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As established in Positronic signature in Nemesis, positronic signatures are detectable at a considerable distance by Starfleet equipment, and Starfleet vessels seem to regularly scan for such signatures. In particular, it was decided that positrons are likely to show up even on passive scans of nearby space. (The motivation was that, in Nemesis, the Enterprise detects B-4's positronic signature from several sectors away.)

In the episodes "Time's Arrow" (Parts 1 and 2), Data's head is knocked off in a catacomb below San Francisco in the 19th Century, and the head spends several hundred years there until it is discovered during an archaeological dig in 2368. This sets in motion a series of events that includes the Enterprise coming back to Earth to investigate and several members of the Enterprise crew travelling to the 19th Century, which in turn completes the loop of Data's head being knocked off. (They also manage to bring Mark Twain to the 24th Century.)

Data's head, sheared off at the neck with some metal and wire internals hanging out, is lying in a brown dropcloth

My question is:

Why wasn't Data's head detectable to Starfleet equipment and other scientific equipment on Earth, or to Starfleet vessels in orbit around Earth?

Comparing to Nemesis, B-4 was in pieces on the surface of Kolarus III and certainly not in any kind of functional state until he was reassembled on the Enterprise. Yet, he was giving off a steady positronic signature.

Was Data's head not emitting a steady signature? If not, then why? If he was emitting one, then why wouldn't the myriad of Starfleet equipment or vessels near Earth have detected the head well before a group of archaeologists working by hand?

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    Presumably because Picard shut it off when he was dickering around in there.
    – Valorum
    Commented Jun 9, 2015 at 18:38
  • @Richard : Ahh...I didn't connect his tinkering to Data's head not giving off a signature. You should post that as an answer.
    – Praxis
    Commented Jun 9, 2015 at 18:39
  • Couldn't it be that they weren't yet searching for such signals at the time? Nemesis is a few years later.
    – user16696
    Commented Jun 9, 2015 at 18:42
  • i havent seen this episode in a while, when they dig data's head up do they already have a current data? Also were dealing with a 10 year time gap, here also the enterprise E is the top of the line for federation tech at the time. personally it may not even be standard to search for positronic signatures, but data and jordi regularly add "new" things to old scans. Data was always personally looking for others like him.
    – Himarm
    Commented Jun 9, 2015 at 18:44
  • @Himarm : They also had a current Data in Nemesis. The technology / 10 year gap speculation is reasonable.
    – Praxis
    Commented Jun 9, 2015 at 18:48

2 Answers 2

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I came up with some possibilities. Note that they're not mutally exclusive. It may be a mix of one, two or all three:

Theory #1

Picard (presumably having an awareness of the propensity of Soong-type android to emit a steady stream of positrons) somehow turned this feature off when he was mucking around inside Data's head. This would allow him to leave the head on Earth, to be found at a more appropriate time:

Using the nail, he begins to fiddle in the back of Data's head. He frowns slightly... he's not sure of this procedure, it is awkward and unfamiliar. He holds the nail up to the light for a moment, squints at it... scrapes some rust from the tip... and begins once more to do something in the back of Data's head.

Theory #2

The cavern itself prevented the positronic emissions from being detected by Starfleet or off-world visitors. Note the similarity in the description between this episode and the cavern in TNG: Silicon Avatar which was supposedly laced with "refractory metals" (and which prevented the crew from scanning for Data):

SCIENTIST: The crew was having a problem with the regulators... something in the cavern was interfering with the phase conditioners... it turned out to be the cavern itself... do you have a tricorder, Commander?

DATA: The composition of the exposed rockface has been altered by exposure to triolic waves...

Theory #3

The head was emitting positrons as normal but eventually just ran out of battery. Note that the head has been placed in some sort of recovery mode and that his blinkies are not blinking, something that in previous episodes was supposed to indicate brain death.

When we join Geordi in the medical lab, he's trying to muck around with the power flow to Data's brain, one assumes to charge him:

Computer... calibrate the power flow polarity selector...

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    We should also consider that Shinzon wanted B4 to be found. Perhaps the Remans did something to make B4 more noticeable?
    – thkala
    Commented Jul 12, 2015 at 7:57
  • @thkala - Also very possible. Shinzon may have amplified his positronic emissions or placed B4 in a place that had an amplifying effect.
    – Valorum
    Commented Dec 10, 2015 at 1:12
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I think we can safely say they were not regularly scanning for positronic signatures at the time of TNG, and did not start scanning until closer to the time of Nemesis. Support to back up this assumption is first, this own question, as to why starfleet did not detect positronic energy on earth, and second is in reference to TNG "Inheritance" in which Data works with someone who claims to be his mother, and who the entire crew assumes to be human, until her arm falls off and she is reveled to be an android like Data. In this episode i believe she was not only teleported on and off of the enterprise with no detection, the planet they were on was getting intensely scanned to find ways of saving it from destruction. Inheritance

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