Here, collisions typically occur at less than a few tens of kilometres per second. (newscientist.com)
They are very hot and very dense, with an average mass of around two solar masses contained within a radius of tens of kilometres. (sciencedaily.com)
The seismic waves produced by the explosions will travel tens of kilometres down before being reflected back to the surface and detected by the 1000 instruments deployed. (newscientist.com)