Question

in deep space, there is very little friction. once they launch a probe into deep space, where there are no external forces acting on it, scientists shut the probes engines off because the scientists want the probe to
move at constant velocity.
speed up.
stop immediately.
slow down.

Explanation:

According to Newton's first - law of motion, an object in motion will stay in motion with a constant velocity in the absence of external forces. In deep space with no external forces and little friction, shutting off the probe's engines means no additional forces to change its motion state. So it will move at a constant velocity.

Answer:

move at constant velocity