Move over, Bruce Willis. Your asteroid-hunting, Earth-saving days may be over. Observations by a now-defunct NASA satellite have given scientists their best guess about the number of asteroids in our Solar System which might someday threaten us. The results are encouraging and terrifying, in equal measure. They suggest that President Obama’s nebulous goal of a human voyage to a near-Earth asteroid by 2025 might be achievable…but also reveal that there are likely to be thousands of lethal space rocks – some the size of football fields – with the capability to hit our planet and wreak unimaginable damage.
A little over two years ago, in December 2009, NASA launched its Wide-Field Infrared Survey Explorer (WISE), a $320 million mission to examine the sky in the infrared portion of the electromagnetic spectrum. Due to a perishable supply of cryogenic hydrogen, needed to keep its infrared detectors cool enough to operate, WISE’s primary mission ended in October of the following year. Although two of its instruments could work without cryogenic coolant, NASA decided not to allocate funding for an extended mission.
However, the interest of the space agency’s Planetary Division was piqued and a one-month, $400,000 proposal was developed to use WISE to find near-Earth objects (NEOs). The project was called ‘NEOWISE’ and it was an endeavour for which the satellite had already made an impressive start. During its active mission, WISE found dozens of new objects every day and identified no fewer than 33,500 previously unknown asteroids and comets.
NEOWISE was so successful that it was extended and ended shortly before the final shutdown of the satellite’s transmitter in February 2011. Its data led to the creation of a new category of near-Earth objects, known as ‘potentially hazardous asteroids’ (PHAs). These doomsday rocks have the closest orbits to our own and come within 8 million km of Earth. Chillingly, they are large enough to survive passage through our atmosphere and could cause damage on regional or even greater scales.
In total, NEOWISE examined 107 PHAs, allowing predictions to be made about their entire population. The results suggest that an average of 4,700 PHAs exist, with diameters larger than 100 m…but barely a quarter of these objects have been found so far. Many of them – twice as many as previously believed – are likely to reside in low-inclination orbits, closely aligned with Earth’s own, and they probably originated from collisions in the main asteroid belt, between Mars and Jupiter. Their intrinsic brightness has led some scientists to speculate that they are stony or metallic in nature.
“The NEOWISE analysis shows us we’ve made a good start at finding those objects that truly represent an impact hazard to Earth,” said Lindley Johnson of the Near-Earth Object Observation Program at NASA Headquarters in Washington, DC. “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.”
Although the precise direction for NASA’s future human space programme remains depressingly unclear, the agency stands by President Obama’s vision to plant astronauts’ bootprints on an asteroid, perhaps as soon as 2025. The NEOWISE data might make that goal more realistic. Asteroids with lower-inclination orbits are more likely to encounter Earth and thus are easier to reach with humans or robots.
“Out team was surprised to find the over-abundance of low-inclination PHAs,” said Amy Mainzer, the principal investigator for NEOWISE at NASA’s Jet Propulsion Laboratory in Pasadena, California. “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.”
Perhaps the next generation will need the caliber of Mr Willis after all.