A mosaic of MSL MAHLI images acquired on Sol 613 (NASA/JPL-Caltech/MSSS. Edit by Jason Major.)
This is very cute — Curiosity’s latest “selfie,” a mosaic I assembled from about a dozen images acquired with the rover’s Mars Hand Lens Imager (MAHLI) instrument on April 27-28, 2014 (Sol 613), with the 5.5-km-high Mount Sharp (Aeolis Mons) rising in the background. Click the image for a high-res version.
While it’s far from a perfect image — there are plenty of discrepancies in the mosaic tiling, I admit — I really like the character it imparts to Curiosity, who almost seems to be giving a toothy (if slightly dusty) grin there on the left with her cylindrical RUHF antenna and a bit of her RTG visible in the lower center. And with almost 21 Earth-months on Mars and lots of discoveries already under her robot belt, Curiosity (and her team) certainly have plenty to smile about!
Composite photo of Lyrid meteor shower and non-Lyrids taken with a NASA All-sky camera April 21-23, 2012. Credit: NASA/MSFC/Danielle Moser
On Friday night/early Saturday May 23-24 skywatchers across the U.S. and southern Canada may witness the birth of a brand new meteor shower. If predictions hold true, Earth will pass through multiple tendrils of dust and pebbly bits left behind by comet 209P/LINEAR, firing up a celestial display on par with the strongest showers of the year. Or better.
Peter Jenniskens of the SETI Institute, who predicted a possible meteor storm associated with comet 209P/LINEAR. Credit: NASA
Earlier predictions called for a zenithal hourly rate or ZHR of 1,000 per hour, pushing this shower into the ‘storm’ category. ZHR is an idealized number based on the shower radiant located at the zenith under ideal skies. The actual number is lower depending on how far the radiant is removed from the zenith and how much light pollution or moonlight is present. Meteor expert Peter Jenniskens of the SETI Instituteand Finland’s Esko Lyytinen first saw the possibility of a comet-spawned meteor storm and presented their results in Jenniskens’ 2006 book Meteor Showers and Their Parent Comets.
Approximate location of the radiant (blue) of the 209P/LINEAR shower at the peak of the brief maximum around 2 a.m. CDT May 24. Between 100-400 meteors may radiate from the dim constellation of Camelopardalis near the North Star. This map shows the sky from the central U.S. Created with Stellarium
Quanzhi Ye and Paul Wiegert (University of Western Ontario) predict a weaker shower because of a decline in the comet’s dust production rate based on observations made during its last return in 2009. They estimate a rate of ~200 per hour.
On the bright side, their simulations show that the comet sheds larger particles than usual, which could mean a shower rich in fireballs. Other researchers predict rates between 200 and 40o per hour. At the very least, the Camelopardalids – the constellation from which the meteors will appear to originate – promise to rival the Perseids and Geminids, the year’s richest showers. Motivation for setting the alarm clock if there ever was.
Comet 209P/LINEAR on April 14, 2014. It’s currently very faint at around magnitude +17. Material shed by the comet during passes from 1898-1919 is expected to contribute to a May 23-24 shower. Credit: Ernesto Guido, Nick Howes, Martino Nicolini
Comet 209P/LINEAR, discovered in Feb. 2004 by the automated Lincoln Laboratory Near-Earth Asteroid Research (LINEAR) sky survey, orbits the sun every 5.04 years with an aphelion (most distant point from the sun) near Jupiter. In 2012, during a relatively close pass of that planet, Jupiter perturbed its orbit, bringing it to within 280,000 miles (450,000 km) of Earth’s orbit.
That set up a remarkably close encounter with our planet on May 29 when 209P will cruise just 5 million miles (8 million km) from Earth to become the 9th closest comet ever observed. Multiple debris trails shed by the comet as long ago as the 18th century will intersect our planet’s path 5 days earlier, providing the material for the upcoming meteor shower/storm.
Shining meekly around magnitude +17 at the moment, 209P/LINEAR could brighten to magnitude +11 as it speeds from the Big Dipper south to Hydra during the latter half of May. Closer to the BIG night, we’ll provide helpful maps for you to track it down in your telescope. Cool to think that both the shower and its parent comet will be on display at the same time.
The shaded area shows where the shower will be visible on May 23-24. North of the red line, the moon (a thick crescent) will be up during shower maximum around 2:10 a.m. CDT. Credit: Mikhail Maslov
The shower’s expected to last only a few hours from about 12:40-3:50 a.m. CDT with the best viewing locations in the U.S. and southern half of Canada. This is where the radiant will be up in a dark sky at peak activity. A thick crescent moon rises around 3-3:30 a.m. but shouldn’t pose a glare problem.
Meteors from 209P/LINEAR are expected to be bright and slow with speeds around 40,000 mph compared to an average of 130,000 mph for the Perseids. Most shower meteoroids are minute specks of rock, but the Camelopardalids contain a significant number of particles larger than 1mm – big enough to spark fireballs.
The dark “finger” represents streams of dust and rocks left behind by 209P/LINEAR during passes made from 1803 to 1924. Earth is shown intersecting the debris on May 23-24, 2014. Credit: Dr. Jeremie Vaubaillon
The farther north you live in the shaded area on the map, the higher the radiant stands in the northern sky and the more meteors you’re likely to see. Skywatchers living in the Deep South will see fewer shooting stars, but a greater proportion will be earthgrazers, those special meteors that skim the upper atmosphere and flare for an unusually long time before fading out.
To see the shower at its best, find a dark place with an open view to the north. Plan your viewing between 12:30 and 4 a.m. CDT (May 24), keeping the 2 a.m. forecast peak in mind. Maximum activity occurs around 3 a.m. Eastern, 1 a.m. Mountain and midnight Pacific time.
No one’s really certain how many meteors will show, but I encourage you to make the effort to see what could be a spectacular show.
This artist’s impression of a supernova shows the layers of gas ejected prior to the final deathly explosion of a massive star. Credit: NASA/Swift/Skyworks Digital/Dana Berry
How can an exploding star appear far brighter than expected? This question vexed astronomers since the discovery of PS1-10afx, supernova that was about 30 times more luminous than other Type 1A supernovas. Astronomers have just confirmed in Science that it was likely due to well-known illusion in space.
The mirage is called a gravitational lens that happens when a huge object in the foreground (like a galaxy) bends the light of an object in the background. Astronomers use this trick all the time to spy on galaxies and even to map dark matter, the mysterious substance believed to make up most of the universe.
Check out some spectacular images below of the phenomenon in action.
Canada-France-Hawaii-Telescope (CFHT) image of the field before the supernova PS1-10afx. (Credit: Kavli IPMU / CFHT)Dark matter in the Bullet Cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;Hubble Space Telescope image shows Einstein ring of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. A. Bolton (UH/IfA) for SLACS and NASA/ESA.The image is made from HST data and shows the four lensed images of the dusty red quasar, connected by a gravitational arc of the quasar host galaxy. The lensing galaxy is seen in the centre, between the four lensed images. Credit: John McKean/HST Archive dataThe HST WFPC2 image of gravitational lensing in the galaxy cluster Abell 2218, indicating the presence of large amount of dark matter (credit Andrew Fruchter at STScI).A picture of the object J1000+0221, which demonstrates the most distant gravitational lens ever discovered. This Hubble picture shows a normal galaxy’s center region (the glow in the picture), but the object is also aligned with a younger, star-creating galaxy that is in behind. The object in the foreground pulls light from the background galaxy with gravity — making rings of pictures. Credit: NASA/ESA/A. van der Wel
Comet C/2012 K1 PANSTARRS displays two tails in this excellent image taken with an 8-inch f/2.8 telescope on April 20, 2014 from Austria. The shorter, brighter spike is the dust tail; the longer is the ion tail with distinct kinks caused by interactions with the solar wind.
Comets often play hard to get. That’s why we enjoy those rare opportunities when they pass close to naked eye stars. For a change, they’re easy to find! That’s exactly what happens in the coming nights when the moderately bright comet C/2012 K1 PANSTARRS slides past the end of the Big Dipper’s handle. I hope Rolando Ligustri’s beautiful photo, above, entices you roll out your telescope for a look.
Comet K1 PANSTARRS glides from northern Bootes up the handle of the Big Dipper this coming week not far from the bright star Alkaid and M51, the Whirlpool Galaxy. This map shows the sky facing east (west is at top, east at bottom) with stars to magnitude +11 and the comet’s position at 10 p.m. CDT daily. Click to enlarge and then print out a copy you can use at the telescope. Created with Chris Marriott’s SkyMap software
If you’ve put off viewing this fuzzball because it’s been lost in the wilds of northern Bootes too long, hesitate no more. I saw it several nights ago through a 15-inch (37-cm) scope and can report a teardrop-shaped coma with a bright, not-quite-stellar nucleus. The comet sports an 8 arc minute long faint tail (1/4 the diameter of the full moon) and glows around magnitude 9-9.5. Granted I observed from dark skies, but K1 PANSTARRS could even be seen faintly in the 10×50 finderscope, putting it within range of ordinary binoculars.
Use this map to get oriented. It shows the sky facing east around 10 o’clock in late April. The comet passes very near Alkaid on April 28-29. Stellarium
Ligustri’s photo shows both gas and dust tails, but most observers will probably pick up the dust tail and strain to see the other. The comet has been moving north and slowly waxing in brightness all winter and spring. Right now, it’s ideally placed for viewing in the early evening sky and remains up all night for northern hemisphere observers. On Monday and Tuesday April 28-29 it’s within 1 degree of Alkaid, the bright star at the end of the Dipper’s handle.
C/2012 K1 PANSTARRS, discovered in May 2012 with the Pan-STARRS 1 telescope from Hawaii, has been under observation a long time. Here on Sept. 13, 2013 it was a still a small, dim object of magnitude ~+13. Credit: Carl Hergenrother
In a 6-inch (15-cm) scope, expect to see a faint puff with a brighter core; observers with 8-inch and larger telescopes will more easily see the tail. K1 PANSTARRS continues to brighten through the spring and summer as it saunters from the Great Bear into Leo. In late July it will be too near the sun to view but re-emerge a month later in Hydra in the morning sky. Southern hemisphere skywatchers will be favored during the fall and early winter, though the comet will continue to hover very low in the southern morning sky for northerners. Predictions call for the PANSTARRS to reach peak brightness around magnitude +6 to +7 in mid-October.
Sounds like old C/2012 K1 will be around a good, long time. Why not get acquainted?
The zodiacal light (left) reaches up from the eastern horizon to "touch" the Milky Way at right on Sept. 23, 2012. Credit: Bob King
Sunrise over the surface of the moon: a series of star tracker images taken by LADEE Saturday, April 12. The lunar horizon is ahead, a few minutes before orbital sunrise. Image Credit: NASA Ames.
[/caption]
NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) literally ‘saw the light’ just days before crashing into the lunar farside last Thursday April 17. Skimming just a few kilometers above the moon’s surface, mission controllers took advantage of this unique low angle to gaze out over the moon’s horizon in complete darkness much like the Apollo astronauts did from lunar orbit more than 40 years ago.
The zodiacal light (left), adorned by the planet Venus, reaches up from the eastern horizon to “touch” the Milky Way before sunrise on Sept. 23, 2012. Credit: Bob King
With the glow of Earth well-hidden, any dust in the moon’s scant atmosphere around the time of orbital sunrise should become visible. Scientists also expected to see the softly luminous glow of the zodiacal light, an extensive cloud of comet and asteroid dust concentrated in the flat plane of the solar system. The zodiacal light gets its name from the zodiac, that familiar band of constellations the planets pass through as they orbit the sun. Back on Earth, the zodiacal light looks like a big thumb of light standing up from the western horizon a couple hours after sunset in spring and before sunrise in fall.
Frame from LADEE’s star tracker camera showing the zodiacal light rising on the moon’s horizon from its extremely low orbit on April 12, 2014. Credit: NASA
So what did LADEE see? As you watch the animation above, comprised of images taken from darkness until sunrise, you’ll see a yellow haze on the horizon that expands into large diffuse glow tilted slightly to the right. This is the zodiacal light along with a smaller measure of light coming from sun’s outer atmosphere or corona. Together they’re referred to as CZL or ‘coronal and zodiacal light’. At the very end, the sun peaks over the lunar horizon.
This is a sketch of the lunar sunrise seen from orbit by Apollo 17 astronaut Eugene Cernan. On the right, the sketches are highlighted in color to show the sources of the scattered light: red indicates coronal and zodiacal light (CZL), blue is a glow along the lunar horizon possibly caused by dust suspended in the moon’s exosphere, and green may be crepuscular rays formed by shadowing and scattered light. Credit: NASA/phys.org
What appears to be missing from the pictures are the mysterious rays seen by some of the Apollo astronauts. The rays, neatly sketched by astronaut Eugene Cernan of Apollo 17, look a lot like those beams of light and shadow streaming though holes in clouds called crepuscular rays.
Crepuscular rays form a crown of rays made of alternating shadows and light scattered by dust and moisture in the atmosphere. Credit: Bob King
Only thing is, Earth’s atmosphere is thick enough for cloud beams. The dust in the moon’s atmosphere appears much too thin to cause the same phenomenon. And yet the astronauts saw rays as if sunlight streamed between mountain peaks and scattered off the dust just like home.
Low level horizon glow photos on the moon captured by the Surveyor 7 unmanned lander in 1968. It’s possible this low altitude glow is caused by larger dust particles that don’t travel as high as the smaller motes. Credit: NASA
It’s believed that dust gets lofted into the spare lunar atmosphere via electricity. Ultraviolet light from the sun knocks electrons from atoms in moon dust, giving them a positive charge. Since like charges repel, bits of dust push away from one another and move in the direction of least resistance: up. The smaller the dust particle, the higher it rises until dropping back down to the surface. Perhaps these “fountains” of lunar dust illuminated by the sun are what the astronauts recorded.
Unlike Cernan, LADEE saw only the expected coronal and zodiacal light but no rays. Scientists plan to look more closely at several sequences of images made of lunar sunrise in hopes of finding them.
Shown here are the actual B-mode polarization patterns provided by the BICEP2 Telescope. Image Credit: Harvard-Smithsonian Center for Astrophysics
Last month astronomers provided evidence that the universe underwent a brief but stupendous expansion at the very beginning of time. It was a landmark discovery. And while the media worldwide gleamed with fantastical headlines, I’m left overwhelmed with the feeling that it didn’t quite get the spotlight it deserves.
The day of the announcement was ablaze with excitement. When I first started to cover the news, I told my mother I was writing on something that was bigger than the Higgs boson. That was the best way I could explain the significance of this monumental discovery to someone with very little physics knowledge in a text message.
But inflation didn’t get the same hype as the Higgs. Why?
Scientific results are mostly tangible. The Higgs boson was created in a 27-kilometer ring of superconducting magnets designed to boost the energy of particles — marking the world’s largest and most powerful particle accelerator. There’s something about this experiment that we can wrap our minds around, even when the particle itself remains elusive. The $10 billion effort has 6,000 researchers working hard to control the system.
But we can’t control the universe. We can’t ask two galaxies to collide; We can’t speed up stellar evolution; And we can’t pull a nearby star a little closer to take a peek at its circling exoplanet. Instead we stand on our cosmic platform and wait for the light from various happenings to reach us. Once it does, we dig through that light — collecting photons in different filters or spreading them across a spectrum of wavelengths — reaping every last bit of knowledge possible.
Astronomical research is complex and abstract. But it’s what we love about it.
The vast cosmic arena — with its unimaginable vistas of time and space — is laid out in the small specks of light on the celestial sphere. By collecting this light we have placed ourselves within the cosmos. We know the universe began with the Big Bang nearly 13.8 billion years ago. We know that dark matter binds massive galaxy clusters together and that dark energy is causing the universe to accelerate rapidly. It’s truly phenomenal that so much can be learned from the study of light.
Still, there’s a fundamental difference between observing the direct light emitted from distant stars and galaxies and observing a slight polarization pattern on the cosmic microwave background — the radiation released 380,000 years after the Big Bang when photons were able to travel freely across the cosmos.
The result threw open a new window on the birth of the universe. To be more precise, it let us peer back at the moment that took place a mere trillionth of a trillionth of a trillionth of a second after the Big Bang. But it takes so many steps (potentially as many steps as seconds from then until now) to grasp this hazy and mind-boggling concept. It will stretch your ideas of space and time to their limits.
Not only does this result succeed in showing the universe in its infancy, explaining the origin of cosmic structure and providing evidence for the last untested prediction of Albert Einstein’s General Theory of Relativity (gravity comes in discrete packets like light), but it makes an even wilder prediction.
The model likely produces not just one universe, but rather an ensemble of universes: an endless series of big bangs that continue to pop up eternally. Our universe may just be one bubble out of a vast cosmic ocean of others.
Astronomy is moving further toward the abstract. Both in how we collect data and the scientific results we carefully and slowly unearth from that data.
I find this awe-inspiring. But while astronomers are finding ingenious and creative methods to further understand the phenomenal universe in which we live, science journalists and educators are going to have to follow suit. We need to act not as translators but as guides who map scientific knowledge, finding paths through vast amounts of abstract information and analyzing key points along the way. Only then will inflation trump the Higgs and the abstract become tangible.
But honestly I’m still ruminating on this question so all additional thoughts are welcome.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has at least 150 moons and moonlets in total, though only 62 have confirmed orbits and only 53 have been given official names.
Most of these moons are small, icy bodies that are little more than parts of its impressive ring system. In fact, 34 of the moons that have been named are less than 10 km in diameter while another 14 are 10 to 50 km in diameter. However, some of its inner and outer moons are among the largest and most dramatic in the Solar System, measuring between 250 and 5000 km in diameter and housing some of the greatest mysteries in the Solar System.
Saturn’s moons have such a variety of environments between them that you’d be forgiven for wanting to spend an entire mission just looking at its satellites. From the orange and hazy Titan to the icy plumes emanating from Enceladus, studying Saturn’s system gives us plenty of things to think about. Not only that, the moon discoveries keep on coming. As of April 2014, there are 62 known satellites of Saturn (excluding its spectacular rings, of course). Fifty-three of those worlds are named.
The Cassini spacecraft observes three of Saturn’s moons set against the darkened night side of the planet. Credit: NASA/JPL/Space Science Institute
Discovery and Naming
Prior to the invention of telescopic photography, eight of Saturn’s moons were observed using simple telescopes. The first to be discovered was Titan, Saturn’s largest moon, which was observed by Christiaan Huygens in 1655 using a telescope of his own design. Between 1671 and 1684, Giovanni Domenico Cassini discovered the moons of Tethys, Dione, Rhea, and Iapetus – which he collectively named the “Sider Lodoicea” (Latin for “Louisian Stars”, after King Louis XIV of France).
n 1789, William Herschel discovered Mimas and Enceladus, while father-and-son astronomers W.C Bond and G.P. Bond discovered Hyperion in 1848 – which was independently discovered by William Lassell that same year. By the end of the 19th century, the invention of long-exposure photographic plates allowed for the discovery of more moons – the first of which Phoebe, observed in 1899 by W.H. Pickering.
In 1966, the tenth satellite of Saturn was discovered by French astronomer Audouin Dollfus, which was later named Janus. A few years later, it was realized that his observations could only be explained if another satellite had been present with an orbit similar to that of Janus. This eleventh moon was later named Epimetheus, which shares the same orbit as Janus and is the only known co-orbital in the Solar System.
Collage of Saturn and its largest moons. Credit: NASA/JPL/SSI
By 1980, three additional moons were discovered and later confirmed by the Voyager probes. They were the trojan moons (see below) of Helene (which orbits Dione) as well as Telesto and Calypso (which orbit Tethys).
The study of the outer planets has since been revolutionized by the use of unmanned space probes. This began with the arrival of the Voyager spacecraft to the Cronian system in 1980-81, which resulted in the discovery of three additional moons – Atlas, Prometheus, and Pandora – bringing the total to 17. By 1990, archived images also revealed the existence of Pan.
This was followed by the Cassini-Huygens mission, which arrived at Saturn in the summer of 2004. Initially, Cassini discovered three small inner moons, including Methone and Pallene between Mimas and Enceladus, as well as the second Lagrangian moon of Dione – Polydeuces. In November of 2004,Cassini scientists announced that several more moons must be orbiting within Saturn’s rings. From this data, multiple moonlets and the moons of Daphnis and Anthe have been confirmed.
The study of Saturn’s moons has also been aided by the introduction of digital charge-coupled devices, which replaced photographic plates by the end of the 20th century. Because of this, ground-based telescopes have begun to discover several new irregular moons around Saturn. In 2000, three medium-sized telescopes found thirteen new moons with eccentric orbits that were of considerable distance from the planet.
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. In 2006, astronomers using Japan’s Subaru Telescope at Mauna Kea reported the discovery of nine more irregular moons. In April of 2007, Tarqeq (S/2007 S 1) was announced, and in May of that same year, S/2007 S 2 and S/2007 S 3 were reported.
The modern names of Saturn’s moons were suggested by John Herschel (William Herschel’s son) in 1847. In keeping with the nomenclature of the other planets, he proposed they be named after mythological figures associated with the Roman god of agriculture and harvest – Saturn, the equivalent of the Greek Cronus. In particular, the seven known satellites were named after Titans, Titanesses and Giants – the brothers and sisters of Cronus.
In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. When in the 20th century, the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology, or giants from other mythologies. All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and Norse ice giants.
Saturn’s Inner Large Moons
Saturn’s moons are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
Saturn’s moon of Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s Inner Large Moons, which orbit within the E Ring (see below), include the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Some people jokingly call Mimas the “Death Star” moon because of the crater on its surface that resembles the machine from the Star Wars universe. The 140 km (88 mi) Herschel Crater is about a third the diameter of the moon itself and could have created fractures (chasmata) on the moon’s opposing side. There are in fact craters throughout the moon’s small surface, making it among the most pockmarked in the Solar System.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 kg, and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved, and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically releases plumes of water ice, gas, and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath its icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core.
Dione’s trailing hemisphere, showing the patches of “whispy terrain”. Credit: NASA/JPL
The moon has at least five different kinds of terrain, a “young” geological surface of less than 100 million years. With a geometrical albedo of more than 140%, which is due to it being composed largely of water ice, Enceladus is one of the brightest known objects in the Solar System.
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep, and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
It’s covered in canyons, crackings, craters, and is coated from dust in the E-ring that originally came from Enceladus. The location of this dust has led astronomers to theorize that the moon was spun about 180 degrees from its original disposition in the past, perhaps due to a large impact.
Saturn’s Large Outer Moons:
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice, and rock. Of these, Rhea is the second-largest – measuring 1,527 km in diameter and 23×1020 kg in mass – and the ninth-largest moon in the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons and takes 4.5 days to complete an orbit.
Views of Saturn’s moon Rhea. Credit: NASA/JPL/Space Science Institute
Like other Cronian satellites, Rhea has a rather heavily cratered surface and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and the Inktomi crater – which measure about 400 and 50 km across, respectively.
Rhea has at least two major sections, the first being bright craters with craters larger than 40 km (25 miles), and a second section with smaller craters. The difference in these features is believed to be evidence of a major resurfacing event at some time in Rhea’s past.
At 5150 km in diameter and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryovolcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System besides Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
Image of Titan’s taken by the Cassini spacecraft, showing light passing through the periphery of the moon’s atmosphere. Credit: NASA/JPL-Caltech/Space Science Institute
Titan is also distinguished for being the only Cronian moon that has ever had a probe land on it. This was the Huygens lander, which was carried to the hazy world by the Cassini spacecraft. Titan’s “Earth-like processes” and thick atmosphere are among the things that make this world stand out to scientists, which include its ethane and methane rains from the atmosphere and flows on the surface.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
The two sides of Iapetus, Saturn’s “yin-yang moon”. Credit: NASA/JPL
Saturn’s Irregular Moons:
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group consists of four prograde outer moons that are named after characters in Gallic mythology – Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second-largest, Ymir, measures 18 km across.
Saturn’s rings and moons Credit: NASA
Within the Inner and Outer Large Moons, there are also those belonging to the Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn. Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Moon Formation:
It is thought that Saturn’s moon of Titan, its mid-sized moons and rings developed in a way that is closer to the Galilean moons of Jupiter. In short, this would mean that the regular moons formed from a circumplanetary disc, a ring of accreting gas, and solid debris similar to a protoplanetary disc. Meanwhile, the outer, irregular moons are believed to have been objects that were captured by Saturn’s gravity and remained in distant orbits.
However, there are some variations to this theory. In one alternative scenario, two Titan-sized moons were formed from an accretion disc around Saturn; the second one eventually broke up to produce the rings and inner mid-sized moons. In another, two large moons fused together to form Titan, and the collision scattered icy debris that formed to create the mid-sized moons.
However, the mechanics of how the moon formed remains a mystery for the time being. With additional missions mounted to study the atmospheres, compositions, and surfaces of these moons, we may begin to understand where they truly came from.
Much like Jupiter, and all the other gas giants, Saturn’s system of satellites is extensive as it is impressive. In addition to the larger moons that are believed to have formed from a massive debris field that once orbited it, it also has countless smaller satellites that were captured by its gravitational field over the course of billions of years. One can only imagine how many more remain to be found orbiting the ringed giant.
Saturn as imaged from Aguadilla, Puerto Rico on April 15th. Credit: Efrain Morales.
Planet lovers can rejoice: one of the finest jewels of the solar system in returning to the evening night sky.
The planet Saturn reaches opposition next month on May 10th. This means that as the Sun sets to the west, Saturn will rise “opposite” to it in the east, remaining well positioned for observation in the early evening hours throughout the summer season. In fact, we’ll have four of the five naked eye planets above the horizon at once for our evening viewing pleasure in the month of May, as Jupiter also rides high to the west at sunset, Mars just passed opposition last month and Mercury reaches greatest eastern elongation on May 25th. Venus is the solitary holdout, spending a majority of 2014 in the dawn sky.
Saturn will shine at magnitude +0.3 this month and its disk spans an apparent 19,” or 44” if you take into account the apparent width of its rings. The rings are currently tipped open 22 degrees with respect to our line of sight. The ring opening is widening, and will reach a maximum of over 25 degrees in 2017 before the trend reverses. Anyone who remembers observing Saturn back in 2009 will recall that its rings were edge on to our view. This widening of Saturn’s rings also lends itself to a curious effect: although we’re in a cycle of oppositions that are getting farther away — Saturn is 12.5 million kilometres or 0.083 Astronomical Units (A.U.s) more distant in 2014 than it was during opposition last year as it’s headed towards aphelion in 2018 — its widening rings are actually making it appear a bit brighter.
The path of Saturn through the constellation Libra from April through October 2014. Created using Starry Night Education Software.
This year’s opposition will find Saturn in the astronomical constellation of Libra, where it’ll spend most of 2014. Oppositions of the ringed planet are set to continue to “head south” until 2018, and won’t occur north of the celestial equator again until 2026. I remember when oppositions of Saturn returned to the constellation Virgo a few years back — where I had first looked at it with my 60mm Jason refractor as a teenager — and realizing that I had now been into observational astronomy for roughly one “Saturnian year.”
The ancients had little knowledge of how unique Saturn was. The faintest and slowest moving of the classical planets, even Galileo knew that something was up when he turned his first primitive telescope towards it. His sketches depict Saturn as something similar to a double handled coffee cup, a testament to how poor his view really was. It wouldn’t be until Christiaan Huygens in 1655 that the true nature of Saturn’s rings was deduced as a flat and separate feature from the disk.
At opposition, the disk of the planet casts a shadow straight back from our point of view. This vantage slowly changes as the planet moves towards eastern quadrature on August 9th and we get a glimpse slightly off to one side of the planet. After opposition, the shadow of the disk can again be seen casting back onto the rings.
An outstanding IPhone 4S capture of Saturn on April 20th, 2014. Credit: Andrew Symes, @FailedProtostar.
Another interesting phenomenon to watch out for near opposition is known as the Seeliger effect. Also sometimes referred to as the “opposition surge,” this sudden brightening of the disk and rings is a subtle effect, as the globe of Saturn and all of those tiny little ice crystals reach 100% illumination. This effect can be noted to the naked eye on successive nights around opposition, and will get more prominent towards 2017. Coherent-backscattering of light has also been proposed as a possible explanation of this phenomenon. Perhaps a video sequence capturing this effect is in order for skilled astro-imagers in 2014.
Through a small telescope, the first feature that becomes apparent is Saturn’s glorious system of rings. Crank up the magnification, and you’ll note a dark groove in the ring system. This is the Cassini Division, first described by Giovanni Cassini in 1675.
Here’s a challenge we came across some years back: can you see the disk of Saturn through the Cassini Division? Right around opposition is a good time to attempt this unusual feat of visual athletics.
A sample simulation depicting the orientation of Saturn’s observable moons on the night of May 9th. Created using Starry Night Education software.
Saturn’s large moon Titan is an easy catch at magnitude +8 in a small telescope. Titan is the second largest moon in the solar system. Place it in a direct orbit about the Sun, and it would be considered a planet, no problem. 7 of Saturn’s 62 known moons are within reach of a small telescope. In addition to Titan, they are, with quoted magnitudes: Mimas (+13), Enceladus (+12), Tethys (+10), Rhea (+10), Dione (+11) and Iapetus. Iapetus is of special interest, as it brightens from +11.9 to magnitude +10.2 as it traces out its 79 day orbit. We always knew there was something unique about this moon, and NASA’s Cassini mission revealed the world to have two distinctly different hemispheres with vastly different albedos during its close 2007 flyby.
The close passage of the Full Moon near Saturn on May 14th. Created using Stellarium.
Also, be sure to check out Saturn on the night of May 14th — just 4 nights after opposition — as the Full Moon sits less than a degree south of the ringed planet. Can you see both in the same telescopic field of view? Can you nab Saturn next to the rising daytime Moon low to the horizon just before local sunset? The Moon will actually occult (pass in front of) Saturn for viewers based in Australia and New Zealand on the 14th. This is only one of 11 occultations — nearly one for each lunation — of Saturn by the Moon in 2014. Unfortunately, the best one for North America occurs in the daytime on August 31st, though it too may be observable telescopically.
The footprint of the May 14th occultation of Saturn by the Moon. Credit: Occult 4.0.
Finally, this evening apparition of the planet runs through northern hemisphere summer and fall until Saturn reaches solar conjunction on November 18th. So get those homemade planetcams out, send those pics in to Universe Today, and be sure to join in to the Virtual Star Party every Sunday Night… Saturn is sure to be featured!
In our Solar System, astronomers often divide the planets into two groups — the inner planets and the outer planets. The inner planets are closer to the Sun and are smaller and rockier. The outer planets are further away, larger and made up mostly of gas.
The inner planets (in order of distance from the sun, closest to furthest) are Mercury, Venus, Earth and Mars. After an asteroid belt comes the outer planets, Jupiter, Saturn, Uranus and Neptune. The interesting thing is, in some other planetary systems discovered, the gas giants are actually quite close to the sun.
This makes predicting how our Solar System formed an interesting exercise for astronomers. Conventional wisdom is that the young Sun blew the gases into the outer fringes of the Solar System and that is why there are such large gas giants there. However, some extrasolar systems have “hot Jupiters” that orbit close to their Sun.
The Inner Planets:
The four inner planets are called terrestrial planets because their surfaces are solid (and, as the name implies, somewhat similar to Earth — although the term can be misleading because each of the four has vastly different environments). They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Below are brief descriptions of each of these planets based on this information from NASA.
Mercury: Mercury is the smallest planet in our Solar System and also the closest. It rotates slowly (59 Earth days) relative to the time it takes to rotate around the sun (88 days). The planet has no moons, but has a tenuous atmosphere (exosphere) containing oxygen, sodium, hydrogen, helium and potassium. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is currently orbiting the planet.
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
Venus: Venus was once considered a twin planet to Earth, until astronomers discovered its surface is at a lead-melting temperature of 900 degrees Fahrenheit (480 degrees Celsius). The planet is also a slow rotator, with a 243-day long Venusian day and an orbit around the sun at 225 days. Its atmosphere is thick and contains carbon dioxide and nitrogen. The planet has no rings or moons and is currently being visited by the European Space Agency’s Venus Express spacecraft.
Earth: Earth is the only planet with life as we know it, but astronomers have found some nearly Earth-sized planets outside of our solar system in what could be habitable regions of their respective stars. It contains an atmosphere of nitrogen and oxygen, and has one moon and no rings. Many spacecraft circle our planet to provide telecommunications, weather information and other services.
Mars: Mars is a planet under intense study because it shows signs of liquid water flowing on its surface in the ancient past. Today, however, its atmosphere is a wispy mix of carbon dioxide, nitrogen and argon. It has two tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours and it takes the planet about 687 Earth days to circle the Sun. There’s a small fleet of orbiters and rovers at Mars right now, including the large NASA Curiosity rover that landed in 2012.
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
The Outer Planets:
The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas. They all have rings and all of plenty of moons each. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers the solar system was bigger than previously thought. Below are brief descriptions of each of these planets based on this information from NASA.
Jupiter: Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot — a raging storm happening for the past 400 years at least (since we were able to view it through telescopes). NASA’s Juno spacecraft is en route and will visit there in 2016.
Saturn: Saturn is best known for its prominent ring system — seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years). Saturn is currently being visited by the Cassini spacecraft, which will fly closer to the planet’s rings in the coming years.
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.
Uranus: Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. There are no spacecraft slated to visit Uranus right now; the last visitor was Voyager 2 in 1986.
Neptune: Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. The only spacecraft to ever visit it was NASA’s Voyager 2 in 1989.
To learn more about the planets and missions, check out these links:
John Houbolt demonstrating Lunar Orbit Rendezvous circa 1962. Credit: NASA.
The space community lost a colossus of the of the Apollo era last week, when John Houbolt passed away last Tuesday just five days after his 95th birthday.
Perhaps the name isn’t as familiar to many as Armstrong or Von Braun, but John Houbolt was a pivotal figure in getting us to the Moon.
Born in Altoona, Iowa on April 10th, 1919, Houbolt spent most of his youth in Joliet, Illinois. He earned a Masters degree in Civil Engineering from the University of Illinois at Urbana-Champaign in 1942 and a PhD in Technical Sciences from ETH Zurich in Switzerland in 1957. But before that, he would become a member of the National Advisory Committee for Aeronautics (NACA) in 1942, an organization that would later become the National Aeronautics and Space Administration or NASA in 1958.
It was 1961 when Houbolt made what would be his most enduring mark on the space program. He was working as an engineer at the Langley Research Center, at a time when NASA and the United States seriously needed a win in the space race. The U.S.S.R. had enjoyed a long string of firsts, including first satellite in orbit (Sputnik 1, October 1957), first spacecraft to photograph the lunar farside (Luna 3 in October 1959) and first human in space with the launch of Yuri Gagarin aboard Vostok 1 in April 1961. A young President Kennedy would make his now famous “We choose to go to the Moon…” speech at Rice University later the next year in late 1962. Keep in mind, in U.S. astronaut John Glenn had just made his first orbital flight months before Kennedy’s speech, and total accumulated human time in space could be measured in mere hours. Unmanned Ranger spacecraft were having a tough time even getting off of the pad, and managing to crash a space probe into the Moon was considered to be a “success”. The task of sending humans “by the end of this decade” was a daunting one indeed…
NASA would soon have a mandate to sent humans to the Moon: but how could they pull it off?
Early ideas for manned lunar missions envisioned a single gigantic rocket that would head to the Moon and land, Buck Rodgers style, “fins first.” Such a rocket would have to be enormous, and carry the fuel to escape Earth’s gravity well, land and launch from the Moon, and return to Earth.
A second approach, known as Earth-orbit rendezvous, would see several launches assemble a mission in low Earth orbit and then head to the Moon. Curiously, though this was an early idea, it was never used in Apollo, though it was briefly resurrected during the now defunct Constellation Program.
Three plans to go to the Moon. Credit: NASA.
But it was a third option that intrigued Houbolt, known as Lunar Orbit Rendezvous. LOR had been proposed by rocket pioneers Yuri Kondratyuk and Hermann Oberth in 1923, but had never been seriously considered. It called for astronauts to depart the Earth in a large rocket, and instead, use a small lander designed only to land and launch from the Moon while the spacecraft for Earth return orbited overhead.
Houbolt became a staunch advocate for the idea, and spent over a year convincing NASA officials. In one famous letter to NASA associate administrator Robert Seamans, Houbolt was known to have remarked “Do we want to go to the Moon or not?”
It’s interesting to note that it was probably only in a young organization like the NASA of the early 1960s that, in Houbolt’s own words, a “voice in in the wilderness” could be heard. Had NASA become a military run organization — as many advocated for in the 1950s — a rigid chain of command could have meant that such brash ideas as Houbolt’s would have never seen the light of day. Thank scientists such as James Van Allen for promoting the idea of a civilian space program that we take for granted today.
Even then, selling LOR wasn’t easy. The idea looked preposterous: astronauts would have to learn how to undock and dock while orbiting a distant world, with no chance of rescue. There was no second chance, no backup option. Early plans called for an EVA for astronauts to enter the Lunar Module prior to descent which were later scrapped in favor of extracting it from atop the third stage and boarding internally before reaching the Moon.
Once Houbolt had sold key visionaries such as Wernher von Braun on the idea in late 1962, LOR became the way we would go to the Moon. And although Houbolt’s estimations of the mass required for the Lunar Module were off by a factor of three, the story is now the stuff of early Apollo era legend. You can see Houbolt (played by Reed Birney) and the tale of the LM and LOR in the From Earth to the Moon episode 5 entitled “Spider”.
The ascent stage of the lunar module on approach to the command module with the Earth in the background. Credit: NASA.
Houbolt was awarded NASA’s medal for Exceptional Scientific Achievement in 1963, and he was in Mission Control When Apollo 11 touched down in the Sea of Tranquility.
He passed away in a Scarborough, Maine nursing home last Tuesday, and joins other unsung visionaries of the early space program such as Mary Sherman Morgan. It’s sad to think that we may soon live in a world where those who not only walked on the Moon, but those who also sent us and knew how to get there, are no longer with us.
