What Keeps Mars Express from Getting Lost in Space?
Posted on: Friday, 30 January 2004, 06:00 CST European Space Agency — Determining the three-dimensional position of Mars Express in space with as much precision as possible, at a distance of 155 million kilometres away from Earth, is no simple business. If the Solar System were shrunk down so that Earth was only the size of a soccer ball, then Mars Express would be a speck of dust floating well over two kilometres away! ESA ground stations have kept track of the spacecraft’s range and velocity ever since launch. By measuring the Doppler shift undergone by the signal coming from the distant spacecraft (like the changing sound of a passing police car siren), they can calculate the spacecraft velocity component along the line of sight to Earth down to a few millimetres per second. Ranging data can be acquired simply by sending a signal to Mars Express then measuring how long it takes to come back. These data are combined together by the mission’s Flight Operations and Navigation team wit
