Pluto comes into focus

Updated

Here's Pluto as never seen before, thanks to NASA's New Horizons spacecraft.

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Did you know that until very recently, the best images we had of Pluto were just a few pixels in size? That's right: those pictures you had in your head of what Pluto looks like were mere artists' impressions.

But that's all changed.

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A piano-sized spacecraft named New Horizons has whizzed past the icy dwarf planet after a journey of more than nine years and 5 billion kilometres.

Its close encounter has already replaced those tiny pixellated images with new ones revealing mystery dark spots, a light heart-shaped patch and a range of icy mountains.

New Horizons has now sent back numerous images from its closest pass by Pluto, which show a range of icy mountains close to Pluto's equator, with peaks rising up to 3,500 metres.

The project's principal investigator, Alan Stern, says the latest imagery is "a scientific bonanza, revealing new details about Pluto’s atmosphere, mountains, glaciers and plains".

The latest images reveal more than a dozen thin haze layers in Pluto's tenuous but extended nitrogen atmosphere, including a thin, low-lying, fog-like haze.

"In addition to being visually stunning, these low-lying hazes hint at the weather changing from day to day on Pluto, just like it does here on Earth," said Will Grundy, from Lowell Observatory.

Scientists and space geeks alike are excited about what's been seen, and awaiting the release of more extensive higher-resolution imagery.

At its closest, New Horizons passed within 12,500 kilometres of Pluto's surface, travelling at 50,000 kilometres an hour.

"Nothing like this has been done in a quarter century and nothing like this is planned by any space agency ever again," says Alan Stern, principal investigator on the New Horizons mission.

His NASA colleague John Grunsfeld calls the flyby "truly a hallmark in human history".

Use the interactive graphic above to see the imagery released from New Horizons so far, and come back in the coming days and weeks to see more images.

New insights into Pluto

New Horizons captured this image of Pluto a day before its closest pass, from a distance of about 740,000 kilometres; it includes a strikingly heart-shaped light patch that NASA referred to as a "love note" sent back to Earth.

It says the heart shape is created by snow, consisting of some combination of nitrogen, carbon monoxide and methane.

The colouring of that image is true colour; NASA says the planet is orangish-red.

This detailed image of Pluto's surface captures a vast, craterless plain north of Pluto's icy mountains, and sitting in the centre-left of the 'heart' feature. NASA says the terrain seen here is not easy to explain.

New Horizons' closest pass also allowed the craft to capture Pluto's largest moon Charon in new detail.

NASA says the image reveals "a swath of cliffs and troughs" stretching about 1,000 kilometres (from left to right) and (on the upper right) a canyon estimated to be four kilometres to six kilometres deep.

A pronounced chasm longer and deeper than Earth's Grand Canyon was found on Charon, as well as a crater nearly 100 kilometres across.

"New Horizons has transformed our view of this distant moon from a nearly featureless ball of ice to a world displaying all kinds of geologic activity," says William McKinnon, part of New Horizons' geology and geophysics team.

Other imagery of icy mountain ranges showed no impact craters.

That has led scientists to conclude the mountains formed no more than 100 million years ago.

"This is one of the youngest surfaces we've ever seen in the solar system," says Jeff Moore of the New Horizons imaging team.

NASA has also released an image of Pluto and Charon that uses exaggerated colours to highlight its surface features.

"These images show that Pluto and Charon are truly complex worlds - there's a whole lot going on here," says New Horizons co-investigator Will Grundy.

"Our surface composition team is working as fast as we can to identify the substances in different regions on Pluto and unravel the processes that put them where they are."

The false colour image reveals the 'heart' seen on Pluto actually consists of two very different regions.

"In the false-colour image, the heart consists of a western lobe shaped like an ice cream cone that appears peach colour," a NASA statement explains. "A mottled area on the right (east) side looks bluish. A mid-latitude band appears in shades ranging from pale blue through red. Even within the northern polar cap, in the upper part of the image, various shades of yellow-orange indicate subtle compositional differences."

Earlier New Horizons images of Pluto revealed four mysterious dark spots, which piqued the interest of scientists and space watchers alike.

Unfortunately, imagery of the spots won't get any better than this - they appear on the side of Pluto that always faces Charon, and that side was invisible to New Horizons when it made its closest pass.

Principal investigator Alan Stern describes the image as "the last, best look that anyone will have of Pluto's far side for decades to come".

The dark areas are believed to be 480 kilometres across, with irregular but sharply defined boundaries between the dark and bright terrain. There are some theories about what the spots may represent but no clear consensus as yet.

"We can't tell whether they're plateaus or plains, or whether they're brightness variations on a completely smooth surface," says Jeff Moore of NASA's Ames Research Centre.

The latest images also capture other surface features such as impact craters, formed when smaller objects struck the dwarf planet.

New Horizons also captured an unusual surface feature on Charon - a depression with a peak in the middle, shown in the upper-left corner of the inset of the following image.

Jeff Moore from NASA's Ames Research Centre calls it a "mountain in a moat" and says the feature "has geologists stunned and stumped"; they don't know what it is.

In January this year, New Horizons captured a series of images of Pluto and Charon locked in a celestial dance. A timelapse animation shows the two bodies orbiting a common centre of gravity about 18,000 kilometres above Pluto's surface. One complete orbit takes about six days and nine hours.

The sequence clearly shows there's a wobble in Pluto's orbit that is caused by Charon's gravity.

The pair's tidal locking means one side of Charon permanently faces Pluto - and Charon never rises or sets in Pluto's skies, appearing pinned over the same spot.

The first of the images from the timelapse series was taken when New Horizons was about 203 million kilometres from Pluto - further than the distance between Earth and the Sun.

Journey to a new world

When it took off in January 2006, New Horizons set a record for the fastest launch speed into space and flew past the moon in just nine hours - a trip that took the Apollo astronauts three days.

But even at those breakneck speeds the spacecraft had to swing past Jupiter for a gravity assist to ensure it could keep its date at the outer edges of our solar system. In other words, the giant planet's gravity helped hurl New Horizons out towards Pluto: NASA says the trip to Pluto would have taken another five years without Jupiter's assistance.

While in Jupiter's neighbourhood New Horizons gave its instruments a test drive and captured some incredible images of the giant planet and its moons.

For most of its journey, the craft has been in extended periods of hibernation to extend the lifespan of essential instruments and keep operational costs down.

But in December 2014 New Horizons woke from its final hibernation and in January 2015 it started sending images back to Earth. Since May it has been capturing images of Pluto that are better than anything we've had before, including from the Hubble Space Telescope. Each new image shows us Pluto and its satellites in ever clearer detail.

New Horizons has schlepped a payload of seven different instruments - including optical and plasma instruments, radio equipment and dust sensors - to gather data so scientists can study the atmospheres, surface features, geology and environments of Pluto, its multiple moons and other objects in the region.

The spacecraft's flyby didn't allow for relaxed sight-seeing: New Horizons doesn't have the fuel it would need to brake and orbit Pluto. So instead, the probe sped by at 14 kilometres per second and spent little more than a day within 1 million kilometres of Pluto.

While the craft will be able to gather significant amounts of information during its rapid flyby, it's going to take months for all that information to be transferred back to scientists on Earth; the bandwidth for getting data back is a roughly equivalent to using an old dial-up modem.

So it's going to take 16 months to send its complete dataset back to Earth. On the plus side, that means we'll be enjoying new images and new revelations about Pluto and its neighbours for some time to come.

Did you know? New Horizons left Earth faster than any other spacecraft to date (at almost 60,000 kilometres an hour).

It took astronauts three days to reach the Moon. New Horizons passed it in nine hours.

Pluto is three-quarters the size of Earth's moon - but has five moons of its own.

Even though it was launched nine years ago, NASA says all the "smartphone tech" wouldn't have helped send back better images because, well, physics.

Oh, and guess what fuel New Horizons has been using to reach Pluto? Plutonium. (Seriously.)

So why have we sent this spacecraft billions of kilometres across the solar system to a tiny ice dwarf that isn't even considered a planet any more?

Surprisingly, it turns out it's not about swimming pools. Scientists are using the images and data gathered to help answer basic questions about what the surface, geology, interior make-up and atmosphere is like on Pluto, and on other objects in the area. This will give them a better understanding of where these objects fit in with the rest of the solar system.

Missions like New Horizons also provide scientists with clues that help them piece together a clearer picture of the origins and evolution of the solar system.

Following its encounter with Pluto and its satellites, New Horizons will continue its one-way journey. The tiny probe will make observations and collect data as it heads into the Kuiper Belt, a frozen ring of debris that sits beyond Neptune and is peppered with millions of icy objects thought to be remnants from the formation of the solar system some 4.6 billion years ago.

Its radioactive power supply will last into the 2030s; NASA wants to focus investigations on two more objects in the Kuiper Belt but will need to secure more funding to make that happen.

What we know about Pluto

Some say the journey to Pluto really began more than 85 years ago when a 24-year-old former farmhand from Kansas discovered the '9th planet'.

Working at Lowell Planetarium in Flagstaff, Arizona, Clive Tombaugh would spend long hours scanning photographic plates of star fields. He eventually found what he was looking for on February 18, 1930, when he spotted a tiny moving point of light in the fixed pattern of the Gemini star constellation.

Some of Tombaugh's ashes are making the journey on New Horizons.

The world was named by an 11-year-old British schoolgirl named Venetia Burney, who suggested the name shared by the Roman god of the underworld.

These days Pluto is considered a dwarf planet; it lost its planet status in 2006 - only months after New Horizons was launched - when the International Astronomical Union clarified its classification system.

It was decided that to be a planet in our solar system, a body must orbit the Sun, must be round (or self-gravitating so as to assume a hydrostatic equilibrium shape), it must clear out its orbital neighbourhood of other large objects, and it can't be a satellite of another planet.

Recent measurements obtained by New Horizons indicate Pluto has a diameter of 2,370 km, which is 18.5 per cent of Earth's, while its moon Charon has a diameter of 1,208 km, 9.5 per cent of Earth's.

New Horizons' measurements appear to settle decades of debate about the size of Pluto, and confirm it is the largest object in the Kuiper Belt, surpassing the size of Eris, another so-called 'dwarf planet'.

While Pluto is round and orbits the Sun, it is just one of an estimated 70,000 similar-sized objects in the Kuiper Belt, accounting for less than 1 per cent of the total mass in the orbit.

But planet or not, there is still much that excites astronomers about Pluto. It follows an 'eccentric' orbit, which is highly elongated and tilted. This is quite different from the planets of our solar system, although it's not at all unusual for objects in the trans-Neptunian region.

This unusual path actually saw Pluto orbiting closer to the Sun than Neptune between 1979 and 1999. It rotates on its side like Uranus and takes 248 Earth years to orbit the Sun.

Pluto is also more reflective than most of the dark and dull objects beyond Neptune, which scientists believe is due to the freezing of its atmosphere as it moves further out from the Sun. This is partly why astronomers thought it was bigger than it was.

It wasn't until 1978 that Pluto's largest moon Charon was discovered, after US Naval Observatory astronomer James Christy noticed a small bump on one side of Pluto when making routine measurements of photographic plates. Charon is about half the size of Pluto and together they form a binary planet system far different from those around traditional planets.

The satellites around other planets are typically far smaller than their host, while Charon is fully half the size of Pluto. The unusual nature of the Pluto-Charon system, like that of the Earth-Moon system, is a vital clue to how those binary objects formed.

Over the following years, further moons were discovered: scientists have found at least four smaller moons orbiting the Pluto-Charon system but there's a chance New Horizons will help spot even more.

It's believed this moon system is the result of a collision between Pluto and another planet-sized body early in the history of the solar system. In 2005, scientists studying images sent back from Hubble in preparation for the New Horizons' mission discovered Pluto had two smaller moons - Nix and Hydra. It was not until the spacecraft was well on its way to Pluto that two other moons were discovered - Kerberos in 2011 and Styx in 2012.

These images released by NASA in 2010 were the first to give us a sense of what Pluto might really like look up-close. Captured by the Hubble Space Telescope, they show a white, dark-orange and charcoal-black terrain undergoing seasonal changes.

At the time, scientists said they believed Pluto's colour was the result of ultraviolet radiation from the distant Sun breaking up methane on the surface, leaving behind a dark and red carbon-rich residue. These images challenged the view that Pluto was simply a ball of rock and ice, and gave a feel for how the surface changes as the atmosphere rises and then freezes out again depending on Pluto's distance from the Sun.

These images were also used to help scientists set the course for New Horizons.

Credits

with expert advice and assistance from astronomer and astrobiologist Dr Jonathon Horner, who's based at University of Southern Queensland

Topics: planets-and-asteroids, astronomy-space, science-and-technology, united-states, australia

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