An artificial moon circles Earth

Clyde Tombaugh found a moving point in the photographic plates he had been comparing.

By comparing images of the same star field, Tombaugh detected the movement of the object later named Pluto.

The discovery itself took place on 18 February; public announcement and naming came later.

The object initially joined the planetary list and was reclassified as a dwarf planet many decades afterward.

The Soviet Union launched the first artificial Earth satellite from a spaceport in the Kazakh Soviet Republic.

Radio signals from Sputnik could be received outside the launch program, making the orbit publicly observable.

The successful launch created a new platform for studying Earth and, later, the wider universe.

The spacecraft became the first successful interplanetary probe to visit another planet.

Mariner 2 measured properties of the planet while passing it without entering orbit.

Mariner 2 showed that interplanetary spacecraft could return measurements from distant worlds.