This image provided by NASA/JPL-Caltech shows a simulation of asteroid 2012 DA14 approaching from the south as it passes through the Earth-moon system on Feb. 15, 2013. The 150-foot object will pass within 17,000 miles of the Earth.

(AP Photo/NASA/JPL-Caltech)

Although the 150-foot (45 meter) rock will zoom harmlessly by our planet on its record near approach this week, it is just one of up to a million "near-Earth objects" which astronomers believe could one day pose a serious threat.

A program set up by NASA to monitor so-called "NEOs" a decade and a half ago has so far tracked fewer than 10,000, meaning the vast majority are still hidden.

Most of the larger specimens, which could have the potential to wipe out entire continents, have been found but even small asteroids like 2012 DA14 could be powerful enough to destroy an entire city if they plunged down to Earth.

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Now experts are calling for greater monitoring of small NEOs measuring less than a kilometre across, and for a contingency plan in the event of a likely impact.

UK Space Agency engineers travelled to the United Nations this week to seek a deal with colleagues from around the world and hope to come to an agreement before Friday.

The first goal is to secure funding for asteroid monitoring from countries other than America, which is currently the only nation carrying out such a program.

This would allow new systems to be built specifically to detect smaller asteroids, and open up more facilities across the world to help in the search.

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Richard Crowther, chief engineer at the UK Space Agency, said: "Our ability to see objects is limited by the number of telescopes on the ground and their ability to see objects of particular sizes.

"The current 'survey' is aimed at kilometre-sized objects...the smaller the object, the more of them there will be but the smaller the proportion we will see with a system not designed to find them.

"The issue is that this is an infrequent but potentially catastrophic event which policy frameworks struggle to address."

As well as increasing the monitoring of NEOs, the UN group hopes to make progress on an emergency plan detailing measures to be taken if an asteroid does become a threat to Earth, such as attempting to destroy or deflect it, or reduce damage to a minimum.

Even if an agreement is reached, experts claim it will be many years before they have a full idea of which asteroids could threaten the Earth, and when.

At a press conference last week Lindley Johnson, chief of Nasa's near-Earth object observations program, said: "It does take quite a bit of capability, both in sensitivity - the ability to detect these small objects - and also time.

"It is an effort that will take another decade or two even if we have the most sophisticated systems that feasible technology will allow us."

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