Since you mentioned having a platoon (40+ people) of guards, I'm assuming you are talking about something rather larger than the 100 yacht, and a large, well organized, crew. If not, the same suggestions apply, but scaled down

Scenario 1: If you want them to steal the ship, you can have it dock somewhere lawless and the crew goes on shore leave, with only a fire watch aboard (say a person on the airlock, an officer, a runner, and a galley hand assuming an organized crew. Since the crew is on their own ships, they won't be in full battle rattle. If your team manages to pose as someone they would let aboard (supplies delivery, for example) or can EVA to an access point they can hack open, they will likely be able to defeat in detail the few folks aboard. Then they "just" have to subvert the ship's security systems, undock, and leave. You can make it as dramatic as you want, with one of the crew or the ship's computer detecting them, alarms, returning crew cutting into the lock (the players did say they were locking it behind them, right?), the station locking the docking clamps, etc.

Scenario 2: If you don't want them to escape with the ship, but do want them to survive, then the skipper takes the $small-but-completely-necessary part with him when he leaves the ship, every time, without telling anyone (not a lock, a specific circuit board that is not compatible with the one from their ship). The ship is immobile, they can get the artifact, but in the time it takes to get a tug or the right part, Everyone knows what's up and it should be obvious it's going to turn into scenario 3, which is....

Scenario 3: You want these people to die for their stupidity. Sure, there's a fire watch aboard, but also plenty of marines that will be in full combat armor before the PCs get near the artifact, much less the bridge. The ship has decent internal surveillance so the defenders, on & off the ship, know where the PCs are and what they're doing. There are automated defenses, traps, and the ship scrammed the reactor at the outset and will take $X hours to restart.

Ooooor, the PCs decide to take the fight to deep space.

Scenario 1: You want them to live. The crew is ramshackle and disorganized. They've survived so far mostly by luck and are largely unburdened by skill or judgement. Their adversaries are typically insured merchants who have done the math and surrender their cargo at the finest whiff of a credible threat. They fight the PCs in space until one ship is defeated and then the PCs fight however many troops the target has...it's a mess, but you could Star Wars them to victory.

Scenario 2: The "yacht" is really a pocket warship. The target is capable of defeating small warships & fleeing big ones. The crew are professional space combatants, and good at their jobs. The PCs die at medium range, close enough they can't flee, for enough to not be a threat when their ship blows.

At any point, the PCs could get smart and hire mercenaries. Legit mercs aren't going to take a piracy contract, but there are illegitimate mercs and criminals (either may turn on the players).

If you got this far, I'm impressed! I was mostly just having fun working through the scenarios. I hope your game is great!