By Rhett Allain
If the International Space Station (ISS) orbits at an altitude of about 410 km above the Earth, why does it sometimes take a vessel like ATV two days – or more – of travelling to get there? If you drove your car 410 km at highway speeds, it would only take about 4 hours. So why does a spacecraft with pretty powerful rocket engine take so long? The answer is that it’s all about orbital mechanics.
Editor's note: In addition to having a knack for science communication, Rhett Allain is Associate Professor of Physics at Southeastern Louisiana University. He writes regularly for Wired's Dot Physics blog and is a bit of a physics fanatic who spends more time than many pondering how daily life intersects with science. With the recently announced development of ATV in cooperation with NASA for Orion, we're delighted to feature a few posts from the far side of the Atlantic. Enjoy!
Suppose you, like ESA, want a spacecraft to orbit the Earth in a circular path. Below is a sketch of that object with the only force acting on it (note that this is not to correct scale). There is just one force on this object, the gravitational force from the interaction with the Earth.