In February 2013, spectacular images of an asteroid burning up over Chelyabinsk in Russia stunned the world. The impact was estimated to be ten times stronger than that of the Hiroshima or Nagasaki atom bombs and nearly 1500 injuries were reported. It stands as the most dramatic recorded strike in over a century but it is nothing new – indeed, asteroids have had a huge influence on Earth’s history. Early bombardments brought water and carbon-based materials, crucial for the development of life, while a huge asteroid or comet 65 million years ago is believed to have wiped out the dinosaurs.
Asteroids are the debris remaining from the formation of our solar system’s planets 4.5 billion years ago and those within 28 million miles of us are considered a potential hazard. They are fascinating, threatening and, until now, have been left well alone by humanity.
Sending humans to asteroids before attempting a manned Martian mission is seen by many as a sensible option and would teach us more about planet formation and how to defend from future asteroids. NASA plans to capture a 500 ton asteroid and drag it back to the Moon, placing it in lunar orbit for astronauts to explore. Human visits to asteroids would take weeks rather than months or years, lowering costs and providing a stepping stone between Low Earth Orbit flights and deep space expeditions.
Attractive and forward-thinking as this sounds, it would require a logistical effort like nothing we have ever attempted. For several decades we have explored, landed on and observed a variety of celestial bodies, but never have we attempted to relocate an object in space. The mechanics of tracking and approaching an asteroid, capturing and stabilising it and then dragging it across the solar system, pose an engineering challenge quite beyond that of simply visiting another world.
The type and size of asteroid selected would be crucial. The Keck study*, which investigated the project last year and concluded it to be “ambitious but do-able”, recommended a seven metre long water-rich carbonaceous asteroid. Such small carbonaceous asteroids have the advantage that they would burn up in the Earth’s atmosphere before reaching the ground, should something go drastically wrong. The trouble with carbonaceous asteroids, though, is that they are dark and small ones are hard to observe unless they are very close to Earth. The chosen asteroid must also have orbital parameters similar to Earth and is thus constrained to encounter Earth approximately once a decade. It is anticipated that we will find perhaps five suitable objects per year. We would need to spot it in the Earth’s vicinity and study its photometry for the slight reddish spectra of the desired carbonaceous chondrites, and its optical light curves to determine its spin. This would require observatories from all over the world to study it for the few days it is nearby, and then track it for a decade until it returns. At this point we could use a craft to relocate it to lunar orbit.
The spacecraft would be delivered to low Earth orbit by a launch vehicle and would then take four years to reach the asteroid, powered by a solar electric propulsion system. The craft would take 90 days to study the asteroid’s size, rotation and surface topography at infrared and visible wavelengths and attempt to match its rotation. A 15 metre wide inflatable drawstring bag would be deployed to capture the asteroid, secure it against a ring to control its centre of mass, and apply forces and torques to stabilise it. If the asteroid was comprised of rubble rather than one individual rock, a “snow blower” could be used to suck the rocks into the bag using a spinning blade. (Unlike a terrestrial snow blower, there would be no escaping air to take into account.) The craft would transport the asteroid to the Earth Moon system and remain with it to support visiting humans in developing ways to extract asteroid resources.
Until now, we have only been able to return up to 100kg of asteroid samples – we would now have access to 500 tons of easily crushable material with a high yield of water and carbon-bearing compounds. This could begin a whole industry in space resources and the material could provide shielding for astronauts from the serious danger of galactic cosmic radiation.
It would be incredibly challenging but is certainly possible. A project like this is the perfect harmony of robotic and manned space exploration and gives us knowledge and development of solar electric propulsion technologies to work towards a manned mission to Mars. It would also unite nations with a goal of learning how to manipulate asteroids for planetary defence. Not since Apollo has there been such an inspiring mission to challenge a new generation of engineers and scientists across the world.