Artist impression of ESA's Rosetta approaching comet 67P/Churyumov-Gerasimenko. ESA / ATG medialab / Rosetta / Navcam

Comets can tell us about the early Solar System, and they may have brought the chemicals of life to ancient Earth. Could we learn more about comets? Yes, said the European Space Agency (ESA). Their mission would meet a comet in deep space and accompany it around the Sun.

The mission and the spacecraft were named Rosetta, and a lander would be Philae.
Rosetta and Philae are places in Egypt where stones with hieroglyphs were found. No one could understand hieroglyphs in the 19th century, but these stones also included writing in known languages. That was the key to decoding the hieroglyphics. ESA hoped that the Rosetta mission would find a key to help us understand the evolution of the Solar System and life on Earth.

Rosetta was launched on March 2, 2004 for a 2014 meeting with Comet 67P/Churyumov-Gerasimenko.
The original plan was to meet comet 46P/Wirtanen in 2003. But the launch had to be postponed because of problems with the launch rocket. This meant that Rosetta couldn't get to comet Wirtanen, so they had to choose another comet.

Help from gravity
ESA couldn't launch Rosetta straight into the outer Solar System to chase the comet. No rockets are powerful enough to do that. Space missions use gravity assists for extra power. A planet's gravity can speed up — or slow down — a passing spacecraft. Scientists plan the spacecraft's course to get extra acceleration while saving fuel. A gravity assist is also called a swing-by or a gravity slingshot.

Rosetta used Earth's gravity three times, and the gravity of Mars once.

1. The first gravity assist from Earth in March 2005 helped Rosetta get to Mars. On the way to Mars, her instruments watched as NASA's Deep Impact probe hit comet Tempel 1. It happened on the Fourth of July!

2. A Mars 2007 gravity assist corrected Rosetta's speed. It had to slow down to make up for the differences in the orbits of Comet Churyumov-Gerasimenko and Comet Wirtanen.

3. In November 2007 there was to be a second gravity assist from Earth. And something funny happened when Rosetta was on the way home.

Early in November, several observatories spotted a “near Earth object” heading towards Earth. It would come closer to Earth than many communications satellites. The Minor Planet Center, which keeps track of asteroids, gave it a number: 2007 VN84.

An asteroid headed towards Earth at the same time as Rosetta could have been a danger to the spacecraft. But maybe you've already guessed that the mystery "asteroid" was Rosetta. A Russian astronomer was the first to realize it, and 2007 VN84 was quickly erased from the minor planets database. If spacecraft could laugh, Rosetta would have been amused. Leaving Earth behind, she made her way to the asteroid belt to see a real asteroid: 2867 Steins.

Rosetta made her last visit home in November 2009. It was to get one last gravity assist to return her to the asteroid belt for a fly-by of asteroid 21 Lutetia. Then a new phase of the adventure would begin.

Rosetta is the first spacecraft to travel farther away than Mars using only solar panels for electricity. After the Lutetia fly-by, Rosetta would be going so far from the Sun that there wouldn't be enough sunlight to power all the instruments.

In June 2011 Rosetta went into hibernation. The small amount of power available was used to run the computer and the heaters that would keep the instruments from freezing. Rosetta spent over two and a half years in deep space out of contact with Earth.

At last, on January 20, 2014, Rosetta's “alarm clock” went off, and the spacecraft woke up. It was time to catch that comet. Rosetta caught up with the comet and went into orbit around it on August 6th.

On November 12, Philae landed on the comet. In fact, it seems to have landed three times with two bounces. That wasn't good, and they didn't know where Philae was. The lander did send data until the battery ran down. There was hope that with more light, Philae might wake up again. On June 14, 2015 that happened. There were more short communications, but the last one was on July 9.

The end of the story. Philae was finally found in September 2016, less than a month before the end of the mission. The mission ended on September 30, 2016 when Rosetta left orbit and landed on the comet. Every six and a half years, Comet 67P comes into our part of the Solar System. It rounds the Sun with Rosetta and Philae on board.