Posted on by Nancy Atkinson

New Horizons Mission Practices Telescopic Imager on Pluto’s Twin

New Horizons image of Neptune and its largest moon, Triton. June 23, 2010. Credit: NASA

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This summer, the New Horizons spacecraft was awoken for its annual systems checkout, and took the opportunity to exercise the long range camera by snapping pictures of Neptune, which at the time, was 3.5 billion km (2.15 billion miles) away. The Long Range Reconnaissance Imager (LORRI) snapped several photos of the gas giant, but Neptune was not alone! The moon Triton made a cameo appearance. And the New Horizons team said that since Triton is often called Pluto’s “twin” it was perfect target practice for imaging its ultimate target, Pluto.

This image gets us excited for 2015 when New Horizons will approach and make the closest flyby ever of Pluto.

“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,” said New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

Weaver pointed out that the solar phase angle (the spacecraft-planet-Sun angle) was 34 degrees and the solar elongation angle (planet-spacecraft-Sun angle) was 95 degrees. Only New Horizons can observe Neptune at such large solar phase angles, which he says is key to studying the light-scattering properties of Neptune’s and Triton’s atmospheres.

“As New Horizons has traveled 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,” said New Horizons Principal Investigator Alan Stern.

Triton is slightly larger than Pluto, 2,700 kilometers (1,700 miles) in diameter compared to Pluto’s 2,400 kilometers (1,500 miles). Both objects 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-400 degrees Fahrenheit. Triton is widely believed to have been a member of the Kuiper Belt (as Pluto still is) that was captured into orbit around Neptune, probably during a collision early in the solar system’s history.

Source: New Horizons

Posted on by Nancy Atkinson

Clearing the Confusion on Neptune’s Orbit

This week Neptune will return to the spot where it was discovered in 1846, in the constellation Capricornus. The planet will complete its first orbit, since being discovered, in 2011. Credit: Starry Night Software, via Space.com

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Last week, Space.com had a great article about how on August 20, 2010, Neptune finally completed one orbit around the Sun since its discovery in 1846, and was now back to its original discovery position in the night sky . The original article was widely quoted, and created a lot of buzz on Twitter, Facebook and other websites. But then, later in the day some contradictory info came out, culminating with Bill Folkner, a technologist at JPL declaring via Twitter: “Neptune will reach the same ecliptic longitude it had on Sep. 23, 1846, on July 12, 2011.” Space.com ended up amending their article, but why the confusion? And could both statements be true? Depending on your perspective, perhaps yes.

“These apparently contradictory statements highlight the problems of defining planetary orbits,” astronomer Brian Sheen from the Roseland Observatory in the UK told Universe Today. “There are two ways of following the progress of a planet around the Sun/night sky.”

The first is from the perspective of being on planet Earth (specifically at the center of our planet) – called geocentric longitude, Sheen said, also known as right ascension.

The second is from the perspective of being on the Sun (specifically at the center of the Sun and indeed our solar system) which is called heliocentric longitude, and also ecliptic longitude.

“The orbital period of a planet is measured with reference to the heliocentric longitude, in the case of Neptune this is 164.8 years,” Sheen explained. “The problem of referencing via geocentric longitude is that the Earth itself is orbiting the Sun and therefore changing its relative position to the other planet, this case, Neptune.”

Neptune was discovered Sept 23, 1846. Adding 164.8 years to that date brings us to July 2011, and specifically 12th July. However taking the Earth’s motion into account we have a number of close approaches. Confusion about this situation is exacerbated by the fact that Neptune retrogrades at opposition.

And so, in April and July of this year (2010), Neptune came very close to returning to its apparent position in the sky at the time of its discovery (in geocentric right ascension and declination), actually much closer than it will be next year when it returns to its 1846 heliocentric longitude. It’s location at discovery and currently is in the constellation Capricornus.

But still, Neptune will not complete its first orbit since being discovered until in 2011.

“Given a discovery date of 23rd Sept 1846 and an orbital period of 164.8 years gives a return date of well into 2011 and a rough check gives 9-13 July,” Sheen said. “This accords well with the given date of 12th July.”

This gives us a celebration to look forward to in 2011!

Posted on by Nancy Atkinson

New Trojan Asteroid Discovered Around Neptune

The green arrow shows the asteroid. The other bright objects are stars in the Milky Way. Credit: Scott Sheppard

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Astronomers have found a new object in a region of Neptune’s orbit, tucked away in a very hard-to-find location, and where no previous object was known to exist. The object, 2008 LC18, is a Trojan asteroid, which refers an asteroid that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability. Six other Trojan asteroids have been located around Neptune’s L4 region, but this is the first one found in Neptune’s L5 region.

Scott Sheppard from the Carnegie Institution’s Department of Terrestrial Magnetism and colleagues used a new observational technique that used large dark clouds to block background light from the galactic plane in order to discover the new Neptune Trojan. They used the discovery to estimate the asteroid population there and find that it is probably similar to the asteroid population at Neptune’s L4 point.

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5,” said Sheppard “It matches the population estimates for the L4 Neptune stability region. This makes the Neptune Trojans more numerous than those bodies in the main asteroid belt between Mars and Jupiter. There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

Jupiter has the most Trojans, 4,076 (as of February 2010) but there are four known Mars Trojans and now seven known Neptune Trojans. So far, searches have failed to uncover any similar objects in the orbits of any other planets.

The five Lagrangian points of stability are shown at Neptune. Credit: Scott Sheppard

“The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively,” said 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 center of our galaxy.”

Sheppard and his team, which included Chad Trujillo from the Gemini Observatory, used images from a digitized all-sky survey to identify places in the stability regions where dust clouds in our galaxy blocked out the background starlight from the galaxy’s plane, providing an observational window to the foreground asteroids. They discovered the L5 Neptune Trojan using the 8.2-meter Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie’s 6.5-meter Magellan telescopes at Las Campanas, Chile.

Because Trojans share their planet’s orbit they are sensitive to the planet’s formation and migration, and astronomers say finding these objects provide clues that may help unlock the answers to fundamental questions about planetary formation and migration.

The region of space is also of interest to the teams from the New Horizon spacecraft, as it will pass through this same area after its encounter with Pluto in 2015.

Read the team’s abstract.

Sources: Carnegie Institute, Science Express.

Posted on by Nancy Atkinson

Comet Whacked Neptune 200 Years Ago

Neptune. Credit: NASA

Researchers studying Neptune’s atmosphere found evidence that a comet may have hit the planet about two centuries ago. Was this a “cold-case” file re-opened, or did they discover a way to travel back in time to witness a long-ago event? To make the discovery, a team from the Max Planck Institute for Solar System Research actually used the Herschel Space Telescope’s PACS (Photodetector Array Camera and Spectrometer) instrument, along with what was learned from observations from when the Shoemaker-Levy 9 hit Jupiter sixteen years ago.
Continue reading “Comet Whacked Neptune 200 Years Ago”

Posted on by Fraser Cain

Neptune Fact Sheet

Neptune

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The following Neptune fact sheet is based on NASA’s excellent planetary fact sheets. Neptune is the 8th planet from the Sun, and it requires a telescope to be able to see it.

Mass: 102.43 x 1024 kg
Volume: 6,254 x 1010 km3
Average radius: 24,622 km
Average diameter: 49,244 km
Mean density: 1.638 g/cm3
Escape velocity: 23.5 km/s
Surface gravity: 11.15 m/s2
Natural satellites: 13
Rings? – Yes
Semimajor axis: 4,495,060,000 km
Orbit period: 60,189 days
Perihelion: 4,444,450,000 km
Aphelion: 4,545,670,000 km
Mean orbital velocity: 5.43 km/s
Orbit inclination: 1.769°
Orbit eccentricity: 0.0113
Sidereal rotation period: 16.11 hours
Length of day: 16.11 hours
Axial tilt: 28.32°
Discovery: 23 September 1846
Minimum distance from Earth: 4,305,900,000 km
Maximum distance from Earth: 4,687,300,000 km
Maximum apparent diameter from Earth: 2.4 arc seconds
Minimum apparent diameter from Earth: 2.2 arc seconds
Maximum visual magnitude: 7.78

We’ve written many articles about Neptune for Universe Today. Here’s an article about the color of Neptune, and here’s an article about the atmosphere of Neptune.

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

We’ve also recorded an entire episode of Astronomy Cast just about Neptune. Listen here, Episode 63: Neptune.

Posted on by Jean Tate

Nereid

Nereid (from Voyager 2; credit JPL)

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Nereid is the name given to the third largest of Neptune’s moons, and the second to have been discovered … by veteran outer solar system astronomer, Gerard P. Kuiper (guess who the Kuiper Belt is named after!), in 1949. Prior to Voyager 2’s arrival, it was the last moon of Neptune to be discovered.

In keeping with the nautical theme (Neptune, Roman god of the sea; Triton, Greek sea god, son of Poseidon), Nereid is named after the fifty sea nymphs, daughters of Nereus and Doris, in Greek mythology … the nautical theme continues with the names of the other 11 moons of Neptune, Naiad (one kind of nymph, Greek mythology; not a Nereid), Thalassa (daughter of Aether and Hemera, Greek mythology; also Greek for ‘sea’), Despina (nymph, daughter of Poseidon and Demeter (Greek); not a Nereid), Galatea (one of the Nereids), Larissa (Poseidon’s lover; Poseidon is the Greek Neptune), Proteus (also a sea god in Greek mythology; Proteus is the Neptune’s second largest moon), Halimede (one of the Nereids), Sao (also one of the Nereids), Laomedeia (guess … yep, another of the Nereids), Psamathe (ditto), and Neso (ditto, all over again).

Almost everything we know about Nereid comes from the images Voyager 2 took of it (83), between 20 April and 19 August, 1989; its closest approach was approximately 4.7 million km.

Nereid’s highly eccentric orbit (eccentricity 0.75, the highest of any solar system moon) takes it from 1.37 million km from Neptune to 9.66 million km (average 5.51 million km); unlike Triton, and like the other inner moons, Nereid’s orbit is prograde. This suggests that it may be a captured Kuiper Belt object, or that its orbit was substantially perturbed when Triton was captured.

For an irregular moon, Nereid is rather large (radius approx 170 km). Its spectrum and color (grey) are quite different from those of other outer solar system bodies (e.g. Chiron), which suggests that it may have formed around Neptune.

For more on Nereid, check out the Jet Propulsion Laboratory’s (JPL) profile of it!

Nereid is a bit of an orphan with regard to Universe Today stories, but there are some! Three new moons discovered for Neptune , and How Many Moons Does Neptune Have?.

Posted on by Fraser Cain

How Far is Neptune’s from the Sun?

Neptune

Neptune’s distance from the Sun is 4.5 billion km; more specifically, it’s 4,503,443,661 km. If you’re still using the Imperial system, that’s the same as 2.8 billion miles.

But this number is actually an average. Like all of the planets in the Solar System, Neptune follows an elliptical orbit around the Sun, so it’s sometimes closer and sometimes further than this average number. When Neptune is at its closest point to the Sun, called perihelion, it’s 4.45 billion km from the Sun. And then when it’s at its most distant point from the Sun, called aphelion, it’s 4.55 billion km from the Sun.

Astronomers also measure distance in the Solar System using a measuring tool called the “astronomical unit”. 1 astronomical unit, or AU, is the average distance from the Earth to the Sun; that’s about 150 million km. So, Neptune’s average distance from the Sun is 30.1 AU. Its perihelion is 29.8 AU, and it’s aphelion is 30.4 AU.

We have written many articles about Neptune for Universe Today. Here’s an article about Neptune’s moons, and here’s an article about how Neptune’s southern pole is the warmest place on the planet.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We’ve also recorded an episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.

Posted on by Fraser Cain

How Long Does it Take Neptune to Orbit the Sun

Neptune

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Neptune orbits much further away from the Sun than the Earth, so its orbit takes much longer. In fact, Neptune takes 164.79 years to orbit around the Sun. That’s almost 165 times longer than Earth takes to orbit the Sun.

Here’s an interesting fact. Neptune was only discovered on September 23, 1846. At the time this article was written (2009), that was only 163 years ago. In other words, since its discovery, Neptune has not even made a single orbit around the Sun.

On July 11, 2011, Neptune will have completed one full orbit around the Sun. Finally, Neptune will be 1 year old.

Just like Earth, Neptune’s axis is tilted away from the Sun’s axis. This means that it experiences seasons as it orbits the Sun. For half of its orbit, Neptune’s northern hemisphere is tilted towards the Sun, and then for the second half of its orbit, its southern hemisphere is tilted towards the Sun. This differential heating creates very powerful winds on Neptune. In fact, Neptune has the strongest sustained winds on the Solar System, with winds measured at 2100 km/hour.

We have written many articles about Neptune for Universe Today. Here’s an article about the atmosphere of Neptune. And here’s an article about who discovered Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have also recorded an entire episode of Astronomy Cast just about Neptune. Listen here, Episode 63: Neptune.

Posted on by Abby Cessna

Solar System Orbits

Take a look at the Solar System from above, and you can see that the planets make nice circular orbits around the Sun. But dwarf planet’s Pluto’s orbit is very different. It’s highly elliptical, traveling around the Sun in a squashed circle. And Pluto’s orbit is highly inclined, traveling at an angle of 17-degrees. This strange orbit gives Pluto some unusual characteristics, sometimes bringing it within the orbit of Neptune. Credit: NASA

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One of the International Astronomical Union’s (IAU) requirements for a celestial body to be classified as a planet (or a dwarf planet) is that it orbits the Sun. All of the planets have different orbits, which affect many of the planets’ other characteristics.

Since Pluto became a dwarf planet, Mercury is the planet with the most eccentric orbit. The eccentricity of an orbit is the measurement of how different the orbit is from a circular shape. If an orbit is a perfect circle, its eccentricity is zero. As the orbit becomes more elliptical, the eccentricity increases. Mercury’s orbit ranges from 46 million kilometers from the Sun to 70 million kilometers from the Sun.

Venus, which is right next to Mercury, has the least eccentric orbit of any of the planet in the Solar System. Its orbit ranges between 107 million km and 109 million km from the Sun and has an eccentricity of .007 giving it a nearly perfect circle for its orbit.

Earth also has a relatively circular orbit with an eccentricity of .017. Earth has a perihelion of 147 million kilometers; the perihelion is the closest point to the Sun in an object’s orbit. Our planet has an aphelion of 152 million kilometers. An aphelion is the furthest point from the Sun in an object’s orbit.

Mars has one of the most eccentric orbits in our Solar System at .093. Its perihelion is 207 million kilometers, and it has an aphelion of 249 million kilometers.

Jupiter has a perihelion of 741 million kilometers and an aphelion of 778 million kilometers. Its eccentricity is .048. Jupiter takes 11.86 years to orbit the Sun. Although this seems a long time compared to the time our own planet takes to orbit, it is only a fraction of the time of some of the other planets’ orbits.

Saturn is 1.35 billion kilometers at its perihelion and 1.51 billion kilometers from the Sun at its furthest point. It has an eccentricity of .056. Since it was first discovered in 1610, Saturn has only orbited the Sun 13 times because it takes 29.7 years to orbit once.

Uranus is 2.75 billion miles from the Sun at its closest point and 3 billion miles from the Sun at its aphelion. It has an eccentricity of .047 and takes 84.3 years to orbit the Sun. Uranus has such an extreme axial tilt (97.8°) that rotates on its side. This causes radical changes in seasons.

Neptune is the furthest planet from the Sun with a perihelion of 4.45 billion kilometers and an aphelion of 4.55 billion kilometers. It has an eccentricity of .009, which is almost as low as Venus’ eccentricity. It takes Neptune 164.8 years to orbit the Sun.

Universe Today has articles on orbits of the planets and asteroid orbits.

For more information, check out articles on an overview of the Solar System and new planet orbits backwards.

Astronomy Cast has episodes on all the planets including Mercury.

References:
NASA: Transits of Mercury
NASA: Solar System Math
NASA: Mars, You’re So Complicated
NASA Solar System Exploration

Posted on by Abby Cessna

Radius of the Planets

Size of the planets compared.

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One way to measure the size of the planets is by radius. Radius is the measurement from the center of an object to the edge of it.

Mercury is the smallest planet with a radius of only 2,440 km at its equator. Mercury is not that much larger than the Moon, and it is actually smaller than some of our Solar System’s larger satellites, such as Titan. Despite Mercury’s small size, it is actually dense with higher gravity than you would expect for its size.

Venus has a radius of 6,052 kilometers, which is only a few hundred kilometers smaller than Earth’s radius. Most planets have a radius that is different at the equator than it is at the poles because the planets spin so fast that they flatten out at the poles. Venus has the same diameter at the poles and at the equator though because it spins so slowly.

Earth is the largest of the four inner planets with a radius of 6,378 kilometers at the equator. This is over two times larger than the radius of Mercury. The radius between the poles is 21.3 km less than the radius at the equator because the planet has flattened slightly since it only takes 24 hours to rotate.

Mars is a surprisingly small planet with a radius of 3,396 kilometers at the equator and 3,376 kilometers at the poles. This means that Mars’ radius is only about half of Earth’s radius.

Jupiter is the largest of all the planets. It has a radius of 71,492 kilometers at the equator and a radius of 66,854 kilometers at the poles. This is a difference of 4,638 kilometers, which is almost twice Mercury’s radius. Jupiter has a radius at the equator 11.2 times Earth’s equatorial radius.

Saturn has an equatorial radius of 60,268 kilometers and a radius of 54,364 kilometers at the poles making it the second largest planet in our Solar System. The difference between its two radiuses is a little more than twice the radius of Mercury.

Uranus has an equatorial radius of 25,559 kilometers and a radius of 24,973 kilometers at the poles. Although this is much smaller than Jupiter’s radius, it is around four times the size of Earth’s radius.

Neptune’s equatorial radius of 24,764 kilometers makes it the smallest of the four outer planets. The planet has a radius of 24,341 kilometers at the poles. Neptune’s radius is almost four times the size of Earth’s radius, but it is only about a third of Jupiter’s radius.

Universe Today has articles on the radius of Neptune and the size of the planets.

If you are looking for more information, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

Astronomy Cast has an episode on Venus and more on all the planets.