Plans to more precisely plot the orbit of an asteroid with a small chance of hitting Earth in 2036 may be badly hit by funding cuts to a US radar facility.
Radar measurements set to be made in January 2013 by the Arecibo Observatory in Puerto Rico, US, could help rule out an impact by asteroid Apophis.
But the cuts mean Arecibo needs an extra $2m-$3m a year to continue.
If not, the observations planned for 2011-2013 will have to be abandoned, the facility's director told BBC News.
Dr Michael Nolan said he was "moderately optimistic" that the money could be found.
But he pointed out that Arecibo was the only observatory in the world sensitive enough for the task.
"If we measure [Apophis] in 2013, there is something like a 95% chance that we'll be able to prove that it can't hit the Earth in 2036," he explained.
Such threats present challenges for policymakers; while the chances of an asteroid strike might be small, the effects would be devastating.
The power of radar for refining the orbits of Near-Earth Objects (Neos) such as Apophis lies in being able to determine the range to the asteroids, by accurate timing of the emitted and returned pulses.
Dr Nolan, who is also the observatory's head of radar astronomy, told BBC News: "If you have a regular telescope, you can tell where it is from left to right in a sense. The radar measures distance, so it is forward to backwards in that same sense.
"If you do all of those at once, you get a very good measurement of where it is."
Professor Richard Crowther, chair of the UN Working Group on Near-Earth Objects (Neos) told BBC News: "Unique facilities such as the Arecibo radar can provide us with clear and reliable data on the trajectory of an object such as Apophis, thus helping us determine accurately whether it will collide with Earth in the future."
Although asteroid 99942 Apophis is one of the most hazardous Neos today, the calculated probability of an impact is small.
Last year, the US space agency (Nasa) lowered the chances of an impact on 13 April 2036 from one in 45,000 to one in 250,000.
The 300m-wide asteroid raised concerns in December 2004, when initial observations indicated a probability of up to 2.7% that it would strike Earth in 2029.
Additional measurements ruled out this possibility, but on 13 April 2029, Apophis will approach Earth at a distance no closer than 29,470 km (18,300 miles).
This is within the distance of geosynchronous satellites - and close enough to be seen by the naked eye.
However, if Apophis passes through a gravitational "keyhole" in space, during this approach, the stage will be set for an impact with Earth in 2036.
This keyhole is a precise region in space several hundred metres across, that could alter the asteroid's course through the influence of Earth's gravity.
Astronomers now say this keyhole is likely to be missed, but the planned campaign of optical and radar measurements in 2011-2013 could eliminate completely the possibility of a hazardous encounter in 2036.
Arecibo's funding struggles are well documented. The observatory currently operates on a budget of approximately $12m a year.
But the US National Science Foundation (NSF), which operates the facility, has indicated it will substantially cut funding starting from 2011, when the telescope will receive an overall budget of $9m.
"If that holds true with nothing else, we would have to drop this programme.
"The radar programme is the only one that would actually save money if you got rid of it," Dr Nolan said.
In its latest budget request, Nasa has set aside money both for Arecibo and for research on Neos like Apophis. If the space agency picked up the difference - estimated to be on the order of $2m-$3m per year - the project would be able to continue.
But Michael Nolan said this would also depend on how much NSF cuts funding to the Arecibo facility: "At some point, you stop doing maintenance to the telescope and stop cutting the grass and then it's done the telescope has to work for us to do these observations," he explained.
'Must do more'
Stephen Lowry, from the University of Kent, UK, commented: "Losing the planetary radar capability at Arecibo would not only affect our ability to monitor the orbits of threatening asteroids, but would also have implications throughout asteroid science, in particular with those studies that make extensive use of precise asteroid shape models that radar observing can provide."
In 2011, Apophis will start to become visible again to optical telescopes and it will make a relatively close flyby in January 2013 - passing by Earth at a distance of 13 million kilometres - which would enable measurements by Arecibo's radar.
Dr Nolan said optical observations alone were likely to rule out an impact in 2036, but that radar measurements increased that likelihood.
Another researcher said that while optical telescopes were very powerful in their own right for calculating the orbits of Neos, asteroids might appear unresolved in optical images due to their small size. This was made worse, the scientist said, by the blurring effects of Earth's atmosphere, affecting efforts to determine its orbit.
In January this year, a report by the National Research Council said the US must do more to safeguard our planet against asteroids.
Dr H Jay Melosh, from Purdue University in Indiana, US, said the best method for deflecting a small asteroid would be to fly a relatively heavy spacecraft into it.
"The momentum from the impact would push it off a collision [course] we have done that - sort of - with the Deep Impact spacecraft; we ran a 300kg spacecraft into a comet," he told BBC News.