On July 14, 2015, NASA's New Horizons flew past Pluto and measured its atmosphere in an unusual way: two dishes of the Deep Space Network sent a 7.2 gigahertz radio signal from Earth, timed to arrive as the spacecraft slipped behind the planet. The bent signal gave the first surface pressure ever measured at Pluto, about 10 microbars, one hundred-thousandth of Earth's, and a temperature high above the ground that nobody expected: much colder than the models said.

In this episode: how fine that ruler was, reading temperature to within three kelvin from four and a half light-hours away. Why the cold mattered for the air escaping to space. The blue haze in the New Horizons photographs, about 20 layers reaching more than 200 kilometers up, and why it is not decoration but, in the words of the 2025 study, the thing that largely controls Pluto's heat balance. The 2017 paper by Xi Zhang and colleagues that proposed it, called a crazy idea at the time, and the prediction it made: Pluto should glow in the mid-infrared. Then the James Webb Space Telescope, which never took a picture of Pluto and instead measured two points of heat, Pluto and Charon, and found the glow between 13 and 20 micrometers. We close with what the sources still leave open, and with New Horizons, awake again since June 2026, counting dust in the Kuiper Belt.

Every figure comes from NASA, Nature, Nature Astronomy and Astronomy and Astrophysics pages, linked in the video description on YouTube. Where the sources don't explain something, we say so.

Watch the video edition: https://www.youtube.com/watch?v=0h-bh8GGWgI

Narration: synthetic voice. Research, writing and editing: human work. Echoes of the Void.