The outer limits?

NASA/JHU APL/SwRI/Steve Gribben NASA/JHU APL/SwRI/Steve Gribben

In fourteen days’ time, humankind will finally get to see a world which was regarded as the outermost planet in the solar system for most of the 85 years since its discovery, yet is a world so incredibly distant that it is no more than a fuzzy blob on the best Hubble Space Telescope images.

Little more than half the size of our own moon, Pluto has always been presumed to be a dead, frozen planet only a few tens of degrees above absolute zero – literally a failed planet, officially demoted to ‘dwarf planet’ a few years ago. But could we be in for a surprise? Could the last dark corner of the solar system throw us a curveball?

NASA’s New Horizons probe has been speeding towards Pluto for almost a decade, getting a gravity assist from Jupiter in 2007 to help it cover the immense 5 billion kilometers to its target. Its cameras started to distinguish Pluto as more than just a speck of light around February of this year, but by April the images showed a hint of two blurry white polar caps, raising the intriguing possibility of ice deposits on its surface.

By early June the images were indicating not simply two polar caps but a more complex pattern of different light and dark regions including bands of lighter terrain as well as a large dark area, suggesting perhaps different geological zones, as well as a lumpy surface possibly indicative of large impact craters or maybe even a large chunk missing from one side.


Image Credit: NASA

Over the coming two weeks, the images should get much more detailed, enough to meet the primary goal of the mission to characterize the surface geology and morphology of both Pluto and its largest moon, Charon. However, travelling at 14 km/s, it will be going too fast to go into orbit around Pluto, so the plan is for it to take as many images and measurements as it can as it zips past the dwarf planet, something it will do in less than 4 minutes at around lunchtime on 14 July.

And then what? New Horizons will fly on into the Kuiper Belt, a huge region of small asteroids about twenty times wider than the asteroid belt between Mars and Jupiter. The ambition is to fly by some of these objects – but with its fuel almost gone, batteries fading and at the limit of its communications, it is pot luck what it will find. Any objects that it does encounter are likely to be no more than 50 km across, too small to see from Earth at that distance.

But Pluto is far from being the outer limit of our solar system. A handful of other dwarf planets have been found in recent years, the so-called Trans-Neptunian Objects (TNOs). TNOs range from a similar size to Pluto down to about a fifth of its diameter. The furthest, Sedna, orbits the sun a massive three times further out than Pluto. Sadly New Horizons will not reach any of them before its batteries run out, so the enigmatic worlds of the colourfully-named Eris, Makemake, Haumea, Sedna, Quaoar and Orcus will have to wait for another generation.

Other surprises have been found in the past few months. Another NASA probe, Dawn, is now in orbit around Ceres, the largest object in the asteroid belt beyond Mars, and roughly half the size of Pluto. For the most part, Ceres has looked much as you would expect an asteroid to look: a heavily cratered miniature version of our own moon. Except for one bizarre and so far unexplained feature – a series of extremely bright spots, so bright they look as though they’ve been painted on with reflective paint. Favourite theories so far are salt deposits or ice, both of which then beg the question of how they got there.

It seems that however far we go in our exploration of the solar system, surprises are always in store.

The Ceres bright spots imaged by Dawn from 4,400 km. This strange group of highly reflective areas are scattered over a region 90 km across. Image credit: NASA.

The Ceres bright spots imaged by Dawn from 4,400 km. This strange group of highly reflective areas are scattered over a region 90 km across. Image credit: NASA.

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