As the ship is preparing to wake the crew there seem to be helmet-like objects -- what are they?

Question

They do look very much like helmets but why would they not be part of a complete spacesuit?

One thing I would add is as the ship is going from its own semi-dormant state, as the computer systems and lights and presumably temperature is adjusted for the humans to awaken to, the "helmets" appear to have text run across the faceplate although this could be a reflection, not something the "helmet" is actually generating.

So perhaps the devices are just helmet-like parts of the computer system although if they are terminals for example, they are atypically shaped.

Answer 1

Nostromo Emergency Helmet ALIEN (1979)

The crew's helmets were left secured to their bridge chairs during the opening scenes of the film

http://alienexplorations.blogspot.com/1979/09/two-robots-chatting.html

Ridley didn't want to do the whole of the first act with people wearing helmet and it got to the point where he was to be asked about the need for these objects

So he was asked "Well, why the helmets then?"

Ridley's answer was "Well if you’re in rough conditions, ok? "

Emergency Helmets

In the beginning of Alien, Ridley wanted to have a computer wake up the other computer and then fundamentally wake the crew up, and this developed into Ridley caming up with the sinister idea of having computer screens reflected in the faces of what were essentially emergency helmets or for when certain conditions arised, perhaps just as one might put the helmets on just when flying a helicopter.

Two Computers Chatting

First of all in the storyboard the helmets were mounted on clips on the seats and later they were positioned on the backs of the two seats. Once Ridley saw the Nostromo's bridge set coming together Ridley had the idea late in the day of the two robot like voices chatting away with one another as if they were asking "Shall we wake them up?" because Terry Rawling the editor had been playing around with Tomita's rendition of "Mars Bringer of War" in the editing room while they were cutting the movie, which featured such computer like voices chatting.

16 millimetre projectors projecting

Ridley developed some little 16 millimetre projectors that simply projected the displays onto the visors of the flight helmets as if it were the small screens of the monitors reflecting relaying alpha numerics and conversation between the two computers, one computer goes "bink bong" and they start to talk. Ridley decided to call the two computers Tom and Jerry. None of it had been scripted but lot of it had been worked out by Jimmy Shields who was his sound designer at the time.

Ridley Scott:

The sinister idea of having the screens reflected in the faces of what were essentially emergency helmets which are always in position on the back of two seats seemed to be a great way of , er, opening the film. Er, I came up with that idea when I actually saw the set coming together, and so the addition of the helmets talking to each other was late in the day, but I think it worked pretty well. (Alien 20th anniversary dvd commentary)

https://uk.propstoreauction.com/view-auctions/catalog/id/44/lot/9342/index.html

In my personal opinion, these are analogous to modern day emergency masks on aircraft or ACES and SOKOL suit helmets (i guess the suits aren't required or are elsewhere, but probably not considered for the movie). Both ACES and SOKOL are not spacesuits in of themselves but rather are emergency suits - keep the wearer alive in the event of an accidental depressurisation of the spacecraft/allow crew to escape the shuttle in certain emergency situations.

Similar to my own gas masks, the thing at the front of the faceplate is to facilitate drinking (with straws, etc).