The ambitious plan is organised by the B612 Foundation. It aims to 'open up the frontier to space exploration and protecting humanity on Earth'. That protection focuses on mapping the orbits of the tens of thousands of near-Earth asteroids with diameters of at least 140m that could strike the Earth with an explosive force of at least 100 megatonnes of TNT. That is 3.5 times the diameter of the object that struck Tunguska, Siberia, in 1908, uprooting up to 80 million trees and shattering windows hundreds of kilometres away.

According to B612, more than 98% of such asteroids remain totally unknown to astronomers. The mission aims to find and track more than 90% of them. The spacecraft, called Sentinel, will take off from NASA's Kennedy Space Center in Florida. A gravitational slingshot manoeuvre off Venus will put the craft into an orbit around the Sun close to that of Venus. The craft will carry an infrared telescope to map the locations and trajectories of Earth-crossing asteroids.

The telescope will scan the entire night half of the sky every 26 days to identify every moving object. Repeated observations of individual asteroids will permit astronomers to calculate their orbits and predict their positions accurately for a century or more in the future.