Clearing the Confusion on Neptune’s Orbit


Last week, 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.” 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!

New Trojan Asteroid Discovered Around Neptune


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.

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”

40 Years of Summer on Triton


If you’re planning a trip to Neptune’s moon Triton, you’ll want to head to the southern hemisphere where it’s now just past mid-summer. Yes, distant Triton actually does have seasons, astronomers at ESO’s Very Large Telescope recently determined. “We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away,” said astronomer Emmanuel Lellouch in an ESO press release. “This icy moon actually has seasons just as we do on Earth, but they change far more slowly.” According to the first ever infrared analysis of Triton’s atmosphere, the seasons last about 40 Earth years. But while summer is in full swing in Triton’s southern hemisphere, there’s no need to pack your bikini. The average surface temperature is about minus 235 degrees Celsius.

Oh, and you’ll also want to bring along a little breathable air. The ESO team also – unexpectedly – discovered carbon monoxide in Triton’s thin atmosphere, mixed in with methane and nitrogen.

The astronomer’s observations revealed that Triton’s thin atmosphere varies seasonally, thickening when warmed. When the distant sun’s rays hits Triton at their best summer angle, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton’s surface sublimates into gas, thickening the icy atmosphere as the season progresses during Neptune’s 165-year orbit around the Sun. Triton passed the southern summer solstice in 2000.

Voyager 2's view of Triton. Credit: NASA

So, while this action increases the thickness of the atmosphere, thus increasing the atmospheric pressure, you’ll still need a pressure suit as well for your visit. 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. The Voyager data indicated the atmosphere of nitrogen and methane had a pressure of 14 microbars, 70,000 times less dense than the atmosphere on Earth. The data from ESO shows the atmospheric pressure is now between 40 and 65 microbars — 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.

“Climate and atmospheric models of Triton have to be revisited now, now that we have found carbon monoxide and re-measured the methane,” said co-author Catherine de Bergh. The team’s results are published in Astronomy & Astrophysics

If we could actually visit Triton, it would likely be a very interesting destination as we know it has geologic activity and a changing surface – plus its unique retrograde motion would offer a unique view of the solar system.

While Triton is the seventh largest moon in our solar system, its distance and position from Earth makes it difficult to observe, and ground-based observations since Voyager 2 have been limited. Observations of stellar occultations (a phenomenon that occurs when a Solar System body passes in front of a star and blocks its light) indicated that Triton’s surface pressure was increasing in the 1990’s. But a new instrument on the VLT, the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES) has provided the chance to perform a more detailed study of Triton’s atmosphere. “We needed the sensitivity and capability of CRIRES to take very detailed spectra to look at the very tenuous atmosphere,” said co-author Ulli Käufl.

These observations are just the beginning for the CRIRES instrument, which will be extremely helpful in studying other distant bodies in our solar system, such as Pluto and other Kuiper Belt Objects. Pluto is often considered a cousin of Triton with similar conditions, and in the light of the carbon monoxide discovery on Triton, astronomers are racing to find this chemical on the even more distant Pluto.

Read the team’s paper.

Source: ESO

New Horizons Spots Neptune’s Moon Triton


New Horizons got a great shot of Neptune’s moon Triton last fall, as it was trucking toward Pluto and the Kuiper Belt. 

The mission was 2.33 billion miles (3.75 billion kilometers) from Neptune on Oct. 16, when its Long Range Reconnaissance Imager (LORRI) locked onto the planet and snapped away. The craft was following a programmed sequence of commands as part of its annual checkout. NASA released the image Thursday afternoon.

Mission scientists say the shot was good practice for imaging Pluto, which New Horizons will do in 2015. Neptune’s moon Triton and Pluto — the former planet retitled in 2006 as the ambassador to the Kuiper Belt — have much in common.

“Among the objects visited by spacecraft so far, Triton is by far the best analog of Pluto,” said New Horizons Principal Investigator Alan Stern. 

Triton is only slightly larger than Pluto, boasting a 1,700-mile (2,700-kilometers) diameter compared to Pluto’s 1,500-mile (2,400-kilometer) girth. Both objects have atmospheres primarily composed of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures. Temperatures average -390 degrees F (-199 degrees C) on Triton and -370 degrees F (-188 degrees C) on Pluto. 

Triton is widely believed to have once been a member of the Kuiper Belt that was captured into orbit around Neptune, probably during a collision early in the solar system’s history. Pluto was the first Kuiper Belt object to be discovered.

Furthermore, “We wanted to test LORRI’s ability to measure a faint object near a much brighter one using a special tracking mode,” said New Horizons Project Scientist Hal Weaver, of Johns Hopkins University, “and the Neptune-Triton pair perfectly fit the bill.”

LORRI was operated in 4-by-4 format (the original pixels are binned in groups of 16), and the spacecraft was put into a special tracking mode to allow for longer exposure times to maximize its sensitivity.

Mission scientists also wanted to measure Triton itself, to follow up on observations made by the Voyager 2 spacecraft during its flyby of Neptune in 1989. Those images revealed evidence of cryovolcanic activity and cantaloupe-like terrain. New Horizons can observe Neptune and Triton at solar phase angles (the Sun-object-spacecraft angle) that are not possible to achieve from Earth-based facilities, yielding new insight into the properties of Titan’s surface and Neptune’s atmosphere.

New Horizons is currently in electronic hibernation, 1.2 billion miles (1.93 billion kilometers) from home, speeding away from the Sun at 38,520 miles (61,991 kilometers) per hour. LORRI will continue to observe the Neptune-Triton pair during annual checkouts until the Pluto encounter in 2015. 

LEAD IMAGE CAPTION: The top frame is a composite, full-frame (0.29° by  0.29°) LORRI image of Neptune taken Oct. 16, 2008, using an exposure time of 10 seconds and 4-by-4 pixel re-binning to achieve its highest possible sensitivity. The bottom frame is a twice-magnified view that more clearly shows the detection of Triton, Neptune’s largest moon. Neptune is the brightest object in the field and is saturated (on purpose) in this long exposure. Triton, which is about 16 arcsec east (celestial north is up, east is to the left) of Neptune, is approximately 180 times fainter.  All the other objects in the image are background field stars. The dark “tails” on the brightest objects are artifacts of the LORRI charge-coupled device (CCD); the effect is small but easily seen in this logarithmic intensity stretch. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Source: NASA

Pluto Spacecraft Gets Brain Transplant


Still seven years away from its rendezvous with Pluto, the New Horizons spacecraft was awoken from hibernation for the second annual checkout of all systems. The spacecraft and its team back on Earth will also undergo three months of operations as the New Horizons will make observations of Uranus, Neptune, and Pluto. But the first order of business was uploading an upgraded version of the software that runs the spacecraft’s Command and Data Handling system. “Our ‘brain transplant’ was a success,” says New Horizons Principal Investigator Alan Stern. “The new software – which guides how New Horizons carries out commands and collects and stores data – is now on the spacecraft’s main computer and operating, over a billion miles from home!”

The mission ops team at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, radioed the software load and the commands to start it earlier this week through NASA’s Deep Space Network of antennas to the spacecraft, now just more than 1.01 billion miles (1.62 billion kilometers) from Earth. In the next 10 days the team will beam up additional new software for both the spacecraft’s Autonomy and Guidance and Control systems.

Space Science Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
Space Science Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Alice Bowman, New Horizons mission operations manager at APL, says the spacecraft and its computers are healthy. “The new software fixes a few bugs and enhances the way these systems operate, based on what we’ve learned in running the spacecraft in the nearly three years since launch,” she says. “They also configure the onboard systems to be ready to support the Pluto-Charon encounter rehearsals scheduled for next summer.”

New Horizons is more than 200 million miles beyond Saturn’s orbit and more than 11 astronomical units (1.02 billion miles) from the Sun, flying about a million miles per day toward Pluto. Annual Checkout 2 (ACO-2) continues through mid-December; follow its progress through frequent updates on the New Horizons Twitter page.

Source: New Horizons Press Release

How Old is Triton’s Surface?


With all of the press going to the moons of Jupiter and Saturn, it’s about time that Neptune got a turn. Triton, one of the moons of Neptune, is curious, with large swaths of the planet resembling the skin of a cantaloupe and a retrograde orbit (opposite that of Neptune’s rotation). Its surface is thought to be rather young, and a new method of counting the craters that pock the moon may push the age of Triton’s surface back even younger than previously thought.

Dr. Paul Schenk of the Lunar and Planetary Institute in Houston, and Kevin Zahnle of the NASA Ames Research Center in California revisited the pictures of Triton’s surface that the Voyager 2 spacecraft took in 1989. By clarifying the images with current technology, they were able to count with very high accuracy the amount of craters, and determine the possible causes of the craters. Their results were published in the July 2007 issue of the journal Icarus, in a paper titled On the negligible age of the surface of Triton.

“Our new crater counts benefit from several improvements in the quality of the Voyager images. Although this does not make invisible craters visible, it does increase the ability to discriminate impact features on Triton,” the researchers wrote.

The images showed that the leading hemisphere — the hemisphere of the planet in the direction of its orbit around Neptune — contains many more craters than that of the trailing hemisphere. Triton is tidally locked to Neptune, which means that – like our Moon – an observer on Neptune would always see the same face of Triton. Thus, the same hemisphere would always face the direction of Triton’s orbit around Neptune.

The researchers propose, “Our map of craters on Triton indicates that all definitive impact craters are on the leading hemisphere. The apparent cratering asymmetry of Triton is extreme. The absence of craters on the trailing hemisphere, and the low frequency of craters near the boundary between leading and trailing hemispheres is unique in the Solar System.”

Since Triton is rotating in the opposite direction of everything else that goes around Neptune, it acts like a giant “vacuum cleaner,” and picks up any debris orbiting the planet in a prograde (the same as Neptune’s rotation) direction.

Triton is thought to have given itself a makeover very recently because it was captured by Neptune long ago; most likely, Triton was one body in a binary system, and when Neptune captured it, the other body was thrown out of the Solar System. After being captured, all of the energy that went into slowing Triton down into orbit around Neptune was transferred into heat that melted the surface and interior of the planet. This heat could have lasted for millions of years, and the tidal energy from Neptune may still warm the interior of Triton today.

Normally, areas that have less craters have been resurfaced more recently, and thus are generally younger than surfaces with lots of craters. By analyzing the density of the craters, and using information about the type and frequency of debris that possibly caused them, the researchers were able to calculate that the terrain on the trailing hemisphere with less craters than that of the leading hemisphere was actually older than the area with more craters.

“Whatever their origin, the paucity of impact craters (and heliocentric craters in particular) suggests that Triton’s surface is very young, younger than 100 million years and possibly as young as a few million years. A return to Neptune and its vigorous, dynamic moon Triton is long overdue,” the researchers wrote.

Source: Icarus

Podcast: Neptune


We’ve reached Neptune, the final planet in our tour through the solar system – but don’t worry! The tour’s not over, but after this week we’ll be all out of planets. Neptune has a controversial story about its discovery, some of the strongest winds in the solar system and some weird moons.
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Neptune – Show notes and transcript

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Neptune’s South Pole is the Warmest Place on the Planet


Here on Earth we think of the poles as cold places, but on Neptune, it’s just the opposite. New images of the planet’s southern pole show that it’s actually 10-degrees warmer than the rest of Neptune. Now don’t pack your bathing suit just yet, Neptune’s average temperature is still -200 degrees Celsius (-328 F); so it’s still really, really cold.

The images were gathered using the European Southern Observatory’s Very Large Telescope (now that’s how you name a telescope). The observatory uses a special mid-infrared camera/spectrometer to reveal the different temperatures across planet.

Obviously Neptune is different from the Earth, but consider this. The planet is located 30 times farther away from the Sun than the Earth. This means only 1/900th the sunlight reaches Neptune than what we get here on Earth. Still, it’s enough sunlight to warm up the southern pole, which is currently tilted towards the Sun.

It’s been receiving the warmth from the Sun for about 40 years now, and the ongoing input of solar energy continuously heats up the polar region to the point that it’s warmer than any other part of Neptune by about 10 degrees Celsius. This heating also whips up the planets winds into some of the strongest in the Solar System. On Neptune, winds can travel more than 2,000 km/hour, faster than any other planet – you definitely don’t want to bring your swimsuit.

The temperatures in the region are high enough that methane gas, normally frozen out of the upper atmosphere, can actually leak out through the region. And this helps explain why scientists have seen abundances of this molecule in the atmosphere.

Original Source: ESO News Release

Are There Oceans on Neptune?

Are There Oceans on Neptune
Smaller and cooler than the gas giants, Neptune and Uranus are classified as ice giants. It’s a good name, since they do have large quantities of water ice mixed in with a largely hydrogen and helium atmosphere. There’s very little water at the cloud tops, but the percentage of water increases as you descend towards the heavier core. Could there be a layer on Neptune with enough pressure and temperature for liquid water to form into vast oceans? And if not Neptune, what about a Neptune-like planet orbiting another star?
Continue reading “Are There Oceans on Neptune?”