Landing on a comet was never going to be easy. But the European Space Agency's attempt to land the first spacecraft on a distant ball of ice just got more complicated.
Agency staff must have thought they were seeing double when they downloaded the latest pictures of their spacecraft Rosetta's destination, a distant comet named 67P/Churyumov-Gerasimenko.
Comet-67P is not one but two comets joined together.
Warwick Holmes, an Australian engineer who helped build Rosetta, said the comet's unusual shape will make highly risky setting down of the satellite's lander, Philae, on the comet surface even harder.
"This is going to be an unbelievable achievement if they can land on such an irregular shaped comet," Mr Holmes said.
Despite the ten-year journey to reach Comet-67P, astronomers had no idea what it looked like.
"On the 11th of July when they got close enough they began to realise that it was a very strange shape," Mr Holmes said.
Three days later, its bizarre formation – shaped like a button mushroom but officially known as an offset binary comet – was revealed.
Two other suspected binary comets have been spied in previous comet fly-by missions, but neither were as dramatic as Comet 67-P, he said.
Rosetta, now less than 4000 kilometres from its destination, will rendezvous with Comet 67-P in about 12 days and come within about 70 kilometres of its surface during its first fly-by.
The satellite will continue past the comet for a few hundred kilometres before firing its thrusters to fly past again.
As the comet does not have enough gravity to pull Rosetta into orbit around itself, the satellite will spend several weeks flying a series of equilateral triangles in front of the comet to map its surface.
"Each time we fire the thrusters we're getting slightly closer and we get to know more about the nature of the comet [including] how much gas is coming out," Mr Holmes said.
Astronomers will use this information to select a site to set down the lander.
For the final landing sequence, scheduled for November 11, Rosetta will come within about 1-2 kilometres above the surface, before releasing Philae.
