The maths, of course, were straightforward. The centuries-old celestial mechanics of Johannes Kepler and Joseph-Louis Lagrange made it clear that Rosetta, a European Space Agency (ESA) mission to intercept Comet 67P/Churyumov-Gerasimenko, should work. But in space, it is the engineering that leaves room for doubt. So for 23 nerve-racking minutes on August 6th, mission scientists waited for a signal of success to travel 405m kilometres from a probe that lifted off more than a decade ago. When the message did arrive, mindful of pan-European involvement in the project, the craft’s caretakers tweeted “Hello, comet!” in 23 languages.
It has been a busy year for Rosetta. The craft was roused from two-and-a-half years of hibernation in January, and has since been putting on the brakes and preparing for this week’s delicate manoeuvres. These have settled it into a roughly triangular orbit 100 km ahead of its target.
This most recent stage of the journey, however, is not the most daring. That will come during a series of exquisitely timed moves over the next few months, as the craft spirals in to approach within 30km or so of its objective. From this vantage, it will survey 67P’s rugged terrain in order to select a landing site for Philae, a lander that it will release in November.
Project Rosetta is the latest in a series of cometary close encounters stretching back to Giotto, another ESA mission, which in 1986 passed within 600km of a more familiar comet, Halley’s. Sadly, the pictures Giotto sent back left much to be desired, but the intervening years have seen further missions with far better images, and in 2006 NASA’s Stardust probe even brought some comet dust back to Earth.
Those who take an interest in comets, however, want more detail than pictures, or even dust, can provide. So, in 2005, Deep Impact (a NASA mission, not the Hollywood film) sent a 370kg “impactor” careering into Comet 9P/Temple 1, to look beneath its surface. Stardust was then redirected to take a peek into the crater the impact created.
All these missions have shown that there is still much to learn from comets. They are now seen as “icy dirtballs”, dominated by rock, in contrast to the “dirty snowballs” hypothesised by Fred Whipple, an astronomer at Harvard University, in 1950. But the idea that the early Earth got its water from ice-laden comets ploughing into its surface is still popular. Indeed, some researchers believe comets also delivered the chemical ingredients of life.
Philae—along with Rosetta, which will continue its orbit and help with the measurements after Philae lands—will ride with the comet as it reaches perihelion, its closest approach to the sun, in about a year’s time. Philae will sample the surface, and also measure radio waves sent through the comet’s bulk to probe its interior in the way that X-rays probe a human body. Then, as 67P arrives at perihelion, more and more icy material will evaporate and escape from it as jets, and Philae should be well-placed to sniff out exactly what lies within.
From the print edition of The Economist