Here’s a picture that could really make an impact. NASA has used fresh data from a space probe to plot the distribution of all asteroids that threaten to hit the Earth. Every orange dot in the image represents a giant chunk of rock 330ft wide (100 metres) or more that is in an orbit which brings it dangerously close to our planet.
They are called potentially hazardous asteroids, or PHAs – and there could be more than 6,000 of them. Worryingly, perhaps, only 20 to 30 per cent of the estimated number of these space missiles have so far been found. A 100-metre wide asteroid is considered the smallest that could cause serious damage if it hit the ground. One that is 1 km wide would cause massive destruction while a rock 10 kilometres across would leave few survivors anywhere.
The Earth’s orbit through this cosmic minefield is shown almost edge-on in green at the centre of the picture. The host of blue dots are other asteroids that come close to us, but which are less of a threat. Those are called Near Earth Objects, or NEOs.
The snapshot of the alarming swarm as it might appear on any given day was produced using observations from a satellite called WISE, short for Wide-field Infrared Survey Explorer.
NASA say the probe has given them their best assessment yet of the number of threatening asteroids, where they came from and how much danger they pose to Earth.
Their results suggest that there are around 4,700 potentially hazardous objects – plus or minus 1,500 – with diameters larger than 330ft. It is the most accurate survey yet and was produced by the asteroid-hunting part of the WISE mission, called NEOWISE.
The survey discovered that twice as many asteroids as thought are orbiting on a similar plane, or level, as the Earth’s path round the Sun, rather than swooping in from above or below. Such “lower-inclination” orbits increase the number of times such asteroids come close, apparently.
Lindley Johnson, of the Near-Earth Object Observation Program at NASA HQ in Washington, said: “The NEOWISE analysis shows us we’ve made a good start at finding those objects that truly represent an impact hazard to Earth.
“But we’ve many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future.”
Amy Mainzer, NEOWISE’s chif scientist at NASA’s Jet Propulsion Laboratory in California, said: “Everything we can learn about these objects helps us understand their origins and fate.
“Our team was surprised to find the overabundance of low-inclination PHAs. Because they will tend to make more close approaches to Earth, these targets can provide the best opportunities for the next generation of human and robotic exploration.”
Jay Tate runs The Spaceguard Centre, a UK observatory at Knighton, Powys, that campaigns to raise awareness about the threat from deadly asteroids.
He told Skymania: “The NEOWISE data emphasises the requirement for search programmes to detect what used to be called sub-critical sized NEOs (smaller than the 1km threshold that would cause a global ctastrophe). There is a substantial threat from smaller objects that would ‘only’ devastate a small country, especially as these smaller objects are more numerous than larger ones and so pose a more frequent hazard.
“The technology required for such a search programme is readily available, and relatively cheap, especially given the actuarial cost of the hazard. While detection is essential we must not forget the subsequent follow up process required to determine accurate orbits. The main participants in this field are amateurs, who do a splendid and vital job. However, smaller asteroids pose significant technical problems to operators of small telescopes that struggle to see faint objects. Any search programme must also address the follow up issue.”
Space scientists believe that many of the PHAs may have been produced in a collision between two asteroids in the main asteroid belt lying between Mars and Jupiter. They were later sent driting into orbits closer to Earth where they began to pose a threat.
The NEOWISE project photographed 600 near-Earth asteroids, of which around 135 were new discoveries. Its heat-sensitive infrared telescope was able to pick up both light and dark objects, resulting in a more representative survey of the entire asteroid population.