An unusually speedy asteroid bigger than a big-city sports stadium will whizz close past the Earth on Halloween.

NASA says the asteroid, known as 2015 TB145, is about 400 metres across, nearly double the diameter of Toronto's Rogers Centre. It will be travelling at a speed of 35 kilometres per second, "which is unusually high," NASA says, when it passes within 490,000 kilometres of the Earth at 7:12 a.m. ET Oct. 31.

Asteroid

NASA says the asteroid, known as 2015 TB145, is about 400 metres across, nearly double the diametre of Toronto's Rogers Centre. (Duk Han Lee/CBC)

That distance is about 1.3 times the distance between the Earth and the moon, and is closer to the Earth than any other object that size is expected to come until August 2027, when another large asteroid called 1999 AN10 will make a flyby.

NASA called the flyby distance "a very safe pass," in response to questions from concerned members of the public directed at its Asteroid Watch Twitter account.

On the other hand, the flyby is close enough that it should provide some of the sharpest radar images of any asteroid this year.

"The flyby presents a truly outstanding scientific opportunity to study the physical properties of this object," NASA said.

2015 TB145 was discovered less than two weeks ago, on Oct. 10, by the Pan-STARRS I survey. It uses a telescope in Hawaii to survey the entire sky several times each month to discover and observe asteroids and comets approaching Earth that could be potentially dangerous.

Comet-like?

The orbit of 2015 TB145 hints that it may be "cometary in nature," NASA says.

While it may theoretically be bright enough to see from North America with small telescopes before sunrise on Oct. 31, that will probably be difficult because it will be close to the moon, which is expected to be quite bright, since it will be full on Oct. 27.

The Halloween flyby will actually be the second asteroid flyby of the week. On Oct. 29, an asteroid called 2009 FD about 300 metres in diameter will pass within 16.3 lunar distances of the Earth.