New Horizons at Pluto

After 9.5 years it flew past Pluto, revealing the heart-shaped plain and flowing nitrogen glaciers.

The moment of New Horizons at Pluto: https://nowsky.org/e/newhorizons-pluto — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

At 11:49 UT on 14 July 2015 New Horizons passed 12,500 km above Pluto at 50,000 km/h. Nine and a half years and five billion kilometres had come down to a single day of flyby — and it was flawless. The spacecraft went silent that day to concentrate on observing, and when the one "all clear" signal arrived 21 hours later the control room erupted.

The Pluto that came back overturned expectations. Nitrogen ice flowed like glaciers across the heart-shaped Sputnik Planitia, water-ice mountains rose 3,500 m, and the atmosphere was wrapped in layer upon layer of blue haze. The near-absence of craters on the plain meant a surface younger than ten million years — the "dead ice ball" was a geologically living world.

Just before the encounter the team fought a "ten seconds of silence": on 4 July, ten days out, a computer overload put the probe into safe mode and cut contact, recovered only after three days. The flyby sequence was rechecked and executed without a single item missing.

Returning the full data set took fifteen months — it had to be sent at one to two kilobits per second from five billion kilometres. New Horizons went on to fly past the Kuiper-belt object Arrokoth in 2019 and is now beyond 60 AU, observing the outer heliosphere. The orbit replay shows the moment of arrival.

Why it matters

With the Pluto flyby every one of the classical nine bodies of the solar system had been visited up close. From Mariner 2 at Venus in 1962 it took 53 years, and that day closed the era of first flybys. What remains are orbiters and landers — and the countless small bodies of the Kuiper belt.

Finding Pluto alive forced a rewrite of heat-source theory for small icy worlds. How Pluto, with no tidal heating, keeps a young surface is still debated, and a subsurface ocean has been proposed. The same question now hangs over Triton, Eris and Makemake.

Pluto's moon Charon showed a red polar cap of tholins and enormous chasms, and the pair was confirmed as a "double dwarf planet" orbiting a barycentre between them, face to face — a formation path unlike Earth and the Moon.

How to watch

At 14th magnitude Pluto needs a 30 cm telescope to show as a point, and telling it from a star requires comparing photographs two nights apart. Select Pluto in the sky view for its current place in Sagittarius and its rise time.

Follow Pluto in the orbit view and move the time to 14 July 2015 to replay New Horizons sweeping past, and see the double system with Charon facing Pluto in a 6.4-day orbit.

New Horizons is now about 62 AU out toward Sagittarius. Switch on the solar-system structure chip in the 3-D interstellar view and Pluto appears inside the Kuiper belt (30–50 AU) with the probe's marker beyond it.

See this for yourself — replay the moment in the orbit view

Background

The flyby trajectory threaded between Pluto and Charon to observe both, then passed into Pluto's shadow to watch the atmosphere refract sunlight. This "eclipse" measurement gave the surface pressure (1 Pa) and the temperature structure.

The Arrokoth flyby on 1 January 2019 was the most distant close encounter ever (44 AU from the Sun). A 35 km contact binary, its two lobes gently joined, is the first physical evidence of the "soft merger" of early planet formation.

Pluto's place names — Sputnik Planitia, Tombaugh Regio, Norgay Montes, Cthulhu Macula — were formally approved by the International Astronomical Union in 2017, which set the rule of drawing names from exploration, myth and literature.

The final weeks before the Pluto encounter were also a time of hazard avoidance. Any unknown small moon or ring in the Pluto system would have made even a grain of dust lethal to a spacecraft passing at 50,000 km/h. During the approach in May and June 2015 the cameras searched the surroundings thoroughly, and only after no new moons or rings were found was the original trajectory kept; alternative paths using the antenna as a shield were ready had a hazard appeared. In the end the Pluto system proved cleaner than expected.

FAQ

How close did it pass?

12,500 km above Pluto at 50,000 km/h, on 14 July 2015.

Why did the data take fifteen months?

From five billion kilometres it could send only one to two kilobits per second.

What is the heart?

Sputnik Planitia — a 1,000 km basin of flowing nitrogen ice with no craters, meaning a young surface.

Where is the probe now?

About 62 AU out toward Sagittarius, observing the outer heliosphere, with power into the late 2030s.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.