New Horizons Reaches Pluto: 14 July 2015
2015 · event
On 14 July 2015 a spacecraft the size of a grand piano passed about twelve thousand five hundred kilometres above the surface of Pluto at almost fifty thousand kilometres an hour. New Horizons had been flying for nine and a half years and had one chance. It could not slow down, so the encounter was a single sweep during which every instrument worked to a script written years earlier. Because a signal takes four and a half hours to reach the Earth from that distance, the team learned only afterwards that everything had worked.
Pluto had been a smudge of a few pixels since its discovery in 1930. The images that came back showed mountains of water ice standing three kilometres high, a smooth plain of frozen nitrogen shaped like a heart and now named Sputnik Planitia, layers of blue haze in a thin atmosphere and a surface with regions of very different ages. A body that had been assumed to be a dead ball of rock and ice turned out to be geologically active.
The data took sixteen months to send home, drop by drop, through a transmitter with less power than a domestic light bulb. Among the objects carried aboard was a small container with the ashes of Clyde Tombaugh, the young astronomer who found Pluto from a hilltop in Arizona.
New Horizons then flew on to a small object in the Kuiper belt called Arrokoth in 2019, the most distant world ever visited closely. The Pluto encounter completed the first reconnaissance of every classical planet of the solar system, a project begun with Mariner 2 in 1962, and it demonstrated that the outer edge of our neighbourhood is far more interesting than anyone expected.