Droplets
Droplets are small unmanned spaceships that originate from the alien planet Trisolaris and they are featured in the Three-Body Problem sci-fi novel trilogy. Despite their miniscule size, these spacecrafts are devastatingly powerful weapons, and even one is more than a match for the entirety of Earth's military forces.
Description
Each Droplet is a small spacecraft with a teardrop shape, hence the name. They are 3.5 meters (11.5 feet) long and mass 10 metric tons (11 short tons, 22,000 lbs). Droplets have perfectly smooth, reflective surfaces with no openings of any kind. They are propelled by a plasma "halo" that forms at their tail during flight; the halo is probably powered by matter-antimatter annihilation.
The mirror-like appearance of the droplets is the result of their unusual composition. They are made of metal with an unknown physics-warping effect applied that allows the strong nuclear force to apply at molecular rather than nuclear distances. This means that the atoms in a droplet's hull are held together by the strong nuclear force, not the electromagnetic force. Droplets are therefore impossibly durable, having the hardness (though not the density) of neutron star material. The surface is smoother, harder, and more reflective than any normal matter could possibly be, and no known weapon is able to scratch it.
The tremendous durability of droplets provides them with the only weapon they truly need. Flying at speeds of hundreds of kilometers per second, they can ram targets in space and on planets with the force of a nuclear strike, all while remaining totally unharmed. Given years of constant acceleration, droplets can reach speeds of at least 15% lightspeed, but they have never been seen to attack targets at this speed.
Droplets can also exercise some control over the radiation they reflect. For instance, when the Doomsday Battle droplet was struck by a gamma-ray laser, it reflected the gamma radiation as a burst of visible light bright enough to blind people for thousands of kilometers around. How they do this is uncertain.
Since droplets' surfaces reflect virtually all incoming radiation of any kind, it is not certain how droplets sense their surroundings. It is possible that sophons communicate data to the droplets via quantum entanglement, which would bypass their impenetrable surfaces, but droplets have been shown to operate even in places where sophons cannot.
History
It is unknown when the droplets were created or under what circumstances, but at the latest they were made fairly early into the Crisis Era. At that time, ten droplets were launched from one of the Trisolaran ships headed to the Solar System, with one due to arrive much earlier than the others. It is never revealed if the other 999 ships in the First Trisolaran Fleet had a similar complement.
The first droplet arrived in the Solar System after two hundred years in flight, coming to a stop near the orbit of Uranus. It was met by the entire Fleet International of humanity in a show of force. The Fleet International and the public at large believed it to be a messenger of peace or, at worst, a bomb. The droplet was taken aboard the ship Mantis for analysis.
Aboard Mantis, the droplet waited until the crew had determined what it was made of and the true threat it posed. At that moment, the droplet activated its propulsion halo, killing everyone on the Mantis and destroying the ship. The droplet then tore through the Fleet International, destroying all but a few ships and killing over a million people. This massacre was known as the Doomsday Battle.
The droplet then headed to near-Earth space, where it broadcast a signal meant to obstruct the use of the sun for a dark forest broadcast. However, scientist Luo Ji figured out another way to send a dark forest broadcast, forcing a peace between Earth and Trisolaris. After this truce, the droplet and its nine companions apparently withdrew from human space.
In fact, the Droplets were lurking just outside the range at which they could be seen from Earth. After a decades-long wait, they struck at Earth and human spacecraft in the Oort Cloud. The droplets that attacked Earth destroyed the dark forest broadcasters there, but the droplets in the Oort Cloud passed through a pocket of four-dimensional space. This allowed the humans to bypass the Droplets' invulnerable exteriors and attack their insides, and these two droplets were successfully disabled. They were the only droplets ever destroyed. In response to this attack, the Oort Cloud ships sent out a dark forest broadcast, condemning both Earth and Trisolaris to destruction by powerful aliens.
The Droplets around Earth were used to enforce the Trisolarans' will, razing cities in response to resistance. However, the Trisolarans evacuated the droplets from the Solar System when the dark forest broadcast reached Earth, since they had no reason to occupy a planet destined for destruction.