New Horizons at Pluto (2015): The First Close Look at Pluto

NASA’s New Horizons gave humanity its first close look at Pluto on 14 July 2015, passing about 12,500 kilometers from the surface. The first results, published by S. A. Stern and colleagues that October, showed a surprisingly varied, geologically active world with a vast, young plain of nitrogen ice.

What was known about Pluto before 2015?

No spacecraft had ever visited Pluto. Since the 1992 discovery of the Kuiper belt, astronomers had come to see it as the largest known member of a population of icy bodies beyond Neptune, and in 2006 it was reclassified as a dwarf planet. The big open question was simple. Is a small, cold world this far from the Sun geologically dead, or can it still change? New Horizons was the first chance to look.

What the paper found

• A diverse surface. Pluto shows a wide range of landforms, terrain ages, colors, compositions and albedos (how much light a surface reflects).
• A young ice plain. A vast, crater-free plain of nitrogen ice, informally called Sputnik Planum and now Sputnik Planitia, points to a geologically young surface. The team also found evidence of ice convection, glacial flow and wind streaks.
• An extended but thin atmosphere. Pluto’s atmosphere reaches far out into space but is less dense than expected, with a surface pressure near 10 microbars, about 100,000 times lower than Earth’s, plus a global haze layer.
• A measured size. Pluto’s diameter came out at about 2,374 kilometers.
• A different moon. Charon has no detectable atmosphere but shows tectonics and puzzling dark terrain at its north pole.

How they did it

New Horizons did not stop at Pluto. It flew past once at high speed, so every instrument had to work during a short window around closest approach. Cameras mapped the surface in visible light and color, spectrometers read the composition of the ices from the light they reflect, and other instruments probed the thin atmosphere.

Reading a surface’s age from craters is a classic planetary trick. Impacts pile up over time, so a heavily cratered surface is old. A broad plain with no craters at all must have been wiped clean recently, in geological terms, which is why Sputnik Planitia stood out.

Why it mattered

New Horizons showed that a small, cold world far from the Sun can still be geologically alive. That raised a question the paper put plainly: how do small planets stay active billions of years after they formed? The flyby also completed the first reconnaissance of the classical planets plus Pluto and opened the exploration of the Kuiper belt, the Solar System’s third zone beyond the rocky and giant planets.

What we know now

Sputnik Planitia is now the formal name of the ice plain, and New Horizons went on to an extended mission in the Kuiper belt. The outer Solar System still holds open questions. One is the hypothesized Planet Nine, proposed in 2016 to explain the clustered orbits of distant Kuiper belt objects, which as of October 2026 has not been detected.

Who wrote it

The paper was led by S. A. Stern with F. Bagenal, K. Ennico and the wider New Horizons team. It appeared in Science on 16 October 2015, three months after the flyby. Semantic Scholar lists about 431 citations (October 2026).

Read the original

Stern, S. A., Bagenal, F., Ennico, K., et al. (2015). “The Pluto system: Initial results from its exploration by New Horizons.” Science 350, aad1815. doi:10.1126/science.aad1815

Free to read: arXiv preprint.