Comets have spun in and out of human history as celestial omens of death and catastrophe for millennia. It was only in the sixteenth century that comets began to be investigated as astronomical bodies. The physical nature of comets as cosmic snowballs of frozen gases, rock and dust was only confirmed recently.
Now, after decades of dreaming and preparation, scientists have finally put a spacecraft into orbit around a comet.
Rosetta launched in 2004 and has travelled 405 million kilometres from Earth (which is about halfway between the orbits of Jupiter and Mars) to arrive at comet 67P/Churyumov-Gerasimenko on 6 August 2014.
“After 10 years, five months and four days travelling towards our destination, looping around the Sun five times and clocking up 6.4 billion kilometres, we are delighted to announce finally ‘we are here’,” says Jean-Jacques Dordain, ESA’s Director General.
Rosetta and the comet are rushing through space at 55 000 km per hour.
Since leaving Earth, Rosetta has taken a convoluted route; it made three flybys of Earth and one of Mars and passed by asteroids Šteins and Lutetia, taking the first ever close-up shots of these astronomical objects. A successful orbit was achieved only after a series of 10 flybys of the comet, all of which had to be perfectly accurate.
The comet is on a 6.5-year orbit of the Sun and Rosetta will join it on its journey for just over a year before spinning close to the Sun and then out towards Jupiter.
Scientists' interest in comets springs from comets' role as a “seed” of life on Earth and as primitive building blocks of the Solar System.
“Today’s achievement is a result of a huge international endeavour spanning several decades,” says Alvaro Giménez, ESA’s Director of Science and Robotic Exploration.
“We have come an extraordinarily long way since the mission concept was first discussed in the late 1970s and approved in 1993, and now we are ready to open a treasure chest of scientific discovery that is destined to rewrite the textbooks on comets for even more decades to come.”
The Visible and Infrared Thermal Imaging Spectrometer, VIRTIS, aboard Rosetta showed that the comet’s temperature was –70ºC, which told scientists that the surface would be “dark and dusty rather than clean and icy”.
Rosetta is now 100 km from the comet but over the next six weeks it will approach even closer. Scientists aim to land Philae, a part of the spacecraft, on the surface of the comet in November.
Landing a craft on a comet is tricky, to say the least. If the chosen spot does not get the perfect amount of sunlight then the craft risks either not being able to recharge or overheating.
The Rosetta has 11 instruments to study the comet and will measure the gravity, mass, density, internal structure and shape, among other characteristics.
Here's a video by the ESA showing the loopy orbit that rosetta is taking around its comet: