spaceSpace and Physics

Pluto and Charon Show Their True Colors


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

945 Pluto and Charon Show Their True Colors
Combined images from New Horizons' Lorri and Ralph imagers. NASA/JHU-APL/SWRI

Revelations about Pluto and its moon Charon are coming thick and fast as New Horizons approaches the pair. Unexpected color differences, strange spots and signs of an atmosphere have all been observed. However, not everything we have learned comes from the spacecraft – observations made in rather unlikely places are proving useful as well.

One recent discovery is obvious from the image above; Pluto and Charon are very different colors.


The finding was unexpected. The dwarf planet and its moon are thought to have formed through similar processes out in the icy wastes at the edge of the solar system. There was little reason to anticipate that the surfaces would be different, and nothing seen from Earth-based telescopes, or Hubble, indicated the stark difference between the pair.

Like Ceres, Pluto has also turned out to have spots, but in this case they are darker than the rest of the surface rather than lighter. The spots look tiny, but are around 500km across. Intriguingly, they are all the same size and are equally spaced.  “It’s a real puzzle – we don’t know what the spots are, and we can’t wait to find out,” said Alan Stern of the Southwest Research Institute.

The spots are located near the equator of the so-called “opposite hemisphere” – the one of which we won't get such a good view during the flyby.

Closest approach will occur on July 14 at a distance of 12,500 kilometers (8000 miles) from Pluto. However, the images collected at that time will be stored to be sent back when things are less hectic, so we may have to wait a fair bit longer for a really good look at many aspects of the two worlds.


New Horizons is still capable of adjusting its path, but so far no sign has been seen of a ring or unknown moon that could pose a menace, so the course has remained unmodified, although a 23-second thruster burst was undertaken to increase speed.

It might be thought that with New Horizons now less than 15 million kilometers, (9 million miles) from its target, observations from Earth would be superfluous. However, on Monday 29th June, NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) flew out of Christchurch, New Zealand, to watch Pluto “occult” (pass in front of) a Sagittarius star.

While SOFIA couldn't see Pluto in anything close to the detail to be recorded by New Horizons, positioning itself to observe the full occultation was crucial. The dimming of the star before Pluto itself passed in front indicated the presence of a thin atmosphere, something SOFIA also picked up in another occultation in 2011.

“SOFIA observations of Pluto demonstrate a capability to make detailed measurements of Pluto’s atmospheric density and structure,” said NASA's Pamela Marcum. A comparison of the differences seen between this event and the previous one will provide information about how Pluto's atmosphere is changing as it moves away from the Sun. New Horizons' mission was brought forward in the hope that it would reach Pluto before the atmosphere froze as the planet moved to a more distant part of its orbit. The findings confirm that we are not too late, with the atmosphere very much still in evidence.


Extraordinarily, while we can predict the paths of eclipses and most occultations thousands of years in advance, astronomers did not know exactly where on Earth one would need to be to witness the occultation beforehand. Interactions with Charon turn Pluto's movements into a complex dance, frustrating such exact forecasts.

Instead, predictions for the place to make observations varied between northern Australia and the coast of Antarctica. SOFIA was forced to divert in response to changing observations, but eventually caught the dead center of the fast-moving shadow in the southeast of New Zealand

Meanwhile, the Royal Astronomical Society of New Zealand had 43 registered observers trying to pick up the event from the ground in order to provide as much information as possible about the exact location of Pluto's shadow.

The findings will be used to corroborate the data collected by New Horizons.





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