Can asteroid collisions with the earth be prevented? After last February's events in Russia - which saw a 10-metre-wide rock explode over the town of Chelyabinsk - asteroid detection has become a hotbed of research, and it's already turning up worrying results. Asteroids as big as Chelyabinsk's are classed as city-killers, but its high-altitude explosion caused only smashed windows - though about 1,500 people were injured. Worryingly, researchers at University of Western Ontario, Canada, reported that the chances of detecting it were almost zero. Meteors of this size are usually spotted by large telescopes about two hours before impact, but for Chelyabinsk - a city of one million - that was impossible because it was approaching from the sun's direction. Scientists painstakingly checked more than six million images for traces of the meteor, and found nothing. They concluded that only about 500 of the estimated 20 million near-earth asteroids the size of the Chelyabinsk impactor have been discovered so far. About 620,000 asteroids in our solar system have been catalogued and are tracked by astronomers, but Nasa estimates that more than six million objects orbit the sun. While some scientists are focused on how to prevent relatively small - though potentially lethal - asteroids like that in Chelyabinsk, others concentrate on one-kilometre (and bigger) sized comets that could wipe out the human race. About 860 big objects capable of such destruction have been identified, although only about 155 are seen as life-threateningly big - that is, about 10 kilometres wide. While current technology can't consistently detect small asteroids, the big epoch-ending-sized comets that swoop through the solar system are generally spotted about two years before they come anywhere near earth. That's down to projects such as Pan-Starrs (Panoramic Survey Telescope and Rapid Response System), which looks for destructive asteroids. The biggest such event in recorded history was in Tunguska, a remote area of Russia, in 1908. Like in Chelyabinsk, the asteroid burst overhead in an explosion estimated to have been about 1,000 times more powerful than the atomic bomb dropped on Hiroshima. The shockwave devastated more than 2,150 square kilometres, knocking down 80 million trees. Plans for coping with impacts in metropolitan areas are beginning to materialise. Since Chelyabinsk there's been an increase in research by astronomers. Governments are becoming interested, too. The United Nations' Action Team on Near Earth Objects proposed in October an International Asteroid Warning Network be created. The Nasa-funded Atlas (Asteroid Terrestrial-Impact Last Alert System), due in 2015, will use eight telescopes to scan the entire night sky for asteroids. Astronomers at the University of Hawaii say the equipment could be situated on its islands. The station could provide a week's notice of a Chelyabinsk-type asteroid. But once you've found an earth-bound asteroid, what do you do? Total evaporation is the thinking behind De-Star - or Directed Energy Solar Targeting of Asteroids and Exploration. It envisages a spacecraft as big as the International Space Station which can store enough solar power to fire laser beams at incoming celestial debris. But even the biggest and costliest array - a six mile-long spacecraft called De-Star 4 - would take an entire year to destroy a 500-metre-wide asteroid. Evaporation isn't strictly necessary. A mere deflection or redirection of an asteroid away from Earth would do just as well. Using lasers to nudge off course any asteroids that threatened earth has been mooted several times, most recently by Dr Richard Fork, an electrical and computer engineering professor at the University of Alabama in Huntsville, the US. Other ideas include using giant solar sails to shield a large asteroid from the sun, thus slightly changing its trajectory. Meanwhile, Nasa advocates simply using a spacecraft's gravitational pull to put an asteroid off-course. This would require a spacecraft to first crash into the object, as Nasa did in 2005 with Comet Tempel 1, part of its Deep Impact mission. Despite such hi-tech ideas, astronomy is perhaps the last science where amateurs still have a vital role to play, and nowhere is that truer than in asteroid-hunting. As part of its Asteroid Grand Challenge project, Nasa is partnering with Planetary Resources to make data from its Catalina Sky Survey available online. The aim is to enlist the help of amateur astronomers in finding, cataloguing and tracking yet-to-be-spotted near-earth objects. Chris Lewicki, president and chief engineer of Planetary Resources, and formerly flight director for Nasa's Spirit and Opportunity Mars rover missions, says: "By harnessing the public's interest in space and asteroid detection, we can more quickly identify the potential threats, as well as the opportunities." Adds Lewicki: "Asteroids hold the resources necessary to enable a sustainable, even indefinite presence in space - for science, commerce and continued prosperity on earth." This is space-mining, but it's not just about making money. Rare-earth metals are becoming more precious by the year, but Nasa knows that long-term space missions in the future will be impossible without a tried-and tested way for astronauts to replenish supplies of water and minerals. Planetary Resources is proposing a series of robotic space missions to identify near-earth asteroids that could be commercially mined. Its ARKYD space telescope, crowd-funded on Kickstarter to the tune of HK$11.7 million, will use the "citizen science" gathered from the Nasa tie-up to help look for dangerous or valuable asteroids. Planetary Resources is a wholly private venture, with name investors, including Virgin Group chairman Richard Branson, and Google chief executive Larry Page. "Our mission is to prospect and mine asteroids to address one of the paramount problems faced on earth and beyond - resource scarcity," Lewicki writes on the company's website. He says our collective impression that space travel is inherently difficult is wrong. "The massive controlled explosions, also known as rockets, used to push every ounce of cargo through our atmosphere into orbit leave a biased impression that all other solutions to developing a thriving space ecosystem must be equally hard," he says. His point is that almost everything else that happens in space - like raising the orbit of the ISS, landing on the moon, and exploring other planets - aren't actually that difficult. "It is getting to the doorstep of outer space that is nearly impossible," he says. But even that is "infinitely more achievable when access is fuelled by the resources present in near-infinite quantities on asteroids," Lewicki says. Whether by using lasers to nudge them closer to us, or even via a giant lasso to bring them into earth's orbit, Planetary Resources plans to physically move asteroids into position for easier access, even though the prospect of bringing such dangerous objects closer to us is a risky business. Get your calculations wrong, and the insurance bill could be sky-high.