There was a big bright flash in the sky over London in the early hours of this morning.
It was a whitish streak with a green tinge, and it was most likely a metallic meteor.
The blast was bright enough to be seen from the south of England to Northern Ireland.
Despite the brightness of the meteor, it probably wasn't very big – perhaps the size of a tennis ball, or a bit bigger.
Dr Karen Masters, an astrophysicist at the University of Portsmouth, told BuzzFeed News that it probably measured "tens of centimetres". "I'm basing that on an estimate of the one that appeared over Scotland recently," she said, "which looked about similar, and [tennis-ball-sized] is the estimate I've seen published."
She's seen claims that last night's meteor was "the size of a bus", but, she said, there's just no way that can be anywhere near true. "Things would hit the ground. This wasn't a grain of dust, but it wasn't the size of a bus either." It definitely wasn't as big, say, as the Chelyabinsk meteor, which exploded over Russia in 2013.
We can't work out exactly how big it was until we know how fast it was travelling.
Big objects glow brighter, but so do faster ones. It's been captured on several cameras, so it may be possible for scientists to work out its orbit, and therefore have a good guess at how fast it was travelling. Then they'll be able to work out its size, pretty accurately, from how bright it was. "It's pretty straightforward physics," said Masters.
But it'll have been moving incredibly fast – tens of thousands of miles an hour.
That's a good ballpark estimate, said Robert Massey, deputy executive director of the Royal Astronomical Society. "About 30km/s [nearly 70,000mph] is pretty common. And a 10cm object is large by these standards – most meteors aren't much bigger than a grain of sand.
"You take an object that size, slam it into the atmosphere at tens of kilometres a second, the air gets superheated. Most of the glow is from the air."
And it was probably metallic, maybe partly made of magnesium.
It seems to have entered the atmosphere over southern England and headed north.
But again, until there's more information, we can't be sure.
The main thing is that this sort of thing is pretty rare, and if you saw it, you're very lucky.
"It's a spectacular, harmless natural phenomenon, and great to see," said Massey. "You're very lucky if you saw it because it's rare to see one so bright."
If you do have footage, or you saw it, he says you should get in touch with the British Astronomical Association.