New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit:
NASA/JHUAPL/SwRI.
It was two years ago this morning that we awoke to see the now iconic image of Pluto that the New Horizons spacecraft had sent to Earth during the night. You, of course, know the picture I’m talking about – the one with a clear view of the giant heart-shaped region on the distant, little world (see above).
This image was taken just 16 hours before the spacecraft would make its closest approach to Pluto. Then, during that seemingly brief flyby (after traveling nine-and-a-half years and 3 billion miles to get there), the spacecraft gathered as much data as possible and we’ve been swooning over the images and pondering the findings from New Horizons ever since.
“This is what we came for – these images, spectra and other data types that are helping us understand the origin and the evolution of the Pluto system for the first time,” New Horizons principal investigator Alan Stern told me last year. “We’re seeing that Pluto is a scientific wonderland. The images have been just magical. It’s breathtaking.”
See a stunning new video created from flbyby footage in honor of the two-year anniversary of the flyby:
All the images have shown us that Pluto is a complex world with incredible diversity, in its geology and also in its atmosphere.
While the iconic “heart” image shows a clear and cloudless view of Pluto, a later image showed incredible detail of Pluto’s hazy atmosphere, with over two dozen concentric layers that stretches more than 200 km high in Pluto’s sky.
With all those layers and all that haze, could there be clouds on Pluto too?
The smooth expanse of the informally named Sputnik Planum (right) is flanked to the west (left) by rugged mountains up to 11,000 feet (3,500 meters) high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. The backlighting highlights more than a dozen layers of haze in Pluto’s tenuous but distended atmosphere. Credit: NASA/JHUAPL/SwRI.
This is a question Stern and his fellow scientists have been asking for a long time, actually, as they have been studying Pluto for decades from afar. Now with data from New Horizons, they’ve been able to look closer. While Stern and his colleagues have been discussing how they found possible clouds on Pluto for a few months, they have now detailed their findings in a paper published last month.
“Numerous planets in our solar system, including Venus, Earth, Mars, Titan, and all four of the giant planets possess atmospheres that contain clouds, i.e., discrete atmospheric condensation structures,” the team wrote in their paper. “This said, it has long been known that Pluto’s current atmosphere is not extensively cloudy at optical or infrared wavelengths.”
They explained that evidence for this came primarily from the “high amplitude and temporal stability of Pluto’s lightcurve,” however, because no high spatial resolution imagery of Pluto was possible before New Horizons, it remained to be seen if clouds occur over a small fraction of Pluto’s surface area.
But now with flyby images in hand, the team set out to do searches for clouds on Pluto, looking at all available imagery from the Long Range Reconnaissance Imager and the Multispectral Visible Imaging Camera, looking at both the disk of Pluto and near and on the limb. Since an automated cloud search was nearly impossible, it was all done by visual inspection of the images by the scientists.
They looked for features in the atmosphere that including brightness, fuzzy or fluffy-looking edges and isolated borders.
Seven Possible Cloud Candidates (PCCs) identified by the New Horizons team. Two of these images (3, 4) were taken by MVIC; the other five (1, 2, 5, 6, 7) were taken by LORRI. Arrows indicate each PCC. Credit: Stern et al, 2017.
In all, they found seven bright, discrete possible cloud candidates. The seven candidates share several different attributes including small size, low altitude, they all were visible either early or late in the day local time, and were only visible at oblique geometry – which is basically a sideways look from the spacecraft.
Also, several cloud candidates also coincided with brighter surface features below, so the team is still pondering the correlation.
“The seven candidates are all similar in that they are very low altitude,” Stern said last fall at the Division of Planetary Sciences meeting, “and they are all low-lying, isolated small features, so no broad cloud decks or fields. When we map them over the surface, they all lie near the terminator, so they occur near dawn or dusk. This is all suggestive they are clouds because low-lying regions and dawn or dusk provide cooler conditions where clouds may occur.”
While haze was detected as high as 220 km, the possible clouds were found at very low altitudes. Stern told Universe Today that these possible, rare condensation clouds could be made of ethane, acetylene, hydrogen cyanide or methane under the right conditions. Stern added these clouds are probably short-lived phenomena – again, likely occurring only at dawn or dusk. A day on Pluto is 6.4 days on Earth.
But all in all, they concluded that at the current time Pluto’s atmosphere is almost entirely free of clouds – in fact the dwarf planet’s sky was 99% cloud free the day that New Horizons whizzed by.
“But if there are clouds, it would mean the weather on Pluto is even more complex than we imagined,” Stern said last year.
The seven cloud candidates cannot be confirmed as clouds because none are in the region where there was stereo imaging or other available ways to cross-check it. They concluded that further modeling would be needed, but specifically a Pluto orbiter mission would be the only way to “search for clouds more thoroughly than time and space and was possible during the brief reconnaissance flyby by New Horizons.”
An artist's illustration of Pluto. Credit: NASA/New Horizons
Once held to be the outermost planet of the Solar System, Pluto‘s designation was changed by the International Astronomical Union in 2006, owing to the discovery of many new Kuiper Belt Objects that were comparable in size. In spite of this, Pluto remains a source of fascination and a focal point of much scientific interest. And even after the historic flyby conducted by the New Horizons probe in July of 2015, many mysteries remain.
What’s more, ongoing analysis of the NH data has revealed new mysteries. For instance, a recent study by a team of astronomers indicated that a survey by the Chandra X-ray Observatory revealed the presence of some rather strong x-rays emissions coming from Pluto. This was unexpected, and is causing scientists to rethink what they thought they knew about Pluto’s atmosphere and its interaction with solar wind.
In the past, many Solar bodies have been observed emitting x-rays, which were the result of interaction between solar wind and neutral gases (like argon and nitrogen). Such emissions have been detected from planets like Venus and Mars (due to the presence of argon and/or nitrogen in their atmospheres), but also with smaller bodies like comets – which acquire halos due to outgassing.
Artist’s impression of New Horizons’ close encounter with the Pluto–Charon system. Credit: NASA/JHU APL/SwRI/Steve Gribben
Ever since the NH probe conducted its flyby of Pluto in 2015, astronomers have been aware that Pluto has an atmosphere which changes size and density with the seasons. Basically, as the planet reaches perihelion during its 248 year orbital period – a distance of 4,436,820,000 km, 2,756,912,133 mi from the Sun – the atmosphere thickens due to the sublimation of frozen nitrogen and methane on the surface.
The last time Pluto was at perihelion was on September 5th, 1989, which means that it was still experiencing summer when NH made its flyby. While studying Pluto, the probe detected an atmosphere that was primarily composed of nitrogen gas (N²) along with methane (CH4) and carbon dioxide (CO²). Astronomers therefore decided to look for signs of x-ray emissions coming from Pluto’s atmosphere using the Chandra X-ray Observatory.
Prior to the NH mission’s flyby, most models of Pluto’s atmosphere expected it to be quite extended. However, the probe found that the atmosphere was less extended and that its rate of loss was hundreds of times lower than what these models predicted. Therefore, as the team indicated in their study, they expected to find x-ray emissions that were consistent with what the NH flyby observed:
“Given that most pre-encounter models of Pluto’s atmosphere had predicted it to be much more extended, with an estimated loss rate to space of ~1027 to 1028 mol/sec of N² and CH4… we attempted to detect X-ray emission created by [solar wind] neutral gas charge exchange interactions in the low density neutral gas surrounding Pluto,” they wrote.
Images sent by NASA’s New Horizons spacecraft show possible clouds floating over the frozen landscape including the streaky patch at right. Credit: NASA/JHUAPL/SwR
However, after consulting data from the Advanced CCD Imaging Spectrometer (ACIS) aboard Chandra, they found that x-ray emissions coming from Pluto were greater than what this would allow for. In some cases, strong x-ray emissions have been noted coming from other smaller objects in the Solar System, which is due to the scattering of solar x-rays by small dust grains composed of carbon, nitrogen and oxygen.
But the energy distribution they noted with Pluto’s x-rays were not consistent with this explanation. Another possibility that the team offered is that they could be due to some process (or processes) that focus the solar wind near Pluto, which would enhance the effect of its modest atmosphere. As they indicate in their conclusions:
“The observed emission from Pluto is not aurorally driven. If due to scattering, it would have to be sourced by a unique population of nanoscale haze grains composed of C, N, and O atoms in Pluto’s atmosphere resonantly fluorescing under the Sun’s insolation. If driven by charge exchange between [solar wind] minor ions and neutral gas species (mainly CH4) escaping from Pluto, then density enhancement and adjustment of the [solar wind] minor ion relative abundance in the interaction region near Pluto is required versus naïve models.”
For the time being, the true cause of these x-ray emissions is likely to remain a mystery. They also highlight the need for more research when it comes to this distant and most massive of Kuiper Belt Objects. Luckily, the data provided by the NH mission is likely to be poured over for decades, revealing new and interesting things about Pluto, the outer Solar System, and how the most distant worlds from our Sun behave.
These two images reveal a moon orbiting the dwarf planet
2007 OR10. NASA/Hubble/ESA/STScI
These two images reveal a moon orbiting the dwarf planet 2007 OR10. NASA/Hubble/ESA/STScI
It isn’t every day we get a new moon added to the list of solar system satellites. The combined observational power of three observatories — Kepler, Herschel and Hubble — led an astronomical detective tale to its climatic conclusion: distant Kuiper Belt Object 2007 OR10has a tiny moon.
The dwarf planet itself is an enigma wrapped in a mystery: with a long orbit taking it out to a distant aphelion 101 astronomical units (AU) from the Sun, back into the environs of Neptune and Pluto for a perihelion 33 AU from the Sun once every 549 years, 2007 OR10 was discovered by Caltech astronomers Megan Schwamb and Mike Brown in 2007. Nicknamed “Snow White” by Mike Brown for its presumed high albedo, 2007 OR10 was 85 AU distant in the constellation Aquarius at the time of discovery and outbound towards aphelion in 2135. 2007 OR10 is about 1,500 kilometers in diameter, the third largest body known beyond Neptune in our solar system next to Pluto and Eris (nee Xena).
See the moon (circled?) at +21st magnitude, it’s a tough catch! NASA/Hubble/STScI
Enter the Kepler Space Telescope, which imaged 2007 OR10 crossing the constellation Aquarius as part of its extended K2 exoplanet survey along the ecliptic plane. Though Kepler looks for transiting exoplanets — worlds around other stars that betray their presence by tiny dips in the brightness of their host as they pass along our line of sight — it also picks up lots of other things that flicker, including variable stars and distant Kuiper Belt Objects. But the slow 45 hour rotational period of 2007 OR10 noted by Kepler immediately grabbed astronomers interest: could an unseen moon be lurking nearby, dragging on the KBO like a car brake?
“Typical rotation periods for Kuiper Belt Objects are under 24 hours,” says Csaba Kiss (Konkoly Observatory) in a recent press release. “We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon.”
And sure enough, digging back through archival data from the Hubble Space Telescope taken during a survey of KBOs, astronomers turned up two images of the faint moon from 2009 and 2010. Infrared observations of 2007 OR10 and its moon by the European Space Agency’s Herschel Space Telescope cinched the discovery, and noted an albedo of 19% (similar to wet sand) for 2007 OR10, much darker than expected. The moon is about 200 miles (320 kilometers) in diameter, in a roughly 9,300 mile (15,000 kilometer) orbit.
The discovery was announced at an AAS meeting just last year, and even now, we’re still puzzling out what little we know about these distant worlds. Just what 2007 OR10 and its moon looks like is any guess. New Horizons gave us our first look at Pluto and Charon two short summers ago in 2015, and will give us a fleeting glimpse of 2014 MU69 on New Year’s Day 2019. All of these objects beg for proper names, especially pre-2019 New Horizons flyby.
This also comes on the heels of two new moons for Jupiter, recently announced last month S/2017 J1 and J2.
What would the skies from the tiny moon look like? Well, ancient 2007 OR10 must loom large in its sky, though Sol would only shine as a bright -15th magnitude star, (a little brighter than a Full Moon) its illumination dimmed down to 1/7,000th the brightness enjoyed here on sunny Earth, which would be lost in its glare.
The current position of 2007 OR10 in the night sky. Stellarium
And looking at the strange elliptical orbits of these outer worldlets, we can only surmise that something else must be out there. Will the discovery of Planet 9 be made before the close of the decade?
One thing’s for sure: this isn’t your parent’s tidy solar system with “Excellent Mothers” serving “Nine Pizzas.”
For decades, we could only imagine what the view of Pluto’s surface might be. Now, we have the real thing.
The images and data from the New Horizons’ mission flyby of Pluto in July 2015 showed us an unexpectedly stunning and geologically active world. Scientists have used words like ‘magical,’ ‘breathtaking’ and ‘scientific wonderland’ to describe the long-awaited close-up views of distant Pluto.
Even though scientists are still analyzing the data from New Horizons, ideas are starting to formulate about sending another spacecraft to Pluto, but with a long-term orbiter mission instead of a quick flyby.
“The next appropriate mission to Pluto is an orbiter, maybe equipped with a lander if we had enough funding to do both,” New Horizons’ principal investigator Alan Stern told Universe Today in March.
This week, Stern has shared on social media that the New Horizons’ science team is meeting. But, separately, another group is starting to talk about a possible next mission to Pluto.
Getting a spacecraft to the outer regions of our solar system as fast as possible provides challenges, particularly in being able to slow down enough to enable going into orbit around Pluto. For the speedy and lightweight New Horizons, an orbital mission was impossible.
What propulsion system might make a Pluto orbiter and/or lander mission possible?
A few ideas are being tossed around.
Space Launch System
One concept takes advantage of NASA’s big, new Space Launch System (SLS), currently under development to enable human missions to Mars. NASA describes the SLS as “designed to be flexible and evolvable and will open new possibilities for payloads, included robotic scientific missions.” Even the first Block 1 version can launch 70 metric tons (later versions might be able to lift up to 130 metric tons.) Block 1 will be powered by twin five-segment solid rocket boosters and four liquid propellant engines, with a proposed 15% more thrust at launch than the Saturn V rockets that sent astronauts to the Moon.
An artist’s interpretation of NASA’s Space Launch System Block 1 configuration with an Orion vehicle. Image: NASA
But an orbiter mission to Pluto might not be the best use of the SLS alone.
It takes a lot of fuel to accelerate a vehicle to fast enough speed to get to Pluto in a reasonable amount of time. For example, New Horizons was the fastest spacecraft ever launched, using a souped-up Atlas V rocket with extra boosters, it performed a big burn when New Horizons departed Earth orbit. The lightweight spacecraft sped away from the Earth at 36,000 miles per hour (about 58,000 km/ hour), then used a gravity assist from Jupiter to boost New Horizons’ speed to 52,000 mph (83,600 km/h), traveling nearly a million miles (1.5 million km) a day in its 3 billion mile (4.8 billion km) journey to Pluto. The flight took nine and a half years.
“To enter Pluto orbit, a vehicle [like SLS] would have to boost up to that same speed, then turn around and decelerate for half the trip to arrive at Pluto with a net velocity of zero relative to the planet,” explained Stephen Fleming, an investor in several alt-space startups including XCOR Aerospace, Planetary Resources and NanoRacks. “Unfortunately, due to the tyranny of the rocket equation, you would have to carry all the fuel/propellant to decelerate with you at launch … which means accelerating the orbiter AND all that fuel in the initial phase. That requires logarithmically more fuel for the initial burn, and it turns out to be a LOT of fuel.”
Fleming told Universe Today that using the multi-billion dollar SLS to launch a Pluto orbiter, you would wind up launching an entire payload full of propellant just to accelerate and decelerate a tiny Pluto orbiter.
“That’s an extraordinarily expensive mission,” he said.
RTG-Ion Propulsion
A better option might be to use a propulsion system of combined technologies. Stern mentioned a NASA study that looked at using the SLS as the launch vehicle and to boost the spacecraft towards Pluto, but then using an RTG (Radioisotope Thermoelectric Generator) powered ion engine to later brake for an orbital arrival.
An RTG produces heat from the natural decay of non-weapons-grade plutonium-238, and the heat is converted into electricity. An RTG ion engine would be a more powerful ion propulsion system than the current solar electric ion engine on the Dawn spacecraft, now orbiting Ceres, in the asteroid belt, plus it would enable operation in the outer solar system, far from the Sun. This nuclear powered ion engine would enable a speeding spacecraft to slow down and go into orbit.
An artist’s illustration of NASA’s Dawn spacecraft with its ion propulsion system approaching Ceres. Image: NASA/JPL-Caltech.
“The SLS would boost you to fly out to Pluto,” Stern said, “and it would actually take two years to do the braking with ion propulsion.”
Stern said the flight time for such a mission to Pluto would be seven and a half years, two years faster than New Horizons.
The proposal uses a Direct Fusion Drive (DFD) engine that has propulsion and power in one integrated device. DFD provides high thrust to allow for a flight time of about 4 years to Pluto, plus being able to send substantial mass to orbit, perhaps between 1000 to 8000 kg.
A Direct Fusion Drive-powered spacecraft in orbit around Pluto, with the lander ready to deploy from the right-hand side. The large wing-like structures are the radiators and the optical communications lasers are on trusses extending from the center. Credits: Princeton Satellite Systems, NASA/JHUAPL/SwRI
DFD is based on the Princeton Field-Reversed Configuration (PFRC) fusion reactor that has been under development for 15 years at the Princeton Plasma Physics Laboratory.
If this propulsion system works as planned, it could launch a Pluto orbiter and a lander (or possibly a rover), and provide enough power to maintain an orbiter and all its instruments, as well as beam a lot of power to a lander. That would enable the surface vehicle to beam back video to the orbiter because it would have so much power, according to Stephanie Thomas from Princeton Satellite Systems, Inc., who is leading the NIAC study.
“Our concept is generally received as, ‘wow, that sounds really cool! When can I get one?’” Thomas told Universe Today. She said her and her team chose a prototype Pluto orbiter and lander mission in their proposal because it’s a great example of what can be done with a fusion rocket.
Their fusion system uses a small linear array of solenoid coils, and their fuel of choice is deuterium helium 3, which has very low neutron production.
Fusion-Enabled Pluto Orbiter and Lander. Credits: Stephanie Thomas.
“It fits on a spacecraft, it fits on a launch vehicle,” Thomas explained in a NIAC symposium talk (her talk starts about 17:30 in the linked video). “There’s no lithium, or other dangerous materials, it produces very few damaging particles. It’s about the size of a minivan or small truck. Our system is cheaper and faster to develop than other fusion proposals.”
The Princeton team has been able to produce 300 millisecond pulses with their plasma heating experiment, orders of magnitude better than any other system.
“The biggest hurdle is the fusion itself,” she said. “We need to build a bigger experiment to finish proving the new heating method, which will require an order of magnitude more resources than the project has been receiving from the Department of Energy so far,” Thomas said via email. “However, it’s still small in the grand scheme of advanced technology projects, about $50 million.”
Thomas said that DARPA has spent much more on many technology initiatives that ended up canceled. And it’s also much less than other fusion technologies require for the same stage of research, since our machine is so small and has a simple coil configuration.” (Thomas said have a look at the budget for ITER, the international nuclear fusion research and engineering megaproject, currently running over $20 billion).
“To put it simply, we know our method heats electrons really well and can extrapolate to heating ions, but we need to build it and prove it,” she said.
Thomas and her team are currently working on the “balance of plant” technology – the subsystems that will be required to operate the engine in space, assuming the heating method works as currently predicted.
In terms of the Pluto mission itself, Thomas said there aren’t any particular hurdles on the orbiter itself, but it would involve scaling up a few technologies to take advantage of the very large amount of power available, such as the optical communications.
“We could dedicate tens or more kW of power to the communication laser, not 10 watts, [like current missions]” she said. “Another unique feature of our concept is being able to beam a lot of power to a lander. This would enable new classes of planetary science instruments like powerful drills. The technology to do this exists but the specific instruments need to be designed and built. Additional technology that will be needed that is under development in various industries are lightweight space radiators, next-generation superconducting wires, and long-term cryogenic storage for the deuterium fuel.”
Thomas said their NIAC research is going well.
“We were selected for the NIAC Phase II study, and are in contract negotiations now,” she said. “We are busy working on higher fidelity models of the engine’s thrust, designing components of the trajectory, and sizing the various subsystems, including the superconducting coils,” she said. “Our current estimates are that a single 1 to 10 MW engine will produce between 5 and 50 N thrust, at about 10,000 sec specific impulse.”
Laser Zapping to Pluto
Another futuristic propulsion possibility is the laser-based systems proposed by Yuri Milner for his Breakthrough Starshot proposal, where small cubesats could be zapped by lasers on Earth, basically “bug zapping” spacecraft to reach incredible speeds (possibly millions of miles/km per hour) to visit the outer solar system or beyond.
“It’s not really in the cards for us to use this kind of technology, because we’d have to wait decades just for this to be developed,” Stern said. “But if you could send lightweight, inexpensive spacecraft at speeds like one-10th the speed of light based on lasers from Earth. We could send these small spacecraft to hundreds or thousands of objects in the Kuiper Belts, and you’d be out there in a matter of two-and-a-half days. You could send a spacecraft past Pluto every day. That would be really game changing.”
The Realistic Future
But even if everyone agrees a Pluto orbiter should be done, the earliest possible date for such a mission is sometime between the early 2020s and the early 2030s. But it all depends on the recommendations put forth by the scientific community’s next decadal survey, which will suggest the most top-priority missions for NASA’s Planetary Science Division.
These Decadal Surveys are 10-year “roadmaps” that set science priorities and provide guidance on where NASA should send spacecraft and what types of missions they should be. The last Decadal Survey was published in 2011, and that set planetary science priorities through 2022. The next one, for 2023-2034, will likely be published in 2022.
The New Horizons mission was the result of the suggestions from the 2003 planetary science Decadal Survey, where scientists said visiting the Pluto system and worlds beyond was a top-priority destination.
So, if you’re dreaming of a Pluto orbiter, keep talking about it.
New Horizon’s July 2015 flyby of Pluto taught us a lot about that planet. For one thing, Pluto is much more geophysically active than thought. Credit: NASA/JHUAPL/SwRI.
New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit:
NASA/JHUAPL/SwRI.
Discovered in 1930 by Clyde Tombaugh, Pluto was once thought to be the ninth and outermost planet of the Solar System. However, due to the formal definition adopted in 2006 at the 26th General Assembly of the International Astronomical Union (IAU), Pluto ceased being the ninth planet of the Solar System and has become alternately known as a “Dwarf Planet”, “Plutiod”, Trans-Neptunian Object (TNO) and Kuiper Belt Object (KBO).
Despite this change of designation, Pluto remains one of the most fascinating celestial bodies known to astronomers. In addition to having a very distant orbit around the Sun (and hence a very long orbital period) it also has the most eccentric orbit of any planet or minor planet in the Solar System. This makes for a rather long year on Pluto, which lasts the equivalent of 248 Earth years!
Orbital Period:
With an extreme eccentricity of 0.2488, Pluto’s distance from the Sun ranges from 4,436,820,000 km (2,756,912,133 mi) at perihelion to 7,375,930,000 km (4,583,190,418 mi) at aphelion. Meanwhile, it’s average distance (semi-major axis) from the Sun is 5,906,380,000 km (3,670,054,382 mi). Another way to look at it would be to say that it orbits the Sun at an average distance of 39.48 AU, ranging from 29.658 to 49.305 AU.
New Horizons trajectory and the orbits of Pluto and 2014 MU69.
At its closest, Pluto actually crosses Neptune’s orbit and gets closer to the Sun. This orbital pattern takes place once every 500 years, after which the two objects then return to their initial positions and the cycle repeats. Their orbits also place them in a 2:3 mean-motion resonance, which means that for every two orbits Pluto makes around the Sun, Neptune makes three.
The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years. The last time this cycle took place was between 1979 to 1999, when Neptune was farther from the Sun than Pluto. Pluto reached perihelion in this cycle – i.e. its closest point to the Sun – on September 5th, 1989. Since 1999, Pluto returned to a position beyond that of Neptune, where it will remain for the following 228 years – i.e. until the year 2227.
Sidereal and Solar Day:
Much like the other bodies in our Solar System, Pluto also rotates on its axis. The time it takes for it to complete a single rotation on its axis is known as a “Sidereal Day”, while the amount of time it takes for the Sun to reach the same point in the sky is known as a “Solar Day”. But due to Pluto’s very long orbital period, a sidereal day and a solar day on Pluto are about the same – 6.4 Earth days (or 6 days, 9 hours, and 36 minutes).
View from the surface of Pluto, showing its large moon Charon in the distance. Credit: New York Time
It is also worth noting that Pluto and Charon (its largest moon) are actually more akin to a binary system rather than a planet-moon system. This means that the two worlds orbit each other, and that Charon is tidally locked around Pluto. In other words, Charon takes 6 days and 9 hours to orbit around Pluto – the same amount of time it takes for a day on Pluto. This also means that Charon is always in the same place in the sky when seen from Pluto.
In short, a single day on Pluto lasts the equivalent of about six and a half Earth days. A year on Pluto, meanwhile, lasts the equivalent of 248 Earth years, or 90,560 Earth days! And for the entire year, the moon is hanging overhead and looming large in the sky. But factor in Pluto’s axial tilt, and you will come to see just how odd an average year on Pluto is.
Seasonal Change:
It has been estimated that for someone standing on the surface of Pluto, the Sun would appear about 1,000 times dimmer than it appears from Earth. So while the Sun would still be the brightest object in the sky, it would look more like a very bright star that a big yellow disk. But despite being very far from the Sun at any given time, Pluto’s eccentric orbit still results in some considerable seasonal variations.
On the whole, the surface temperature of Pluto does not change much. It’s surface temperatures are estimated to range from a low of 33 K (-240 °C; -400 °F ) to a high of 55 K (-218 °C; -360°F) – averaging at around 44 K (-229 °C; -380 °F). However, the amount of sunlight each side receives during the course of a year is vastly different.
Compared to most of the planets and their moons, the Pluto-Charon system is oriented perpendicular to its orbit. Much like Uranus, Pluto’s very high axial tilt (122 degrees) essentially means that it is orbiting on its side relative to its orbital plane. This means that at a solstice, one-quarter of Pluto’s surface experiences continuous daylight while the other experiences continuous darkness.
This is similar to what happens in the Arctic Circle, where the summer solstice is characterized by perpetual sunlight (i.e. the “Midnight Sun”) and the winter solstice by perpetual night (“Arctic Darkness”). But on Pluto, these phenomena affect nearly the entire planet, and the seasons last for close to a century.
Even if it is no longer considered a planet (though this could still change) Pluto still has some very fascinating quarks, all of which are just as worthy of study as those of the other eight planets. And the time it takes to complete a full year on Pluto, and all the seasonal changes it goes through, certainly rank among the top ten!
For more information, be sure to check out NASA’s Solar System Exploration page on Pluto, and the New Horizon’s mission page for information on Pluto’s seasons.
A new definition of what is a planet would mean there are at least 110 planets in our Solay System. Image Courtesy of Emily Lakdawalla of the Planetary Society, Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla. https://creativecommons.org/licenses/by-nc-sa/3.0/
Pluto’s status as a non-planet may be coming to an end. Professor Mike Brown of Caltech ended Pluto’s planetary status in 2006. But now, Kirby Runyon, a doctoral student at Johns Hopkins University, thinks it’s time to cancel that demotion and restore it as our Solar System’s ninth planet.
Pluto’s rebirth as a planet is not just all about Pluto, though. A newer, more accurate definition of what is and what is not a planet is needed. And if Runyon and the other people on the team he leads are successful, our Solar System would have more than 100 planets, including many bodies we currently call moons. (Sorry elementary school students.)
This composite of enhanced color images of Pluto (lower right) and Charon (upper left), was taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015. Credits: NASA/JHUAPL/SwRI
In 2006, the International Astronomical Union (IAU) changed the definition of what a planet is. Pluto’s demotion stemmed from discoveries in the 1990’s showing that it is actually a Kuiper Belt Object (KBO). It was just the first KBO that we discovered. When Pluto was discovered by Clyde Tombaugh in 1930, and included as the ninth planet in our Solar System, we didn’t know much about the Kuiper Belt.
But in 2005, the dwarf planet Eris was discovered. It was like Pluto, but 27% more massive. This begged the question, Why Pluto and not Eris? The IAU struck a committee to look into how planets should be defined.
In 2006, the IAU had a decision to make. Either expand the definition of what is and what is not a planet to include Eris and other bodies like Ceres, or shrink the definition to omit Pluto. Pluto was demoted, and that’s the way it’s been for a decade. Just enough time to re-write text books.
But a lot has happened since then. The change to the definition of planet was hotly debated, and for some, the change should never have happened. Since the New Horizons mission arrived at Pluto, that debate has been re-opened.
“A planet is a sub-stellar mass body that has never undergone nuclear fusion…” – part of the new planetary definition proposed by Runyon and his team.
The group behind the drive to re-instate Pluto have a broader goal in mind. If the issue of whether Pluto is or is not a planet sounds a little pedantic, it’s not. As Runyon’s group says on their poster to be displayed at the upcoming conference, “Nomenclature is important as it affects how we compare, think, and communicate about objects in nature.”
Runyon’s team proposes a new definition of what is a planet, focused on the geophysics of the object: “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters.”
The poster highlights some key points around their new planetary definition:
Emphasizes intrinsic as opposed to extrinsic properties.
Can be paraphrased for younger students: “Round objects in space that are smaller than stars.”
The geophysical definition is already in use, taught, and included in planetological glossaries.
There’s no need to memorize all 110 planets. Teach the Solar Systems zones and why different planet types formed at different distances from the Sun.
Their proposal makes a lot of sense, but there will be people opposed to it. 110 planets is quite a change, and the new definition is a real mouthful.
“They want Pluto to be a planet because they want to be flying to a planet.” – Prof. Mike Brown, from a BBC interview, July 2015.
Mike Brown, the scientist behind Pluto’s demotion, saw this all coming when New Horizons reached the Pluto system in the Summer of 2015. In an interview with the BBC, he said “The people you hear most talking about reinstatement are those involved in the (New Horizons) mission. It is emotionally difficult for them.”
Saying that the team behind New Horizons find Pluto’s status emotionally difficult seems pretty in-scientific. In fact, their proposed new definition seems very scientific.
This image from New Horizons shows the true nature of Pluto. What for a long time was just a blurry, round, blob in space, was revealed as a geologically active planet with a seasonal atmosphere. Image: NASA/JPL/New Horizons
There may be an answer to all of this. The term “classical planets” might be of some use. That term could include our 9 familiar planets, the knowledge of which guided much of our understanding and exploration of the Solar System. But it’s a fact of science that as our understanding of something grows more detailed, our language around it has to evolve to accommodate. Look at the term planetary nebula—still in use long after we know they have nothing to do with planets—and how much confusion it causes.
“It is official without IAU approval, partly via usage.” – Runyon and team, on their new definition.
In the end, it may not matter whether the IAU is convinced by Runyon’s proposed new definition. As their poster states, “As a geophysical definition, this does not fall under the domain of the IAU, and is an alternate and parallel definition that can be used by different scientists. It is “official” without IAU approval, partly via usage.”
It may seem pointless to flip-flop back and forth about Pluto’s status as a planet. But there are sound reasons for updating definitions based on our growing knowledge. We’ll have to wait and see if the IAU agrees with that, and whether or not they adopt this new definition, and the >100 planet Solar System.
You can view Runyon and team’s poster here.
You can view Emily Lakdawalla’s image of round objects in our Solar System here.
You can read the IAU’s definition of a planet here.
Looking back at an overexposed Charon and Plutoshine. Credit: NASA/JPL/New Horizons
Sometimes, its not the eye candy aspect of the image, but what it represents. A recent image of Pluto’s large moon Charon courtesy of New Horizons depicting what could only be termed ‘Plutoshine’ caught our eye. Looking like something from the grainy era of the early Space Age, we see a crescent Charon, hanging against a starry background…
So what, you say? Sure, the historic July 14th , 2015 flyby of New Horizons past Pluto and friends delivered images with much more pop and aesthetic appeal. But look closely, and you’ll see something both alien and familiar, something that no human eye has ever witnessed, yet you can see next week.
We’re talking about the reflected ‘Plutoshine‘ on the dark limb of Charon. This over-exposed image was snapped from over 160,000 kilometers distant by New Horizons’ Ralph/Multispectral imager looking back at Charon, post flyby. For context, that’s just shy of half the distance between the Earth and the Moon. “Bigger than Texas” (Cue Armageddon), Charon is about 1200 kilometers in diameter and 1/8th the mass of Pluto. Together, both form the only true binary (dwarf) planetary pair in the solar system, with the 1/80th Earth-Moon pair coming in at a very distant second.
Earthshine on the Moon. Credit: Dave Dickinson
We see reflected sunlight coming off of a gibbous Pluto which is just out of frame, light that left the Sun 4 hours ago and took less than a second to make the final Pluto-Charon-New Horizons bounce. You can see a similar phenomenon next week, as Earthshine or Ashen Light illuminates the otherwise dark nighttime side of the Earth’s Moon, fresh off of passing New phase this weekend. Snow and cloud cover turned Moonward can have an effect on how bright Earthshine appears. One ongoing study based out of the Big Bear Solar observatory in California named Project Earthshine seeks to characterize long-term climate variations looking at this very phenomenon.
The view on the evening of January 28th looking west at dusk. Credit: Stellarium.
Standing on Pluto, you’d see a 3.5 degree wide Charon, 7 times larger than our own Full Moon. Of course, you’d need to be standing in the right hemisphere, as Pluto and Charon are tidally locked, and keep the same face turned towards each other. It would be a dim view, as the Sun shines at -20 magnitude at 30 AU distant, much brighter than a Full Moon, but still over 600 times fainter than sunny Earth. Dim Plutoshine on the nightside of Charon would, however, be easily visible to the naked eye.
A small 6 cm instrument, Ralph images in the visual to near-infrared range. Ralph compliments New Horizons larger LORRI instrument, which has a diameter and very similar optical configuration to an amateur 8-inch Schmidt-Cassegrain telescope.
Charon as seen from Pluto. Credit: Starry Night.
Don’t look for Pluto now; it just passed solar conjunction on the far side of the Sun on January 7th, 2017. Pluto reaches opposition and favorable viewing for 2017 on July 10th, one of the 101 Astronomical Events for 2017 that you’ll find in our free e-book, out from Universe Today.
And for an encore, New Horizons will visit the 45 kilometer in diameter Kuiper Belt Object 2014 MU69 on New Year’s Day 2019. From there, New Horizons will most likely chronicle the environs of the the distant solar system, as it joins Pioneer 10 and 11 and Voyagers 1 and 2 as human built artifacts cast adrift along the galactic plane.
A pretty pair: Pluto and Charon. Credit: NASA/JPL/New Horizons
And to think, it has taken New Horizons about 18 months for all of its flyby data to trickle back to the Earth. Enjoy, as it’ll be a long time before we visit Pluto and friends again.
Color mosaic map of Pluto's surface, created from the New Horizons many photographs. Credit: NASA/JHUAPL/SwRI
On July 14th, 2015, the New Horizons mission made history by conducting the first flyby of Pluto. This represented the culmination of a nine year journey, which began on January 19th, 2006 – when the spacecraft was launched from the Cape Canaveral Air Force Station. And before the mission is complete, NASA hopes to send the spacecraft to investigate objects in the Kuiper Belt as well.
To mark the 11th anniversary of the spacecraft’s launch, members of the New Horizons team took part in panel a discussion hosted by the Johns Hopkins University Applied Physics Laboratory (JHUAPL) located in Laurel, Maryland. The event was broadcasted on Facebook Live, and consisted of team members speaking about the highlights of the mission and what lies ahead for the NASA spacecraft.
The live panel discussion took place on Thursday, Sept. 19th at 4 p.m. EST, and included Jim Green and Alan Stern – the director the Planetary Science Division at NASA and the principle investigator (PI) of the New Horizons mission, respectively. Also in attendance was Glen Fountain and Helene Winters, New Horizons‘ project managers; and Kelsi Singer, the New Horizons co-investigator.
Artist’s concept of the New Horizons spacecraft encountering a Kuiper Belt object, part of an extended mission after the spacecraft’s July 2015 Pluto flyby. Credits: NASA/JHUAPL/SwRI
In the course of the event, the panel members responded to questions and shared stories about the mission’s greatest accomplishments. Among them were the many, many high-resolution photographs taken by the spacecraft’s Ralph and Long Range Reconnaissance Imager (LORRI) cameras. In addition to providing detailing images of Pluto’s surface features, they also allowed for the creation of the very first detailed map of Pluto.
Though Pluto is not officially designated as a planet anymore – ever since the XXVIth General Assembly of the International Astronomical Union, where Pluto was designated as a “dwarf planet” – many members of the team still consider it to be the ninth planet of the Solar System. Because of this, New Horizons‘ historic flyby was of particular significance.
As Principle Investigator Alan Stern – from the Southwestern Research Institute (SwRI) – explained in an interview with Inverse, the first phase of humanity’s investigation of the Solar System is now complete. “What we did was we provided the capstone to the initial exploration of the planets,” he said. “All nine have been explored with New Horizons finishing that task.”
Other significant discoveries made by the New Horizons mission include Pluto’s famous heart-shaped terrain – aka. Sputnik Planum. This region turned out to be a young, icy plain that contains water ice flows adrift on a “sea” of frozen nitrogen. And then there was the discovery of the large mountain and possible cryovolcano located at the tip of the plain – named Tombaugh Regio, (in honor of Pluto’s discovered, Clyde Tombaugh).
New Horizons path from the inner Solar System to Pluto and the Kuiper Belt. Credit: NASA/JHUAPL
The mission also revealed further evidence of geological activity and cryovolcanism, the presence of hyrdocarbon clouds on Pluto, and conducted the very first measurements of how Pluto interacts with solar wind. All told, over 50 gigabits of data were collected by New Horizons during its encounter and flyby with Pluto. And the detailed map which resulted from it did a good job of capturing all this complexity and diversity. As Stern explained:
“That really blew away our expectations. We did not think that a planet the size of North America could be as complex as Mars or even Earth. It’s just tons of eye candy. This color map is the highest resolution we will see until another spacecraft goes back to Pluto.”
After making its historic flyby of Pluto, the New Horizons team requested that the mission receive an extension to 2021 so that it could explore Kuiper Belt Objects (KBOs). This extension was granted, and for the first part of the Kuiper Belt Extended Mission (KEM), the spacecraft will perform a close flyby of the object known as 2014 MU69.
This remote KBO – which is estimated to be between 25 – 45 km (16-28 mi) in diameter – was one of two objects identified as potential targets for research, and the one recommended by the New Horizons team. The flyby, which is expected to take place in January of 2019, will involve the spacecraft taking a series of photographs on approach, as well as some pictures of the object’s surface once it gets closer.
Before the extension ends in 2021, it will continue to send back information on the gas, dust and plasma conditions in the Kuiper Belt. Clearly, we are not finished with the New Horizons mission, and it is not finished with us!
Alan Stern’s 2006 Nissan 350Z and Percival Lowell’s 1912 car in front of the Lowell Observatory. Percival Lowell’s car nicknamed his car “Big Red,” and Stern’s car is nicknamed “New Red.” Credit: Lowell Observatory
Many of the rocket and space flight enthusiasts I know are also car buffs. If you fit into that category, here’s an opportunity you won’t want to miss: a chance to own the car that New Horizons principal investigator Alan Stern drove all the way to Pluto.
Well, technically, he drove his shiny red Nissan 350Z the entire time the New Horizons’ spacecraft was making a beeline for the icy dwarf planet. But Stern has now donated this car to the Lowell Observatory, the facility where Pluto was discovered. The car is being auctioned off on eBay, with proceeds going to support “Lowell’s mission of scientific research and education.” You can make your bid now, as bids are being accepted from December 15-24, and the winner will not only have the privilege of owning the car, but also enjoy a dinner with Stern.
New Horizons Principal Investigator Alan Stern and the Nissan sports car he has donated to the Lowell Observatory for a fundraiser. Credit: Lowell Observatory.
Stern bought the car in 2006, the year New Horizons launched (it has a bumper sticker that says “My other vehicle is on its way to Pluto”) and he continued driving it until earlier this year, well past the spacecraft’s flyby of Pluto in July 2015.
It is a two-door model with red exterior and carbon interior, and has just over 77,000 miles on it, which, as Stern points out, is almost 10 times fewer miles than New Horizons clocked on its first day of flight. A November 9, 2016 appraisal states the vehicle is in excellent shape and has a life expectancy of 300,000 miles.
“It was Percival Lowell’s perseverance and dedication that resulted in the discovery of Pluto and, ultimately, resulted in the flight of New Horizons to explore this distant, small planet,” Stern said in a press release from the Lowell Observatory. “New Horizons was, and is, the best aspect of my career so far, so I wanted to donate this car to Lowell Observatory as a fundraising vehicle to recognize the fact that New Horizons could not have happened without the historic and pioneering work that took place at Lowell Observatory early in the last century.”
Bumper sticker on Alan Stern’s car. Credit: Lowell Observatory.
Stern was the impetus behind New Horizons, billed as the fastest spacecraft ever launched, so he calls the Nissan 350Z his “second fastest vehicle.” He still oversees the New Horizons mission, as the spacecraft continues on its journey through the Kuiper Belt. It will fly past another object, named 2014 MU69, which Stern said is an ancient KBO that formed where it orbits now.
“It’s the type of object scientists have been hoping to study for decades, and this will be the most distant world we’ve ever been able to see up close,” Stern told me during an interview for my upcoming book, “Incredible Stories From Space.” Chapter 1 tells the stories of the New Horizons mission, including many stories from Stern.
With a penchant for both creating and driving state-of-the-art vehicles, Stern revealed earlier this year that his new car is a Tesla.
Lowell director Jeff Hall said, “It’s been a real pleasure working with Alan over the past few years leading up to and past the Pluto flyby. He’s been tremendously supportive of Lowell, and his donation of his car for us to auction is a sterling example of this. We’re thrilled by this gesture, and we look forward to meeting the lucky winner.”
The Lowell Observatory was founded in 1894 by Percival Lowell and has been home to many important discoveries including the detection of the large recessional velocities (redshift) of galaxies by Vesto Slipher in 1912-1914 (a result that led to the realization the universe is expanding), and the discovery of Pluto by Clyde Tombaugh in 1930. Today, Lowell’s 14 astronomers use ground-based telescopes around the world, telescopes in space, and NASA planetary spacecraft to conduct research in astronomy and planetary science. Lowell is a private, non-profit research institution and is located near Flagstaff, Arizona.
New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit:
NASA/JHUAPL/SwRI.
The evidence keeps growing for a large subsurface ocean at Pluto, which also provides clues how the iconic ‘heart’ of Pluto was formed.
We reported in early October that thermal models of Pluto’s interior and tectonic evidence suggest an ocean may exist beneath Pluto’s heart-shaped Sputnik Planitia. Now, new research on data from the New Horizons mission shows more indications of an ocean just below Pluto’s surface that consists of a slushy, viscous liquid, kept warm from Pluto’s interior and a hint of anti-freeze.
“As far as we can tell, there’s no tidal heating helping to keep the ocean liquid,” Francis Nimmo from UC Santa Cruz told Universe Today. He is the first author of a paper on the new findings published today in Nature. “The main heat source keeping the ocean liquid is radioactive decay in Pluto’s rocky interior, although it certainly helps if there is an ‘antifreeze’ present.”
This cutaway image of Pluto shows a section through the area of Sputnik Planitia, with dark blue representing a subsurface ocean and light blue for the frozen crust. Artwork by Pam Engebretson, courtesy of UC Santa Cruz.
Nimmo said he suspects the ocean is mostly water with ammonia acting as an antifreeze. This subsurface ocean is also bulging, similar to the ‘mascons’ on the Moon, putting stress on Pluto’s icy outer shell, causing fractures consistent with features seen in the New Horizons images.
High-resolution images of Pluto taken by NASA’s New Horizons spacecraft just before closest approach on July 14, 2015, reveal features as small as 270 yards (250 meters) across, from craters to fractures and faulted mountain blocks, to the textured surface of the vast basin informally called Sputnik Planitia. Credit: NASA/JHUAPL/SWRI
Sputnik Planitia forms one side of the prominent heart-shaped feature seen in some of the first close-up images from New Horizons July 2015 flyby. It was likely created by the impact of a giant meteorite, which would have blasted away a huge amount of Pluto’s icy crust.
But a deep basin is just a “big, elliptical hole in the ground,” Nimmo said, that would not provide the extra mass needed to cause that kind of reorientation. “So, the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that.”
These schematic diagrams show how the gravity anomaly at Sputnik Planitia is affected by an uplifted ocean and the thickness of the nitrogen layer. Either a nitrogen layer more than 40 km thick (panel b) or an uplifted ocean (panel c) could result in a present-day positive gravity anomaly at Sputnik Planitia; otherwise, the gravity anomaly will be strongly negative (panel a). (Image from Nimmo et al., Nature, 2016)
But Pluto is cold, with temperatures ranging from -387 to -369 Fahrenheit (-233 to -223 Celsius). How could there be an ocean?
“Pluto is small enough that it’s just about almost cooled off but still has a little heat, and it’s about 2 percent the heat budget of the Earth, in terms of how much energy is coming out,” said co-author Richard Binzel, from MIT. “So we calculated Pluto’s size with its interior heat flow, and found that underneath Sputnik Planitia, at those temperatures and pressures, you could have a zone of water-ice that could be at least viscous. It’s not a liquid, flowing ocean, but maybe slushy. And we found this explanation was the only way to put the puzzle together that seems to make any sense.”
The massive basin also appears extremely bright relative to the rest of the planet, and the data from New Horizons suggest it is filled with frozen nitrogen ice.
Previous research from the the mission showed evidence that the liquid nitrogen may be constantly refreshing, or convecting, as a result of a weak spot at the bottom of the basin, and this weak spot may let heat rise through Pluto’s interior to continuously refresh the ice.
Additionally, the extra weight of an underground ocean could help explain the longstanding question of why Pluto’s heart aligns almost exactly opposite from Charon. Nimmo said this alignment is “suspicious” and that the likelihood of this being just a coincidence is only 5 percent. Therefore, the alignment suggests that extra mass in that location interacted with tidal forces between Pluto and Charon to reorient Pluto, putting Sputnik Planitia directly opposite the side facing Charon.
A thick, heavy ocean, the new data suggest, may have served as a “gravitational anomaly,” which would factor heavily in Pluto and Charon’s gravitational tug-of-war, the researchers said. Over millions of years, the planet would have spun around, aligning its subsurface ocean and the heart-shaped region above it, almost exactly opposite along the line connecting Pluto and Charon.
While scientists are still studying the data from New Horizons, it is safe to say that Pluto keeps surprising everyone, even the scientists who know it best.
“Pluto is hard to fathom on so many different levels,” said Binzel.
