Tom Servo of Mystery Science Theater 3000 gets around the Satellite of Love by a device called a hoverskirt. With it, he can hover a few inches above the ground, or even slowly descend large distances! We can see this effect clearly in Mystery Science Theater 3000: The Movie. As Crow pounds away at the Satellite of Love with a pickaxe singing It's a long way to Tipperary, Tom hovers down the ladder. When Crow breaches the hull, Tom has to adjust (right before he is pushed towards the hole by the difference in pressure):

“Woah, gaining maximum RPM, adjust pitch and yaw thrusters, stabilized... There, that should do it. Woah!”
But if Tom can hover, it raises the question: “Why does Joel or Mike have to carry him into the theater?”

The “on-screen” reason is discussed in experiment #110 – Robot Holocaust. As the movie ends, Tom Sevo is left behind and quips:

"Hey Joel, you gotta come carry me over this heating grid."
Mike was informed of this same fact during his first episode as a host, experiment #513 - The Brain that Wouldn’t Die.

It seems that there is a ventilation shaft at or near the entrance to the theater, and Tom’s hoverskirt has a difficult time getting over it. For this reason, and taking into account the incident in the movie, it can be assumed that Tom’s hoverskirt works with thrusters of some sort and not by electromagnetism.

The real-life reason that Tom has to be carried is that he is the left-most of the three characters. Because the characters entered from the right, Tom’s puppeteer (Josh Weinstein or Kevin Murphy) had to already be in position when the scene began. As Mike or Joel walked in, they would have to carry Tom with them, and hand the Tom puppet over. Crow’s puppeteer (Trace Beaulieu or Bill Corbett) could just slide in from the side with the puppet in hand. This process was reversed at the end of the scene.

The Mst3k FAQ @
Mystery Science Theater 3000: The Movie
Experiment #110 – Robot Holocaust
Experiment #513 - The Brain that Wouldn’t Die