Rights: © Copyright 2015. University of Waikato. All Rights Reserved. Published 27 August 2015 Download

Comets move extremely quickly, and they’re an immensely long way away on the edge of our Solar System. At present, rockets do not have the capacity to travel at the speeds required or carry the fuel needed to catch a comet. Avionics engineer Warwick Holmes explains how the Rosetta Mission was able to achieve what appeared impossible by using a process called gravity assist.



How do we get out to a comet? Now, the problem is comets are moving very fast, and they’re very far from the Sun. So to catch the comet, which we planned the mission for, we had to get an extra 50 000 kilometres an hour of speed, and the largest rocket we have and all the fuel in the spacecraft was only going to get 8800 kilometres of extra speed, which was nowhere near enough to catch the comet.

So we had to design a mission that used what we call gravity assist, and that is we fly around the Sun, and we synchronise with planets to get the gravity of those planets to pull us forward – a bit like a skateboarder grabbing the back of a bicycle to get an acceleration to go down the road, but we have to wait for the right bicycle to come past.

OK, so this is the launch on the 2nd of March 2004, more than 11 years ago. The white line is the spacecraft. We left the Earth, but we came around and we synchronised with the velocity of the Earth to accelerate us forward by 15 000 kilometres an hour. We went out a bit further then because of that extra boost in speed. We came back in, and this time, we synchronised with Mars, here, on the 25th of February 2007.

And this changed our direction again to get the gravity assist of the Earth, and we had another very large acceleration – another 15 000 kilometres an hour. Each time, the circles are getting larger and larger. This whole sequence of gravity assist took more than 10 years to finally get the speed to get out to the comet.

This is the comet out here. The problem is, we then had to go into hibernation, because we were getting so far from the Sun, the solar panels could not generate enough electricity to keep the spacecraft alive. So when we came out of hibernation, we were out here in January 2014,  and then we follow the comet for 8 months until we do our first orbit around the comet on 6th of August 2014.

So today, this is where we are, looking down on the Solar System, so this is the orbit of Jupiter out here. Here is Mars and Earth, and we’re outside the orbit of Mars at the moment travelling at 92 000 kilometres an hour, 387 million kilometres from Earth, 270 million from the Sun. And it takes 21 and a half minutes for images from the comet to get to Earth travelling at the speed of light – 21 and a half minutes to wait for the speed of light.

We’ve travelled a total of 6.9 billion kilometres, almost 7 billion kilometres, going four times round the Sun, doing the gravity assist to catch the comet to get to this point today.

The Science Learning Hub would like to acknowledge the following for their contribution to this resource:
Warwick Holmes
Lecture video footage courtesy of the University of Waikato
Rocket launch and gravity assist footage courtesy of the ESA – European Space Agency