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Happy birthday Neptune!


Neptune, the eighth planet from the Sun, has now completed one 165-Earth-year-long orbit of the Sun since it's discovery in 1846.

THE EIGHTH PLANET is celebrating today, having completed one orbit around the Sun since its discovery in 1846. Neptune’s year is 164.8 Earth years long, so it has taken until now for it to make one full circle of the Solar System.

Neptune was the first planet to be found via a mathematical prediction. Astronomers had noted that Uranus—the next planet inwards to the Sun—was not following its predicted path, and the gravitational pull of an as-yet-undiscovered planet was thought to the culprit.

Predictions were made for where in the sky this mystery planet might be found, and sure enough, there it was—Neptune.

The story of the prediction and discovery has lots of twists and turns—read more about it here.

And the story of how the planet then got its name is equally complex—and you can read more about that here.

Neptune orbits the Sun at an average distance of 30.1 AU (one AU being the distance between Earth and Sun), or about 4.5 billion kilometres.

The following video (courtesy NASA, ESA, G. Bacon, and Z. Levay (STScI)) shows a speeded up view of Neptune rotating, using images taken every four hours by the Hubble Space Telescope:

Neptune is the fourth-largest planet, with a radius at the equator of 24,764 kilometres—about four times wider than Earth.

The giant blue world is the most distant Solar System body visited by a spacecraft. NASA’s Voyager 2 probe flew past Neptune in 1989.

Here’s a fascinating video (courtesy NASA, ESA, and G. Bacon and M. Estacion (STScI)) which puts that one Neptunian orbit into an Earth timeline perspective:

Story by Jonathan Nally. Images courtesy NASA.

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Voyager – the journey continues

AFTER 33 YEARS, NASA’s twin Voyager spacecraft are still going strong and still sending home information. This video features highlights of the Voyager journeys to the outer planets, and looks at their current status, at the edge of our Solar System, poised to cross over into interstellar space.

Adapted from information issued by NASA / JPL.

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Planets all in a row

EARLY-RISING AUSTRALASIAN SKYWATCHERS are in for a treat this coming Saturday morning, April 30, as four of the naked-eye planets and the crescent Moon all come together in the morning sky before dawn.

In the video above, Melbourne Planetarium’s marvellous astronomer, Tanya Hill, explains when and where to see the spectacle.

And you can keep up-to-date with sky happenings with’s monthly Whats’ Up? section.

Video courtesy of Museum Victoria.

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Pluto mission spots planet’s twin

New Horizons image of Neptune Triton

New Horizons image of the planet Neptune. Its largest moon, Triton, can be seen be seen just off to one side. The spacecraft was almost 3.5 billion kilometres away from the pair when it took this image!

  • New Horizons spots Neptune and its moon Triton
  • Triton orbits Neptune the “wrong way”
  • Triton is often considered to be a twin of Pluto

NASA’s Pluto-bound New Horizons spacecraft—now a little over halfway there—has turned its attention to the planet Neptune and its largest moon, Triton.

Mission controllers periodically test the spacecraft’s cameras by aiming them at other Solar System bodies.

New Horizons’ Long Range Reconnaissance Imager (LORRI) snapped several images of Neptune during the latest annual systems checkout, which ended July 30. Neptune was 23.2 astronomical units (about 3.48 billion kilometres!) from New Horizons when LORRI took aim at the gas giant planet—and Triton made a cameo appearance.

Because Neptune and Triton were so far away, they are hard to tell apart in the images. But Triton can be seen as a dot or blob just off to one side.

New Horizons

New Horizons spacecraft prior to launch in 2006.

“That we were able to see Triton so close to Neptune, which is approximately 100 times brighter, shows us that the camera is working exactly as designed,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

“As New Horizons has travelled outward across the Solar System, we’ve been using our imagers to make just such special-purpose studies of the giant planets and their moons because this is a small but completely unique contribution that New Horizons can make—because of our position out among the giant planets,” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute.

Triton is often called Pluto’s twin. Only slightly larger than Pluto—2,700 kilometres diameter compared to Pluto’s 2,400 kilometers—both worlds have atmospheres composed mostly of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures approaching minus -240 degrees Celsius.

Pluto’s twin: an enigma

Triton was discovered on October 10, 1846 by English astronomer William Lassell, just 17 days after Neptune itself had been discovered (by German astronomer Johann Gottfried Galle).

Geyser on Triton

Dark streaks show where Triton's ice geysers have been active.

Neptune’s big moon is very unusual, in that it is the only large moon that goes around its planet backwards. That is, Neptune rotates from west to east (as does Earth), but Triton orbits in the planet from east to west (unlike our Moon). This is called a retrograde orbit.

The only plausible explanation is that Triton’s didn’t form along with Neptune, but rather was captured as it wandered past. Given that it is almost a twin of Pluto, it is supposed by most astronomers that Triton was a member of the Kuiper Belt—the swarm of small icy worlds that orbit the Sun beyond Neptune.

Triton also is quite big—its diameter of 2,700 kilometres makes it the seventh largest moon in the Solar System.

Its surface is a frozen crust of mostly nitrogen, underneath which is a core thought to be composed of rock and metals and making up two-thirds the moon’s mass.

One of the amazing things about Triton is that it has active geological features. When NASA’s Voyager 2 probe flew past in 1989, it spotted dark geysers shooting up from the surface, and dark streaks on the surface downwind of the geysers. The only other Solar System bodies confirmed to have volcanic activity are Earth, Jupiter’s moon Io, and Saturn’s moon Enceladus.

Unlike many moons—which are covered in craters—Triton has few impact craters. Scientists put this down to the geological activity, such as tectonic processes and volcanoes, which can reshape the landscape and wipe out any traces of craters. But unlike the volcanoes on Earth, on this frozen world the lava consists of water and ammonia!

Voyager 2 also sensed a thin atmosphere as it went past—observations made from Earth in 1990s indicated that the atmosphere was, at that time, thicker than when Voyager was there.

Pluto, here we come!

New Horizons was launched on January 19, 2006, on a trajectory and with a velocity that to reach Pluto in the minimum possible time. In consequence, New Horizons is the fastest spacecraft to leave Earth, having reached a velocity of 58,536 km/h after launch.

The spacecraft reached the orbit of Jupiter in February 2007, passed the orbit of Saturn in June 2008, and is not far away from the orbital distance of Uranus.

New Horizons is due to reach Pluto on July 14, 2015, and conduct a fly-by. It is not equipped with a rocket system to slow down and go into orbit around Pluto; instead, it will go sailing past.

But for around 200 days leading up to the encounter, it will start taking images that are better than best images we currently have of the icy world, so there will be plenty of time to make new discoveries.

Following the encounter, New Horizons will continue into deep space. There is a strong chance that mission controllers will be able to target the spacecraft to do a subsequent fly-by of one of the other icy worlds that inhabit the Kuiper Belt.

Adapted from information issued by NASA / JHU APL.

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Neptune’s dead zone is not empty

Neptune and some of its moons

Neptune and some of its moons. Stable points along Neptune's orbit, called Lagrangian points, have been found to harbour asteroids that probably were captured as they wandered through.

  • Stable points where gravity from Sun and a planet balance out
  • Asteroids and other material can gather in these points
  • First asteroid has been found in one of Neptune’s stable points

There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable there. These places are called Lagrangian points.

Three Lagrangian points are found inline with a planet and the Sun—one beyond the planet (called L2), one between the planet and the Sun (L1), and one on other side of the Sun (L3)

There are also two stable points along the planet’s orbit, but at an angle of 60 degrees to the Sun. These are the L4 and L5 points (see the diagram at right).

Diagram showing Neptune's Lagrangian points

Neptune's Lagrangian points, where the gravitational tug of the planet and the Sun balances out. Asteroids had been found at L4, and now one has been found at L5.

So-called Trojan asteroids have been found in some of these stable spots near Jupiter and Neptune.

Trojans share their planet’s orbit and help astronomers understand how the planets formed and how the Solar System evolved.

Scott Sheppard at the Carnegie Institution’s Department of Terrestrial Magnetism and Chad Trujillo at the Gemini Observatory have discovered the first Trojan asteroid, 2008 LC18, in the difficult-to-study L5 point at Neptune.

They used the discovery to estimate the asteroid population there and suggest that it is similar to the asteroid population at Neptune’s L4 point.

“The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively,” explains Sheppard. “Unlike the other three Lagrangian points, these two areas are particularly stable, so dust and other objects tend to collect there.”

“We found 3 of the 6 known Neptune Trojans in the L4 region in the last several years, but L5 is very difficult to observe because the line-of-sight of the region is near the bright centre of our galaxy.”

This means that it is very hard to pick out a faint asteroid from amongst the myriad stars in the background.

Silver lining to these dark clouds

So the scientists devised a unique observing strategy. They used images from a digitised all-sky survey to identify places in the stability regions where dust clouds in our galaxy blocked out the background stars, making it easier to spot the foreground asteroids.

Discovery images of the L5 trailing Neptune Trojan 2008 LC18

Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008. The Trojan is seen moving from right to left near the centre of the image. Each image is separated by about one hour in time. The background stars are stationary.

They discovered the L5 Neptune Trojan using the 8.2-metre Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie’s 6.5-metre Magellan telescopes at Las Campanas, Chile.

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometres and that there are about 150 Neptune Trojans of similar size at L5,” Sheppard said. “It matches the population estimates for the L4 Neptune stability region.”

“This makes 100-km-wide Neptune Trojans more numerous than similar-sized bodies in the main asteroid belt between Mars and Jupiter.”

Probably captured

There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

The Trojan 2008 LC18 has an orbit that is very tilted to the plane of the Solar System, just like several in L4. This suggests they were captured into these stable regions during the very early Solar System when Neptune was moving on a much different orbit than it is now.

Capture was either through a slow, smooth planetary migration process or as the giant planets settled into their orbits, their gravitational attraction could have caught and “frozen” asteroids into these spots.

The Solar System was likely a much more chaotic place during that time with many bodies stirred up onto unusual orbits.

Adapted from information issued by Carnegie Institution / NASA, ESA, E. Karkoschka (University of Arizona), and H.B. Hammel (Space Science Institute, Boulder, Colorado).

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Unique view of Jupiter

Image of Jupiter taken by New Horizons

A New Horizons spacecraft Long Range Reconnaissance Imager (LORRI) image of Jupiter and two of its moons, taken on June 24 when the spacecraft was 2.4 billion kilometres from the giant planet.

  • New Horizons spacecraft bound for Pluto
  • Has taken images of Jupiter and Neptune
  • Pluto rendezvous set for July 14, 2015

NASA’s Pluto-bound spacecraft, New Horizons—which in December 2009 passed the halfway mark in its 10-year journey to the distant world—has turned around to take a look back into the middle part of the Solar System.

On June 24, the spacecraft’s Long Range Reconnaissance Imager (LORRI) was trained on Jupiter, the largest planet in the Solar System.

Artist's impression of New Horizons at Pluto

Artist's impression of how the New Horizons spacecraft will look during its fly-by of Pluto in July 2015.

A little over three years ago, New Horizons made a close fly-by of Jupiter to pick up speed using a “gravitational slingshot”. Now, in July 2010, the spacecraft is 2.4 billion kilometres from the planet—1,000 times further than it was at the moment of closest approach during that fly-by.

“The picture is a dramatic reminder of just how far New Horizons, moving about a million miles [1.6 million kilometres] a day, has travelled,” says mission Principal Investigator Alan Stern, of the Southwest Research Institute.

Despite the huge distance, the disc of Jupiter is readily apparent due to the planet’s large size. And because from LORRI’s perspective there was a good angle between Jupiter and the Sun, the planet looks a bit like a half full Moon.

And speaking of moons, two of Jupiter’s can faintly be seen—Ganymede on the left and Europa on the right.

Calibration image

The main aim of the Jupiter image was to test LORRI’s susceptibility to sunlight. LORRI is designed to operate in the distant, pitch black environment of Pluto, where sunlight is hundreds of times dimmer than it is here on Earth. So the camera is very sensitive; too much light would damage it. Which is why the Jupiter image was made with an exposure of only 0.009 second, and why the two moons appear so faint.

“We wanted to see how much stray sunlight would creep into these Jupiter pictures, especially since we’ll make observations of the Pluto system in a similar geometry after the spacecraft passes Pluto in 2015,” says Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory.

“We generally prefer to look at targets in the opposite direction from the Sun. In fact, LORRI is calibrated for the low light we’ll see in the Pluto system and Kuiper belt. Pointing too close to the Sun could damage the camera, but we decided it was safe to try to observe Jupiter.”

“The observations were successfully executed and the images look great.”

Image of Neptune taken by New Horizons of almost 3.5 billion kilometres.

Neptune as seen by New Horizons from a distance of around 3.4 billion kilometres.

A glance at Neptune

LORRI also was pointed toward another of the Solar System’s planets, Neptune. The huge blue world is around six times further from the Sun than Jupiter, so in the LORRI image it looks just like a fuzzy star.

The 100-millisecond exposure, made on June 23 when New Horizons was still 23 astronomical units from Neptune (one astronomical unit is the distance between the Sun and the Earth), was part of a navigation system test. New Horizons is equipped with “optical navigation”, a fancy term that means trajectory corrections during the final stages of its approach to Pluto will be aided by taking pictures of the planet and comparing its apparent position with its calculated position.

Even though no detail can be made out on Neptune’s disc, scientists can still use the images to learn more about the planet’s atmosphere, by seeing how sunlight scatters off the molecules.

On course for Pluto

At the beginning of July, the New Horizons team instructed the spacecraft to make a 35.6-second burn of its thruster. Calculations had shown the craft drifting slightly off course, due to a tiny amount of force applied by heat coming from the spacecraft’s radioisotope power source and reflecting off the main antenna!

With the burn correctly executed, New Horizons is on target for its closest approach to Pluto at 7:49am US EDT on July 14, 2005.

Due to mass constraints, the craft is not equipped with a braking rocket, so it will not be able to stop. Instead, it will go sailing straight past the tiny dwarf planet at a great rate of knots, and then head out into deep space. Mission planners are hoping they can find one or more candidate Kuiper Belt Objects—the small, icy worlds that inhabit the outer Solar System—and steer New Horizons to another rendezvous.

Hubble Space Telescope images of Pluto's surface

Our very best views of Pluto's surface, made from multiple Hubble Space Telescope images taken in 2002-03. From 200 days out, New Horizons will begin to take better photos than these.

Even though New Horizons will be unable to stop at Pluto, that doesn’t mean it won’t get a good look at the small planet and its three known moons. From roughly 200 days before the encounter right through until many days after, the spacecraft will take images that are better than the Hubble Space Telescope can produce.

Story by Jonathan Nally, Editor,

Images courtesy of NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute / ESA/ M. Buie.

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Neptune’s big moon is heating up

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface. The distant Sun appears at the upper-left and the blue crescent of Neptune right of centre.

Summer is in full swing in the southern hemisphere of Neptune’s moon Triton, say astronomers who performed the first-ever infrared analysis of its atmosphere.

The European observing team used the European Southern Observatory’s (ESO) Very Large Telescope to detect carbon monoxide and make the first ground-based detection of methane in Triton’s thin atmosphere.

These observations revealed that the thin atmosphere varies seasonally, thickening when warmed.

“We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away. This icy moon actually has seasons just as we do on Earth, but they change far more slowly,” says Emmanuel Lellouch, the lead author of the scientific paper reporting the results.

On Triton, where the average surface temperature is about minus 235 degrees Celsius, it is currently summer in the southern hemisphere and winter in the northern.

A Voyager 2 image of Triton, with Neptune in the distance

Combined images of Neptune's moon Triton (foreground), and Neptune.

As Triton’s southern hemisphere warms up, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton’s surface turns into gas, thickening the icy atmosphere as the season progresses during Neptune’s 165-year orbit around the Sun.

A season on Triton lasts a little over 40 years, and Triton passed the southern summer solstice in 2000.

Triton’s thickening atmosphere

Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton’s atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon.

But despite that increase, the atmospheric pressure on Triton is still only between 40 and 65 microbars — that’s 20,000 times less than on Earth.

Carbon monoxide was known to be present as ice on the surface, but Lellouch and his team discovered that Triton’s upper surface layer is enriched with carbon monoxide ice by about a factor of ten compared to the deeper layers, and that it is this upper “film” that feeds the atmosphere.

While the majority of Triton’s atmosphere is nitrogen (much like on Earth), the methane in the atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well.

Is Triton a twin of Pluto?

Of Neptune’s 13 moons, Triton is by far the largest, and, at 2,700 kilometres in diameter (or three quarters the Earth’s Moon), is the seventh largest moon in the Solar System.

Since its discovery in 1846, Triton has fascinated astronomers thanks to its geologic activity, and its many different types of surface ices, such as frozen nitrogen as well as water and dry ice (frozen carbon dioxide).

Triton is also the only large moon in the Solar System with a retrograde motion, which is an orbital direction in the opposite direction to its planet’s rotation. This is one of the reasons why Triton is thought to have been captured from the Kuiper Belt, and thus shares many features with the dwarf planets, such as Pluto.

Pluto, often considered a cousin of Triton and with similar conditions, is receiving renewed interest in the light of the carbon monoxide discovery, and astronomers are racing to find this chemical on the even more distant dwarf planet.

Adapted from information issued by ESO / L. Calçada.