Space and astronomy news
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Every planet in the Solar System takes a certain amount of time to complete a single orbit around the Sun. Here on Earth, this period lasts 365.25 days – a period that we refer to as a year. When it comes to the other planets, we use this measurement to characterize their orbital periods. And what we have found is that on many of these planets, depending on their distance from the Sun, a year can last a very long time!
Consider Saturn, which orbits the Sun at a distance of about 9.5 AU – i.e. nine and a half times the distance between the Earth and the Sun. Because of this, the speed with which it orbits the Sun is also considerably slower. As a result, a single year on Saturn lasts an average of about twenty-nine and a half years. And during that time, some interesting changes happen for the planet’s weather systems.
Saturn orbits the Sun at an average distance (semi-major axis) of 1.429 billion km (887.9 million mi; 9.5549 AU). Because its orbit is elliptical – with an eccentricity of 0.05555 – its distance from the Sun ranges from 1.35 billion km (838.8 million mi; 9.024 AU) at its closest (perihelion) to 1.509 billion km (937.6 million mi; 10.086 AU) at its farthest (aphelion).
With an average orbital speed of 9.69 km/s, it takes Saturn 29.457 Earth years (or 10,759 Earth days) to complete a single revolution around the Sun. In other words, a year on Saturn lasts about as long as 29.5 years here on Earth. However, Saturn also takes just over 10 and a half hours (10 hours 33 minutes) to rotate once on its axis. This means that a single year on Saturn lasts about 24,491 Saturnian solar days.
It is because of this that what we can see of Saturn’s rings from Earth changes over time. For part of its orbit, Saturn’s rings are seen at their widest point. But as it continues on its orbit around the Sun, the angle of Saturn’s rings decreases until they disappear entirely from our point of view. This is because we are seeing them edge-on. After a few more years, our angle improves and we can see the beautiful ring system again.
Another interesting thing about Saturn is the fact that its axis is tilted off the plane of the ecliptic. Essentially, its orbit is inclined 2.48° relative to the orbital plane of the Earth. Its axis is also tilted by 26.73° relative to the ecliptic of the Sun, which is similar to Earth’s 23.5° tilt. The result of this is that, like Earth, Saturn goes through seasonal changes during the course of its orbital period.
For half of its orbit, Saturn’s northern hemisphere receives more of the Sun’s radiation than the southern hemisphere. For other half of its orbit, the situation is reversed, with the southern hemisphere receiving more sunlight than the northern hemisphere. This creates storm systems that dramatically change depending on which part of its orbit Saturn is in.
For staters, winds in the upper atmosphere can reach speeds of up to 5oo meters per second (1,600 feet per second) around the equatorial region. On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval).
This unique but short-lived phenomenon occurs once every Saturnian year, around the time of the northern hemisphere’s summer solstice. These spots can be several thousands of kilometers wide, and have been observed on many occasions throughout the past – in 1876, 1903, 1933, 1960, and 1990.
Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed, which was spotted by the Cassini space probe. Given the periodic nature of these storms, another one is expected to happen in 2020, coinciding with Saturn’s next summer in the northern hemisphere.
Similarly, seasonal changes affect the very large weather patterns that exist around Saturn’s northern and southern polar regions. At the north pole, Saturn experiences a hexagonal wave pattern which measures some 30,000 km (20,000 mi) in diameter, while each of it six sides measure about 13,800 km (8,600 mi). This persistent storm can reach speeds of about 322 km per hour (200 mph).
Thanks to images taken by the Cassini probe between 2012 and 2016, the storm appears to undergo changes in color (from a bluish haze to a golden-brown hue) that coincide with the approach of the summer solstice. This was attributed to an increase in the production of photochemical hazes in the atmosphere, which is due to increased exposure to sunlight.
Similarly, in the southern hemisphere, images acquired by the Hubble Space Telescope have indicated the existence of large jet stream. This storm resembles a hurricane from orbit, has a clearly defined eyewall, and can reach speeds of up to 550 km/h (~342 mph). And much like the northern hexagonal storm, the southern jet stream undergoes changes as a result of increased exposure to sunlight.
Cassini was able to captured images of the south polar region in 2007, which coincided with late fall in the southern hemisphere. At the time, the polar region was becoming increasingly “smoggy”, while the northern polar region was becoming increasingly clear. The reason for this, it was argued, was that decreases in sunlight led to the formation of methane aerosols and the creation of cloud cover.
From this, it has been surmised that the polar regions become increasingly obscured by methane clouds as their respective hemisphere approaches their winter solstice, and clearer as they approach their summer solstice. And the mid-latitudes certainly show their share of changes thanks to increases/decreases in exposure to solar radiation.
Much like the length of a single year, what we know about Saturn has a lot to do with its considerable distance from the Sun. In short, few missions have been able to study it in depth, and the length of a single year means it is difficult for a probe to witness all the seasonal changes the planet goes through. Still, what we have learned has been considerable, and also quite impressive!
We have written many articles about years on other planets here at Universe Today. Here’s The Orbit of the Planets. How Long Is A Year On The Other Planets?, The Orbit of Earth. How Long is a Year on Earth?, The Orbit of Mercury. How Long is a Year on Mercury?, The Orbit of Venus. How Long is a Year on Venus?, The Orbit of Mars. How Long is a Year on Mars?, The Orbit of Jupiter. How Long is a Year on Jupiter?, The Orbit of Uranus. How Long is a Year on Uranus?, The Orbit of Neptune. How Long is a Year on Neptune?, The Orbit of Pluto. How Long is a Year on Pluto?
If you’d like more information on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.
We have also recorded an entire episode of Astronomy Cast that’s just about Saturn. Listen here, Episode 59: Saturn.
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If you took geology in high school, then chances are you remember learning something about how the Earth’s crust – the outermost layer of Earth – is arranged into a series of tectonic plates. These plates float on top of the Earth’s mantle, the semi-viscous layer that surrounds the core, and are in constant motion because of convection in the mantle. Where two plates meet, you have what it is known as a boundary.
These can be “divergent” or “convergent”, depending on whether the plates are moving apart or coming together. Where they diverge, hot magma can rise from below, creating features like long ridges or mountain chains. Interestingly enough, this is how one of the world’s largest geological features was formed. It called the Mid-Atlantic Ridge, which run from north to south along the ocean floor in the Atlantic.
The Mid-Atlantic Ridge (MAR) is known as a mid-ocean ridge, an underwater mountain system formed by plate tectonics. It is the result of a divergent plate boundary that runs from 87° N – about 333 km (207 mi) south of the North Pole – to 54 °S, just north of the coast of Antarctica.
Like other ocean ridge systems, the MAR developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates. In the North Atlantic, it separates the Eurasian and North American Plates; whereas in the South Atlantic, it separates the African and South American Plates.
The MAR is approximately 16,000 km (10,000 mi) long and between 1,000 and is 1,500 km (620 and 932 mi) wide. The peaks of the ridge stand about 3 km (1.86 mi) in height above the ocean floor, and sometimes reach above sea level, forming islands and island groups. The MAR is also part of the longest mountain chain in the world, extending continuously across the oceans floors for a total distance of 40,389 km (25,097 mi).
The MAR also has a deep rift valley at is crest which marks the location where the two plates are moving apart. This rift valley runs along the axis of the ridge for nearly its entire length, measuring some 80 to 120 km (50 to 75 miles) wide. The rift marks the actual boundary between adjacent tectonic plates, and is where magma from the mantle reaches the seafloor.
Where this magma is able to reach the surface, the result is basaltic volcanoes and islands. Where it is still submerged, it produces “pillow lava”. As the plates move further apart, new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process, known as “sea floor spreading”, is happening at an average rate of about 2.5 cm per year (1 inch).
In other words, North America and Europe are moving away from each other at a very slow rate. This process also means that the basaltic rock that makes up the ridge is younger than the surrounding crust.
As noted, the ridge (while mainly underwater) does have islands and island groups that were created by volcanic activity. In the Northern Hemisphere, these include Jan Mayen Island and Iceland (Norway), and the Azores (Portugal). In the Southern Hemisphere, MAR features include Ascension Island, St. Helena, Tristan da Cunha, Gough Island (all UK territories) and Bouvet Island (Norway).
Near the equator, the Romanche Trench divides the North Atlantic Ridge from the South Atlantic Ridge. This narrow submarine trench has a maximum depth of 7,758 m (25,453 ft), one of the deepest locations of the Atlantic Ocean. This trench, however, is not regarded an official boundary between any of the tectonic plates.
The ridge was initially discovered in 1872 during the expedition of the HMS Challenger. In the course of investigating the Atlantic for the sake of laying the transatlantic telegraph cable, the crew discovered a large rise in the middle of the ocean floor. By 1925, its existence was confirmed thanks to the invention of sonar.
By the 1960s, scientists were able to map the Earth’s ocean floors, which revealed a seismically-active central valley, as well as a network of valleys and ridges. They also discovered that the ridge was part of a continuous system of mid-ocean ridges that extended across the entire ocean floor, connecting all the divergent boundaries around the planet.
This discovery also led to new theories in terms of geology and planetary evolution. For instance, the theory of “seafloor spreading” was attributed to the discovery of the MAR, as was the acceptance of continental drift and plate tectonics. In addition, it also led to the theory that all the continents were once part of subcontinent known as “Pangaea”, which broke apart roughly 180 million years ago.
Much like the “Pacific Ring of Fire“, the discovery of the Mid-Atlantic Ridge has helped inform our modern understanding of the world. Similar to convergent boundaries, subduction zones and other geological forces, the process that created it is also responsible for the world as we know it today.
Basically, it is responsible for the fact that the Americas have been drifting away from Africa and Eurasia for millions of years, the formation of Australia, and the collision between the India Subcontinent and Asia. Someday – millions of years from now – the process of seafloor spreading will cause the Americas and Asia to collide, thus forming a new super continent – “Amasia”.
We have written many interesting articles about Earth here at Universe Today. Here’s 10 Interesting Facts About Earth, What are Plate Boundaries?, What are Divergent Boundaries?, Mountains: How are they Formed?, What is a Subduction Zone?, What is an Earthquake?, What is the Pacific Ring of Fire?, and How Many Continents are There?
For more information, check out the Geological Society’s page on the Mid-Atlantic Ridge.
Astronomy Cast also has episodes that are relevant to the subject. Here’s Episode 51: Earth and Episode 293: Earthquakes.
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If you’ve got really good eyesight and can find a place where the light pollution is non-existent, you might be able to see Uranus without a telescope. It’s only possible with the right conditions, and if you know exactly where to look. And for thousands of years, scholars and astronomers were doing just that. But given that it was just a tiny pinprick of light, they believed Uranus was a star.
It was not until the late 18th century that the first recorded observation that recognized Uranus as being a planet took place. This occurred on March 13th, 1781, when British astronomer Sir William Herschel observed the planet using a telescope of his own creation. From this point onwards, Uranus would be recognized as the seventh planet and the third gas giant of the Solar System.
The first recorded instance of Uranus being spotted in the night sky is believed to date back to Classical Antiquity. During the 2nd century BCE, Hipparchos – the Greek astronomer, mathematician and founder of trigonometry – apparently recorded the planet as a star in his star catalogue (completed in 129 BCE).
This catalog was later incorporated into Ptolemy’s Almagest, which became the definitive source for Islamic astronomers and for scholars in Medieval Europe for over one-thousand years. During the 17th and 18th centuries, multiple recorded sightings were made by astronomers who also catalogued it as being a star.
This included English astronomer John Flamsteed, who in 1690 observed the star on six occasions and catalogued it as a star in the Taurus constellation (34 Tauri). During the mid-18th century, French astronomer Pierre Lemonnier made twelve recorded sightings, and also recorded it as being a star. It was not until March 13th, 1781, when William Herschel observed it from his garden house in Bath, that Uranus’ true nature began to be revealed.
On the evening in question – March 13th, 1781 – William Herschel was surveying the sky with his telescope, looking for binary stars. His first report on the object was recorded on April 26th, 1781. Initially, he described it as being a “Nebulous star or perhaps a comet”, but later settled on it being a comet since it appeared to have changed its position in the sky.
When he presented his discovery to the Royal Society, he maintained this theory, but also likened it to a planet. As was recorded in the Journal of the Royal Society and Royal Astronomical Society on the occasion of his presentation:
“The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.”
While Herschel would continue to maintain that what he observed was a comet, his “discovery” stimulated debate in the astronomical community about what Uranus was. In time, astronomers like Johann Elert Bode would conclude that it was a planet, based on its nearly-circular orbit. By 1783, Herschel himself acknowledged that it was a planet to the Royal Society.
As he lived in England, Herschel originally wanted to name Uranus after his patron, King George III. Specifically, he wanted to call it Georgium Sidus (Latin for “George’s Star”), or the Georgian Planet. Although this was a popular name in Britain, the international astronomy community didn’t think much of it, and wanted to follow the historical precedent of naming the planets after ancient Greek and Roman gods.
Consistent with this, Bode proposed the name Uranus in a 1782 treatise. The Latin form of Ouranos, Uranus was the grandfather of Zeus (Jupiter in the Roman pantheon), the father of Cronos (Saturn), and the king of the Titans in Greek mythology. As it was discovered beyond the orbits of Jupiter and Saturn, the name seemed highly appropriate.
In the following century, Neptune would be discovered, the last of the eight official planets that are currently recognized by the IAU. And by the 20th century, astronomers would discovery Pluto and other minor planets within the Kuiper Belt. The process of discovery has been ongoing, and will likely continue for some time to come.
We have written many articles about planetary discovery here at Universe Today. Here’s Who Discovered Mercury?, Who Discovered Venus?, Who Discovered Earth?, Who Discovered Mars?, Who Discovered Jupiter?, Who Discovered Saturn?, Who Discovered Neptune?, and Who Discovered Pluto?
Here’s an article from the Hubble educational site about the discovery of Uranus, and here’s the NASA Solar System Exploration page on Uranus.
We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.
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On March 30, 2017, SpaceX performed a pretty routine rocket launch. The payload was a communications satellite called SES-10, owned by a company in Luxembourg. And if all goes well, the satellite will eventually make its way to a high orbit of 35,000 km (22,000 miles) and deliver broadcasting and television services to Latin America.
For all intents and purposes, this is an absolutely normal, routine, and maybe even boring event in the space industry. Another chemical rocket blasted off another communications satellite to join the thousands of satellites that have come before.
Of course, as you probably know, this wasn’t a routine launch. It was the first step in one of the most important achievements in space flight – launch reusability. This was the second time the 14-story Falcon 9 rocket had lifted off and pushed a payload into orbit. Not Falcon 9s in general, but this specific rocket was reused.
In a previous life, this booster blasted off on April 8, 2016 carrying CRS-8, SpaceX’s 8th resupply mission to the International Space Station. The rocket launched from Florida’s Cape Canaveral, released its payload, re-entered the atmosphere and returned to a floating robotic barge in the Atlantic Ocean called Of Course I Still Love You. That’s a reference to an amazing series of books by Iain M. Banks.
Why is this such an amazing accomplishment? What does the future hold for reusability? And who else is working on this?
Developing a rocket that could be reused has been one of the holy grails of the space industry, and yet, many considered it an engineering accomplishment that could never be achieved. Trust me, people have tried in the past.
Portions of the space shuttle were reused – the orbiter and the solid rocket boosters. And a few decades ago, NASA tried to develop the X-33 as a single stage reusable rocket, but ultimately canceled the program.
To reuse a rocket makes total sense. It’s not like you throw out your car when you return from a road trip. You don’t destroy your transatlantic airliner when you arrive in Europe. You check it out, do a little maintenance, refuel it, fill it with passengers and then fly it again.
According to SpaceX founder Elon Musk, a brand new Falcon 9 first stage costs about $30 million. If you could perform maintenance, and then refill it with fuel, you’d bring down subsequent launches to a few hundred thousand dollars.
SpaceX is still working out what a “flight-tested” launch will cost on a reused Falcon 9 will cost, but it should turn into a significant discount on SpaceX’s already aggressive prices. If other launch providers think they’re getting undercut today, just wait until SpaceX really gets cranking with these reused rockets.
For most kinds of equipment, you want them to have been re-used many times. Cars need to be taken to the test track, airplanes are flown on many flights before passengers ever climb inside. SpaceX will have an opportunity to test out each rocket many times, figuring out where they fail, and then re-engineering those components. This makes for more durable and safer launch hardware, which I suspect is the actual goal here – safety, not cost.
In addition to the first stage, SpaceX also re-used the satellite fairing. This is the covering that makes the payload more aerodynamic while the rocket moves through the lower atmosphere. The fairing is usually ejected and burns up on re-entry, but SpaceX has figured out how to recover that too, saving a few more million.
SpaceX’s goals are even more ambitious. In addition to the first stage booster and launch fairing, SpaceX is looking to reuse the second stage booster. This is a much more complicated challenge, because the second stage is going much faster and needs to lose a lot more velocity. In late 2014, they put their plans on hold for a second stage reuse.
SpaceX’s next big milestone will be to decrease the reuse time. From almost a year to under 24 hours.
Sometime this year, SpaceX is expected to do the first launch of the Falcon Heavy. A launch system that looks like it’s made up of 3 Falcon-9 rockets bolted together. Since that’s basically what it is.
The center booster is a reinforced Falcon-9, with two additional Falcon-9s as strap-on boosters. Once the Falcon Heavy lifts off, the three boosters will detach and will individually land back on Earth, ready for reassembly and reuse. This system will be capable of carrying 54,000 kilograms into low Earth orbit. In addition, SpaceX is hoping to take the technology one more step and have the upper stage return to Earth.
Imagine it. Three boosters and upper stage and payload fairing all returning to Earth and getting reused.
And waiting in the wings, of course, is SpaceX’s huge Interplanetary Transport System, announced by Elon Musk in September of 2016. The super-heavy lift vehicle will be capable of carrying 300,000 kilograms into low Earth orbit.
For comparison, the Apollo era Saturn V could carry 140,000 kg into low Earth orbit, so this thing will be much much bigger. But unlike the Saturn V, it’ll be capable of returning to Earth, and landing on its launch pad, ready for reuse.
SpaceX just crossed a milestone, but they’re not the only player in this field.
Perhaps the biggest competitor to SpaceX comes from another internet entrepreneur: Amazon’s Jeff Bezos, the 2nd richest man in the world after Bill Gates. Bezos founded his own rocket company, Blue Origin in Seattle, which had been working in relative obscurity for the last decade. But in the last few years, they demonstrated their technology for reusable rocket flight, and laid out their plans for competing with SpaceX.
In April 2015, Blue Origin launched their New Shepard rocket on a suborbital trajectory. It went up to an altitude of about 100 km, and then came back down and landed on its launch pad again. It made a second flight in November 2015, a third flight in April 2016, and a fourth flight in June 2016.
That does sound exciting, but keep in mind that reaching 100 km in altitude requires vastly less energy than what the Spacex Falcon 9 requires. Suborbital and orbital are two totally milestones. The New Shepard will be used to carry paying tourists to the edge of space, where they can float around weightlessly in the vomit of the other passengers.
But Blue Origin isn’t done. In September 2016, they announced their plans for the follow-on New Glenn rocket. And this will compete head to head with SpaceX. Scheduled to launch by 2020, like, within 3 years or so, the New Glenn will be an absolute monster, capable of carrying 45,000 kilograms of cargo into low Earth orbit. This will be comparable to SpaceX’s Falcon Heavy or NASA’s Space Launch System.
Like the Falcon 9, the New Glenn will return to its launch pad, ready for a planned reuse of 100 flights.
A decade ago, the established United Launch Alliance – a consortium of Boeing and Lockheed-Martin – was firmly in the camp of disposable launch systems, but even they’re coming around to the competition from SpaceX. In 2014, they began an alliance with Blue Origin to develop the Vulcan rocket.
The Vulcan will be more of a traditional rocket, but some of its engines will detach in mid-flight, re-enter the Earth’s atmosphere, deploy parachutes and be recaptured by helicopters as they’re returning to the Earth. Since the engines are the most expensive part of the rocket, this will provide some cost savings.
There’s another level of reusability that’s still in the realm of science fiction: single stage to orbit. That’s where a rocket blasts off, flies to space, returns to Earth, refuels and does it all over again. There are some companies working on this, but it’ll be the topic for another episode.
Now that SpaceX has successfully launched a first stage booster for the second time, this is going to become the new normal. The rocket companies are going to be fine tuning their designs, focusing on efficiency, reliability, and turnaround time.
These changes will bring down the costs of launching payloads to orbit. That’ll mean it’s possible to launch satellites that were too expensive in the past. New scientific platforms, communications systems, and even human flights become more reasonable and commonplace.
Of course, we still need to take everything with a grain of salt. Most of what I talked about is still under development. That said, SpaceX just reused a rocket. They took a rocket that already launched a satellite, and used it to launch another satellite.
It’s a pretty exciting time, and I can’t wait to see what happens next.
Now you know how I feel about this accomplishment, I’d like to hear your thoughts. Do you think we’re at the edge of a whole new era in space exploration, or is this more of the same? Let me know your thoughts in the comments.
Since ancient times, astronomers have organized the stars into various constellations. We have the Big Dipper (Ursa Major), Orion the Hunter, and his “Greater Dog” and “Lesser Dog”(Canis Major and Canis Minor). And those are just some of the better-known ones. But have you ever wondered if the Sun belongs to one of these collections of stars?
The simple answer is that – in accordance with both ancient astrological tradition and modern astronomy – the Sun technically has no constellation. But if you were to change locations and travel to a new star system, you would then be able to view the Sun as we do other distant collection of stars. Unfortunately, depending on where you are, the answer would change.
First, let us consider the astrological answer to this question. Unless you were born prior to the Scientific Revolution – during which time Nicolaus Copernicus proposed the heliocentric model of the Solar System – you know that the Earth revolves around the Sun. Over the course of a year, the position of the stars changes as the Earth’s position relative to the Sun changes.
During the year, the Sun passes through each of the constellations of the Zodiac. For example, in August, the Sun is in Leo, and then in September, the Sun is in Virgo. Your astrological sign is based on this. What this means is that the Sun is part of each constellation of the Zodiac over the course of a single year, so it can’t be said to be in any single constellation.
However, astrology is an obsolete and entirely unscientific practice. And if someone were to ask which constellation the Sun is in, surely they are seeking an answer that was astronomical (and not astrological) in nature. For that, we must consider what the constellations are in scientific terms.
Since ancient times, astronomers and scholars have been keeping track of “asterisms” (aka. constellations) in the night sky. By definition, these are collections of stars that, when viewed from Earth, appear in the same general area as each other night after night. In reality, they are actually located in very different locations, and can sometimes be up to thousands of light-years away from each other.
During the 2nd century CE, Hellenistic astronomer Claudius Ptolemaeus (Ptolemy) organized the constellations into a single treatise. This treatise, known as the Almagest, was the definitive source on Greek astronomy, and contained the names and meanings of the then-known 48 constellations. For over a thousand years, this work would remain canon for European and Islamic Astronomers.
Thanks to the Scientific Revolution and “Age of Exploration” – ca. 15th to 18th centuries CE – astronomers became aware of many more constellations. This was due to extensive overseas exploration, which brought European traders, explorers and waves of colonization to the Southern Hemisphere, East Asia and the Americas.
By 1922, the International Astronomical Union (IAU) officially divided the celestial sphere into 88 constellations. Of these, 36 lie predominantly in the northern sky while the other 52 lie predominantly in the southern. While it would take years to work out the exact delineation between these constellations, and many corresponded to their Greco-Roman predecessors, these 88 modern constellations would remain in use until this day.
However, these constellations divide up the night sky based on how it is viewed from Earth. Once again, our Sun cannot be considered to lie in any one of them because – relative to the Earth-bound observer – it passes through them. Alas, the only way to answer this question is to change our perspective.
If you could move away to another star, then our Sun would indeed appear to be part of the background stars. For example, if you were to travel to a planet orbiting the nearest star to the Solar System – Alpha Centauri (aka. Rigil Kentaurus) – then the Sun would indeed appear to be part of a constellation.
To be scientifically accurate, let us consider a planet that we actually know of. This would be the rocky extrasolar planet recently discovered around Proxima Centauri, which is known as Proxima b. Viewed from the surface of this planet, the Sun would appear to be part of the Cassiopeia constellation. However, rather than forming a W shape, our Sun would form a sixth point on its “western” end, making it look like a mountain chain (or a scribbled line).
But if you went to a different star system, the Sun’s position would change, depending on the direction. As such, the Sun really isn’t in any constellation per se. But then again, none of the other stars that make up the Milky Way are either. Much like what Einstein’s Theory of Relativity teaches us about space and time, the constellations themselves are relative to the observer.
We have written many interesting articles about the Sun and the constellations here at Universe Today. Here’s What are the Constellations?, Zodiac Signs and their Dates?, Where is the Sun?, and Earth’s Orbit Around the Sun.
For more information on how our Sun looks from Alpha Centauri, be sure to check out this page from Learn Astronomy. SAnd here’s an article about all 88 recognized constellations.
Astronomy Cast also has episodes on the subject. Here’s Episode 30: The Sun, Spots and All and Episode 157: Constellations.
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Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the open galactic star cluster known as Messier 39. Enjoy!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these objects is known as Messier 39, an open star cluster located in the direction of the Cygnus constellation. Because of its proximity to Deneb and its size – it is actually larger in the night sky than a full Moon – it is easily observed using binoculars and small, low magnification telescopes.
Positioned only about 800 light years away from our solar system, this 300 million year old group of about 30 stars may look like they are spread fairly far apart in the sky. But as clusters go, they are close, really close! This group is gathered in space in only a 7 light year neighborhood! All of its stars are main sequence and the very brightest of them are just about to evolve into the red giant star phase.
In a study done by Jean Claude Mermilliod (et al), they conducted a long-term monitoring of solar-type dwarfs with CORAVEL – a study which took 19 years. While most individual radial velocities were never published – apart from a small number of spectroscopic binaries – the stars themselves and their properties were well documented in the works of B. Uyaniker and T. L. Landecker of the National Research Council, Herzberg Institute of Astrophysics.
As Uyaniker and Landecker claimed in their 2002 study, “A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium“:
“We describe a Faraday rotation structure in the interstellar medium detected through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane Survey (CGPS). The structure, at l = 918,b = -25, has an extent of ~2°, within which polarization angle varies smoothly over a range of ~100°. Polarized intensity also varies smoothly, showing a central peak within an outer shell. This region is in sharp contrast to its surroundings, where low-level chaotic polarization structure occurs on arcminute scales. The Faraday rotation structure has no counterpart in radio total intensity and is unrelated to known objects along the line of sight, which include a Lynds Bright Nebula, LBN 416, and the star cluster M39 (NGC 7092). It is interpreted as a smooth enhancement of electron density. The absence of a counterpart, in either optical emission or total intensity, establishes a lower limit to its distance. An upper limit is determined by the strong beam depolarization in this direction. At a probable distance of 350 ± 50 pc, the size of the object is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of ionized gas is 23 M. It has a very smooth internal magnetic field of strength 3 UG, slightly enhanced above the ambient field. G91.8-2.5 is the second such object to be discovered in the CGPS, and it seems likely that such structures are common in the magneto-ionic medium.”
So where do these gases come from? Perhaps they are there all along. As Yu N. Efremov and T.G. Sitnik wrote in their 1988 study:
“It is found that about 90% of young clusters o-b2 and OB-associations situated within 3 kpc from the Sun are united into complexes with diameters from 150 to 700 pc. Almost all complexes contain giant molecular clouds with masses. A number of complexes (mostly large ones)-are connected with giant H I clouds; a few of small complexes are situated in the H I-caverns. Older (>b2) cluster avoid the regions occupied by young star groups. Complexes often have an hierarchic structure; some neighbouring complexes may be united into supercomplexes with diameters about 1.5 kpc.”
Does this mean it’s possible that M39 could be more than one cluster combined? As H. Schneider wrote in his 1987 study:
“Early-type stars up to 12.0 mag and spectral type F2 in two young northern clusters were investigated by means of Stromgren and H-beta photometry. The distance and reddening of the clusters were estimated, and the membership of the stars discussed. In the case of NGC 7039 a distance of 675 pc and a color excess of E(b-y) = 0.056 were found; the respective values for NGC 7063 were 635 pc and E(b-y) = 0.062. The reality of NGC 7039 is somewhat puzzling: it seems that there exists a loose star aggregate called NGC 7039, containing about six to nine stars, and in the background another cluster at a distance of about 1500 pc. Besides this, variable reddening across the cluster area is probable.”
While it is possible this bright star cluster was remarked upon by Aristotle as a cometary appearing object about 325 BC, and it is also possible that it may have been discovered by Le Gentil in 1750, the fact remains M39 is most frequently attributed to be an original discovery of Charles Messier. As he recorded in his notes:
“In the night of October 24 to 25, 1764, I observed a cluster of stars near the tail of Cygnus: One distinguishes them with an ordinary (nonachromatic) refractor of 3 and a half feet; they don’t contain any nebulosity; its extension can occupy a degree of arc. I have compared it with the star Alpha Cygni, and I have found its position in right ascension of 320d 57′ 10″, and its declination of 47d 25′ 0″ north.”
Because Sir William Herschel did not publish his findings on Messier’s works, very few have read his observations of the object -“Consists of such large and straggling stars that I could not tell where it began nor where it ended. It cannot be called a cluster.” However, it would later go on to receive a New General Catalog (NGC) designation by Sir John Herschel who would describe it as “A star of 7th mag [position taken], one of a large loose cluster of stars of 7th to 10th magnitude; very coarsely scattered, and filling many fields.”
Even as accomplished as historic observers were, they sometimes didn’t always do the right thing. In the case of Messier 39, it is so close to us that it appears large dimensionally in the sky – and therefore needs less magnification instead of more to be properly studied as a whole. However, don’t always put away the magnfication, because as Admiral Smyth reports:
“A loose cluster, or rather splashy galaxy field of stars, in a very rich visinity between the Swan’s tail and the Lizard, due south of Beta Cephei, and east-north-east of Deneb [Alpha Cygni]. This was picked up by Messier in 1764, with his 3 1/2 foot telescope, and registered as being a degree in diameter. Among them there are several pairs, of which a couple were slightly estimated; the first being the brightest star (7m) and its comes, and the second a pretty pair of 10th-magnitudes.”
This coarse open star cluster is easily found in small optics. Start first by identifying the very large constellation of Cygnus and pinpointing its brightest, northernmost star. Aim you binoculars there. You’ll find M39 about 9 degrees east and a bit north of Deneb (Alpha Cygni). If at first you don’t succeed, try looking at Deneb from a dark sky location and see if you can spot a small, hazy patch about a fist width away to the east. There’s your star cluster!
It will also show easily in the telescope finderscope as a hazy patch and even begin resolution with larger aperture finders. M39 is very well suited to light polluted skies and moonlit observing and will even hold up well to less than ideal sky conditions. Small instruments will easily see a bright handful of stars while larger telescopes will resolve many more faint members and pairs. Because of its large apparent size, you’ll enjoy viewing M39 far more if you use the least amount of magnification possible.
Enjoy this star-studded cluster and the great Milky Way field that frames it!
And here are the quick facts on this Messier Object to help get you started:
Object Name: Messier 39
Alternative Designations: M39, NGC 7092
Object Type: Galactic Open Star Cluster
Constellation: Cygnus
Right Ascension: 21 : 32.2 (h:m)
Declination: +48 : 26 (deg:m)
Distance: 0.825 (kly)
Visual Brightness: 4.6 (mag)
Apparent Dimension: 32.0 (arc min)
We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.
Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.
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As neighboring planets, Earth and Mars have a few things in common. Both are terrestrial in nature (i.e. rocky), both have tilted axes, and both orbit the Sun within its circumstellar habitable zone. And during the course of their orbital periods (i.e. a year), both planets experience variations in temperature and changes in their seasonal weather patterns.
However, owing to their different orbital periods, a year on Mars is significantly longer than a year on Earth – almost twice as long, in fact. And because their orbits are different, the distance between our two planets varies considerably. Basically, every two years Earth and Mars will go from being “at conjunction” (where they are farther from each other) to being “at opposition” (where they are closer to each other).
Earth orbits the Sun at an average distance (semi-major axis) of 149,598,023 km (92,955,902 mi; or 1 AU), ranging from 147,095,000 km (91,401,000 mi) at perihelion to 152,100,000 km (94,500,000 mi) at aphelion. At this distance, and with an orbital velocity of 29.78 km/s (18.5 mi/s) the time it take for the planet to complete a single orbit of the Sun (i.e. orbital period) is equal to about 365.25 days.
Mars, meanwhile, orbits the Sun at an average distance of 227,939,200 km (141,634,850 mi; or 1.523679 AU), ranging from 206,700,000 km (128,437,425 mi) at perihelion to 249,200,000 km (154,845,700 mi) at aphelion. Given this difference in distance, Mars orbits the Sun at a slower speed (24.077 km/s; 14.96 mi/s) and takes about 687 Earth days (or 668.59 Mars sols) to complete a single orbit.
In other words, a Martian year is almost 700 days long, which works out to being 1.88 times as long as a year on Earth. This means that every time Mars completes a single orbit around the Sun, the Earth has gone around almost twice. During the moments when they are on opposite sides of the Sun, this is known as a “conjunction”. When they are on the same side of the Sun, they are at “opposition”.
By definition, a “Mars opposition” occurs when planet Earth passes in between the Sun and planet Mars. The term refers to the fact that Mars and the Sun appear on opposite sides of the sky. Because of their orbits, Mars oppositions happens about every 2 years and 2 months – 779.94 Earth days to be precise. From our perspective here on Earth, Mars appears to be rising in the east just as the Sun sets in the west.
After staying up in the sky for the entire night, Mars then sets in the west just as the Sun begins to rise in the east. During an opposition, Mars becomes one of the brightest objects in the night sky, and is easy to see with the naked eye. Through small telescopes, it will appear as a large and bright object. Through larger telescopes, Mars’ surface features will even become apparent, which would include its polar ice caps.
An opposition can also occur anywhere along Mars’ orbit. However, opposition does not necessary mean that the two planets are at their closest overall. In truth, it just means that they are are at their closest point to each other within their current orbital period. If Earth and Mars’ orbits were perfectly circular, they would be closest to each other whenever they were at opposition.
Instead, their orbits are elliptical, and Mars’ orbit is more elliptical than Earth’s – which means the difference between their respective perihelion and aphelion is greater. Gravitational tugging from other planets constantly changes the shape of our orbits too – with Jupiter pulling on Mars and Venus and Mercury affecting Earth.
Lastly, Earth and Mars do not orbit the Sun on the exact same plane – i.e. their orbits are slightly tilted relative to each other. Because of this, Mars and Earth become closest to each other only over the long-term. For instance, every 15 or 17 years, an opposition will occur within a few weeks of Mars’ perihelion. When it happens while the Mars is closest to the sun (called “perihelic opposition”), Mars and Earth get particularly close.
And yet, the closest approaches between the two planets only take place over the course of centuries, and some are always closer than others. To make matters even more confusing, over the past few centuries, Mars’ orbit has been getting more and more elongated, carrying the planet even nearer to the Sun at perihelion and even farther away at aphelion. So future perihelic oppositions will bring Earth and Mars even closer.
On August 28th, 2003, astronomers estimated that Earth and Mars were just 55,758,118 km (34,646,488 mi; 0.37272 AU) apart. This was the closest the two planets had come to each other in almost 60,000 years. This record will stand until August 28th, 2287, at which point the planets will be an estimated 55,688,405 km (34,603,170.6 mi; 0.372254 AU) from each other.
Want to organize your schedule for the next time Mars will be close to Earth? Here are some upcoming dates, covering the next few decades. Plan accordingly!
And in case your interested, Mars will be making close approaches on two occasions this century. The first will take place on August 14th, 2050, when Mars and Earth will be 55.957 million km (34.77 million mi; or 0.374051 AU) apart; and on September 1st, 2082, when they will be 55,883,780 km (34,724,571 mi; 0.373564 AU) apart.
There’s a reason missions to Mars depart from Earth every two years. Seeking to take advantage of shorter travel times, rovers, orbiters and landers are launched to coincide with Mars being at opposition. And when it comes time to send crewed mission to Mars (or even settlers) the same timing will apply!
We have written many interesting articles about Mars here at Universe Today. Here’s How Far is Mars from Earth?, How Long Does it Take to Get to Mars?, How Long is a Year on Mars?, How Far is Mars from the Sun?, and How Long Does it Take Mars to Orbit the Sun?
For more information, here’s a comprehensive schedule of upcoming Mars oppositions.
Astronomy Cast also has some wonderful episodes on the Red Planet. Here’s Episode 52: Mars, and Episode 91: The Search for Water on Mars.
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Uranus is a most unusual planet. Aside from being the seventh planet of our Solar System and the third gas giant, it is also classified sometimes as an “ice giant” (along with Neptune). This is because of its peculiar chemical composition, where water and other volatiles (i.e. ammonia, methane, and other hydrocarbons) in its atmosphere are compressed to the point where they become solid.
In addition to that, it also has a very long orbital period. Basically, it takes Uranus a little over 84 Earth years to complete a single orbit of the Sun. What this means is that a single year on Uranus lasts almost as long as a century here on Earth. On top of that, because of it axial tilt, the planet also experiences extremes of night and day during the course of a year, and some pretty interesting seasonal changes.
Uranus orbits the Sun at an average distance (semi-major axis) of 2.875 billion km (1.786 billion mi), ranging from 2.742 billion km (1.7 mi) at perihelion to 3 billion km (1.86 billion mi) at aphelion. Another way to look at it would be to say that it orbits the Sun at an average distance of 19.2184 AU (over 19 times the distance between the Earth and the Sun), and ranges from 18.33 AU to 20.11 AU.
The difference between its minimum and maximum distance from the Sun is 269.3 million km (167.335 mi) or 1.8 AU, which is the most pronounced of any of the Solar Planets (with the possible exception of Pluto). And with an average orbital speed of 6.8 km/s (4.225 mi/s), Uranus has an orbital period equivalent to 84.0205 Earth years. This means that a single year on Uranus lasts as long as 30,688.5 Earth days.
However, since it takes 17 hours 14 minutes 24 seconds for Uranus to rotate once on its axis (a sidereal day). And because of its immense distance from the Sun, a single solar day on Uranus is about the same. This means that a single year on Uranus lasts 42,718 Uranian solar days. And like Venus, Uranus’ rotates in the direction opposite of its orbit around the Sun (a phenomena known as retrograde rotation).
Another interesting thing about Uranus is the extreme inclination of its axis (97.7°). Whereas all of the Solar Planets are tilted on their axes to some degree, Uranus’s extreme tilt means that the planet’s axis of rotation is approximately parallel with the plane of the Solar System. The reason for this is unknown, but it has been theorized that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus and tilted it onto its side.
A consequence of this is that when Uranus is nearing its solstice, one pole faces the Sun continuously while the other faces away – leading to a very unusual day-night cycle across the planet. At the poles, one will experience 42 Earth years of day followed by 42 years of night.
This is similar to what is experienced in the Arctic Circle and Antarctica. During the winter season near the poles, a single night will last for more than 24 hours (aka. a “Polar Night”) while during the summer, a single day will last longer than 24 hours (a “Polar Day”, or “Midnight Sun”).
Meanwhile, near the time of the equinoxes, the Sun faces Uranus’ equator and gives it a period of day-night cycles that are similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7th, 2007. During the Voyager 2 probe’s historic flyby in 1986, Uranus’s south pole was pointed almost directly at the Sun.
Uranus’ long orbital period and extreme axial tilt also lead to some extreme seasonal variations in terms of its weather. Determining the full extent of these changes is difficult because astronomers have yet to observe Uranus for a full Uranian year. However, data obtained from the mid-20th century onward has showed regular changes in terms of brightness, temperature and microwave radiation between the solstices and equinoxes.
These changes are believed to be related to visibility in the atmosphere, where the sunlit hemisphere is thought to experience a local thickening of methane clouds which produce strong hazes. Increases in cloud formation have also been observed, with very bright cloud features being spotted in 1999, 2004, and 2005. Changes in wind speed have also been noted that appeared to be related to seasonal increases in temperature.
Uranus’ “Great Dark Spot” and its smaller dark spot are also thought to be related to seasonal changes. Much like Jupiter’s Great Red Spot, this feature is a giant cloud vortex that is created by winds – which in this case are estimated to reach speeds of up to 900 km/h (560 mph). In 2006, researchers at the Space Science Institute and the University of Wisconsin observed a storm that measured 1,700 by 3,000 kilometers (1,100 miles by 1,900 miles).
Interestingly enough, while Uranus’ polar regions receive more energy on average over the course of a year than the equatorial regions, the equatorial regions have been found to be hotter than the poles. The exact cause of this remains unknown, but is certainly believed to be due to something endogenic.
Yep, Uranus is a pretty weird place! On this planet, a single year lasts almost a century, and the seasons are characterized by extreme versions of Polar Nights and Midnight Suns. And of course, an average year brings all kinds of seasonal changes, complete with extreme winds, massive storms, and thickening methane clouds.
We have written many articles about the length of a year on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, How Long is a Year on Mercury?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Neptune? and How Long is a Year on Pluto?
If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.
We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.
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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!
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.
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.
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).
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.
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!
We have written many interesting articles about a year on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, Which Planet has the Longest Day?, How Long is a Year on Mercury?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Uranus?, and How Long is a Year on Neptune?.
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.
Astronomy Cast also has some great episodes on the subject. Here’s Episode 1: Pluto’s Planetary Identity Crisis and Episode 64: Pluto and the Icy Outer Solar System.
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