The origin of an all-blue asteroid "rock comet" that recently flew by the Earth remains a mystery, despite an international effort to figure it out.
Phaethon came within 10 million kilometres of us in December, giving astronomers a good look at it - and they've come away even more confused than before.
For example, since its discovery in 1983 it's been suspected Phaethon is a fragment of another blue asteroid, the much-larger Pallas.
"However, Pallas' albedo [reflectivity] is about twice what we found for Phaethon," said Teddy Kareta, a University of Arizona graduate who led the study into Phaethon.
Phaethon is also evenly blue all around, unlike Pallas, because of its regular close encounters with the sun when it gets cooked. Its eccentric orbit takes it within 21 million kilometres of the sun - much closer than Mercury - then all the way out past Mars.
"[It's] so hot that metals on the surface turn to goo," said Mr Kareta.
It's also not clear whether Phaethon has always been an asteroid. It's the only asteroid known to have created a meteor shower, the Geminids, which shower Earth in December each year; every other meteor shower has been linked to comets, which regularly shed material.
Phaethon doesn't shed anywhere near enough to sustain a meteor shower, said Mr Kareta. He suspects Phaethon used to be a comet, but lost all its ice, dust and gas, becoming an asteroid.
"If it was cometary at some point in the past, maybe it made the meteor shower the normal way and left behind those comet crumbs - but since then, it's been cooked through and turned off and it just looks like a rock."
Or perhaps it had a catastrophic collision sometime in the distant past, and the Geminids are "kind of like blood splatter, to be gruesome, from this really violent event".
Phaethon's unique properties mean it remains the only object in space so far to be designated a "rock comet".
Japan plans to send a probe to Phaethon in 2022 to find out once and for all what this blue thing really is.
Newshub.
