Animation of Tethys passing in front of Dione from Cassini's point of view. (CLICK TO PLAY)
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Saturn’s moon Tethys passes in front of its slightly larger sister Dione in this animation made from 25 raw images acquired by Cassini on March 14, 2012. Pretty cool! (Click the image to play.)
Tethys and Dione (NASA/JPL/SSI)
Tethys and Dione are similar in diameter, being 1,062 kilometers (660 miles) wide and 1,123 kilometers (698 miles) wide, respectively. Both are heavily cratered, ice-rich worlds.
In this view, Tethys’ enormous Odysseus crater can be seen on its northern hemisphere. 400 km (250 miles) across, Odysseus is two-fifths the diameter of Tethys itself, suggesting that it was created early in the moon’s history when it was still partially molten — or else the impact would have shattered the moon apart entirely.
The more extensively-cratered trailing side of Dione is visible here, its signature “wispy lines” rotated out of view. Since it makes sense that a moon’s leading face should be more heavily cratered, it’s thought that Dione has been spun around by an impact event in the distant past.
If you look closely, a slight rotation in Tethys can also be discerned from the first frame to the last.
Color-composite image from Cassini raw data acquired on March 12, 2012. (NASA/JPL/SSI/J. Major)
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Little Enceladus and enormous Titan are seen on either side of Saturn’s rings in this image, a color-composite made from raw images acquired by Cassini on March 12, 2012. The original images were taken in red, green and blue color channels, and with a little Photoshop editing I combined them into a roughly true-color view of what Cassini saw as it passed within 1,045,591 km of Enceladus.
It may be one of the best images from Cassini yet this year! Cloud-covered Titan and tiny Prometheus (can you see it just above the rings on the right?) are literally dwarfed by their parent Saturn in an image captured on Jan. 5, 2012.
Prometheus’ pinpoint shadow can also be seen on Saturn’s cloud tops, just inside the thin, outermost F ring shadow at bottom left.
The two moons themselves couldn’t be more different; Titan, 3,200 miles (5,150 km) wide, is wrapped in a nitrogen and methane atmosphere ten times thicker than Earth’s and is covered with vast plains of dark hydrocarbon dunes and crisscrossed by rivers of liquid methane.
Prometheus imaged by Cassini in Dec. 2009.
Prometheus, on the other hand, is a potato-shaped shepherd moon 92 miles long and 53 miles wide (148 x 53 km) that orbits Saturn just inside the narrow, ropy F ring. While it doesn’t have an atmosphere, it does create some impressive effects on the icy material in the ring!
Another moon, Pandora, casts its shadow onto Saturn just outside the F ring shadow at bottom center. 50 miles (80 km) wide, Pandora shepherds the outer edge of the F ring but is itself not visible in this image. Watch an animation here.
This image was featured on the Cassini Imaging Central Laboratory for Operations (CICLOPS) website on Feb. 28, 2012. The view looks toward the southern, unilluminated side of the rings from about 1 degree below the ringplane.
Image credit: NASA / JPL / Space Science Institute.
This radar image of asteroid 2005 YU55 was obtained on Nov. 7, 2011. Credit: NASA/JPL/Caltech.
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Look up in a clear night sky. How many moons do you see? Chances are, you’re only going to count to one. Admittedly, if you count any higher and you’re not alone, you may get some funny looks cast in your direction. But even though you may not be able to actually see them, there may very well be more moons out there orbiting our planet.
For the time being, anyway.
Today, Earth has one major moon in orbit around it. (Technically the Earth-Moon system orbits around a common center of gravity, called the barycenter, but that’s splitting hairs for the purpose of this story.) At one time Earth may have had two large moons until the smaller eventually collided into the larger, creating the rugged lump we now call the farside highlands. But, that was 4 billion years ago and again not what’s being referred to here.
Right now, at his moment, Earth may very well have more than the one moon we see in the night sky. Surprise.
Of course, it would be a very small moon. Perhaps no more than a meter across. But a moon nonetheless. And there could even be others – many others – much smaller than that. Little bits of solar system leftovers, orbiting our planet even farther out than the Moon we all know and love, coming and going in short-lived flings with Earth without anyone even knowing.
This is what has been suggested by researcher Mikael Granvik of the University of Helsinki in Finland. He and his colleagues have created computer simulations of asteroids believed to be occupying the inner solar system, and what the chances are that any number of them could be captured into Earth orbit at any given time.
Orbit of 2006 RH120, a confirmed TCO identified in 2006.
The team’s results, posted Dec. 20 in the science journal Icarus, claim it’s very likely that small asteroids would be temporarily captured into orbit (becoming TCOs, or temporarily captured objects) on a regular basis, each spending about nine months in up to three revolutions around Earth before heading off again.
Some objects, though, might hang around even longer… in the team’s simulations one TCO remained in orbit for 900 years.
“There are lots of asteroids in the solar system, so chances for the Earth to capture one at any time is, in a sense, not surprising,” said co-author Jeremie Vauballion, an astronomer at the Paris Observatory.
In fact, the team suspects that there’s most likely a TCO out there right now, perhaps a meter or so wide, orbiting between 5 and 10 times the distance between Earth and the Moon. And there could be a thousand smaller ones as well, up to 10 centimeters wide.
So if these moons are indeed out there, why don’t we know about them?
Put simply, they are too small, too far, and too dark.
At that distance an object the size of a writing desk is virtually undetectable with the instruments we have now.. especially if we don’t even know exactly where to look. But in the future the Large Synoptic Survey Telescope (LSST) may, once completed, be able to spot these tiny satellites with its 3200-megapixel camera.
Once spotted, TCOs could become targets of exploration. After all, they are asteroids that have come to us, which would make investigation all the easier – not to mention cheaper – much more so than traveling to and back from the main asteroid belt.
“The price of the mission would actually be pretty small,” Granvik said. And that, of course, makes the chances of such a mission getting approved all the better.
Read more on David Shiga’s article on New Scientist here.
Titan's thick atmosphere shines in backlight sunlight
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Made from one of the most recent Cassini images, this is a color-composite showing a backlit Titan with its dense, multi-layered atmosphere scattering sunlight in different colors. Titan’s atmosphere is made up of methane and complex hydrocarbons and is ten times as thick as Earth’s. It is the only moon in our solar system known to have a substantial atmosphere.
Titan’s high-level hydrocarbon haze is nicely visible as a pale blue band encircling the moon.
Color image of Titan and sister moon Dione, seen by Cassini on Dec. 10. (NASA/JPL/SSI and J. Major)
At 3,200 (5,150 km) miles wide, Titan is one of the largest moons in the solar system – even larger than Mercury. Its thick atmosphere keeps a frigid and gloomy surface permanently hidden beneath opaque clouds of methane and hydrocarbons.
This image was made from three raw images acquired by Cassini on December 13. The raw images were in the red, green and blue visible light channels, and so the composited image you see here approximates true color.
This particular flyby of Titan (designated T-79) gave Cassini’s instruments a chance to examine Titan in many different wavelengths, as well as map its surface and measure its atmospheric temperature. Cassini passed by the giant moon at a distance of about 2,228 miles (3,586 kilometers) traveling 13,000 mph (5.8 km/sec). Read more on the flyby page here.
Credit: NASA / JPL / Space Science Institute. Edited by Jason Major.
See more color-composite images of Titan and other moons of Saturn on my Flickr set here.
Enceladus and Epimetheus as seen by Cassini on October 1, 2011.
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Taken during the Cassini spacecraft’s October 1 flyby of Saturn’s ice-spewing moon, this image – released today – shows a crescent-lit Enceladus with southern geysers in action… and the much smaller Epimetheus peeking out from behind!
Epimetheus
The 70-mile (113-km) -wide Epimetheus is dwarfed by its larger sibling Enceladus, which is 313 miles (504 km) in diameter… about the width of the state of Arizona.
One of the most reflective objects in the solar system, Enceladus appears to be casting some reflected light onto Epimetheus as well. (Image processors at the Cassini Imaging Lab have brightened the moons by a factor of 1.8 relative to the rings in order to bring out detail.)
Some bright clumps of material can also be seen orbiting within Saturn’s rings at upper left, possibly stirred up by the movement of the shepherd moon Pan.
An artist impression of an exomoon orbiting an exoplanet, could the exoplanet's wobble help astronomers? (Andy McLatchie)
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It wasn’t that long ago that astronomers began discovering the first planets around other stars. But as the field of exoplanetary astronomy explodes, astronomers have begun looking to the future and considering the possibility of detecting moons around these planets. Surprisingly, the potential for doing so may not be that far off.
Before exploring how we might detect satellites of distant planets, astronomers must first attempt to get an understanding of what they may be looking for. Fortunately, this question ties in well with the rapidly developing understanding of how solar systems form.
In general, there are three mechanisms by which planets may obtain satellites. The simplest is for them to simply form together from a single accretion disk. Another is that a massive impact may knock material off of a planet which forms into a satellite as astronomers believe happened with our own Moon. Some estimates have indicated that such impacts should be frequent and as many as 1 in 12 Earth like planets may have formed moons in this way. Lastly, a satellite may be a captured asteroid or comet as is likely for many of the moons of Jupiter and Saturn.
Each of these cases produces a different range of masses. Captured bodies are likely to be the smallest and therefor are unlikely to be detectable in the near future. Impact generated moons are expected to only be able to form bodies with 4% of the total mass of the planet and as such, are rather limited as well. The largest moons are thought to form in the disks around forming Jupiter like planets. These are the most likely to be detectable.
The first method by which astronomers may detect such moons is by the changes they would make in the wobble of the star that has been used to detect many extrasolar planets to date. Astronomers have already studied how a pair of binary stars may affect a binary star system may have on a third star it orbits. If the binary star is swapped out for a planet and a moon it turns out that the easiest systems to detect are massive moons that are distant from the planet, but close to the parent star. However, except in extreme cases, the amount of wobble that the pair could induce in the star is so small that it would be swamped by the convective motion of the star’s surface, making detection through this method nearly impossible.
Astronomers have begun detecting large numbers of exoplanets by transits, where the planet causes minor eclipses. Could astronomers also detect the presence of moons this way? In this case, the limit on detection would again be based on the size of the moon. Currently, the Kepler satellite is expected to detect planets similar in mass to Earth. If moons exist around a super-Jovian planet that are also similar in size to Earth, they too should be detected. However, forming moons this large is difficult. The largest moon in the solar system in Ganymede which is 40% of the diameter of Earth, putting it modestly below current detection thresholds, but potentially in reach of future exoplanet missions.
However, direct detection of the eclipses caused by transits isn’t the only way transits could be used to discover exomoons. In the past few years, astronomers have begun using the wobble of other planets on the ones they had already discovered to infer the existence of other planets in the system in the same way the gravitational tug of Neptune on Uranus allowed astronomers to predict Neptune’s existence before it was discovered. A sufficiently massive moon could cause detectable variations in when the transit of the planet would begin and end. Astronomers have already used this technique to place limits on the mass of potential moons around exoplanets HD 209458 and OGLE-TR-113b at 3 and 7 Earth masses respectively.
The first discovered exoplanet was discovered around a pulsar. The tug of this planet caused variation of the regular pulsation of the pulsar’s beat. Pulsars often beat hundreds to thousands of times per second and as such, are extremely sensitive indicators of the presence of planets. The pulsar PSR B1257+12 is known to harbor one planet that is a mere 0.04% the mass of Earth, which is well below the mass threshold of many moons. As such, variations in these systems, caused by moons would be potentially detectable with current technology. Astronomers have already used it to search for moons around the planet orbiting PSR B1620-26 and ruled out moons more than 12% the mass of Jupiter within half an Astronomical Unit (the distance between the Earth and Sun or 93 million miles) of the planet.
The last method by which astronomers have detected planets that could potentially be used for exomoons is direct observation. Since direct imaging of exoplanets has only become realized in the past few years, this option is likely still a ways off, but future missions like the Terrestrial Planet Finder Coronagraph may put it into the realm of possibility. Even if the moon is not fully resolved, the offset of the center of the dot of the pair may be detectable with current instruments.
Overall, if the explosion of knowledge on planetary systems continues, astronomers should be capable of detecting exomoons within the near future. The possibility already exists for some cases, like pulsar planets, but due to their rarity, the statistical likelihood of finding a planet with a sufficiently large moon is low. But as equipment continues to improve, making detection thresholds lower for various methods, the first exomoons should come into view. Undoubtedly, the first ones will be large. This will beg the question of what sorts of surfaces and potentially atmospheres they may have. In turn, this would inspire more questions about what life may exist.
Have you seen a very bright star rising in the East every night the past few months? If you’re a night owl, you may have noticed it moves across they sky from the East into the West, shining brightly throughout the night. However this object is not a star! It’s the planet Jupiter and it is the brightest object in the night sky at the moment, apart from the Moon.
At the end of October Jupiter will be at opposition. This means the mighty planet (the largest in our solar system) will be directly opposite the sun as seen from Earth and it will also be at its closest point to Earth in the two planets’ orbits around the Sun. This makes Jupiter or any other object at opposition appear brighter and larger. The opposition of Jupiter occurs on October 29, 2011.
But Jupiter has been gracing our night sky for several months, and will continue to shine brightly as it moves in and out of opposition. But enjoy the view now, as this will be the closest opposition until 2022!
Visually, even with the naked eye, Jupiter is stunning! A burning yellowish-white star-like object, many times brighter than any other stars.
But through a pair of ordinary binoculars or a small telescope, Jupiter comes to life. Not only is it possible to see the disc of the Planet, you can also see the four Galilean moons.
The Galilean moons, Callisto, Ganymede, Europa and Io were discovered by Galileo over 400 years ago and are amazing worlds in their own right.
Callisto is the outermost moon with a very ancient and heavily cratered surface. It is the second largest of the four moons, but does not interact tidally with an “orbital resonance” unlike the other three moons. Callisto. Image credit: NASA/JPL
Ganymede is the largest of the four moons and is also the largest moon in the Solar system, being larger than the Planet Mercury. The bizarre surface is a mix of two types of terrain – highly cratered dark regions and younger, but still ancient regions with a large array of grooves and ridges. Ganymede is the only moon in the solar system to have its own magnetosphere.
Ganymede Credit: NASA
Europa is the second closest moon and is also the smallest. It has one of the smoothest and newest surfaces in the solar system, being covered purely with ice. Europa is likely a water world and it is believed that below its icy surface, lies a deep moon-wide ocean surrounding a warm mantle. It is one of the most likely places to harbour life in the solar system.
Europa from Galileo
Io is the innermost of the four Galilean moons of Jupiter and third largest. It is the most geologically active body in the solar system with over 400 active volcanoes and an ever changing and hostile surface of sulphur and silicates.
Io Credit: NASA
When you look up tonight and stare at Jupiter, or you are looking at it through binoculars or a telescope, just think – Jupiter and the four Galilean moons are a very interesting place, almost a mini solar system with our larger solar system!
Occasionally you will see Jupiter’s “Great Red Spot” or the shadow of one of the moons on Jupiter’s surface. The Jupiter system is always changing.
If you want to find out what the positions are for the moons, use planetarium software such as Stellarium and then have a look yourself.
A quintet of Saturn's moons come together in the Cassini spacecraft's field of view for this portrait. From left to right: Janus, Pandora, Enceladus, Mimas and Rhea. Credit: NASA/JPL-Caltech/Space Science Institute
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Check out this gorgeous new portrait of a Saturnian moon quintet taken by Earths’ emissary – NASA’s Cassini Orbiter. The moons are majestically poised along a backdrop of Saturn’s rings, fit for an artist’s canvas.
Janus, Pandora, Enceladus, Mimas and Rhea are nearly lined up (from left to right) in this view acquired by Cassini at a distance of approximately 684,000 miles (1.1 million kilometers) from Rhea and 1.1 million miles (1.8 million kilometers) from Enceladus.
The newly released image was taken by Cassini’s narrow angle camera on July 29, 2011. Image scale is about 4 miles (7 kilometers) per pixel on Rhea and 7 miles (11 kilometers) per pixel on Enceladus.
Cassini will stage a close flyby of Enceledus – Satarn’s geyser spewing moon – in about two weeks, swooping within 99 km
Moon Facts from JPL:
Janus (179 kilometers, or 111 miles across) is on the far left. Pandora (81 kilometers, or 50 miles across) orbits between the A ring and the thin F ring near the middle of the image. Brightly reflective Enceladus (504 kilometers, or 313 miles across) appears above the center of the image. Saturn’s second largest moon, Rhea (1,528 kilometers, or 949 miles across), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometers, or 246 miles across) can be seen beyond Rhea also on the right side of the image.
This view looks toward the northern, sunlit side of the rings from just above the ring plane. Rhea is closest to Cassini here. The rings are beyond Rhea and Mimas. Enceladus is beyond the rings.
The simple graphic below shows dozens of Saturn’s moons – not to scale. So far 62 have been discovered and 53 have been officially named.
Saturn’s moons. Click on link below to learn more about each moon. Credit: NASA/JPL
Learn more about Saturn’s moons at this link
Phobos, one of the two natural satellites of Mars silhouetted against the Martian surface. Credit: ISRO
Mars Express images of Phobos from January 9, 2011 flyby
If someone were to ask you when fear was first discovered, you could tell them August 11, 1877. That’s when, 134 years ago today, Asaph Hall identified Phobos, the larger of Mars’ two moons. But even though it’s named after the Greek god of fear, there’s nothing to be afraid of…
