extract each impact's temperature and an estimate for the colliding object's mass.

But such calculations are tricky — partly because there's no way to know exactly how fast these interplanetary bullets are striking the Moon and partly because the amount of kinetic energy that goes into creating the flash (its luminous efficiency) is guesswork.

Composite lunar flash

A sequence of frames, taken every 33 milliseconds (left to right), shows how a lunar impact flash disappears quickly in visible light (top pair) but lingers longer when recorded in near-infrared light.
NELIOTA project

So far, the flashes have varied from 1,770 to 3,730 Kelvins, a range that fits theoretical predictions well. Avdellidou isn't convinced that these blackbody temperatures are telling the whole story, however. So she wants to conduct a series of hypervelocity laboratory experiments in simulated lunar materials to see how the target's composition affects the intensity and duration of each lunar flash.

In the meantime, she's using mapping data from NASA's Lunar Reconnaissance Orbiter to try to determine the composition of each impact site. This spacecraft is also very good at spotting fresh impacts on the Moon. So, with luck, LRO scientists can use NELIOTA's high-quality images to track down where some of the larger strikes have occurred — the "smoking gun" that would provide crucial links between an impactor's kinetic energy and the brightness of its flash.

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