Posted on by Nancy Atkinson

New Hubble Images Show Pluto is Changing

New images of Pluto from the Hubble Space Telescope. Credit: NASA, ESA, and M. Buie (Southwest Research Institute).

Pluto-philes (and astronomers, too) have always bemoaned the fact that the best image of the principal dwarf planet wase just a fuzzy, pixelized haze. Bemoan no more. The most detailed look to date of the entire surface of Pluto has been constructed from hundreds of images taken by the Hubble Space Telescope. The images were taken during 2002 to 2003, and it took four years of computer crunching and software tweaking to create the global images. Surprisingly, the images show Pluto changed noticeably during the two-year photo shoot; the dwarf planet’s color became “redder,” and astronomers could see Pluto’s ice sheets were shifting.

“These Hubble pictures represent a true-color appearance of what you would see if you were near Pluto, comparable to looking at our own Moon with the naked eye,” said principal investigator Marc Buie of the Southwest Research Institute. “We now know we’re looking at something that has the biggest surface changes of any object in our solar system.”

The pictures show nitrogen ice growing and shrinking, getting brighter in the north and darker in the south.

Buie and planet hunter Mike Brown from Caltech introduced the Hubble images during a teleconference with reporters today, and emphasized how surprised they were with the changes seen on Pluto in just a relatively short period of time. Even accounting for seasonal changes, seasons can last 120 years in some regions of Pluto.

The top picture was taken in 1994 by the European Space Agency’s Faint Object Camera. The bottom image was taken in 2002-2003 by the Advanced Camera for Surveys. The dark band at the bottom of each map is the region that was hidden from view at the time the data were taken. Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

They said the images underscore that Pluto is not simply a ball of ice and rock but a dynamic world that undergoes dramatic atmospheric changes. While they believe the changes are driven by the seasons, it may mostly come from how quickly things can change on Pluto. The seasons are propelled as much by the planet’s 248-year elliptical orbit as its axial tilt — unlike Earth where the tilt alone drives seasons. On Pluto spring transitions to polar summer quickly in the northern hemisphere because Pluto is moving faster along its orbit when it is closer to the Sun.

“If Earth had such an extreme orbit, and we were experiencing a nice springtime day with 60-70 degree F temperatures, as the orbit changed it could suddenly drop to -90 degrees F,” said Brown.

There is also a mysterious bright spot on the center of Pluto, which has been observed in earlier images. But the spot is unusually rich in carbon monoxide frost.

Click here to see a video of Pluto rotating.

The astronomers said Pluto is so small and distant that the task of resolving the surface is as challenging as trying to see the markings on a soccer ball 40 miles away. Buie said we won’t have a better look at Pluto until the New Horizon’s spacecraft is six months away from the dwarf planet in 2015.

The images were taken with the Advanced Camera for Surveys on HST, and the 348 images taken in 2002 and 2003 were the last ones taken of Pluto with high enough resolution to be useful. “I had time allocated two years ago to look at Pluto, which came just three or four weeks after the high resolution camera failed,” Buie. “That was very disappointing.”

But the images do show Pluto is significantly redder than it had been for the past several decades. Astronomers use the word “red” to mean it reflects more red light than blue or green light. To the human eye, Pluto has a yellowish-orange color, and is about 20% redder than it used to be. “It’s not as red as the surface of Mars, but more red than Io,” Buie said.

Red is usually associated with carbon. The astronomers said there is also methane, which is not usually stable in an environment like Pluto’s.

“This business about the color change had me scared for awhile,” Buie confessed. “I got the result years ago, but it was so hard to understand and believe. I’m still nervous about it. It could be that I completely screwed this up, but I can tell you Charon is on the same images, and Charon has the same color throughout but Pluto changed. I don’t’ know how the camera system on HST could have given me the wrong colors on Pluto.”

This was previously the best image of Pluto, taken in 2000 by HST. Credit: Eliot Young (SwRI) et al., NASA

Someone suggested that Pluto is reddening because of its recent demotion from full planethood. “Yes, people have said that Pluto is mad at me,” said Brown, who has the nickname of the “Pluto killer” because he discovered other Kuiper Belt objects which led to the new class of dwarf planets.

“For a long time Pluto was this lonely oddball that we didn’t have anything else to compare it with,” said Brown. “Understanding this all as a new class of objects is a much more interesting way of looking at the solar system and it is quite a bit of fun, too.”

More information from Buie’s webpage on the Pluto images.

The paper about the images isn’t posted online yet, but it will be up on this webpage soon.

Source: Conference call.
Additional images and info from NASA

Posted on by Nancy Atkinson

New Horizons Spacecraft Now Closer to Pluto Than Earth

Fastest Spacecraft
Artist concept of the New Horizons spacecraft. Credit: NASA/JPL

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The New Horizons spacecraft crossed a milestone boundary today: it is now closer to its primary destination, Pluto, than to Earth. But New Horizons –the fastest man-made object — is not yet halfway to the dwarf planet. That won’t happen until February 25, 2010. New Horizons is now 1,440 days into its 9.5-year journey and well past 15 AU (astronomical units) from the Sun. But there is a long haul yet to go: there are still 1,928 days until operations begin for the close encounter, and 2022 days until the spacecraft reaches the closest point to Pluto in the summer of 2015. It is exciting to think what we will learn about Pluto and the Kuiper Belt in five and a half years. Will our perspectives change? Hard to believe they won’t.

New Horizons is currently traveling at about 50,000 kph (31,000 mph) (relative to the Sun) and is located about 2.4 billion kilometers (1.527 billion miles) from Earth.

The spacecraft launched in January 2006.

New Horizons will be taken out of hibernation in early January to repoint the communications dish antenna to keep up with the changing position of the Earth around the Sun. It was last awoken in November to download several months of stored science data from the Venetia Burney Student Dust Counter, to correct a recently discovered bug in the fault protection system software, (last thing anyone wants is to have the spacecraft go into safe mode at closest approach), and to upload instructions to run the spacecraft through early January. Telemetry shows that New Horizons is in very good health and almost exactly on its planned course.

Principal Investigator Alan Stern wrote in his last PI’s Perspective notes that the science team will meet in January to discuss which Kuiper Belt Objects they hope to “fly by and reconnoiter after Pluto. Those searches will begin next summer and continue through 2011 and 2012. Hopefully, they’ll net us four to 10 potential targets.”

Posted on by Fraser Cain

Pluto Fact Sheet

Pluto's temperature makes it one of the coldest places in the Solar System.

The following Pluto fact sheet is based on NASA’s excellent planetary fact sheets. Pluto is no longer a planet, but a dwarf planet.

Mass: 0.0125 x 1024 kg
Volume: 0.715 x 1010 km3
Average radius: 1,195 km
Average diameter: 2,390 km
Mean density: 1.750 g/cm3
Escape velocity: 1.2 km/s
Surface gravity: 0.58 m/s2
Natural satellites: 3
Rings? – No
Semimajor axis: 5,906,380,000 km
Orbit period: 90,465 days
Perihelion: 4,436,820,000 km
Aphelion: 7,375,930,000 km
Mean orbital velocity: 4.72 km/s
Orbit inclination: 17.16°
Orbit eccentricity: 0.2488
Sidereal rotation period: 153.2928 hours
Length of day: 153.2820 hours
Axial tilt: 122.53°
Discovery: 18 February 1930
Minimum distance from Earth: 4,284,700,000 km
Maximum distance from Earth: 7,528,000,000 km
Maximum apparent diameter from Earth: 0.11 arc seconds
Minimum apparent diameter from Earth: 0.06 arc seconds
Maximum visual magnitude: 13.65

We’ve written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here’s an article about the distance to Pluto.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded an entire episode of Astronomy Cast just about Pluto. Listen here, Episode 64: Pluto.

Posted on by Fraser Cain

When Was Pluto Discovered?

Clyde Tombaugh

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Five of the planets are visible with the unaided eye and have been known about for thousands of years. Uranus was discovered in 1781, and Neptune was found in 1846. but when was Pluto discovered?

Pluto was discovered February 18th, 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. Now that Uranus and Neptune had been discovered, astronomers were certain that there were more planets in the outer Solar System. The director of the Lowell Observatory, Vesto Melvin Slipher, handed the job of finding this next planet, dubbed “Planet X” to the 23-year old Tombaugh.

Tombaugh used a tool called a “blink comparator” to compare two photographs of the night sky. He worked methodically, comparing two photographic plates, looking for any object that jumped from one night to another night. And finally on February 18th, 1930, Tombaugh discovered Pluto; a faint object in the right orbit.

The name for Pluto was chosen by an 11-year old British school girl named Venetia Burney. This continued the tradition of naming planets after Roman gods. Pluto was the Roman god of the underworld, the same as Hades in Greek mythology. It also matched the first initials of Percival Lowell, who the observatory was named after. Lowell died in 1916, and so he never saw the discovery of Pluto.

We have written many articles about the discovery of planets in the Solar System for Universe Today. Here’s an article about when Uranus was discovered, and here’s an article about when Neptune was discovered.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded a couple of episodes of Astronomy Cast about Pluto. Here’s a good one, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

Who Discovered Pluto?

Clyde Tombaugh

Pluto is incredibly faint. You need a powerful backyard telescope to even see it as a dot, so it’s not surprising that Pluto wasn’t discovered until the modern age. Who discovered Pluto? That was the astronomer Clyde Tombaugh, who found Pluto on February 18, 1930.

Tombaugh worked as an astronomer at the Lowell Observatory in Flagstaff, Arizona. He was given the task of finding a trans-Neptunian object which was predicted by Percival Lowell and William Pickering – the search for Planet X. Tombaugh used a tool called a blink comparator to study two images of the same region of the sky taken several nights apart. He would display one image and then blink to the second image to see if any objects had moved from night to night.

And so on February 18, 1930, Tombaugh turned up just such an object moving at the right speed to be the unknown Planet X. The name “Pluto” was suggested by Venetia Burney, and 11-year old English school girl. Pluto was the name of the Roman god of the underworld, and Percival Lowell liked it because the first two letters started with PL, after his own initials.

Pluto was considered the 9th planet in the Solar System until 2006, when the International Astronomical Union reclassified Pluto as a dwarf planet, joining Eris and Ceres as the Solar System’s 3 dwarf planets. There are now only 8 planets in the Solar System.

We have written many articles about the discovery of planets in the Solar System. Here’s an article about who discovered Uranus, and here’s an article about who discovered Neptune.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded several episodes of Astronomy Cast about Pluto. Start here, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

How Far Away is Pluto From the Sun?

The Pluto system seen from the surface of Hydra. Credit: NASA

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How far away is Pluto from the Sun? Pluto’s average distance from the Sun is 5.9 billion km or 3.7 billion miles.

But Pluto actually follows an elliptical orbit around the Sun. Sometimes it’s much closer to the Sun, and other times it’s further away. At its closest point, Pluto measures only 4.4 billion km from the Sun. This is close enough that a thin layer of frost evaporates from its surface, becoming a thin atmosphere around the planet. And then as it continues its journey around the Sun, Pluto gets colder again and this atmosphere refreezes onto the planet. It continues to travel out to a distance of 7.4 billion km from the Sun.

Astronomers use another method of measuring distances in the Solar System called the astronomical unit. 1 astronomical unit or AU is the average distance from the Earth to the Sun; approximately 150 million km. So we can use this to describe Pluto’s distance from the Sun. At its closest point, Pluto measures 29.7 AU. And then at its furthest point, Pluto is 49.3 AU.

We have written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here are some pictures of Pluto.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded several episodes of Astronomy Cast about Pluto. Listen here, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

Pluto’s Distance from the Sun

The Pluto system seen from the surface of Hydra. Credit: NASA

Pluto’s distance from the Sun is 5.9 billion km – the exact number is 5,906,376,272 km. Need that figure in miles? Pluto’s distance from the Sun is 3.67 billion miles.

Keep in mind that this distance is an average. Pluto follows a highly elliptical orbit around the Sun. At the closest point of its orbit, called perihelion, Pluto gets to within 4.44 billion km from the Sun. And then at its most distant point of its orbit, called aphelion, Pluto gets to within 7.38 billion km of the Sun.

Astronomers use another term to measure distance in the Solar System called “astronomical units”. 1 astronomical unit, or AU, is the average distance from the Earth to the Sun – about 150 million km. Pluto’s perihelion is 29.7 AU, and its aphelion is 49.3 AU. Pluto’s average distance, or semi-major axis, is 39.5 AU.

We have written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here’s an article about methane in Pluto’s atmosphere.

Want more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve recorded several episodes of Astronomy Cast about Pluto. Here’s one, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

How Long Does it Take Pluto to Orbit the Sun?

Hubble image of Pluto and some of its moons, Charon, Nix and Hydra. Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

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Because Pluto orbits much further from the Sun than Earth, it takes much longer to orbit the Sun. In fact, Pluto takes 248 years to orbit the Sun. That’s because Pluto orbits at an average distance of 5.9 billion km from the Sun, while Earth only orbits at 150 million km. In fact, it takes so long for Pluto to orbit that Sun, that the dwarf planet hasn’t even completed a third of an orbit from when it was discovered back in February 18th, 1930.

Pluto has a highly elliptical orbit. Its distance from the Sun varies from 4.4 billion km to 7.4 billion km. And during this orbital period, Pluto goes through a few interesting changes. You might be surprised to learn that Pluto has an atmosphere. When it’s at its closest point to the Sun, Pluto’s atmosphere evaporates from the surface and surrounding the dwarf planet. And then when it gets further away, the atmosphere freezes again, coating the surface in a thin layer.

Pluto was only discovered in 1930 by Clyde W. Tombaugh. Because it takes 248 years to orbit the Sun, Pluto won’t have completed a full orbit until the year 2178.

We have written many articles about Pluto for Universe Today. Here’s an article about why Pluto isn’t a planet any more, and here’s an article about methane discovered in Pluto’s atmosphere.

If you’d like more information on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded several episodes of Astronomy Cast just about Pluto. Listen here, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

How Big is Pluto?

Pluto used to be the smallest planet in the Solar System, but now it isn’t a planet any more, thanks to a recent decision from the International Astronomical Union. But now it’s one of the largest dwarf planets, so that’s a good thing. How big is Pluto?

The diameter of Pluto is only 2,390 km across. Just for comparison, that’s about 70% the diameter of the Moon. And it’s a fraction of the size of the Earth; about 18% of the Earth’s diameter.

In terms of volume, Pluto only has 6.39 x 109 km3. That sounds like a huge number, but it’s only 0.59% of the volume of the Earth. In other words, you could put almost 170 objects the size of Pluto inside the Earth.

The mass of Pluto is 1.3 x 1022 kg, which is only 0.2% the mass of the Earth, or 18% the mass of the Moon. Needless to say, Pluto doesn’t have very much mass at all.

The surface area of Pluto is 1.67 x 107 square kilometers. That’s only 3.3% the surface area of Earth, and about the same surface area as Russia.

If you could stand on the surface of Pluto, you would experience only 6.7% the gravity you enjoy on Earth.

We have written many article about Pluto for Universe Today. Here’s an article that explains why Pluto isn’t a planet any more, and here are some interesting facts about Pluto.

Want more information on Pluto? Here’s Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve recorded an entire episode of Astronomy Cast about Pluto and the rest of the icy outer Solar System. You can find it here.

Posted on by Abby Cessna

Mythology of the Planets

Planets and other objects in our Solar System. Credit: NASA.

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Thousands of years ago, ancient civilizations turned to the heavens, marveling at their wonders. These ancient people worshipped various gods and often linked their gods with planets in the sky, which they considered to be “wandering stars.”

Mercury gets its name from the winged messenger of the gods. He was also the god of thievery, commerce, and travel. Most likely, the planet got its name from the rate at which it spins.

Venus was the Roman goddess of love and beauty, so it is a fitting name for this brightly shining planet. The only objects in our Solar System brighter than Venus are the Sun and the Moon. Ancient civilizations thought that Venus was two different objects – the Morning Star and the Evening Star. Other civilizations have also associated the planet with love. The Babylonians called the planet Ishtar after their goddess of womanhood and love.

Earth is the only planet not named after a Roman god or goddess, but it is associated with the goddess Terra Mater (Gaea to the Greeks). In mythology, she was the first goddess on Earth and the mother of Uranus. The name Earth comes from Old English and Germanic. It is derived from “eor(th)e” and “ertha,” which mean “ground.” Other civilizations all over the world also developed terms for our planet.

Mars is named after the Roman god of war. The planet got its name from the fact that it is the color of blood.  Other civilizations also named the planets for its red color.

Jupiter was the Roman king of the gods. Considering that Jupiter is the largest planet in our Solar System, it makes sense that the planet was named after the most important god.

Saturn was named after the Roman god of agriculture and harvest. While the planet may have gotten its name from its golden color, like a field of wheat, it also had to do with its position in the sky. According to mythology, the god Saturn stole the position of king of the gods from his father Uranus. The throne was then stolen by Jupiter.

Uranus was not discovered until the 1800’s, but the astronomers in that time period continued the tradition of naming planets after Roman gods. In mythology, Uranus was the father of Saturn and was at one time the king of the gods.

While Neptune almost ended up being named after one of the astronomers credited with discovering it – Verrier – that was greatly disputed, so it was named after the god of the sea. The name was probably inspired by its blue color.

Pluto is no longer a planet, but it used to be. The dark, cold, former planet was named after the god of the underworld. The first two letters of Pluto are also the initials of the man who predicted  its existence, Percival Lowell.

Universe Today has articles on names of the planets and all the planets.

For more information on the planets check out all about the planets and mythology of the planets.

Astronomy Cast has episodes on all the planets including Saturn.