Simon Lauder reported this story on Saturday, November 5, 2011 08:12:00
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ELIZABETH JACKSON: Astronomers are keeping a very close eye on an asteroid which will pass between Earth and the moon in a few day's time. It's about 400 metres in diameter, and nothing that big has come quite as close to hitting earth for decades.
Experts say it's a close call but nothing to worry about, although another asteroid which is due to pass earth within the next 20 years will come a lot closer.
From Melbourne, Simon Lauder reports.
SIMON LAUDER: It's called 2005 YU55 and its orbit around the sun will bring a close encounter for Earth on Wednesday morning.
Glen Nagle is from the CSIRO NASA tracking station in Canberra.
GLEN NAGLE: About 400 metres in size and it'll be travelling at about 13.7 kilometres a second.
SIMON LAUDER: Will it actually pass between the Earth and the moon?
GLEN NAGLE: Yeah actually this is going to pass about 324,000 kilometres from our planet so inside the orbit of the moon. This is the closest this object's been in about the last 200 years.
SIMON LAUDER: The asteroid will be watched very closely by radar telescopes and Glen Nagle says the observations will help come up with a plan in case another asteroid gets too close for comfort.
GLEN NAGLE: There's an interesting one coming up towards the end of the 2020s, a 300-metre wide asteroid called Apophis, which won't hit us at that stage but there's some interesting little gravitational effects that it comes by - if it hits a certain sweet spot then it has a higher risk by about 2035 but things like this then gives us a couple of decades to figure out what to do about it if they are ever going to be a threat.
SIMON LAUDER: Nothing as big as 2005 YU55 has come close to earth since the late 1970s, but that asteroid went unnoticed at the time. Glen Nagle says YU55 won't pose a threat to earth.
GLEN NAGLE: But there are other objects out there that could come our way and if they strike down on the ground they can certainly wipe out vast areas, they can have effects on climate and so forth.
SIMON LAUDER: You say this one will pass within 324,000 kilometres of planet Earth. What's the margin of error on those calculations, just roughly?
GLEN NAGLE: Actually they're pretty good. We're down to just you know a few tens of kilometres either side of that mark.
SIMON LAUDER: Astronomically speaking, is that about as close as you get to a hit?
GLEN NAGLE: Oh yeah, in space terms yeah, that's a hair's breadth away but the one that they're talking about for the late 2020s, the Apophis asteroid, that'll actually pass much closer. Some calculations suggest about
30,000 to 36,000 kilometres 
away. 
And there is some talk about actually landing a small probe on that particular asteroid so that in coming years we can actually more precisely track its orbit through the solar system and see what threat it potentially does pose to us.
ELIZABETH JACKSON: That's Glen Nagle from the CSIRO NASA tracking station in Canberra, speaking there to our reporter Simon Lauder.