Solar System Archives

July 30, 2012January 6, 2016 by Ken Kremer

Curiosity’s Grand Entrance with Star Trek’s William Shatner and Wil Wheaton – Video Duet

Video Caption: Star Trek’s Captain Kirk, actor William Shatner, guides viewers through the video titled, “Grand Entrance,” showing NASA’s Curiosity Mars Science Lab mission from atmospheroic entry through descent, and after landing on the Red Planet on August 6 2012.

As NASA engineers and scientists make final preparations for the Red Planet landing of NASA’s most difficult planetary science mission to date – the Curiosity Mars Science Lab – inside Gale Crater on the night of August 5/6, Star Trek actors William Shatner and Wil Wheaton lend their voices to a pair of new mission videos titled “Grand Entrance”

The video duet describes the thrilling story of how Curiosity will touch down on Mars and guides viewers through the nail biting “7 Minutes of Terror” – from entry into the Martian atmosphere at over 13,000 MPH and then how the rover must slow down through descent, set down for a soft and safe landing and ultimately how Curiosity will search for signs of life. Continue reading “Curiosity’s Grand Entrance with Star Trek’s William Shatner and Wil Wheaton – Video Duet”

July 30, 2012December 23, 2015 by Ken Kremer

Curiosity Completes Crucial Course Correction – 1 Week from Mars !

Image Caption: Course correcting thruster firings on July 29 successfully placed Curiosity on target to touchdown beside Mount Sharp inside Gale Crater on Mars on Aug 6 in search of signs of a habitable environment. Credit: NASA

Now just 1 week out from landing beside a 3 mile high (5 km) layered Martian mountain in search of life’s ingredients, aiming thrusters aboard the cruise stage of NASA’s car sized Curiosity Mars Science Lab successfully fired to set the rover precisely on course for a touchdown on Mars at about 1:31 a.m. EDT (531 GMT) early on Aug. 6 (10:31 p.m. PDT on Aug. 5).

Two precise and brief thruster bursts lasting about 7 seconds were successfully carried out just hours ago earlier today at 1 a.m. on July 29, EDT (10 p.m. PDT on July 28). The effect was to change the spacecraft’s velocity by about 1/40 MPH or 1 cm/sec as it smashes into Mars at about 13,200 mph (5,900 meters per second).

This was the fourth and possibly last of 6 interplanetary Trajectory Correction Manuevers (TCM’s) planned by mission engineers to steer Curiosity since departing Earth for the Red Planet.

If necessary, 2 additional TCM’s could be implemented in the final 48 hours next Saturday and Sunday before Curiosity begins plunging into the Martian atmosphere late Sunday night on a do or die mission to land inside the 100 mile wide Gale Crater with a huge mountain in the middle. All 6 TCM maneuvers were preplanned long before the Nov 26, 2011 liftoff from Cape Canaveral, Florida.

Without this course correction firing, MSL would have hit a point at the top of the Martian atmosphere about 13 miles (21 kilometers) east of the target entry point. During the preprogrammed Entry, Descent and Landing (EDL) sequence the vehicle can steer itself in the upper atmosphere to correct for an error amounting to a few miles.

On landing day, MSL can steer enough during its flight through the upper atmosphere to correct for missing the target entry aim point by a few miles and still land on the intended patch of Mars real estate. The mission’s engineers and managers rated the projected 13-mile miss big enough to warrant a correction maneuver.

“The purpose of this maneuver is to move the point at which Curiosity enters the atmosphere by about 13 miles,” said Tomas Martin-Mur of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., chief of the mission’s navigation team. “The first look at telemetry and tracking data afterwards indicates the maneuver succeeded as planned.”

Image Cation: Curiosity Mars Science Laboratory Rover – inside the Cleanroom at KSC, with robotic arm extended prior to encapsulation and Nov. 26, 2011 liftoff. Credit: Ken Kremer/kenkremer.com

As of today (July 30), Curiosity has traveled about 97% of the overall journey to Mars or about 343 million miles (555 million kilometers) of its 352-million-mile (567-million-kilometer) total flight distance.

“I will not be surprised if this was our last trajectory correction maneuver,” Martin Mur said of the TCM-4 firing. “We will be monitoring the trajectory using the antennas of the Deep Space Network to be sure Curiosity is staying on the right path for a successful entry, descent and landing.”

Curiosity will use an unprecedented rocket powered descent stage and a helicopter like sky crane to set down astride the sedimentary layers of Mount Sharp.

She will then conduct a minimum 2 year prime mission with the most sophisticated science instrument package ever dispatched to Mars to determine if a habitable zone ever existed on this region of Mars.

Curiosity will search for the ingredients of life in the form of organic molecules – the carbon based molecules which are the building blocks of life as we know it. The one-ton behemoth is packed to the gills with 10 state of the art science instruments including a 7 foot long robotic arm, scoop, drill and laser rock zapper.

As Curiosity dives down to Mars surface on Aug. 6, 3 spacecraft from NASA and ESA are now positioned in orbit around the Red Planet and are ready to relay and record signals from the “7 Minutes of Terror” – Read the details in my article – here

Watch NASA TV online for live coverage of the Curiosity landing on Aug 5/6:
mars.jpl.nasa.gov or www.nasa.gov

Ken Kremer

July 28, 2012December 23, 2015 by Ken Kremer

T Minus 9 Days – Mars Orbiters Now in Place to Relay Critical Curiosity Landing Signals

Image Caption: NASA’s Mars Odyssey will relay near real time signals of this artist’s concept depicting the moment that NASA’s Curiosity rover touches down onto the Martian surface. NASA’s Mars Reconnaissance Orbiter (MRO) and ESA’s Mars Express (MEX) orbiter will also record signals from Curiosity for later playback, not in real time. Credit: NASA

It’s now just T minus 9 Days to the most difficult and complex Planetary science mission NASA has ever attempted ! The potential payoff is huge – Curiosity will search for signs of Martian life

The key NASA orbiter at Mars required to transmit radio signals of a near real-time confirmation of the August 5/6 Sunday night landing of NASA’s car sized Curiosity Mars Science Lab (MSL) rover is now successfully in place, and just in the nick of time, following a successful thruster firing on July 24.

Odyssey will transmit the key signals from Curiosity as she plunges into the Martian atmosphere at over 13,000 MPH (21,000 KPH) to begin the harrowing “7 Minutes of Terror” known as “Entry, Descent and Landing” or EDL – all of which is preprogrammed !

Engines aboard NASA’s long lived Mars Odyssey spacecraft fired for about 6 seconds to adjust the orbiters location about 6 minutes ahead in its orbit. This will allow Odyssey to provide a prompt confirmation of Curiosity’s landing inside Gale crater at about 1:31 a.m. EDT (531 GMT) early on Aug. 6 (10:31 p.m. PDT on Aug. 5) – as NASA had originally planned.

Without the orbital nudge, Odyssey would have arrived over the landing site about 2 minutes after Curiosity landed and the signals from Curiosity would have been delayed.

A monkey wrench was recently thrown into NASA relay signal plans when Odyssey unexpectedly went into safe mode on July 11 and engineers weren’t certain how long recovery operations would take.

“Information we are receiving indicates the maneuver has completed as planned,” said Mars Odyssey Project Manager Gaylon McSmith of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Odyssey has been working at Mars longer than any other spacecraft, so it is appropriate that it has a special role in supporting the newest arrival.”

Odyssey has been in orbit at Mars since 2001 conducting orbital science investigations.

Read my review article on Odyssey’s science discoveries – here

Odyssey serves as the primary communications relay for NASA’s other recent surface explorers – Opportunity, Spirit and Phoenix. Opportunity recently passed 3000 Sols of continuous operations.

Two other Mars orbiters, NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express, also will be in position to receive radio transmissions from the Mars Science Laboratory during its descent. However, they will be recording information for later playback, not relaying it immediately, as only Odyssey can.

“We began optimising our orbit several months ago, so that Mars Express will have an orbit that is properly “phased” and provides good visibility of MSL’s planned trajectory,” says Michel Denis, Mars Express Spacecraft Operations Manager.

Mars Express has been orbiting the planet since December 2003.

Image Caption: Mars Express supports Curiosity MSL. Credit: ESA

“NASA supported the arrival of Mars Express at Mars in 2003, and, in the past few years, we have relayed data from the rovers Spirit and Opportunity,” says ESA’s Manfred Warhaut, Head of Mission Operations.

“Mars Express also tracked the descent of NASA’s Phoenix lander in 2008 and we routinely share our deep space networks.

“Technical and scientific cooperation at Mars between ESA and NASA is a long-standing and mutually beneficial activity that helps us both to reduce risk and increase the return of scientific results.”

Watch NASA TV online for live coverage of Curiosity landing: mars.jpl.nasa.gov or www.nasa.gov

Ken Kremer

July 19, 2012December 23, 2015 by Jason Major

Bolt from the Blue: Giant Flash of Lightning Seen in Saturn’s Storm

An enormous storm that wrapped its way around Saturn’s northern hemisphere during the first half of 2011 wasn’t just a churning belt of high-speed winds; it also generated some monster flashes of lightning as well — one of which was captured on camera by the Cassini spacecraft!

Check it out…

The image above was created from Cassini raw images acquired in red, green, and blue color channels and assembled to create a somewhat “true-color” image of Saturn. The image shows the storm as it looked on February 25, 2011, a couple of months after it was first noticed by amateur astronomers on the ground. (The circle at upper left illustrates the comparative size of Earth.)

Read: Studying Saturn’s Super Storm

These images were acquired by Cassini almost two weeks later, on March 6, the first showing a bright blue flash of lightning within the storm, along the eastern edge of a large eddy. The second image, taken 30 minutes later, does not have any visible flash.

Because the flash was only visible in blue light (and there was no red channel data) the images are false color. Near-infrared replaced the visible red channel.

Based on the image resolution (12 miles/20 km per pixel) the size of the lightning flash is estimated to be about 120 miles (200 km) wide — as large as the strongest lightning seen on Earth. And like on Earth, Saturn’s lightning is thought to originate deeper in the atmosphere, at the level where water droplets freeze.

Although the 2011 northern storm was a great feature to observe, this wasn’t the first time lightning had been spotted on Saturn. Cassini had observed flashes on the ringed planet in August of 2009 as well, allowing scientists to create the first movie of lightning flashing on another planet.

Since its arrival at Saturn in 2004, Cassini has detected 10 lightning storms on Saturn — although with up to 10 flashes per second and eventually covering an area of 2 billion square miles (4 billion sq. km) the 2011 storm was by far the largest ever seen.

Image credits: NASA / JPL-Caltech / Space Science Institute. Top composite by J. Major. Video: JPL

July 18, 2012December 23, 2015 by Jason Major

Why Doesn’t Earth Have More Water?

Water, water everywhere… Coleridge’s shipbound ancient mariners were plagued by a lack of water while surrounded by a sea of the stuff, and while 70% of Earth’s surface is indeed covered by water (of which 96% is salt water, hence not a drop to drink) there’s really not all that much — not when compared to the entire mass of the planet. Less than 1% of Earth is water, which seems odd to scientists because, based on conventional models of how the Solar System formed, there should have been a lot more water available in Earth’s neck of the woods when it was coming together. So the question has been floating around: why is Earth so dry?

According to a new study from the Space Telescope Science Institute in Baltimore, MD, the answer may lie in the snow.

The snow line, to be exact. The region within a planetary system beyond which temperatures are cold enough for water ice to exist, the snow line in our solar system is currently located in the middle of the main asteroid belt, between the orbits of Mars and Jupiter. Based on conventional models of how the Solar System developed, this boundary used to be closer in to the Sun, 4.5 billion years ago. But if that were indeed the case, then Earth should have accumulated much more ice (and therefore water) as it was forming, becoming a true “water world” with a water mass up to 40 percent… instead of a mere one.

As we can see today, that wasn’t the case.

“Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn’t have much water, and that has always been a puzzle.”

– Rebecca Martin, Space Telescope Science Institute

A study led astrophysicists Rebecca Martin and Mario Livio of the Space Telescope Science Institute took another look at how the snow line in our solar system must have evolved, and found that, in their models, Earth was never inside the line. Instead it stayed within a warmer, drier region inside of the snow line, and away from the ice.

“Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth’s orbit,” Livio said. “Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, our model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry. ”

Read: Rethinking the Source of Earth’s Water

The standard model states that in the early days of a protoplanetary disk’s formation ionized material within it gradually falls toward the star, drawing the icy, turbulent snow line region inward. But this model depends upon the energy of an extremely hot star fully ionizing the disk — energy that a young star, like our Sun was, just didn’t have.

“We said, wait a second, disks around young stars are not fully ionized,” Livio said. “They’re not standard disks because there just isn’t enough heat and radiation to ionize the disk.”

“Astrophysicists have known for quite a while that disks around young stellar objects are NOT standard accretion disks (namely, ones that are ionized and turbulent throughout),” added Dr. Livio in an email to Universe Today. “Disk models with dead zones have been constructed by many people for many years. For some reason, however, calculations of the evolution of the snow line largely continued to use the standard disk models.”

Without fully ionized disk, the material is not drawn inward. Instead it orbits the star, condensing gas and dust into a “dead zone” that blocks outlying material from coming any closer. Gravity compresses the dead zone material, which heats up and dries out any ices that exist immediately outside of it. Based on the team’s research it was in this dry region that Earth formed.

The rest, as they say, is water under the bridge.

The team’s results have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Read the release on the Hubble news site here, and see the full paper here.

Lead image: Earth as seen by MESSENGER spacecraft before it left for Mercury in 2004. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Disk model image: NASA, ESA, and A. Feild (STScI). Earth water volume image: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.

July 17, 2012April 26, 2016 by Jason Major

Postcards From The (Inner) Edge

As the world turns its gaze outward in anticipation of the arrival of Mars Science Laboratory — with its hair-raising “seven minutes of terror” landing — let’s take a moment to look back inward, where MESSENGER is still faithfully orbiting the first rock from the Sun, Mercury, and sending back images that could only have been imagined just a few years ago.

The image above shows the graben-gouged terrain around Balanchine crater, within Mercury’s vast Caloris Basin impact crater. Named for the co-founder of the New York City Ballet, Balanchine crater is 41 km (25.5 miles) in diameter and filled with the curious erosion features known as hollows. Graben — basically sunken troughs in the surface — are the result of extensional forces that have pulled sections of the planet’s upper crust apart.

This image shows the peak-ring structure located within the much larger crater Rustaveli, which is 180 km (112 miles) in diameter. One of the more recently-named craters (the IAU convention for new features on Mercury has them titled after renowned artists, writers and composers from history) Rustaveli is named for a 12th-century Georgian poet who wrote the epic “The Knight in the Panther’s Skin”. The crater that now bears his namesake is located on Mercury’s northern hemisphere.

These two craters — also located within Caloris Basin — don’t yet have names but are no less interesting. Their overlapping positions works like an optical illusion, making the newer,sharper-edged crater on the right seem to almost float above the surface. The false-color of the image highlights the difference in surface composition of the two craters, which are both about 40 km (24 miles) wide. (The Caloris Basin in which they reside, however, is one of the largest known impact sites in our solar system, measuring at 1550 km — 963 miles — across!)

Now we zoom out for a wider view of our solar system’s second-densest planet (Earth is the first) and take a look at an image that’s night and day — literally! This is Mercury’s terminator, the twilit dividing line between night and day. More than just making a pretty picture, data on this transition is valuable to scientists as some atmospheric phenomena can only be observed at the terminator, such as the interaction between surface dust and charged particles from the Sun (which, at less than half the distance to the Sun than we are, Mercury is constantly bathed in.)

And now to zoom back in, we get a good look at an unnamed central-peaked crater about 85 km (52 miles) across in an oblique view that highlights the hollows and depressions within its floor. Acquired as part of what’s called a “targeted observation”, high-resolution images like this (79 meters/pixel) allow scientists to closely investigate specific features — but sadly there’s just not enough mission time to image all of Mercury at this level of detail.

On March 17, 2011 (March 18, 2011, UTC), MESSENGER became the first spacecraft ever to orbit Mercury. The mission has provided the first data from Mercury since Mariner 10, over 30 years ago. After over 1,000 orbits, 98 percent of Mercury is now imaged in detail, allowing us to know more about our solar system’s innermost world than ever before.

Keep up with MESSENGER updates (and the latest images) on the mission website here.

Image credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

July 12, 2012December 23, 2015 by John Williams

Rethinking the Source of Earth’s Water

Artist's impression of an asteroid impact on early Earth (credit: NASA)

Earth, with its blue hue visible from space, is known for its abundant water – predominately locked in oceans – that may have come from an extraterrestrial source. New research indicates that the source of Earth’s water isn’t from ice-rich comets, but instead from water-bearing asteroids.

Looking at the ratio of hydrogen to deuterium, a heavy isotope of hydrogen, in frozen water, scientists can get a pretty good idea of the distance the water formed in the solar system. Comets and asteroids farther from the Sun have a higher deuterium content than ice formed closer to the Sun. Scientists, led by the Carnegie Institution for Science’s Conel Alexander, compared water from comets and from carbonaceous chondrites. What they found challenges current models in how the solar system formed.

Primeval Earth was a hot and dry place. Any water that may have formed with Earth was boiled away from the scorching crust. Ultraviolet light from the newly formed Sun stripped hydrogen atoms from the water molecules leaving no rain to fall back on the surface. Scientists believe that both comets and carbonaceous asteroids formed beyond the orbit of Jupiter, perhaps at the very fringes of the solar system, then moved inward bringing both water and organic material to Earth. If this were true, Alexander and his colleagues suggest that ice found in comets and the remnants of ice preserved in carbonaceous chondrites in the form of clays would have similar isotopic composition.

After studying 85 carbonaceous chondrites, supplied by Johnson Space Center and the Meteorite Working Group, they show in a paper released today by Science Express that they likely did not form in the same regions of the solar system as comets because they have much lower deuterium content. They formed closer to the Sun, perhaps in the asteroid belt between Mars and Jupiter. And its that material that rained on early Earth to create the wet planet we know today.

“Our results provide important new constraints for the origin of volatiles in the inner solar system, including the Earth,” Alexander said. “And they have important implications for the current models of the formation and orbital evolution of the planets and smaller objects in our solar system.”

Image caption: Artist impression of an asteroid impact on early Earth (credit: NASA)

Image caption 2: This is a cross-section of a chondritic meteorite.

July 12, 2012April 13, 2022 by Ken Kremer

Dawn’s Vestan Endeavour Exceptionally Exciting near End of Year-Long Super Science Survey

Image Caption: Divalia Fossa equatorial trough at Vesta pictured in side by side images showing apparent brightness and topography. The trough encircles most of Vesta and is located just south of the equator. It is about 10 kilometers (6 miles) wide. Rubria and Occia craters straddle Divalia Fossa. The image was snapped on Oct 16, 2011 from an altitude of 700 km (435 mi) from the HAMO mapping orbit. Image Credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA

“NASA’s Dawn mission to Asteroid Vesta is going exceptionally well”, Dr. Marc Rayman, the mission’s Chief Engineer, told Universe Today in an exclusive interview as the revolutionary spacecraft nears the end of its more than 1 year long super science survey orbiting the giant space rock.

“The Dawn mission is not only going better than we had expected but even better than we had hoped.”

Dawn is Earth’s first mission ever to orbit and explore Vesta up close.

“We have acquired so much more data than we had planned even in late 2011! We have conducted a tremendous exploration of Vesta – the second most massive body between Mars and Jupiter, a giant of the main asteroid belt.”

“Now we are in our second high altitude mapping orbit (HAMO2), which is the final intensive campaign of the Vesta mission,” Rayman told me.

Image Caption: Dawn Orbiting Vesta above the “Snowman” craters. This artist’s concept shows NASA’s Dawn spacecraft orbiting the giant asteroid Vesta above the Snowman craters. The depiction of Vesta is based on images obtained by Dawn’s framing cameras. Dawn is an international collaboration of the US, Germany and Italy. Credit: NASA/JPL-Caltech

Indeed Dawn’s science and maneuvering endeavour’s at Vesta have proceeded so flawlessly that NASA has granted the science team a bonus of 40 days additional time in orbit split between the lower and higher science orbits known as LAMO and HAMO or the Low Altitude Mapping Orbit and the High Altitude Mapping Orbit respectively.

“Our original Vesta departure date was July 17, and now it is about August 26.” Rayman explained.

The bonus time at LAMO has already been completed. Now the team is about to begin the bonus time at HAMO – consisting of two additional mapping cycles beyond the four originally planned.

Each mapping cycle in HAMO2 consists of 10 orbits. Each orbit is about 12.5 hours.

“On July 14, we will complete mapping cycle 4 and begin 5 (of 6). On July 25 we will leave HAMO2 and escape from orbit on August 26. We will stop thrusting several times before escape to take more neat pictures, mostly of the northern hemisphere,” Rayman told me.

“As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours.”

When Dawn arrived in orbit at Vesta in July 2011 the northern polar region was in darkness as the southern hemisphere basked in summer’s glow. Now as Dawn departs Vesta in August, virtually all of the previously unseen and unphotographed northern polar region is illuminated and will be mapped in exquisite detail.

Coincidentally on July 13/14 as HAMO2 Cycle 4 ends, I’ll be presenting a free public lecture about Dawn and NASA’s Planetary and Human Spaceflight programs at the Adirondack Public Observatory.

Image Caption: Asteroid Vesta and Mysterious Equatorial Grooves – from Dawn Orbiter. This full view of the giant asteroid Vesta was taken by NASA’s Dawn spacecraft on July 24, 2011, at a distance of 3,200 miles (5,200 kilometers). This view shows impact craters of various sizes and mysterious grooves parallel to the equator. The resolution of this image is about 500 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Why has Dawn been granted an extended mission ?

“Dawn has gone so well that we had consumed not even one day of our 40 days of operations margin,” Rayman stated .

“That allowed us to spend more time in LAMO. We had had some unexpected events to be sure, but we managed to deal with all of them so expeditiously that the entire margin remained intact. Then we received the (entirely unrelated) 40 day extension, which allowed us to leave Vesta later. That came about because of our being able to shorten the flight from Vesta to Ceres, so we could still reach Ceres on schedule in 2015.”

“That 40 days allowed us to spend still ~ 30 more days in LAMO and increase HAMO2 by 10 days to a total of six cycles. We got still more time by finding ways to make the trip from HAMO2 to escape a little more efficiently, and that’s what allowed HAMO2 to be even longer, with the additional eight days of VIR-only observations I described in my most recent Dawn Journal.”

“The summary is that every investigation has been more productive than we could have imagined, and because the exploration of Vesta has gone so well, we have been able to apply our unused margin to get even more out of the mission. It is very very gratifying and exciting.”

So we have a few more weeks to enjoy the wondrous sights of Vesta before Dawn fires up her revolutionary ion thrusters to escape the gravitational tug of Vesta and head off to the dwarf planet Ceres, the largest asteroid in the main belt of our Solar System – and which some have speculated may hold vast caches of water and perhaps even liquid oceans suitable for sustaining life.

Ken Kremer

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July 13/14: Free Public Lectures about NASA’s Mars, Vesta and Planetary Exploration, the Space Shuttle, SpaceX , Orion and more by Ken Kremer at the Adirondack Public Observatory in Tupper Lake, NY.

July 10, 2012April 26, 2016 by Jason Major

The Return of the Rings!

Now that Cassini has gone off on a new trajectory taking it above and below the equatorial plane of Saturn, we’re back to getting some fantastic views of the rings — the likes of which haven’t been seen in over two and a half years!

The image above shows portions of the thin, ropy F ring and the outer A ring, which is split by the 202-mile (325-km) -wide Encke gap. The shepherd moon Pan can be seen cruising along in the gap along with several thin ringlets. Near the A ring’s outer edge is a narrower space called the Keeler gap — this is the home of the smaller shepherd moon Daphnis, which isn’t visible here (but is one of my personal favorites!)

The scalloped pattern on the inner edge of the Encke gap downstream from Pan and a spiral pattern moving inwards from that edge are created by the 12.5-mile-wide (20-km-wide) moon’s gravitational influence.

Other features that have returned for an encore performance are the so-called propellers, spiral sprays of icy ring material created by tiny micro-moons within the rings. Individually too small to discern (less than half a mile in diameter) these propeller moons kick up large clumps of reflective ring particles with their gravity as they travel through the rings, revealing their positions.

The three images above show a propeller within the A ring. Nicknamed “Sikorsky” after Russian-American aviator Igor Sikorsky, the entire structure is about 30 miles (50 km) across and is one of the more well-studied propellers.

Scientists are eager to understand the interactions of propellers in Saturn’s rings as they may hold a key to the evolution of similar systems, such as solar systems forming from disks of matter.

See a video of a propeller orbiting within the rings here, and here’s an image of one that’s large enough to cast a shadow!

“One of the main contributing factors to the enormous success we on the Cassini mission have enjoyed in the exploration of Saturn is the capability to view the planet and the bodies around it from a variety of directions,” Cassini Imaging Team Leader Carolyn Porco wrote earlier today. “Setting the spacecraft high into orbit above Saturn’s equator provides us direct views of the equatorial and middle latitudes on the planet and its moons, while guiding it to high inclination above the equator plane affords the opportunity to view the polar regions of these bodies and be treated to vertigo-inducing shots of the planet’s glorious rings.”

As always, keep up with the latest Cassini news on the mission site here, and read more about these images on the CICLOPS imaging team page here.

Image credits: NASA / JPL / Space Science Institute.

July 8, 2012December 23, 2015 by Fraser Cain

Why Planets Orbit the Sun

Why Do Planets Orbit the Sun — The Solar System

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In ancient times, astronomers thought that all celestial objects – the Sun, Moon, planets and stars – orbited around the Earth in a series of crystal spheres. But as modern science developed, astronomers were better able to understand our place in the cosmos. They discovered that all the planets, including the Earth, actually orbit around the Sun.

Not only did scientists discover that the simple fact that the planets orbit the Sun, they uncovered the underlying reasons for why. What chain of events led us to our current Solar System, with planets orbiting the Sun?

Astronomers Used to Think the Earth was the Center of the Solar System
Because we live on Earth, and we see objects passing across our view of the skies, it’s natural to assume that the Earth is the center of the Universe. In fact, this perspective – known as geocentrism – was the default for all ancient civilizations. The Sun, the Moon, the planets and the stars appeared to move around the Earth each day. And because the Earth itself didn’t seem to be moving, astronomers like Ptolemy assumed that Earth was the center of the Universe. In fact, they went so far as to create very detailed models for predicting the motions of objects with a high degree of accuracy, using this completely inaccurate model of the Solar System. The predictions made by Ptolemy were used to make astrological predictions for more than 1500 years, until a much better model came along.

Actually, the Sun is the Center of the Solar System
A new, more accurate model of the Solar System didn’t come around until the 16th century, when the Polish astronomer Nicolai Copernicus published his Universe-changing book: On the Revolutions of the Heavenly Bodies. Copernicus accurately reorganized the Solar System, putting the Sun at the center in a heliocentric model. And the Earth took its proper place, as just another planet orbiting the Sun – one of the 6 known to astronomers at the time.

Copernicus’ model helped answer two questions which had troubled astronomers for centuries: why the planets brighten and dim over the course of several months (because they’re getting closer and further away), and why the planets seem to reverse and move in a retrograde direction. Easily explained because of the changing positions of the Earth, planets and the background stars.

But Why Do They Orbit the Sun?
Once they could accurately describe the nature of the planetary motion in the Solar System, they were left with a more fundimental question: Why do the planets orbit the Sun? What sequence of events led to the current motions of the planets around the Sun?

To explain this, we need to look back 4.6 billion years ago, before there was even a Solar System. In our place instead, there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity.

Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning. Imagine two spinning skydivers colliding with one another in mid-air; after their collision, they’ll have a new rotation speed and direction based on the addition of their original directions.

Eventually all of this hydrogen gas was collected together into a massive spinning ball of gas that continued to collapse under its own gravity. As it collapsed, it began to spin faster and faster, just like a figure skater pulling in her arms increases her rotation speed.

The spinning cloud of gas and dust flattened out because of the rotational force, with the Sun at the center, and then a pancake-shaped disc of material surrounding it. The planets formed out of this disk of material, collecting together particles of dust into larger and larger rocks until planet-sized objects had accumulated together.

The Planets are in Perfect Balance

The planets orbit the Sun because they’re left over from the formation of the Solar System. Their current motion depends on the gravitational attraction of the Sun at the center of the Solar System. In fact, they’re in perfect balance.

There are two opposing forces acting on the planets: gravity pulling them inward, and the inertia of their orbit driving them outwards. If gravity was dominant, the planets would spiral inward. If their inertia was dominant, the planets would spiral outward into deep space.

The planets are trying to fly out into deep space, but the gravity of the Sun is pulling them into a curved orbit.

Research further:
Cornell Astronomy
The Universe of Aristotle and Ptolemy
Copernical Model: A Sun-Centered Solar System
The Solar Nebula
On the Revolution of the Heavenly Bodies
The Copernican Revolution