Introduction To Rover Driving

You might think that driving a rover on another planet is similar to driving a radio controlled car on Earth. In some ways it is - control signals are sent from Earth as radio waves, which the rover receives and follows.

However, the large distances involved make things very different. In the vacuum of space, radio waves can travel at the speed of light, but even at this high speed it will still take some time to make the journey. The image below shows the distances and communication delays for some interesting locations in the solar system (partly taken from here - some websites suggest different values for the distances, so just take these as approximates).

Distances and time delays in communicating between planets One-way distances and time delays in communicating between planets/moons

The range of distances given for Mars and Europa are the theoretical minimum and maximums, depending on whether they are both on the same side of the Sun, or on opposite sides of the Sun.Note that the figures are for a one-way trip. If you wanted to send a message, and then wait for the response, then you will need to at least double the values.

For this reason rovers are actually sent a sequence of instructions to act out. For example the rover might be sent the full set of instructions to move from its landing point to the nearest boulder. The images below show two simulations provided by NASA that you can try in your web browser (the links are provided under each picture). The first is more game-like, while the second is a slightly more accurate simulation of the Sojourner rover.

The MarsQuest site also gives an idea of how much pressure the rover control team were under. The original plan was for the Sojourner rover to last only 7 days, with just one chance per day to send instructions. If you are going to spend US$250 Million to get seven chances of sending control signals, then you need to make sure each one counts. Although Sojourner actually exceeded expectations, and managed to survive for 83 sols (Martian days).

One way to test your rover driving in the real world is to get hold of a Big Track Jr (or the original 1980's BigTrack). You can program a sequence of actions using the keypad on the back, and them watch them acted out. It even comes with some plastic craters to name and run over.

Big Track Jr - The programmable rover Big Track Jr - The programmable rover