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In VOY Homestead, Voyager has detected Talaxian life signs 4.9 lightyears away. In VOY Workforce, Kim finds life signs that are 'less than 3 days away at maximum warp', which for Voyager would probably mean 25 light years (source)

That struck me as being very far away. Their sensors would need to be extremely powerful, even accounting for the incorporated Borg technology.

But have we seen larger distances at which a Starfleet vessel has detected life signs (in canon)?

dennis_vok
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  • http://scifi.stackexchange.com/questions/54153/what-is-the-range-of-the-enterprise-ds-sensors I would refer you to this discussion – Vanja Vasiljevic Oct 27 '16 at 11:17
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    What's a "life sign" anyway? – ths Oct 27 '16 at 12:13
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    Are (active) scanners ftl? Or were there lifesigns 4.9 or 25 years, respectively, ago? – I'm with Monica Oct 27 '16 at 12:51
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    Interesting point. All episodes seem to suggest that the scanners are faster-than-light, because the detected life-signs are always there now. E.g. in the Workforce example there were (presumably) no human life signs at the delta quadrant planet 25 years ago. – dennis_vok Oct 27 '16 at 12:56
  • @ths - That's covered in the Technical manual, as is the fact that the scanners work at FTL speeds – Valorum Oct 27 '16 at 13:03
  • @AlexanderKosubek - That's covered in the Technical manual. The scanners work at FTL speeds – Valorum Oct 27 '16 at 13:03
  • "Less than three days at maximum warp" means 25 lightyears at most. Normally, saying something takes less than three days means it takes more than two days. Seeing the supposed sciency nature of statements made it would have to mean more than 2.5 days. That gives you a distance between 21 and 25 lightyears. –  Oct 27 '16 at 14:22
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    Related, not really a dupe; How Do Starfleet Sensors Detect Life Signs?. – Valorum Oct 27 '16 at 17:32
  • Thought I had an answer but it turns out that it was detection of an M-class planet and this question is about lifesigns. But it was at 132 lightyears' distance. VOY season six Friendship One. –  Nov 13 '16 at 22:23
  • there is probably some variation based on other factors, we know for instance that nebulae, geological phenomena and certain alloys (I seem to remember the maintenance tubes on DS9 being sensor shielded) effect it. as well as intensity of source (life signs are often described as being "weak" when characters are injured). so if there was a M-class planet with a large population that could be detected probably hundreds of light years away meanwhile a wounded person in a wreckage might be unnoticed even just a few hundred meters away. – Ummdustry Jan 05 '18 at 20:33

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In general, there is not an established distance at which the arbitrary term 'life signs' can be detected.

Indeed, given that it seems to be a colloquial term (I doubt they have a graph on their sensor readout stating 'life signs') that is based upon the interpretation of the person reading the sensors, different people could detect 'life signs' at very different ranges.

Voyager suffers from this wild difference more than most ships would, given that the crew is a mix of Starfleet, Maquais, and random aliens picked up along the way. Once they start heavily incorporating alien technology into the ship, it becomes an even bigger gray area.

It's entirely possible that someone who is monitoring passive sensors and detects regular subspace communications from a location (which could also be referred to colloquially as 'a signal') could report it as 'life signs', as it would be potential signs of life. The same could be said for lightspeed communications such as radio, or an analysis of a planet's atmosphere's chemical makeup.

There are numerous ways to detect signs of life, and Star Trek's sensors (which do have FTL capability when used actively) can likely report on all of them. When close enough (and no, I do not have a definitive answer for 'how close') they can likely detect life signs in a number of ways including heat signatures, bio-electrical signatures, optical sensors (coupled with effective computerized visual recognition), or even remote chemical analysis of the atmosphere around a being.

Out-of-Universe, of course, the answer is "They can detect them at the range the plot requires." Voyager was especially bad about this, possibly due to a more lax adherence to a strict technical manual/series bible or the necessities of creating engaging episodes when the show format practically required 'problem of the week' episodes instead of longer overarching narratives.

Jeff
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    I think it's narrative necessity, not that TNG or DS9 were light on "problems of the week", but because they could have other ships/stations/colonies notify them of problems. Voyager had to find all its problems on its own. – Cadence Nov 14 '19 at 16:38