Based on the postulate that a cubesat is a low-power transmitter without a directional antenna, then the most suitable example would be a Soviet balloon in the atmosphere of Venus.
Its transmitter had a power of 4.5 watts. The signal was received by the American and Soviet Deep Space Network and a number of other radiotelescopes in several ...
Based on some research, I could not find a cubesat that has communicated with the Deep Space Network yet.
However, there are two interesting findings:
The Lunar IceCube Cubesat, planned to launch on Artemis-1 on 2021, will validate the use of the Delay Tolerant Network (DTN) for the first time on a cubesat in deep space. For that purpose, the Morehead State ...
AFAIK it is "0.0 kg".
Orbital launch vehicles with pressure-fed first stages are few and far between. Diamant had that with its Vexin engine, but solid upper stage.
I don't know of any successful orbital vehicle with all pressure-fed stages, but maybe there's an obscure one out there.
See also Some examples of pressure fed engines used on launch ...
a hydrogen-oxygen fuel cells were first used on the manned spacecraft Gemini in 1965.
Image source: Smithsonian
More info: On the Shoulders of Titans Chapter 7
Soviet designers had no problems with the additional mass of energy sources and were forced to use fuel cells only in the Buran spacecraft.
The U.S. Vanguard rocket reached orbit three times with a first stage thrust of only 125 kN.
The first stage of the three-stage Vanguard Test vehicle was powered by a GE X-405 28,000 pound (~125,000 N) thrust liquid rocket engine.
Vanguard TV3 — NASA NSSDCA
Note: When this answer was written, the question read as follows:
What is the lowest first stage rocket engine thrust for a rocket that reached Earth orbit?
The first stage vernier engines on early Atlas boosters were 526 lbf (2.3 kN).
@2012campion's answer shows that this was not the lowest thrust
The smallest rocket to reach orbit is the Japanese SS520-5. It had a peak thrust of $185 kN$ according to the same web page:
Firing up its first stage, SS-520-5 shot up from its launch rail at 2:03:00 p.m. local time on Saturday with its aft fins sending the climbing rocket into a spin to ...
The definition of "reaching space", in contrast to orbit is a bit arbitrary. The Kármán line of 100km is often used.
An amateur rocket launch, but the USC rocket team reached 103.6km with their Traveler IV. They list their rocket as 13 foot tall, or around 4 metres.
That gives an upper bound.
If you consider 72km sufficient, the Black Brant VI and ...
It's very likely that inhabitants of the atoll Makatea in the South Pacific were the closest to Mars !
(As was discovered and announced by @SE-stop firing the good guys in one of his comments to this answer.)
Now that one of the other answers has ruled out any Apollo astronaut, and the ISS crew could be excluded too, we can focus on determining the exact ...
The Lambda 4S from Mark Adlers' answer has later been surpassed by another Japanese rocket, the SS-520, which successfully delivered a 4kg payload to LEO in 2018.
The total mass of the vehicle is 2,600kg, which with propellant subtracted is 610kg of dry mass.
As you correctly guessed, it's indeed a repurposed sounding rocket.
That is as far as I can tell the SS-520, placing a 4kg payload into LEO in 2018, with the total length of the launch vehicle being 9.54 meters.
If this record has not been surpassed after 2018, it still stands.