The above video shows the largest meteor strike on the Moon’s surface ever caught on film. It’s not as spectacular as the Chelyabinsk meteor that exploded over Russia in February 2013, but it’s still quite the sight. The Moon’s craters form an uneroded, unweathered record every impact since the Solar System formed - this is a rare chance to see one of those impacts as it happened.
This impact on 11 September was seen by a team of astronomers led by José M Madiedo at the Spanish universities of Huelva and Sevilla and the Institute of Astrophysics of Andalusia , as part of the Moon Impacts Detection and Analysis System , or Midas. The glow of the impact was as bright as some of the brightest stars in the night sky, and lasted for at least eight seconds until fading. “At that moment I realised that I had seen a very rare and extraordinary event,” Madiedo said.
There’s a really good reason to monitor the Moon for impacts: it’s a lot easier than keeping track of impacts on Earth. Most of the stuff that hits us in space is tiny, no more than a few centimetres across at most, but there are always larger objects that could threaten us. Ideally, scientists would like to know how many large rocks there are relative to the small ones - but if the smaller ones burn up in the atmosphere before hitting the ground, that kind of survey is made difficult.
However, the Moon has no atmosphere, and so the impacts of even the smallest objects on its surface are visible to telescopes down on Earth. The Moon therefore makes a useful sampling tool - relative to the size of the Solar System, it’s right next door to Earth, and that makes the things that hit it indicative of what will also, on average, hit us.
In a paper published in the Monthly Notices of the Royal Astronomical Society, Mediedo and his team estimate the object - which was between 60cm and 1.4m wide - was travelling faster than 60,000km/h. Despite its tiny size, it left a crater 40m wide, and the explosive energy was the equivalent of more than 15 tonnes of TNT. The press release  accompanying this paper describes the 400kg object as having “the mass of a small car”, but that’s a bit misleading - most cars weigh more than a tonne. A more accurate mental image, considering its height and mass, would be of three average-sized adult American men holding each other in a circle. Only, you know, made of rock.
It also reached an apparent magnitude of 2.9. Apparent magnitude is a somewhat strange measurement, as things get brighter the lower the value. The Sun is the brightest object in the sky, and has an apparent magnitude of -26.7, while Mars might get as bright as -2.8; the dimmest stars visible to the eye, though, are of magnitude 6, while the dimmest objects seen by Hubble have magnitudes around 30. This meteor's 2.9 makes it brighter than two 3 magnitude flashes seen during the Leonid meteor shower in 1999, and the light burst also lasted much longer, not fading away until eight seconds after impact. That's because dust and tiny bits of rock get vapourised in the collision, and take a few seconds to cool down again.
There's no danger to humans from meteorites of this size, however - anything around a metre in size would almost entirely burn up while falling. There are, however, some "implications" for the assessing meteor strike risk on Earth, the scientists write in the paper. Across their study, they can average that the Moon should received an average of 126 events per years with an energy release of more than 15 tonnes of TNT - and, scaled up to the Earth, that should give 1680 such hits per year. That's a much higher estimate than some other studies have given.
They write: "This event exemplifies that Earth impact hazard estimations were not well constrained... Thus, a systematic monitoring of moon impact ﬂashes but also of ﬁreballs in the Earth’s atmosphere would provide a more reliable impact frequency, especially if the luminous efﬁciency is well calibrated."