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Miniaturisation has many advantages, and in recent times, nanosatellites, picosatellites, and femosatellite concepts have emerged. The Wikipedia article on miniaturised satellite has some pointers.

What is the smallest operational artificial satellite that exists or has existed? By operational, I mean any satellite that performs or has performed a particular function, however modest, but at least communication with Earth (either through a relay, or directly to a ground station), thereby excluding orbital debris (that would also classify as artificial satellites).

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    $\begingroup$ (CLASSIFIED)... $\endgroup$
    – Deer Hunter
    Commented Aug 1, 2013 at 16:19
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    $\begingroup$ @DeerHunter Actually, from this popular science article spy sats are rather large, huge antennas/telescopes to look at small details happening on Earth. I don't know of any applications of tiny satellites that should be secret. $\endgroup$
    – gerrit
    Commented Aug 1, 2013 at 16:21
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    $\begingroup$ gerrit - en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pebbles :) you can take this as a starting point. $\endgroup$
    – Deer Hunter
    Commented Aug 1, 2013 at 16:24
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    $\begingroup$ Aah, water-melon sized bullets. But those don't satisfy my communication with Earth criterion. I also believe some "proper" artificial satellites are actually smaller already. $\endgroup$
    – gerrit
    Commented Aug 1, 2013 at 16:26
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    $\begingroup$ I doubt this is the type of answer you are looking for, but the West Ford Needles were < 2cm long and about 20 μm thick, and they were (temporarily) a way to relay communications between points on Earth. $\endgroup$
    – user29
    Commented Aug 1, 2013 at 17:00

6 Answers 6

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Update: First KickSat is now officially in orbit since today, April 18, 2014 and the 104 Sprites will be deployed in 16 days after the launch giving KickSat CubeSat enough time to complete its system checks. The 3U CubeSat unit was deployed onboard the SpaceX Falcon 9 during its Dragon SpX-3 mission:

The 3U CubeSat consists of a 1U avionics bus that provides electrical power, active attitude control, command and control systems and communications for telemetry downlink and command uplink. The remaining 2 CubeSat units are dedicated to the deployment mechanism that holds and secures the Sprites during launch and in orbit using a lid. The mechanism can hold up to 200 Sprites and uses a spring-loaded pusher and by a nichrome burn wire system to deploy all the Sprites. The prototype mission launching on SpX-3 carries 104 Sprites. The KickSat CubeSat weighs 5.5 Kilograms.

Source: Spaceflight 101 article on Dragon SpX-3 Cargo Overview

And this is how each of these 104 Sprite ChipSat femtosatellites look like:

       enter image description here

       enter image description here

And the whole 3U KickSat with the ChipSat trays, release mechanisms and main bus unit looks like this:

       enter image description here

All images credit: KickSat on HitHub (includes more info and a video of KickSat sprite deployment). All 104 Spite spacecraft will be deployed at intervals on a timer starting at 4:00 PM EDT (20:00 GMT) on May 4th.

Not yet flying on their own (but already used since 2011 when they were attached to the ISS external platform for testing), so this is answer is a bit premature for this question, but in December 2013 the now successfully crowd-funded KickSat project will launch first free-flying Sprite femtosatellites in low orbit, presumably onboard the SpaceX CRS-3 ISS resupply mission:

               Sprite femtosatellite

               KickSat's Sprite femtosatellite design prototype in hand, the size of a couple of postage stamps.

Three prototype "chip satellites" were launched to the ISS on Space Shuttle Endeavor on its final mission in May 2011. They were attached to the ISS external platform Materials International Space Station Experiment (MISSE-8) for testing.

As far as future plans go, a bit confusingly also named KickSat (maybe KickSat 1 rather?) will be a 3U P-POD CubeSat (housing a 1U avionics bus and a 2U Sprite deployer) carrying 128 of Sprite femtosatellites into space, launched as auxiliary payloads through the NASA's ELaNa program, and will be deployed in low Earth orbit (between 300 and 350 kilometers).

       KickSat deploying Sprite femtosatellites

       Artist's impression of the KickSat deploying Sprite femtosatellites (source: KickSat on GitHub)

After being released from the P-POD, KickSat will perform a de-tumble maneuver and establish communication with Cornell’s ground station. After check-out, the spacecraft will be put in a sun-pointing attitude and spun up to maintain that attitude. A command signal from the ground station will then trigger the deployment and the Sprites will be released as free-flying spacecraft.

After deployment, telemetry and sensor measurements from the individual Sprites will be received through Cornell’s ground station in Ithaca, NY, as well as several other amateur radio ground stations throughout the world.

More information on KickSat's Sprite femtosatellites can be found on the KickSat's KickStarter and KickSat's GitHub pages.

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    $\begingroup$ This makes me twitchy, as I do orbital debris work for the ISS for a living. These things are exactly the right size to cause maximum damage -- too small to track, too big to defend against. $\endgroup$
    – Tristan
    Commented Sep 3, 2013 at 13:31
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    $\begingroup$ @ZoltánSchmidt They can, and you probably have some in your smartphone, gaming controller,... And they're getting even smaller, for example this STMicro A3G4250D (PDF) can already be bought and is 4x4x1.1 mm in size, packaged as 16-pin LGA and has all three axis angular sensors. There are some even smaller ones coming out soon, e.g. L2G2IS 2-axis MEMS gyro at 2.3x2.3x0.7 mm. $\endgroup$
    – TildalWave
    Commented Apr 19, 2014 at 11:30
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    $\begingroup$ @TildalWave Oh, yes, I can remember now. I feel myself so outdated now... $\endgroup$
    – Zoltán Schmidt
    Commented Apr 19, 2014 at 13:06
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    $\begingroup$ From the Github page: "making it easy enough and affordable enough for anyone to explore space" - how is putting debris in earth orbit space exploration?! Can someone explain me the point of all this, besides "because we can and it's cool"? $\endgroup$
    – SAnderka
    Commented Apr 22, 2014 at 7:25
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    $\begingroup$ @SAnderka I don't understand your point. First, your question is misplaced because I'm not some KickSat evangelist, I'm just answering the question. Second, these are not debris but functional devices. Third, they lack ative propulsion and their orbit will naturally decay fast enough via atmospheric drag and lack of momentum (small mass, relatively large cross-sectional area). Their orbit will be well known, can be easily tracked, and have small relative Δv to anything else in LEO. The whole Sprites concept also isn't new. $\endgroup$
    – TildalWave
    Commented Apr 22, 2014 at 13:19
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There are only very few projects, which are smaller than cubesats (1U @ 10x10x10cm). One of the key guys behind the original cubesat standard, Bob Twiggs, is now promoting "pocket cubes". The new standard specifies satellites at a size of about 5x5x5cm, 1/8 of a cubesat in volume. A future launch including 'myPocketQub 442' is currently scheduled in October 2013, after some delays. Most projects appear to be related to amateur radio, see for example this description.

There are currently no pocket cubes orbiting the Earth and there have not been any yet. This leaves 1U cubesats as the smallest known satellites in orbit at the present time.

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  • $\begingroup$ There is no "Pocket Cubes" listed on en.wikipedia.org/wiki/OSCAR $\endgroup$
    – PearsonArtPhoto ♦
    Commented Aug 1, 2013 at 19:51
  • $\begingroup$ @PearsonArtPhoto Yes, I can confirm this. $\endgroup$
    – s-m-e
    Commented Aug 1, 2013 at 22:27
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    $\begingroup$ "Scheduled in October 2013" -- how did it go? $\endgroup$
    – user
    Commented Oct 1, 2014 at 8:56
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The Pocketqube 50dollarsat (eagle-2) was successfully launched as part of the UNISAT-5 mission on Nov. 21, 2013. The satellite is in orbit now and can be heard on 437.505 MHz. This femtosatellite is 2x2x3 inches and can be received by anyone with a handheld radio and a small directional antenna. More info is available here.

This satellite is in a 600+km orbit and will be around for a while (we hope).

  • Howie AB2S
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The smallest satellites seem to be the Cubesat design, at 10cmx10cmx10cm. There are a number of these in orbit, perhaps around a dozen, with about the same number failing during the launch phase. For a list of them (And other amateur satellites) see Wikipedia.

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  • $\begingroup$ Nice list, but there at least two operational satellites / cubesats, that I can think of, which carry amateur radio equipment and which are not in the list. This list is just too short ..... (?) $\endgroup$
    – s-m-e
    Commented Aug 1, 2013 at 20:06
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KickSat did not deploy unfortunately.

The other candidates for the smallest satellite, one that was capable of two way communications, would have been the WREN PocketQube or the POCKET-PUCP satellite launched out of PUCP-SAT-1. Neither of these have been reported as operational.

So the current candidate for the world's smallest (communications) satellite would appear to be $50SAT, launched on November 21st 2013 and still operational.

Unless anyone knows of another smaller satellite that actually went operational.

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Last answer is from 2014. I'll add an answer that will address 2019 and beyond.

SpaceBEEs are ¼U (quarter-U) cubesat format with long antennas that extend upon deployment and special radar reflectors that make them more visible.

Rather than copy/paste other posts here, I'll refer you to these questions and their answers:

From tweet:

Fourteen Swarm satellites built in a week, with vibration, shock, and thermal vacuum testing complete!

enter image description here

From Swarm Wants to Send Hundreds of Tiny CubeSats Into Orbit; A notorious startup asks FCC for permission to launch a 150-strong constellation of small IoT satellites

SpaceBEEs from Swarm Technology

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