## Rosetta in final approach to 67P/Churyumov–Gerasimenko

After three years of hibernation Rosetta finally woke up and is still on it’s way to comet 67P/Churyumov–Gerasimenko. But it’s not like there’s not much to do; in the next few days Rosetta will adapt it’s orbit to match that of 67P and since yesterday ESA started bringing up Rosetta’s and Philae’s science instruments online.

As the BBC reported yesterday, OSIRIS and Alice were successfully recomissioned so I’m quite sure that we’ll see some first-light images from OSIRIS pretty soon.

However, what I find really interesting is how the final approach of Rosetta to 67P really works. As of today, Rosetta is about 4.2 million km away from 67P. Rosetta, along with Philae, only has a mass of about $3000\,kg$, so we can’t really talk about gravity here; 67P’s mass is around $3.14*10^{12}\,kg$ and doing the math there’s only very little force at work:

$G\,*\,\frac{3.14 * 10^{12}\,kg * 3000\,kg}{(4.2 * 10^6 m)^2} \approx 3.56 * 10^{-8} N$

Rosetta orbiting the comet; ESA

So it’s not really a matter of “bringing Rosetta into an orbit around 67P” but matching Rosetta’s orbit to 67P/Churyumov–Gerasimenko’s orbit around the sun. However, the closer Rosetta get’s, the greater gravity gets. In the final stage – let’s say when Rosetta is only around 5 km away from the comet’s center of gravity, the force increases to around $25.15\, mN$ – not very much, so approach will be tricky :-)

Rosetta has a Reaction Control System (RCS) on board which is used to get the right attitude; I haven’t found any specifics so far, but as soon as I find more information, I’ll cover this in another posting.

Update: Adapted $m_{Rosetta}$ for it’s mass is 3 tons, rather than 192 kg.