Why NASA couldn't just use Hubble telescope to see Pluto

As NASA continues to release highly detailed, unprecedented images of the dwarf planet Pluto captured by the New Horizons probe, it raises the question: Why couldn't it have just used the powerful Hubble telescope to capture the same scenes, instead of sending a spacecraft across the solar system on a $700-million (US) mission? Why is Hubble able to get extremely detailed images of galaxies and nebulae millions of light years away, but when it comes to taking pictures of Pluto, it shows up as a blurry ball?

The answer is straightforward but perhaps not intuitive. Pluto may be close, but it is very small. Galaxies millions of light years away appear larger (as seen from Earth), and that is why the Hubble is able to photograph them in more detail, and why NASA had to send a spacecraft to get good photos of Pluto.

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The best images of Pluto taken by the Hubble Space Telescope are a series of blurred shots just a few pixels across. Compare these with the stunning images of galaxies millions of light years away.

"There's a good reason Hubble can take brilliant sharp images of distant galaxies millions of light years away, but ends up with blurry pictures of Pluto which is just a few light hours away," Chris Lidman of the Australian Astronomical Observatory said.

The issue is to do with how large the object actually appears to be, and this is based on its actual size and how far away it is.

This is called the angular size and it effectively refers to the amount of the sky covered by the object from where you're positioned.

"It comes down to the simple fact that even though galaxies are much further away than Pluto, they're also hundreds of trillions of times bigger, so despite their greater distance, they still wind up covering much more of the sky," Lidman said.

Sun and moon provide an example

Astronomers use a process called angular separation to measure celestial objects in the sky in terms of degrees, arcminutes and arcseconds.

"You can divide a circle into degrees, and there's 360 degrees in a circle," he said.

A degree can further be divided into smaller segments called arcminutes, with 60 arcminutes to one degree; and 60 arcseconds in one arcminute.

The sun and the moon both have the same angular size, which is a good example to keep in mind.

"The sun is 400 times larger than the moon, but it's also 400 times further away, so it covers the same angular size as the moon, about half a degree of the sky," Lidman said.

"We are fortunate that they have the same angular size. Otherwise, total solar eclipses would not be as spectacular as they are."

Being both smaller and a lot further away, Pluto covers a far smaller angle in the sky than the moon, just one-20th of an arcsecond.

"So in terms of the circle, that's one-20th of one-3600th of one-360th of a circle — so it's a very small angle," he said.

"In comparison, a galaxy that Hubble might observe could cover a full arcsecond on the sky, so that's 20 times larger."

'Just a pin prick'

NASA sent its New Horizons spacecraft to take close up images of the mysterious world of Pluto and its moon Charon.

The stunning photos have changed our understanding of this distant realm on the dark outer rim of the solar system.

New Horizons flew less than 12,500 kilometres above the dwarf planet's frozen surface, providing the most detailed view ever of a Kuiper Belt object.

"Before New Horizons, Pluto was just a pin prick against a starry backdrop," Lidman said.

"NASA's amazing spacecraft has allowed us to see its frozen plains and towering mountain peaks, forcing a rewrite [of] the textbooks."