rock drive

when a material is heated it expands 

and when heated enough it becomes gas or even plasma 

so rock could be taken from an asteroid 

and could be used as reaction mass 

which could be heated enough to produce exhaust from a rocket engine 

an asteroid without any gas could be chosen 

among all the asteroids in the asteroid belt 

rocket engines could be placed on that asteroid 

and robots could land on that asteroid 

robots would dig rock material from asteroid 

and place that rock material in the rocket engines 

the rocket engines would heat the rock material 

so that the rock material is exhausted from rocket engines 

propelling the asteroid forward  

the rocket engines would use electricity to heat the rocks 

and would get electricity from nuclear reactors 

since solar panels would get too little starlight 

during interstellar travel between stars 

such an asteroid drone could be large enough 

to carry lots of reaction mass for it's rocket engines 

which would enable it to travel at constant acceleration 

for very long distances 

which would enable it to reach a high top speed 

and since it's a drone it wouldn't need to slow down 

unless it finds something extremely interesting 

but the asteroid drone would need nuclear materials 

to power it's nuclear reactors 

unless it would use solar panels 

but if it used solar panels 

then it wouldn't be able to accelerate constantly 

since it wouldn't get any starlight when it traveled between stars 

if there is gas on the asteroid 

then the gas could be used as reaction mass 

or the gas could be used to create heat 

that would heat the rocks 

that would create exhaust from the rocket engines 

the asteroid drone could have many missions 

but primarily it would be exploration