Astronomers have witnessed the largest lunar impact to date. With an impact energy equivalent to 15 tons of TNT — approximately three times as great as the previous record-holder — the flash was visible even to the naked-eye.
In astronomy there’s an ongoing joke: if you can’t easily explain a specific phenomenon, then it was probably the result of a collision. The Moon likely formed when a Mars-sized object slammed into Earth billions of years ago, dinosaurs probably became extinct when an asteroid hit Earth at the end of the Mesozoic era, and chances are elliptical galaxies are created when two or more galaxies collide.
Another great example is easily seen when one simply peers up at the full Moon. Even with the naked eye, you can see a vast number of impact craters — marking collisions that span the majority of the Moon’s 4.5-billion-year history. Without an atmosphere, small rocks from space can effortlessly leave their marks on the Moon’s surface.
But rarely do we actually see these collisions take place. So scientists have set up networks of telescopes that detect them by looking for bright flashes on the Moon’s surface. One such automated system worked remarkably well on September 11th, 2013, when two telescopes in Seville, Spain, picked up a flash so intense it took eight seconds to fade.
As brilliant as the famous SN 1987A in the Large Magellanic Cloud, September's flare was undoubtedly the result of a rock crashing into the lunar surface. Astronomer José M. Madiedo (University of Huelva) saw this footage of the strike once the telescopes’ software had processed the impact:
Impacts take place at huge speeds, causing the meteorites to become vaporized at the impact site instantaneously. This produces a thermal glow that can easily be detected in any automated system.
Typically these flashes last a fraction of a second. But the much longer flash produced last September occurred when a meteorite with an impact energy equivalent to 15 tons of TNT — approximately three times as great as the previous record-holder — slammed into the unlit side of the Moon.
Madiedo and his collaborators believe the meteorite weighed 880 pounds (roughly half the mass of a small car) and crashed at a speed of 40,000 miles per hour. It likely punched a fresh crater roughly 150 to 200 feet wide into the ancient lava-filled basin known as Mare Nubium.
The event was recorded under a five-year-long automated project entitled the Moon Impacts Detection and Analysis System (MIDAS). It captures scores of much smaller lunar impacts every day as tiny meteorites weighing only a few grams hit the Moon roughly every 3 hours. By collecting statistics on these collisions, Madiedo hopes to learn about similar threats to the Earth.
However, even meteorites this large don’t pose a threat to Earth, as most would burn up in our upper atmosphere. The meteorite that triggered the mass extinction of the dinosaurs 65 million years ago, for instance, was thousands of times larger than this one.
José M. Madiedo, José L. Ortiz, Nicolás Morales and Jesús Cabrera-Caño “A large lunar impact blast on 2013 September 11” Monthly Notices of the Royal Astronomical Society, in press.