The final frontier

ALL good things come to an end. And July 14th will see the finale of the Heroic Age of space exploration. On that day a visitor from Earth will fly past Pluto and head off into the Kuiper belt—the icy, rubble-strewn fringe of the sun’s sphere of influence. In doing so this visitor, an American craft called New Horizons, will fulfil an aspiration that began a mere 60 years ago, to turn the planets from being little more than night lights, whose surface features were visible fuzzily, if at all, in telescopes, into palpable worlds of known geography.

The picky may object that Pluto is not a planet. Technically, they are right. It was regarded as one when New Horizons took off, in January 2006, but was downgraded to the status of “dwarf planet” seven months later by a meeting of astronomers who decided, ironically, that the knowledge which earlier spacecraft had brought allowed them to refine their classifications of what did and did not constitute “planetness”. Pluto, to the dismay of many, fell on the wrong side of the cut.

Planet or not, Pluto completes a collection that began with the Moon, went on to the inner planets (Mercury, Venus and Mars, though they were not first visited in that order), the outer planets (Jupiter, Saturn, Uranus and Neptune), and various moons, asteroids and comets. With Pluto in the bag, the photo album of this grandest of tours will be full.

One thing which led to Pluto’s demotion was that its mass turned out to be less than a quarter of one per cent of Earth’s. Even seen through the mighty Hubble space telescope, the place was therefore little more than a pixelated blob. Basic data such as its radius, the size of its core and its internal composition could not be nailed down precisely.

That is changing fast. New Horizons’ cameras have already displayed the place in unprecedented detail (the picture above, which shows Pluto accompanied by its largest moon, Charon, is one such image). On April 29th NASA reported that the probe had found what looks like a polar ice cap. There are also suggestions of ice volcanoes akin to those found on Neptune’s moon, Triton, which resembles Pluto in many ways. There was a glitch on July 4th, which resulted in the loss of a few pictures. But everything now seems set up for the moment—11:50 GMT on the 14th—when the craft will pass within 12,500km (7,800 miles) of Pluto’s surface.

Brief encounter

Photographs of Pluto, though, are only one of New Horizons’ pay-offs. Of equal scientific interest will be the spectrographs. These should show precisely what Pluto’s surface, and its atmosphere, are made of.

Observations from Earth suggest the surface is mostly frozen nitrogen, but there is an intriguing, ever-changing contrast between its dark and light areas. And the whole planet is tinted slightly orange, a hue thought to be the result of ultraviolet light from the sun causing some of the nitrogen to react with methane that is also present on the surface, to create a class of ruddy chemicals called tholins.

A nitrogen-rich surface means the atmosphere will be mostly nitrogen, too. And, as luck would have it, now is almost as good a time as is possible to observe Pluto’s atmosphere, since the orb’s most recent perihelion (its point of closest approach to the sun during its highly elliptical, 248-year orbit) was in 1989. Pluto is thus just about as warm as it ever becomes, and its atmosphere, fed by sublimation from its surface, at its thickest.

New Horizons will also look at Pluto’s moons, of which there are five. Charon was found in 1978. It is about half the diameter of its parent planetoid. Nix and Hydra were first observed in 2005, and Kerberos and Styx were discovered in 2011 and 2012 respectively, whilst New Horizons was en route. The whole six-body system is thought to have been the result of a collision between two precursor objects early in the solar system’s history—a similar explanation to that often advanced to explain the Earth and its own satellite.

New Horizons’ data will allow scientists to refine their models of what Pluto looks like internally. The present best guess is of a rocky core surrounded by a mantle of ice. Radioactivity in the core could, conceivably, generate enough heat to keep some of the mantle liquid and give Pluto a subsurface ocean similar to those thought to exist on Europa and Enceladus (moons of Jupiter and Saturn respectively).

All these observations should help turn the present pencil-sketch of Pluto into something resembling a proper portrait. And that, in turn, may permit researchers to engage in a bit of planetary genealogy.

Part of the process that led to Pluto being kicked out of the planetary club was the realisation that it resembled the objects of the Kuiper belt more than it did the rest of the proper planets. Indeed, one of these orbs, called Eris, is 27% more massive than Pluto. It just happened, because Pluto is on the belt’s inner edge, that it was the first Kuiper-belt object to swim into the field of vision of an earthly telescope. It was spotted in 1930 by an American astronomer called Clyde Tombaugh.

Like the members of the asteroid belt, which lurk between the orbits of Mars and Jupiter, those of the Kuiper belt are thought to be leftovers from the disc of dust and rock that surrounded the young sun, most of which eventually condensed into planets. Studying these belts ought therefore to shed light on how the solar system formed, and particularly how its planets came to occupy the orbits they do. Some astronomers, for instance, think Ceres—the largest body in the asteroid belt, and one that is currently under scrutiny by another NASA probe, Dawn—is actually an immigrant to the inner solar system from the Kuiper belt.

With luck, New Horizons will help settle such questions. NASA hopes to keep it reporting from the Kuiper belt for many years to come, and has already selected three other Kuiper-belt objects that lie close to its post-Pluto trajectory as possible targets for similar flying visits. Perhaps there is room in the album for a few more pictures after all.