The requirement of a Ph.D to design robots that explore other planets really depends on what part of the robot design you would like to do. To put a robot on another planet requires MANY different kinds of jobs.
For instance, the instruments that are selected to go on a particular robot are often designed at Universities by researchers and professors or at private companies. Usually the instrument teams are lead by people who have Ph.Ds in their specific area of research because it is highly specialized. However, those who actually build the instrument may not.
Others design particular parts of the robot, like the propulsion system, the wheels or landing system or the materials that the robot is made of like thermal protection material or materials that have particular properties that can survive the extreme conditions on other planets. People in all of these positions do not necessarily have Ph.Ds but some do. Usually those who do have a Ph.D specialize in a very specific aspect of the design.
As for myself, I work as part of the team who designs rovers to go to other planets. I do not have a Ph.D but I do have a Masters in Mechanical Engineering. I work specifically on the robots flight through the atmosphere. For example, I help to design parachutes used for landing the robots by using models of the robot and the atmosphere in computer simulations to determine how fast it has to slow down to land safely. That translates into how big the parachute has to be and when it needs to be deployed. Those results are provided to the parachute makers and the rover builders. Everything we do is interconnected but many people work on a tiny aspect of the whole thing.
All of these are just a few examples of what aeronautical engineers do.
In summary, many engineering jobs do not require a Ph.D, many scientific research jobs do. My advice is to try different things, intern at different places, then decide which is best for you.
Hope this helps,