How do I enter the alien ship?

You will need to find where it is first.

Any clue where it might be?

You don't know about "alien," but there is another ship in town.

Or a sculpture of one.

Could maybe one be a decoy for the other?

If you haven't done so already, examine the ship sculpture in more detail.

The era when the sculpture was still being built might offer some clues.

Time travel to the 50s and look at the ship sculpture under construction.

The anchor in particular.

Aha! There's a KEYPAD on the anchor!

Of course, it's not yet connected to anything.

But now you'll know where to find it once the sculpture is fully constructed and everything is set up.

Time travel to the Present and tap the sculpture's anchor to see the now working keypad.

Getting past the keypad will get you into the ship.

You'll only need to insert a KEYCARD, successfully be approved by both CAMERAS, and enter a 3-digit alien passcode. Piece of cake.

When you've done all that successfully, step back to see an alien elevator coming up out of the ground.

Enter it to be taken to the alien ship.