Was it a UFO, meteor . . . or an out-of-control Russian satellite?

Was it a comet, UFO, space junk, meteor or burning plane?

According to a local expert the object seen flying through our sky last night was probably the remains of a Russian satellite - and it there's a chance, a tiny one, that a small part of it might be sitting in someone's backyard.

Thousands of excited Kiwis from Whangarei to Nelson quickly took to social media last night to post pictures and video of the bright object that streaked through our dusk sky, and then seemed to disappear.

There's been much speculation since then about what the object was - and what happened to it once it vanished from view - but according to a leading space expert it was almost certainly the remains of an out-of-control Russian missile defence satellite.

Theoretical cosmologist Professor Richard Easther, the head of physics at the University of Auckland, said he was 99 per cent certain that it was the Russian Kosmos 2430 satellite - and that the Russians appeared to have lost control of it.

He said it was among several satellites sent up to Earth orbit by Russia to protect against missile attacks, primarily by the United States.

"The US has a similar constellation of satellites."

Easther said it was most likely that any remains of the nearly 2 tonne satellite that had survived the intense heat of its dive through the atmosphere landed in the sea.

It was conceivable, however, that debris such as large chunks of glass or scorched metal had landed in someone's backyard.

"In the remote [case] ... that you discover some 'space junk', some spacecraft have small manoeuvring engines that use toxic propellants, so anything that looked like a gas bottle or tank should be treated with caution.

"But pieces of metal are almost certainly safe, apart from the sharp edges."

Asked if there was any obligation to report such finds, he said, "I guess the fire brigade would be your first call; they handle chemical risks as well.

"And I have no idea who owns debris," he said, adding, tongue in cheek, that if it did any damage, the Russian President could be approached: "you could send Vladimir Putin the bill."

Easther said several satellites returned to Earth each year. Notable ones included the US Skylab and a Russian satellite that contaminated part of Canada.

Debris from Skylab was found in Western Australia after the US space station crashed back to Earth in 1979.

The previous year, the Soviet reconnaissance satellite Kosmos 954 scattered radioactive debris over northern Canada when it re-entered the Earth's atmosphere. A fault before re-entry had prevented the safe separation of its onboard nuclear reactor.

Easther said Kosmos 2430 was known to be passing over New Zealand at the time of the bright object last night. The proof that it was the cause would be in skywatchers reporting its absence.

If it had been a controlled descent, it would have been manoeuvred to splash into the Southern Ocean.

The fact its re-entry occurred over New Zealand implied that "the Russians lost control of it".