Argthtjtr

I am currently creating a Spacefaring Race that regularly travels between planetary systems in order to harvest resources, but due to the fact that the conditions required for them to Breed are fairly specific, they must regularly return to their homeworld in order to reproduce, in much the same way Salmon on Earth do. This has been a fairly constant theme throughout their species' evolutionary history, and since they are driven by instinct to return to their birthplace on their own planet they have developed remarkable navigational skills that have served them well as they ventured out into space.

However, on their own planet, at least part of their success in accurately navigating complex long-distance migration paths came through their acute sense of smell (Again, much like Salmon). While I do have a means by which they can fairly accurately navigate in space (in addition to Extremely Advanced Spacial Awareness Skills and a natural penchant for gauging the distances/positions/speeds/sizes of Objects, they also have developed Spectroscopic Vision that allows them to use stars of particular compositions as Landmarks or Waypoints), I was wondering if there would be a way for them to incorporate Scent, or if this would have be limited specifically to their exploration of planets and moons.

If they could somehow sniff the vacuum of space without suffocating... Is there even anything to smell, if there's no air to carry it? What would space even smell like? Could things leave a reliable scent-trail in a vacuum?

If not, is there another alternative that they might find similarly intuitive?

Most likely not

To simplify smell, a creature is taking in atoms or molecules into a sensor, and those atoms or molecules stimulate the sensor.

Searching the web it seems it's generally accepted that there are approximately 100,000 - 1,000,000 atoms per cubic meter in Space. Contrast this with Earth's atmosphere at sea level, which has about 300,000,000,000,000,000,000 atoms per cubic meter (3*10^22).

Which means for every 1 atom in space you'll find 60 quadrillion atoms here on Earth. Your animals' senses of smell would have to be extraordinarily more powerful than any Earth creature's.

If I wanted to "smell" space I'd use a mass spectrometer

with such a long mean free path, smelling would be highly directional with all the limitations that implies (like line of sight)

Smelling is almost the exact same as tasting with your tongue and they are very closely linked. It happens because particles react with certain cells, and those cells send a signal telling you what you are tasting/smelling.

Because it's space, there's almost no particles to interact with. You also can't create a lower-pressure area in your body to attract those particles into your nose.

The solution, if you want it, would be to have large sail-like pieces around your body that use their surface area to get enough particles to interact and give you an idea of your surroundings. Using Nex Terrans answer you would need to make it large and travel fast to interact with more volume of space. Since you are traveling interplanetary distances you should be encountering enough particles. Using this article: https://www.google.com/url?sa=t&source=web&rct=j&url=https://tools.niehs.nih.gov/wetp/Public/DOE_Trainers/13_HANDOUT_SENSE_OF_SMELL.pdf&ved=2ahUKEwjJ1NLS09bfAhUIJVAKHcmVDUQQFjAAegQIAhAB&usg=AOvVaw3BDR52z0714Pyk4lO_U5o9 you can see that some particles can already be smelled at 0.008 parts per million, combine that with traveling through space at interplanetary speeds and you'll be scooping up more than the 3*10^22 particles per m3 every second to smell.

You would have to know the particular (average) composition of particles of different solar systems and solar winds to make sense of them. I'm not saying it's impossible, I'm not saying it's possible, but it's not going to be a very easy or aestetically pleasing thing I think.

There's a YT video of a former space station astronaut. Amongst the things he discusses is what he believes is the "smell" of outer space. Basically he said if you broke seal in the airlock before the space properly re-pressurized, you would smell "slightly burned meat." He explains why he thinks that odor is detected.

https://www.youtube.com/watch?v=t6rHHnABoT8

Believe it or not, there are aromatic molecules in space, and these molecules come in a number of fun flavors which vary depending on where you are. There is a dust cloud near the center of our galaxy that contains ethyl formate, the same chemical that gives raspberries their smell. Other chemicals found in space include familiar smells like ammonia, hydrogen sulfide, and formaldehyde.

Nothing could breathe vacuum, but fortunately breathing and smell don't have to go together. Plenty of Earth animals smell through organs completely unrelated to their breathing tubes, in fact the best sniffers on Earth - moths - smell using their wide, feathery antennae. Space doesn't have a lot of molecules to go around, but I'd expect any space-dwelling creature to have wide solar "sails" to provide it with energy and it's not an altogether bad idea to put some chemosensing organs on the same sails.

That being said, smell is probably not a great way of navigating the void. While smells don't need air to carry them (smells are simply gas molecules and gas can spread out through space once it is released) it tends to fade away to nothingness pretty quick unless the smells are created in vast quantities by something like an exploding star. But a spacefaring species that could smell in space for whatever reason could very well recognize the scent of its home system.

You could have them smell the solar wind.

https://www.sciencefocus.com/space/what-is-the-solar-wind-made-of/

The solar wind is a stream of energetic particles ejected by the Sun.
These include electrons and protons from hydrogen, along with atomic
nuclei like helium, otherwise known as alpha particles.

There are also traces of ‘heavy ions’ and atomic nuclei of carbon,
nitrogen, oxygen, neon and magnesium. Recent missions have even
detected tiny amounts of potassium, titanium and nickel.

Probably the solar wind produced by each star is characteristic of that star. Maybe just as salmon can taste their own home stream even far out at sea, your creatures can taste the particular components of their home star even far away in space. Of course this wind is made of charged particles moving very fast through the vacuum - but they are particles, not electromagnetic radiation or vibrations, and so smell or taste would be the most relevant sensory modality.