Pluto revealed as KING of the Kuiper belt

Still too small to be a planet, even though Eris was measured in error

New Horizons spacecraft approaching Pluto

As astroboffins prepare themselves for the arrival of data from New Horizons' nearest approach in its Pluto fly-by, the little probe has already revised opinion about how big/small the planet/dwarf planet really is.

Pluto won't get to recover its lost status as a planet, but NASA's New Horizons team has confirmed that at 2,370 km diameter, it's definitely the largest object we know about in the Kuiper belt.

The measurement also settles the debate about whether Pluto or Eris was the larger. Eris, which has nearly no atmosphere, was relatively easy to measure, but Pluto's atmosphere meant measurements of the size of the object underneath was fuzzy.

Since Eris' discovery in 2005, it was most often regarded as the larger – one reason that Pluto got downgraded from being regarded as a “planet”. However, the New Horizons measurement gives Pluto an edge of around 30 km.

That's still smaller than Earth's moon, at 3,475 km – so sorry, Pluto fans, you still won't persuade academic astronomy to upgrade it back to being called a planet.

At the moment, pixellation means that estimate is plus-or-minus 20 km, but NASA says the flyby will narrow the error bars down to a couple of kilometres. The measurements were made quite simply: images from the spacecraft were fitted to circular disks.

Mission scientist Bill McKinnon of Washington University in St Louis says in the statement that Pluto's size has been subject to debate since the 1930s, when it was first discovered.

Since the dwarf planet is bigger (a little) than expected, it's a little less dense than previously calculated. That leads NASA to suggest it will have more ice in its interior, and a shallower troposphere (the lowest layer of its atmosphere).

Don't bother staying up or changing your plans to watch the flyby at 1150 GMT / 0740 EDT on July 14. New Horizons will be out of contact at the time; mission controllers will just have to wait until it resumes transmission afterwards, and start transmitting images from as close as 12,500 km (which will have a resolution of about 100 m per pixel). ®

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