How do the people on Earth control the Mars probe on Mars?

They have a remote control probe vehicle on Mars. How to do they control it from so far away?|||It's a pretty interesting question, with a quite long-winded answer.





The MER rovers (Spirit and Opportunity) themselves have two antennas - one omnidirectional low-gain antenna, and one steerable high-gain dish that can be pointed towards the Earth. The low-gain antenna is also used to send data to the orbiters around Mars, including all the data gathered by the rovers. This data can then be relayed back to Earth by the orbiters.





NASA uses the Deep Space Network facility to receive data and send commands to the rovers. The network consists of three sites distributed about 120 degrees from each other along the Earth's surface, to ensure that at least one site is available at any time.





However, the DSN isn't used only for the rovers, but also for all the other probes that NASA are operating. The rovers have vastly outlasted their intended service time, so there isn't really room in the time budget for more than a few minutes per day to operate them. Mars is rotating about the same rate as the Earth, so the window of opportunity has to be carefully chosen. Signals also take between 8 and 40 minutes to travel back and forth between the planets.





This means that for the most part, the rovers operate on their own. The operations team typically design the intended tasks for an entire Mars day, and send the whole list of commands to the rovers all at once. The rovers have a pretty advanced software, capable of checking the terrain ahead of them and carefully navigating obstacles. Maximum speed is 50 mm/s (2 in/s), so any collisions would be pretty gentle. The software has been upgraded once, and is now capable of making some choices about sending images, probing rocks and such.