This is designed to probe the nature of "dark energy", which is thought to be responsible for the quickening expansion of the universe that Hubble helped bring to the world's attention.

Numerous astronomical surveys will probe space-stretching dark energy, although figuring out what it is could take much longer than another decade.

In the absence of the harpoons, they think, the lander's flexible legs managed to absorb most of the impact energy, but not enough to stop the probe bouncing.