# Do the Mars rovers have to be stationary to turn?

I saw a video of the mars rover curiosity turning in place. It seemed really stable by turning its wheels it was able to easily reorient itself even on a slope while not moving forward. This made me think back to a lot of the odd track patterns in rover pictures.

My main question is:

Does the rover have to be stationary to change direction? Or can it turn and move like a car? If it can- what's the maximum degree it can turn while in motion? I'm sure they have something in place to avoid hairpin turns. If it can't- what was the reason behind this design choice? Stability?

According to this description of curiosity's wheels it is able to "swerve and curve" -- it seems the bogie can boogie (emphasis mine).

The Mars Science Laboratory has six wheels, each with its own individual motor.

The two front and two rear wheels also have individual steering motors (one each). This steering capability allows the vehicle to turn in place, a full 360 degrees. The four-wheel steering also allows the rover to swerve and curve, making arching turns.

In practice, the rover is driven very cautiously, usually with each movement sent as a command in advance. The simplest way to do this is to have it always move in a straight line, then stop and turn a specified amount. It also normally takes a picture at each step.

When curiosity moves autonomously, it identifies obstacles with its camera and swerves around them. The process is slower, so most movement will be a combination of straight-line moves and in-place turns.

driving on Mars

• I don't see any reason for a hard limit on how tight of a turn it would make while moving. Perhaps we can find something about tests of the extremes on the Earthbound prototype. Practical reasons for limiting turns would be to avoid slippage and possibly to avoid jolting sensitive components if they are in use while moving. Oct 12, 2019 at 15:35
• It's moving at less than 1 km/h. You can basically crank the wheels to 90º and not fall over. Oct 12, 2019 at 17:43

This picture from Opportunity (public domain) shows plenty of rover tracks in Eagle Crater. Most of the "turns" appear to be the "stop and turning place" variety. However, there do appear to be arcing motions in the center of the crater and as the rover exits, suggesting that when it made the track the rover had been moving around a center of curvature outside the span of its tracks. Possibly the arcing turns may have been simpler to implement on the curved, sloping surface of the crater.

The tracks do not, of course, give any indication of the speed of the rover, and as indicated in a comment by Hobbes the rover was certainly moving quite slowly as it made the complex maneuvers.

• Thanks for catching that. Oct 13, 2019 at 22:41
• Could be that during photographs its more stationary. Survivors bias, photographing less of the wheel arcs while in motion-- as capturing photographs isnt the primary concern. Oct 13, 2019 at 23:28