The risk of a sizeable asteroid colliding with the earth has always been purported to be extremely low, with scientists reporting that such an event was only likely to occur every twenty million years or so.
Unfortunately, our collective sigh of relief could be a little premature, as recent evidence suggests that, not only is the risk much higher than previously thought, but that such events are occurring regularly in our atmosphere. The new information has been released by an organisation known as the B612 foundation, a private collective whose mission is to detect and deflect dangerous asteroids.
"There is a popular misconception that asteroid impacts are extraordinarily rare ... that's incorrect," said former astronaut Ed Lu, who now heads the California-based B612 Foundation.
The US-based B612 group was founded in 2002 by Clark Chapman, Piet Hut, Ed Lu, and Rusty Schweickart, who realised that the threat to Earth from an asteroid hit was very real and serious action needed to be taken in order to address the issue. During its first ten years, the volunteer organisation focused primarily on deflection techniques, but latterly it was recognised that detection of potential threats was even more important, as it would be impossible to deflect an unknown and unexpected object.
In July 2012 on the anniversary of the Tunguska Asteroid impact, the B612 Foundation announced the launch of the Sentinel Mission, the first private-sector space plan to build and operate a deep space telescope. The telescope will be fitted with an infrared lens, designed to detect and track the millions of asteroids known to have the potential to cause significant damage on Earth.
The Sentinel mission, a privately funded venture costing around $250 million, is a way off completion yet, with a launch date not predicted until 2018; the latest information released by the B612 foundation has therefore been compiled from another source, the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). The CTBTO comprises a network of sensors implemented with the aim of detecting covert atom bomb detonations.
The system recorded 26 major blasts on Earth between the years 2000 and 2013, ranging in force from one to 600 kilotons. To provide some perspective on these figures, the atomic explosion that obliterated the Japanese city of Hiroshima in 1945 was caused by a 15 kiloton bomb, so it can be seen that a 600 kiloton event is of very major significance.
So why have such major blasts apparently passed unnoticed?
It seems that Earth has been spared the full fury of these cataclysmic blasts merely because the explosions occurred high up in the atmosphere, or over expanses of ocean.The data from the CTBTO system suggests that such events are actually far more common than previously thought, and that Earth has escaped serious damage in recent years more by luck than low probability.
One of the most recent recorded asteroid impacts, which occurred in February 2013 in Chelyabinsk, Russia, pales into insignificance when compared to the massive explosions revealed by the new data. Though the object caused widespread damage and minor injuries, its effects were relatively negligible compared to the type of asteroid impact that B612 is hoping to spot, which could cause mass extinction-level destruction.
Russia has been in the firing line before, when an even larger asteroid struck Tunguska in 1908, but fortunately this touched down in a very remote area and caused little damage.In a more heavily populated area, however, the outcome may have been very different.
Another fact that has become apparent from the CTBTO data is of even more concern: of the 26 major explosions detected, only one was detected in advance of the explosion, highlighting the future need for priority to be given to early detection devices. It is possible that over 90% of the really massive asteroids out in space are as yet undetected, with data from Nasa's Wise telescope indicating that that there may be up to 20,000 objects in the 100-1,000m range, the vast majority of which have yet to be identified and tracked.
The CTBTO data has been utilised to help predict the regularity and size of future impacts, however, and suggests that Earth is likely to be smacked by a multi-megaton asteroid around once every 100 years.
"This is a bit like earthquakes," explains Ed Lu, former shuttle astronaut and CEO of the B612 Foundation.
"In the cities that have a major danger - Tokyo, Los Angeles, San Francisco - they know the odds of big earthquakes by observing how many small earthquakes there are. Because there's a known distribution of earthquakes, meaning that earthquakes come in all sizes, small to large - if I can measure the small ones, I know how many big ones they're going to be. And you can do this with asteroids."
The new evidence has validated the cost of the Sentinel project, which would help to spot objects in the inner Solar System that normally escape detection due to the glare of the Sun.
"Picture trying to spot something that's only the size of a small apartment building, that's tens of millions of miles from Earth, and that's black against a black background. That's incredibly hard. That's what requires the technological advances of Sentinel."
So, if a massive asteroid is identified and shown to be on a collision course with Earth, what can be done to deflect it?
One of the simplest methods of mitigation would be to strike the asteroid and change its trajectory. This efficacy of this course of action would be difficult to predict, and would need to be done well in advance of impact, highlighting the need for early detection.
Another "simple" approach being mooted is known as the "gravity tractor". This method would involve positioning a spacecraft close to an asteroid, and using long-lived ion thrusters to maintain the distance between the two. The gravitational attraction between the spacecraft and the asteroid should make it possible to pull the rock off its trajectory. Once again, the process would require time to put into place, and prior knowledge of the threat would be essential.
"These types of mission are arguably less difficult than building Sentinel. The hard part is finding these things," says Lu.
All hopes now appear to be pinned on Sentinel, whose implementation is surely critical to detect future hazards. It seems inconceivable that such a vital project is not receiving any government funding; let us hope that the Earth's luck continues to hold out until after 2018.