This picture of Pluto’s atmosphere, backlit by the sun, is a Parthian shot of the place taken on July 15th (but transmitted to Earth a week later) by New Horizons, NASA’s probe to the dwarf planet. It is a last, backward glance as the spacecraft hurtles away into the Kuiper belt of frozen asteroids that lies beyond the part of the solar system inhabited by planets. New Horizons’ mission has been a revelation. The Pluto it portrays is a russet-pink body of mountains kilometres high, of giant icy plains and of glaciers of frozen nitrogen. Altogether, the craft has been able to photograph about half of Pluto’s surface in reasonable detail, recording features as little as 1km across. What the other half looks like will remain a mystery for a long time. No future mission is planned and, for a telescope in orbit around Earth to achieve a resolution equivalent to that of New Horizons’ cameras, it would need a main mirror 3km across.
