Cassini image of Saturn's rings from Dec. 20, 2012 (NASA/JPL-Caltech/SSI)
As Saturn steadily moves along its 29.7-year-long orbit toward summertime in its northern hemisphere NASA’s Cassini spacecraft is along for the ride, giving astronomers a front-row seat to seasonal changes taking place on the ringed planet.
One of these fluctuations is the anticipated disappearance of the “spokes” found in the rings, a few of which can be seen above in an image captured on Dec. 20 of last year.
First identified by Voyager in 1980, spokes are ghostly streaks of varying size and brightness that stretch radially across Saturn’s ring system. They orbit around the planet with the ring particles and can last for hours before fading away.
Under the right lighting conditions spokes can appear dark, as seen in this image from Jan. 2010 (NASA/JPL/SSI)
One of the most elusive and transient of features found on Saturn, spokes are thought to be made up of larger microscopic particles of ice — each at least a micron or more — although exactly what makes them gather together isn’t yet known.
They are believed to be associated with interactions between ring particles and Saturn’s electromagnetic field.
“The spokes are most prominent at a point in the rings where the ring particles are moving at the same speed as Saturn’s electromagnetic field,” said Brad Wallis, Cassini rings discipline scientist. “That idea and variations of it are still the most prominent theories about the spokes.”
Other researchers have suggested that they may be caused by electron beams issuing outwards along magnetic field lines from lightning storms in Saturn’s atmosphere.
Regardless of how they are created, spokes are more often observed when sunlight is striking the rings edge-on — that is, during the spring and autumn equinoxes. Perhaps the increased solar radiation along Saturn’s equator increases the formation of lightning-generating storms, in turn creating more spokes? It’s only a guess, but Cassini — and astronomers — will be watching to see if these furtive features do in fact fail to appear during Saturn’s northern summer, the height of which arrives in 2016.
Raw Cassini image captured on 26 Feb. 2013 (NASA/JPL/SSI)
Freshly delivered from Cassini’s wide-angle camera, this raw image gives us another look at Saturn’s north pole and the curious hexagon-shaped jet stream that encircles it, as well as the spiraling vortex of clouds at its center.
Back in November we got our first good look at Saturn’s north pole in years, now that Cassini’s orbit is once again taking it high over the ringplane. With spring progressing on Saturn’s northern hemisphere the upper latitudes are getting more and more sunlight — which stirs up storm activity in its atmosphere.
The bright tops of upper-level storm clouds speckle Saturn’s skies, and a large circular cyclone can be seen near the north pole, within the darker region contained by the hexagonal jet stream. This could be a long-lived storm, as it also seems to be in the images captured on November 27.
About 25,000 km (15,500 miles) across, Saturn’s hexagon is wide enough to fit nearly four Earths inside!
The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)
The hexagon was originally discovered in images taken by the Voyager spacecraft in the early 1980s. It encircles Saturn at about 77 degrees north latitude and is estimated to whip around the planet at speeds of 354 km/h (220 mph.)
Color map of Mercury's varied surface. The 1,550-km-wide Caloris Basin can be seen at upper right.
Created by the MESSENGER mission team at the Johns Hopkins University Applied Physics Laboratory and the Carnegie Institution of Washington, this animation gives us a look at the spinning globe of Mercury, its surface color-coded to reflect variations in surface material reflectance.
Thousands of Wide Angle Camera images of Mercury’s surface were stitched together to create the full-planet views.
While the vibrant colors don’t accurately portray Mercury as our eyes would see it, they are valuable to scientists as they highlight the many different types of materials that make up the planet’s surface. Young crater rays surrounding fresh impact craters appear light blue or white. Medium- and dark-blue “low-reflectance material” (LRM) areas are thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. Small orange spots are materials deposited by explosive volcanic eruptions.
At this point, over 99% of the Solar System’s innermost planet has been mapped by MESSENGER. Read more about the ongoing mission here.
Image/video credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Venus, Earth, Jupiter, Saturn, Uranus and Neptune as seen by Voyager 1 on Valentine's Day in 1990
On February 14, 1990, after nearly 13 years of travel through the outer Solar System, NASA’s Voyager 1 spacecraft crossed the orbit of Pluto and turned its camera around, capturing photos of the planets as seen from that vast distance. It was a family portrait taken from over 4.4 billion kilometers away — the ultimate space Valentine.
Who says astronomy isn’t romantic?
Full mosaic of Voyager 1 images taken on Feb. 14, 1990 (NASA/JPL)
“That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives… There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world.”
– Carl Sagan
It was the unique perspective above provided by Voyager 1 that inspired Carl Sagan to first coin the phrase “Pale Blue Dot” in reference to our planet. And it’s true… from the edges of the solar system Earth is just a pale blue dot in a black sky, a bright speck just like all the other planets. It’s a sobering and somewhat chilling image of our world… but also inspiring, as the Voyager 1 and 2 spacecraft are now the farthest human-made objects in existence — and getting farther every second. They still faithfully transmit data back to us even now, over 35 years since their launches, from 18.5 and 15.2 billion kilometers away.
The Voyagers sure know the value of a long-term relationship.
Pluto's known system of moons (NASA/ESA/M. Showalter))
Today marks seven months since the announcement of Pluto’s fifth moon and over a year and a half since the discovery of the one before that. But both moons still have letter-and-number designations, P5 and P4, respectively… not very imaginative, to say the least, and not really fitting into the pantheon of mythologically-named worlds in our Solar System.
Today, you can help change that.
According to the New Horizons research team, after the discovery of P4 in June 2011 it was decided to wait to see if any more moons were discovered in order to choose names that fit together as a pair, while a*lso following accepted IAU naming practices. Now, seven months after the announcement of P5, we think a decision is in order… and so does the P4/P5 Discovery Team at the SETI Institute.
“Hey, I can be democratic about all this!”
Today, SETI Senior Research Scientist Mark Showalter revealed a new poll site, Pluto Rocks, where visitors can place their votes on a selection of names for P4 and P5 — or even write in a suggestion of their own. In line with IAU convention these names are associated with the Greek and Roman mythology surrounding Pluto/Hades and his underworld-dwelling minions.
“In 1930, a little girl named Venetia Burney suggested that Clyde Tombaugh name his newly discovered planet ‘Pluto.’ Tombaugh liked the idea and the name stuck. I like to think that we are doing honor to Tombaugh’s legacy by now opening up the naming of Pluto’s two tiniest known moons to everyone.”
– Mark Showalter, SETI Institute
As of the time of this writing, the ongoing results look like this:
Results of Pluto Rocks voting as of Feb. 11, 2013 at 10 am EST (15:00 UT)
Do you like where the voting is headed? Are you hellishly opposed? Go place your vote now and make your opinion count in the naming of these two distant worlds!
(After all, New Horizons will be visiting Pluto in just under two and a half years, and she really should know how to greet the family.)
Voting ends at noon EST on Monday, February 25th, 2013.
The SETI team welcomes you to submit your vote every day, but only once per day so that voting is fair.
UPDATE: On Feb. 25, the final day of voting, the tally is looking like this:
PlutoRocks results as of Feb. 25, 2013 – Vulcan is in the lead, thanks to publicity from Mr. William Shatner
Thanks in no small part to a bit of publicity on Twitter by Captain Kirk himself, Mr. William Shatner (and support by Leonard Nimoy) “Vulcan” has made the list and warped straight to the lead. Will SETI and the IAU honor such Trek fan support with an official designation? We shall soon find out…
Image of Uranus’ crescent taken by a departing Voyager 2 on January 25, 1986 (NASA/JPL)
27 years ago today, January 24, 1986, NASA’s Voyager 2 spacecraft sped past Uranus, becoming simultaneously the first and last spacecraft to visit the blue-tinged gas giant, third largest planet in the Solar System.
The image above shows the crescent-lit Uranus as seen by Voyager 2 from a distance of about 965,000 km (600,000 miles.) At the time the spacecraft had already passed Uranus and was looking back at the planet on its way outwards toward Neptune.
Although composed primarily of hydrogen and helium, trace amounts of methane in Uranus’ uppermost atmosphere absorb most of the red wavelengths of light, making the planet appear a pale blue color.
Image of the 1,500-km-wide Oberon acquired by Voyager 2 on Jan. 24, 1986 (NASA/JPL)
The second of NASA’s twin space explorers (although it launched first) Voyager 2 came within 81,800 kilometers (50,600 miles) of Uranus on January 24, 1986, gathering images of the sideways planet, its rings and several of its moons. Voyager 2 also discovered the presence of a magnetic field around Uranus, as well as 10 new small moons.
Three moons discovered by Voyager 2 in 1986 (NASA/JPL)
Data gathered by Voyager 2 revealed that Uranus’ rate of rotation is 17 hours, 14 minutes.
At the time of this writing, Voyager 2 is 15,184,370,900 km from Earth and steadily moving toward the edge of the Solar System at a speed of about 3.3 AU per year. At that distance, signals from Voyager take just over 14 hours and 4 minutes to reach us.
See images from Voyager 2’s visit of Uranus here, and check out a video of the August 20, 1977 launch below along with more images from the historic Voyager mission’s “Grand Tour” of the outer Solar System.
If you’ve ever wanted to see what it’s like to buzz Venus like only a spacecraft can, here’s your chance: this is a video animation of images taken by ESA’s Venus Express as it makes a pole-to-pole orbit of our neighboring world.
Captured in ultraviolet wavelengths, the images were acquired by the spacecraft’s Venus Monitoring Camera last January over a period of 18 hours. It’s truly a “day in the life” of Venus Express!
From ESA’s description of the video:
We join the spacecraft from a staggering 66,000 km above the south pole, staring down into the swirling south polar vortex. From this bird’s-eye view, half of the planet is in darkness, the ‘terminator’ marking the dividing line between the day and night sides of the planet.
Intricate features on smaller and smaller scales are revealed as Venus Express dives to just 250 km above the north pole and clouds flood the field of view, before regaining a global perspective as it climbs away from the north pole.
The observed pattern of bright and dark markings is caused by variations in an unknown absorbing chemical at the Venus cloud tops.
False-color image of cloud features on Venus. Captured by Venus Express from a distance of 30,000 km (18,640 miles) on December 8, 2011. (ESA/MPS/DLR/IDA)
A view of Mercury from MESSENGER’s October 2008 flyby (NASA / JHUAPL / Gordan Ugarkovic)
Every now and then a new gem of a color-composite appears in the Flickr photostream of Gordan Ugarkovic, and this one is the latest to materialize.
This is a view of Mercury as seen by NASA’s MESSENGER spacecraft during a flyby in October 2008. The image is a composite of twenty separate frames acquired with MESSENGER’s narrow-angle camera from distances ranging from 18,900 to 17,700 kilometers and colorized with color data from the spacecraft’s wide-angle camera. (North is to the right.)
Click the image for a closer look, and for an even bigger planet-sized version click here. Beautiful!
The images that made up this mosaic were taken two and a half years before MESSENGER entered orbit around Mercury on March 19, 2011 UT, becoming the first spacecraft ever to do so and making Mercury the final “classical” planet to be orbited by a manmade spacecraft.
Since that time MESSENGER has completed well over 1,000 orbits and taken more than 100,000 images of the first planet in the Solar System, which filled in most of our gaps in Mercury’s map and showed us many never-before-seen features of the planet’s Sun-scoured surface. And just this past year MESSENGER’s extended mission helped confirm what could be called its most important discovery of all: water ice on Mercury’s north pole.
With all that’s been achieved by MESSENGER in 2011 and 2012, 2013 is looking to be an interesting year!
“We learned a great deal about Mercury over the past year,” said MESSENGER Principal Investigator Sean Solomon of Columbia University’s Lamont-Doherty Earth Observatory. “The team published three dozen scientific and technical papers and delivered more than 150 presentations at national and international meetings. New measurements continue to stream back from our spacecraft, and we can look forward with excitement to many additional discoveries in 2013.”
When the Moon was receiving its highest number of impacts, so was Earth. Credit: Dan Durda
Some questions about our own planet are best answered by looking someplace else entirely… in the case of impact craters and when, how and how often they were formed, that someplace can be found shining down on us nearly every night: our own companion in space, the Moon.
By studying lunar impact craters both young and old scientists can piece together the physical processes that took place during the violent moments of their creation, as well as determine how often Earth — a considerably bigger target — was experiencing similar events (and likely in much larger numbers as well.)
With no substantial atmosphere, no weather and no tectonic activity, the surface of the Moon is a veritable time capsule for events taking place in our region of the Solar System. While our constantly-evolving Earth tends to hide its past, the Moon gives up its secrets much more readily… which is why present and future lunar missions are so important to science.
Take the crater Linné, for example. A young, pristine lunar crater, the 2.2-km-wide Linné was formed less than 10 million years ago… much longer than humans have walked the Earth, yes, but very recently on lunar geologic terms.
It was once thought that the circular Linné (as well as other craters) is bowl-shaped, thus setting a precedent for the morphology of craters on the Moon and on Earth. But laser-mapping observations by NASA’s Lunar Reconnaissance Orbiter (at right) determined in early 2012 that that’s not the case; Linné is actually more of a truncated inverted cone, with a flattened interior floor surrounded by sloping walls that rise up over half a kilometer to its rim.
On our planet the erosive processes of wind, water, and earth soon distort the shapes of craters like Linné, wearing them down, filling them in and eventually hiding them from plain sight completely. But in the Moon’s airless environment where the only weathering comes from more impacts they retain their shape for much longer lengths of time, looking brand-new for many millions of years. By studying young craters in greater detail scientists are now able to better figure out just what happens when large objects strike the surface of worlds — events that can and do occur quite regularly in the Solar System, and which may have even allowed life to gain a foothold on Earth.
Most of the craters visible on the Moon today — Linné excluded, of course — are thought to have formed within a narrow period of time between 3.8 and 3.9 billion years ago. This period, called the Late Heavy Bombardment, saw a high rate of impact events throughout the inner Solar System, not only on the Moon but also on Mars, Mercury, presumably Venus and Earth as well. In fact, since at 4 times its diameter the Earth is a much larger target than the Moon, it stands to reason that Earth was impacted many more times than the Moon as well. Such large amounts of impacts introduced material from the outer Solar System to the early Earth as well as melted areas of the surface, releasing compounds like water that had been locked up in the crust… and even creating the sorts of environments where life could have begun to develop and thrive.
(It’s been suggested that there was even a longer period of heavy impact rates nicknamed the “late late heavy bombardment” that lingered up until about 2.5 billion years ago. Read more here.)
In the video below lunar geologist David Kring discusses the importance of impacts on the evolution of the Moon, Earth and eventually life as we know it today:
“Impact cratering in Earth’s past has affected not only the geologic but the biologic evolution of our planet, and we were able to deduce that in part by the lessons we learned by studying the Moon… and you just have to wonder what other things we can learn by going back to the Moon and studying that planetary body further.”
It’s these sorts of connections that make lunar exploration so valuable. Keys to our planet’s past are literally sitting on the surface of the Moon, a mere 385,000 km away, waiting for us to just scoop them up and bring them back. While the hunt for a biological history on Mars or resource-mining an asteroid are definitely important goals in their own right, only the Moon holds such direct references to Earth. It’s like an orbiting index to the ongoing story of our planet — all we have to do is make the connections.
Learn more about lunar research at the LPI site here, and see the latest news and images from LRO here.
Artist’s depiction of the Kepler 10 system, which contains planets 2.2 and 1.4 times the size of Earth. (NASA/Ames/JPL-Caltech)
Kepler mission scientists announced today the discovery of literally hundreds of new exoplanet candidates — 461, to be exact — orbiting distant stars within a relatively small cross-section of our galaxy, bringing the total number of potential planets awaiting confirmation to 2,740. What’s more, at least 4 of these new candidates appear to be fairly Earth-sized worlds located within their stars’ habitable zone, the orbital “sweet spot” where surface water could exist as a liquid.
Impressive results, considering that NASA’s planet-hunting spacecraft was launched a little under 4 years ago (and watching 150,000 stars to spot the shadows of planets is no easy task!)
“… the ways by which men arrive at knowledge of the celestial things are hardly less wonderful than the nature of these things themselves.”
— Johannes Kepler
Since the last official announcement of Kepler candidates in Feb. 2012 the number of smaller Earth- and super-Earth-sized worlds observed has risen considerably, as well as the identification of multi-planet systems that are organized more-or-less along a flat plane… just like ours.
“There is no better way to kickoff the start of the Kepler extended mission,” said Kepler scientist Christopher Burke, “than to discover more possible outposts on the frontier of potentially life bearing worlds.”
Since the last Kepler catalog was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets orbiting 2,036 stars. The most dramatic increases are seen in the number of Earth-size and super Earth-size candidates discovered, which grew by 43 and 21 percent respectively.
The new data increases the number of stars discovered to have more than one planet candidate from 365 to 467. Today, 43 percent of Kepler’s planet candidates are observed to have neighbor planets.
The most dramatic increases are seen in the number of Earth-size and super Earth-size candidates discovered, which grew by 43 and 21 percent respectively. (NASA)
Although some of the new candidates announced today are large Neptune-sized planets, more than half are Earth- to super-Earth sized worlds less than twice the radius of our own planet.
In order for Kepler candidates to be “officially” called exoplanets, they must be observed 3 times on a regular orbit — that is, their signature dimming of the light from their home star must occur as predicted once their presence and then orbital period is calculated. Only then is an exoplanet confirmed.
To date Kepler has confirmed 105 exoplanets.
The longer the mission continues, the better the chance that Kepler will be able to confirm smaller Earth-sized worlds in longer-period orbits.
“The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits — orbital periods similar to Earth’s,” said Steve Howell, Kepler mission scientist. “It is no longer a question of will we find a true Earth analogue, but a question of when.”
Scientists analyzed more than 13,000 transit-like signals called ‘threshold crossing events’ to eliminate known spacecraft instrumentation and astrophysical false positives, phenomena that masquerade as planetary candidates, to identify the potential new planets. Watch the video below to see how Kepler observes the light-curve of transit events.
It was the unique perspective above provided by Voyager 1 that inspired Carl Sagan to first coin the phrase 
