MSL Curiosity is busy investigating the surface of Mars, to see if that planet could have harbored life. Image: NASA/JPL/Cal-Tech
The recent announcement by NASA confirming the presence of liquid water on Mars pulls planetary protection into the spotlight and is causing some serious head-scratching in the scientific community. On the one hand, having existing liquid water on the Red Planet is a cause for wonder, excitement, and a strong desire to investigate it in a great deal more depth to look for the possibility of life. On the other hand, there is the dilemma of protecting a potential biosphere from contamination from Earthly bugs. As keen as the Curiosity mission team is to take advantage of the rover to have a much closer look at recurring slope lineae (RSL), the rover itself is just not clean enough. Continue reading “The Puzzle of Planetary Protection”
A relic of the early Space Age turns 44 years old this week.
The United Kingdom’s first and only successful space launch using a UK-built rocket is still visible in low Earth orbit today, if you know exact where and how to look for it.
Launched atop a 3-stage Black Arrow R3 rocket on October 28th, 1971 from the Woomera launch station in the Australian outback, Prospero (sometimes also referred to simply as the X-3) was designed to test key communications satellite technologies.
Prospero wasn’t the first satellite fielded by the United Kingdom–that credit goes to the Ariel 1 satellite launched atop a Thor DM-19 Delta rocket by the United States from Cape Canaveral on April 26th, 1962—but Prospero was notable as part of a program cut short in its early stages.
The Black Arrow project from which Prospero was born was cancelled shortly after the launch, making the X-3 the only successful mission fielded by the program (X-2 failed to achieve orbit due to an early shut-down of the stage 2 rocket). Prospero almost didn’t make it as well, as the final Waxwing stage hit the satellite upon deployment, taking one of Prospero’s four radio antennae clean off!
How to spot fainter satellites
Unlike watching for bright passes of naked eye objects in low Earth orbit such as the International Space Station, hunting for binocular satellites such as Prospero takes careful planning. Our tried and true technique is not unlike the method recently described on Universe Today to hunt for near Earth grazers such as the Halloween asteroid 2015 TB 145. In stakeout mode, you’ll need to know exactly when Prospero passes near a bright object, such as a star or planet.
Heavens-Above is a great resource, and catalogs every satellite back through the early Space Age. And what’s really nifty is that Heavens-Above will plot the passage of the satellite showing the timing of the pass against the sky against the background of constellations and stars for your specific location.
A screen capture of a satellite pass from Heavens-Above. Image credit: Chris Peat/Heavens Above.com
If you have Space-Track access, you can also download the TLEs (Two Line Elements) for a particular satellite for manual entry into a program such as Starry Night or Orbitron to forecast passes. You’ll be aiming your binoculars at the target star Project Moonwatch-style at the appointed time, and simply waiting for the satellite to drift by. For precise timing, we like to listen to WWV radio broadcasting the time (in Universal or Greenwich Mean/Zulu Time) out of Fort Collins Colorado on AM shortwave 5000, 10000, 15000 and 20000 Hz. WWV radio calls out the time at the top of each minute, with time ticks for each second, allowing the observer to keep eyes on the sky continuously. Just which WWV station comes in clearest can vary after sunset, as the ionosphere changes in terms of radio reflectivity at dusk.
The orbital trace of Prospero. Image Credit: Orbitron
We tracked down a good pass on the errant ‘space tool bag’ lost by International Space Station astros back in 2008 using this method once it was assigned an individual NORAD ID number… there it was, a lost tool satchel with a date with a fiery reentry destiny, drifting right by the bright star Spica at the appointed time.
Prospects for Prospero
Radio operators tracked Prospero for decades on transmission frequency 137.560 MHz until 2004, eight years past its official deactivation in 1996. As of this writing, there aren’t any official future attempts to contact Prospero in the works, though it’s certainly possible for a motivated party to do in theory… Prospero isn’t expected to reenter until 2070, and perhaps it’ll last until its centenary in space.
For latitudes 30-40 degrees north, good viewing prospects for Prospero start up again around December 20th of this year at dusk. At its brightest on a pass straight overhead through the observer’s zenith, expect Prospero to reach about +8 magnitude in brightness, well within range of binoculars. Prospero orbits Earth once every 103 minutes in a 527 by 1,304 kilometre orbit, inclined 82 degrees relative to the Earth’s equator. Prospero’s NORAD ID COSPAR designator is 1971-093A catalog number (05580).
Our favorite tool for satellite hunting… Image credit: Dave Dickinson
Other relics of the Space Age are also visible in backyard near you, including:
The Vanguards: launched in starting in 1958 by the United States, The three Vanguard satellites represent the oldest bits of human artifacts in Earth orbit, and they aren’t due for reentry for another two centuries.
Tracking relics of the Space Age brings home the personal relevance of early space history. Looking further out towards satellites in geostationary orbit, we are seeing artifacts that may long withstand the tests of time and become the solitary testaments of our current civilization to a far off future era.
-Got a favorite relic of the Space Age you’d like us to track down? Let us know!
Map showing TB145's approximate path starting at 4 hours UT on Oct. 31 (11 p.m. CDT Oct. 30). This view faces east. Tick marks show its hourly position. This map provides context for the detailed maps above. Credit: Chris Marriott's SkyMap
Trick or treat! I think we’re definitely in for a treat. 2015 TB145 will fly past Earth at a safe distance slightly farther than the moon’s orbit on Oct. 31 at 12:05 p.m. CDT (17:05 UT). Estimated at 1,300 feet (400-meters) across, this Great Pumpkin of an asteroid will be big enough and close enough to show in small telescopes.
Do I hear the doorbell ringing already?
Shining faintly at 18th magnitude on October 22, 2015 TB145 is already under study by amateur and professional astronomers. Its close approach will make for an excellent opportunity to learn a great deal about its surface properties and orbit. Watch for it to brighten up to magnitude +10.1 at peak, bright enough to see in a 4.5-inch telescope. Credit: Gianluca Masi
The close approach of such of TB145 will make for great science opportunities, too. Several optical observatories and the radar capabilities of the agency’s Deep Space Network at Goldstone, California will be tracking this flying mountain as will many amateur astronomers. The 110-foot (34-meter) Goldstone antenna will ping the asteroid with radio waves; the returning echoes will be collected by dishes in West Virginia and Puerto Rico and used to construct images showing the object’s surface features, shape and dimensions. NASA scientists hope to obtain radar images of the asteroid as fine as about 7 feet (2 meters) per pixel.
“The close approach of 2015 TB145 at about 1.3 times the distance of the moon’s orbit, coupled with its size, suggests it will be one of the best asteroids for radar imaging we’ll see for several years,” said Lance Benner, of JPL, who leads NASA’s asteroid radar research program. “We plan to test a new capability to obtain radar images with two-meter resolution for the first time and hope to see unprecedented levels of detail.”
View of the orbit of asteroid 2015 TB145. Its orbit is inclined about 39° to the plane of the Solar System. Credit: P. Chodas (NASA/JPL – Caltech)
Astronomers first nabbed asteroid 2015 TB145 on Oct. 10, 2015, using the University of Hawaii’s Pan-STARRS-1 (Panoramic Survey Telescope and Rapid Response System) telescope atop Mt. Haleakala in Maui. According to the catalog of near-Earth objectskept by the Minor Planet Center, this is the closest currently known approach by an object this large until asteroid 1999 AN10 (about 2,600 feet or 800-m in size) zips by at about 1 lunar distance in August 2027.
The gravitational influence of the asteroid is so small it will have no detectable effect on the Moon or anything here on Earth, including our planet’s tides or tectonic plates. But the planet will certainly have an effect on the asteroid. Earth’s gravity will deflect TB145’s path during the close approach, making it tricky this far out to create an accurate map of its flight across the sky. That’s why the two maps I’ve included with this article are only approximate. As we get closer to Halloween, further refinements in the asteroid’s orbit will allow for more accurate path-making.
TB145’s path starting at 4 hours UT on Oct. 31 (11 p.m. CDT Oct. 30). This view faces east. Tick marks show its hourly position. At the start of the path, the asteroid will shine around magnitude 11.4 but will gradually brighten through the night. To convert from UT, subtract 4 hours for EDT, 5 for CDT, 6 for MDT and 7 for PDT. Click for a large version. Credit: Chris Marriott’s SkyMap
Because the asteroid passes so near Earth, parallax will shift its path north or south up to 1/2°. Parallax is the apparent shift in an object’s position against the more distant background stars depending on the observer’s location on Earth. You can see how parallax works using your eyes and a finger. Stick your arm straight out in front of you and hold up your index finger. Open and close your right and then your left eye in a back and forth blinking pattern and watch your finger jump back and forth across the more distant background. Each eye sees the thumb from a slightly different perspective, causing it to shift position against the distant scene.
Graphic depicting the orbit of asteroid 2015 TB145. The asteroid will safely fly past Earth slightly farther out than the moon’s orbit on Halloween. Credit: P. Chodas (NASA/JPL – Caltech)
This happens all the time with the Moon. You might see it conjunct with a bright planet where skywatchers on the opposite side of the planet see an occultation. That’s why it’s best to make your own map of TB145’s wild ride across the sky. When closest to Earth, the asteroid will cover a Full Moon diameter about every 3 minutes as it tears by us at 22 miles per second (35 km/sec). Without a good map, it’ll get away from you.
Method #1: Using Stellarium
Download the free sky-plotting program Stellarium. Once you’ve set your location, either hit F2 or click on the Configuration icon in the lower left corner of your screen. Now select the Plugins tab then Solar System Editor. Click on Configure at the bottom of the tab, choose Solar System and click Import orbital elements in MPC format.
Next, select the Asteroids option and then from the bookmarks list, choose MPCORB: near-Earth asteroids (NEAs) and then Get orbital elements. Allow the list — a very large one — to load then scroll through it until you find 2015 TD145 and put a check mark in the box. Then click Add objects.
Stellarium view of the sky and featured asteroid seen from northern, Minnesota at 11:55 p.m. October 30, 2015. Notice that a bright, waning gibbous Moon will be nearby during the best viewing opportunities for the Americas, which will make 2015 TB145 a little harder to spot.
Still with me? OK, close the Solar System editor and press F3 or select the magnifying glass icon in the lower left corner of your screen, then type in the asteroid’s name exactly as 2015 TD145. Hit enter and you’ll see a set of rotating red crosshairs. Bingo! This where the asteroid will be at the time you chose. You can adjust your magnitude range, field of view and even download additional files of fainter stars and deep sky objects. Unfortunately, Stellarium can’t draw an arc showing TB145’s changing position with time. Cross your fingers that appears in the next iteration.
Method #2: Download up-to-date orbital elements into your sky-charting program
2015 TB145 belongs to the Apollo family of asteroids, whose orbits cross that of Earth. Amor asteroids approach but don’t cross, while Atens also cross Earth’s path but spend most of their time inside our orbit. Credit: ESA
Let’s say you already have a sky-charting program like Guide, Dance of the Planets, MegaStar or Starry Night. Go to the Minor Planet &Comet Ephemeris Serviceand type in 2015 TB145 in the big, blank box. Next, scroll down and select your program from the list and click on Get Ephemerides/HTML page. Save the file of orbital elements that pops up and place into the appropriate folder in your program. Open your program, select 2015 TB145 and make a chart!
Method #3: Manually input orbital elements into your program
You can also go to JPL’s Horizons site for the very latest orbital elements you can manually input in your program. 2015 TB145 is expected to be as bright as magnitude +10.1 (no problem in a 4.5-inch scope) but that occurs during the afternoon for the Americas. The Middle East and Asia are the place to be for closest approach. Peak brightness over the U.S. will occur before dawn on Halloween, so you can begin observation around 11 p.m. local time Friday evening October 30 when Orion comes up in the east. The asteroid starts shines at around magnitude +11-11.5 that evening and brightens overnight to around +10.3-10.5 before dawn for the Americas.
A word about tracking fast-moving asteroids. I’ve found that the best way to catch sight of one is to “camp” at the place they’ll pass at a certain time. Say you want to see TB145 at 1:15 a.m. October 31. Make a chart that shows its position every 15 minutes. Five minutes before it arrives at the 1:15 a.m. spot, point your telescope there and wait for a “moving star” to enter the field of view. If you don’t see it right way, wait a few minutes and pan around to the north and south of the location. By the way, the asteroid will pass less than a degree northwest of the Crab Nebula (M1) in Taurus around 10:30 UT (5:30 a.m. CDT).
Be aware that the bright, waning gibbous Moon will be within 10° of the asteroid when it’s best visible in the Americas. While this will make observing the asteroid more challenging, don’t let it stop you from trying. If bad weather gets in the way, Gianluca Masi has you covered. He’ll live-stream the flyby on his Virtual Telescope sitebeginning at 0:00 UT (7 p.m CDT) on October 31st.
One way or another, we’ll all have a shot at seeing the Great Pumpkin asteroid this Halloween.
2015 TB145 looks stellar in this photo taken on October 24th when it glowed at only 16th magnitude. Credit: Peter Lake
UPDATE Oct. 27, 2015: There’s been some discussion about TB145’s orbit resembling that of a comet along with speculation it might be a dead or dormant comet. Amateur and professional astronomers have been watching it closely, looking for hints of activity such as a fuzzy coma. So far, photos show the asteroid as completely stellar.
I also wanted to update you on its visibility. Those with 10-inch or larger telescopes can begin looking for the object Thursday night Oct. 29th when it reaches magnitude +13.5. The following night it leaps to +11.5 with a peak brightness of +10.0 occurring around 14:00 UT (9 a.m. CDT) on Halloween. TB145 fades rapidly thereafter – down to 15th magnitude just 8 hours later.
A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
Much has been learned about Saturn’s system of moons in recent decades, thanks to the Voyager missions and the more recent surveys conducted by the Cassini spaceprobe. Between its estimated 150 moons and moonlets (only 53 of which have been identified and named) there is no shortage of scientific curiosities, and enough mysteries to keep astronomers here on Earth busy for decades.
Consider Mimas, which is often referred to as Saturn’s “Death Star Moon” on a count of its unusual appearance. Much like Saturn’s moons Tethys and Rhea, Mimas’ peculiar characteristics represents something of a mystery. Not only is it almost entirely composed ice, it’s coloration and surface features reveal a great deal about the history of the Saturnian (aka. Cronian) system. On top of that, it may even house an interior, liquid-water ocean.
Discovery and Naming:
Saturn’s moon Mimas was discovered by William Herschel in 1789, more than 100 years after Saturn’s larger moons were discovered by Christian Huygens and Giovanni Cassini. As with all the seven then-known satellites of Saturn, Mimas’ name was suggested by William Herschel’s son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.
Mimas takes its name from one of the Titans of Greek mythology, who were the sons and daughters of Cronus (the Greek equivalent to Jupiter). Mimas was an offspring of Gaia, born from the blood of the castrated Uranus, who eventually died during the struggle with the Olympian Gods for control of the universe.
A replica of the telescope which William Herschel used to observe Uranus. Credit: Alun Salt/Wikimedia Commons
Size, Mass and Orbit:
With a mean radius of 198.2 ± 0.4 km and a mass of about 3.75 ×1019 kg, Mimas is equivalent in size to 0.0311 Earths and 0.0000063 times as massive. Orbiting Saturn at an average distance (semi-major axis) of 185,539 km, it is the innermost of Saturn’s larger moons, and the 8th moon orbiting Saturn. It’s orbit also has a minor eccentricity of 0.0196, ranging from 181,902 km at periapsis and 189,176 km at apoapsis.
With an estimated orbital velocity of 14.28 km/s, Mimas takes 0.942 days to complete a single orbit of Saturn. Like many of Saturn’s moons. Mimas rotation period is synchronous to its orbital period, which means it keeps one face constantly pointing towards the planet. Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora.
Composition and Surface Features:
Mimas’ mean density of 1.1479 ± 0.007 g/cm³ is just slightly higher than that of water (1 g/cm³), which means that Mimas is mostly composed of water ice, with just a small amount of silicate rock. In this respect, Mimas is much like Tethys, Rhea, and Dione – moon’s of Saturn that are primarily composed of water ice.
Due to the tidal forces acting on it, Mimas is noticeably prolate – i.e. its longest axis is about 10% longer than the shortest, giving it its egg-shaped appearance. In fact, with a diameter of 396 km (246 mi), Mimas is just barely large and massive enough to achieve hydrostatic equilibrium (i.e. to become rounded in shape under the force of its own gravitation). Mimas is the smallest known astronomical body to have achieved this.
Mosaic image of Mimas, created from images taken by NASA’s Cassini spacecraft, showing the Herschel crater in the center. Credit: NASA/JPL
Three types of geological features are officially recognized on Mimas: craters, chasmata (chasms) and catenae (crater chains). Of these, craters are the most common, and it is believed that many of them have existed since the beginning of the Solar System. Mimas surface is saturated with craters, with every part of the surface showing visible depressions, and newer impacts overwriting older ones.
Mimas’ most distinctive feature is the giant impact crater Herschel, named in honor of William Herschel (the discoverer of Uranus, its moons Oberon, and Titania, and the Cronian moons Enceladus and Mimas). This large crater gives Mimas the appearance of the “Death Star” from Star Wars. At 130 km (81 mi) in diameter, Herschel’s is almost a third of Mimas’ own diameter.
Its walls are approximately 5 km (3.1 mi) high, parts of its floor measure 10 km (6.2 mi) deep, and its central peak rises 6 km (3.7 mi) above the crater floor. If there were a crater of an equivalent scale on Earth, it would be over 4,000 km (2,500 mi) in diameter, which would make it wider than the continent of Australia.
The impact that made this crater must have nearly shattered Mimas, and is believed to have created the fractures on the opposite side of the moon by sending shock waves through Mimas’s body. In this respect, Mimas’ surface closely resembles that of Tethys, with its massive Odysseus crater on its western hemisphere and the concentric Ithaca chasma, which is believed to have formed as a result of the impact that created Odysseus.
Color map of Mimas, created using data provided by the Cassini spaceprobe. Credit: NASA/JPL/Caltech/SSI/LPI
Mimas’ surface is also saturated with smaller impact craters, but no others are anywhere near the size of Herschel. The cratering is also not uniform, with most of the surface being covered with craters larger than 40 km (25 mi) in diameter. However, in the south polar region, there are generally no craters larger than 20 km (12 mi) in diameter.
Data obtained in 2014 from the Cassini spacecraft has also led to speculation about a possible interior ocean. Due to the planet’s libration (oscillation in its orbit), scientists believe that the planet’s interior is not uniform, which could be the result of a rocky interior or an interior ocean at the core-mantle boundary. This ocean would likely be maintained thanks to tidal flexing caused by Mimas’ orbital resonances with Tethys and Pandora.
A number of features in Saturn’s rings are also related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, which is the gap between Saturn’s two widest rings – the A Ring and B Ring. The repeated pulls by Mimas on the Cassini Division particles, always in the same direction, forces them into new orbits outside the gap.
Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. In other words, they orbit Saturn twice for each orbit competed by Mimas. The boundary between the C and B ring is meanwhile in a 3:1 resonance with Mimas; and recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas.
This figure illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn’s small inner moon Mimas. Credit: NASA/JPL/GSFC/SWRI/SSI
Exploration:
The first mission to study Mimas up close was Pioneer 11, which flew by Saturn in 1979 and made its closest approach on Sept. 1st, 1979, at a distance of 104,263 km. The Voyager 1 and 2 missions both flew by Mimas in 1980 and 1981, respectively, and snapped pictures of Saturn’s atmosphere, its rings, its system of moons. Images taken by Voyager 1 probe were the first ever of the Herschel crater.
Mimas has been imaged several times by the Cassini orbiter, which entered into orbit around Saturn in 2004. A close flyby occurred on February 13, 2010, when Cassini passed Mimas at a distance of 9,500 km (5,900 mi). In addition to providing multiple images of Mimas’ cratered surface, it also took measurements of Mimas’ orbit, which led to speculation about a possible interior ocean.
The Saturn system is truly a wonder. So many moons, so many mysteries, and so many chances to learn about the formation of the Solar System and how it came to be. One can only hope that future missions are able to probe some of the deeper ones, like what might be lurking beneath Mimas’ icy, imposing “Death Star” surface!
We’ve written many great articles about Mimas and Saturn’s moons here at Universe Today. Here’s one about the Herschel Crater, one about the first detailed look Cassini made, and one about it’s “Death Star” appearance.
Another great resource about Mimas is Solar Views, and you can get even more info from the Nine Planets.
Saturn's moon Tethys, imaged by Cassini on April 14, 2012.
Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.
This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.
Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.
Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain
Size, Mass and Orbit: With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.
Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.
Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding
Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.
Composition and Surface Features: Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.
It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).
This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.
The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI
The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.
The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.
Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.
Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.
Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.
Exploration: Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.
Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.
On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.
Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute
The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.
At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.
We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.
Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.
Saturn's moon Rhea, as imaged by the Cassini-Huygens spaceprobe. Credit: NASA/JPL-Caltech
The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study. Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter.
For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System.
Discovery and Naming:
Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea (“the stars of Louis”) in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.
In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the “mother of the gods” and one the sisters of Cronos (Saturn, in Roman mythology).
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Size, Mass and Orbit:
With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).
With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn’s moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it.
Composition and Surface Features:
With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.
In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter’s moon Callisto).
Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute
Models of Rhea’s interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core.
Rhea’s surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller.
Another difference between Rhea’s leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters. It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea’s history when its interior was still liquid.
However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon’s surface. Rhea also has a very faint “line” of material at its equator which was thought to be deposited by material deorbiting from its rings (see below).
Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI
Rhea has two particularly large impact basins, both of which are situated on Rhea’s anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across. The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its “Death-Star” appearance).
Atmosphere:
Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent.
The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn’s magnetosphere (aka. radiolysis). These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon’s interior.
Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012. Credit: NASA/JPL
Rhea may also have a tenuous ring system, which was inferred based on observed changes in the flow of electrons trapped by Saturn’s magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as the impact points of deorbiting ring material).
However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea’s equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.
Exploration:
The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it’s arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances.
The Cronian system is definitely a fascinating place, and we’ve really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn’s moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other “Death Star Moon”, Dione.
Ever wonder what happens on the surface of other stars?
An amazing animation was released this week by astronomers at the Leibniz Institute for Astrophysics (AIP) in Potsam Germany, showing massive sunspot activity on the variable star XX Trianguli (HD 12545). And while ‘starspot’ activity has been seen on this and other stars before, this represents the first movie depicting the evolution of stellar surface activity beyond our solar system.
“We can see our first application as a prototype for upcoming stellar cycle studies, as it enables the prediction of a magnetic-activity cycle on a dramatically shorter timescale than usual,” says Leibniz Institute for Astrophysics Potsdam astronomer Andreas Kunstler in a recent press release.
The images were the result of a long term analysis of the star carried out using the twin STELLA (STELLar Activity) robotic telescopes based on Tenerife in the Canary Islands. The spectroscopic data was gathered over a period of six years, and this video demonstrates that, while other stars do indeed have sunspot cycles similar to our Sun, those of massive stars such as XX Tri are much more intense than any we could imagine here in our own solar system.
STELLA on the hunt. Image credit:
Even the largest and closest of stars have a minuscule angular diameter –measured in milliarcseconds (mas, our 1/1,000ths of an arc second)—in size. For example, we know from lunar occultation timing experiments that the bright star Antares at 550 light years distant and 5 times the radius of our Sun is about 41 mas in size. At an estimated 910 to 1,500 light years distant and 10 times the radius of our Sun, XX Tri is probably comparable, at about 20 mas in size.
That’s tiny from our perspective, though the massive starspot depicted must be truly gigantic to see up close.
To image something on that scale, astronomers use a technique known as Doppler tomography gathered from high-resolution spectra. Over said six year span covering a period from July 2006 to April 2012, 667 viable spectra were gathered, covering 86 total rotational periods for the star. Incidentally, that’s not much longer than the average equatorial rotational period of our Sun—remember, as a ball of gas, the rotational period of our Sun varies with solar latitude—at about 22 days.
Our relatively sedate host star. Image credit: Dave Dickinson
The views compiled by the team show a pole facing, Mercator projection, and a spherical ‘real view’ of the star. Of course, to see XX Tri up close would be amazing, if a not a little intimidating with those massive, angry spots dappling its surface.
Watch the animation, and you can see the changing morphology of the spots, as they decay, merge and defuse again. Just how permanent is that massive pole spot? Why are we seeing spots across the pole of a star like XX Tri at all, something we never see on the Sun? Do other stars follow something analogous to Spörer’s Law and their own version of the 11-year sunspot cycle that we see on Sol?
Capabilities such as those demonstrated by STELLA may soon crack these questions wide open. Composed of two 1.2 meter robotic telescopes jointly operated by the Institute for Astrophysics at Potsdam and the Instituto de Astrofísica de Canarias (IAC), STELLA combines the capability of a wide-field photometric imager with that of a high-resolution spectrograph, ideal for this sort of analysis of remote stellar surfaces.
A diagram featuring the twin STELLA instruments. Image credit: Leibniz Institute for Astrophysics Potsdam (AIP)
Hey, here’s a crazy idea: turn STELLA loose on KIC 8462852 and see if the hypothesized ‘exo-comets’ or ‘alien mega-structures’ turn up… though it weighs in much smaller than XX Tri at 1.4x solar masses, KIC 8462852 is also about 1,400 light years distant, perhaps just doable using high resolution spectroscopy…
The location of XX Tri (also known as HIP 9630) in the northern sky. Image credit: created by the author using Stellarium planetarium software
Want to see XX Tri for yourself? An RS Canum Venaticorum variable orange giant star (spectral type K0 III) located in the constellation of Triangulum the Triangle, XX Tri shines at magnitude +8.5 and varies over about half a magnitude in brightness. Its coordinates are:
Right Ascension: 2 hours 3 minutes 47 seconds
Declination: 35 North 35 minutes 29 seconds
The more we learn about other stars, the more we understand about how to live with our very own sometimes placid, sometimes tempestuous host star.
Does XX Trianguli look familiar? That might be because it was featured as the Astronomy Picture of the Day as ‘imaged’ by the Coude Feed Telescope on Kitt Peak way back when on November 2nd, 2003.
On October 8th, the waning crescent Moon occulted (passed in front of) the bright planet Venus for observers in the southern hemisphere. And while such occurrences aren’t at all rare—the Moon occults Venus 3 times in 2015, and 25 times in this decade alone worldwide—the particulars were exceptional for observers in Australia, with a -4.5 magnitude, 40% illuminated Venus 30” in size emerging under dark skies 45 degrees west of the Sun from behind the dark limb of the Moon.
David and Joan Dunham rose to the challenge, and caught an amazing sequence featuring a brilliant Venus reappearing from behind the Moon as seen from the Australian Outback. When I first watched the video posted on You Tube by International Occultation Timing Association (IOTA) North American coordinator Brad Timerson, I was a bit perplexed, until I realized we were actually seeing the dark nighttime side of a waning Moon, with the bright crescent just out of view. Venus fully emerges in just under a minute after first appearing, and its -4th magnitude visage shines like a spotlight when revealed in its full glory.
A simulation of Venus on the limb of the Moon on October 8th. Image credit: Stellarium
“Joan and I observed the reappearance of Venus from behind the dark side of the 15% sunlit waning crescent Moon, from a dark and wide parking area on the east side of the Stuart Highway that afforded a low (1-2 degree) horizon to the east,” Dunham said. “Since the past observations of ashen light were visual, I decided that it would be best to use the 25mm eyepiece with the 8-inch visually rather than just make a redundant video. Neither the real-time visual observation, nor close visual inspection of the video recording, showed any sign of the dark side of Venus.”
Dunham’s ‘box scope’ imaging set up Image credit: David Dunham
Reports by visual observers of ashen light on the dark limb of Venus over the centuries remain a mystery. On the crescent Moon, it’s easy to explain, as the Earth illuminates the nighttime side of our natural satellite; no such nearby illumination source exists in the case of Venus. Ashen light on Venus is either an illusion—a trick of the dazzling brilliance of a crescent Venus fooling the eye of the observer—or a real, and not as yet fully described phenomenon. Over the years, suggestions have included: lightning, airglow, volcanism, and aurora. A good prime candidate in the form of an ‘auroral nightglow” was proposed by New Mexico State University researchers in 2014. 19th century astronomers even proposed we might be seeing the lights of Venusian cities, or perhaps forest fires!
Could we ever separate the bright crescent of Venus from its nighttime side? A lunar occultation, such as the October 8th event provides just such a fleeting opportunity. Though it’s hard to discern in the video, Dunham also watched the event visually through the telescope, and noted that, in his words, “the dark side of Venus remains dark,” with no brief appearance prior to sighting the crescent shining through the lunar valleys.
A tentative light curve made by Mr. Timerson seems to support this assertion, as the appearance of Venus quickly over-saturates the view:
A rough light curve of the event. Photon counts are along the vertical axis, each tick mark along the horizontal equals one second. Image credit: Brad Timerson
Of course, this is far from conclusive, but seems to support the idea that the ashen light of Venus noted by ground observers is largely an optical illusion. Not all occultations of Venus by the Moon are created equal, and the best ones to test this method occur when Venus is less than half illuminated and greater than 40 degrees from the Sun against a relatively dark sky. Compounding problems, the ‘dark’ limb of the Moon has a brightness of its own, thanks to Earthshine. Dunham notes that observers in southern Alaska may have another shot at seeing this same phenomenon on December 7th, when the 13% illuminated crescent Moon occults a -4.2 magnitude 69% illuminated Venus, 42 degrees west of the Sun… the rest of North and South America will see this occultation in the daytime, still an interesting catch.
The occultation footprint for the Dec 7 event. The dashed lines indicate where the event happens during daylight. Image credit: Occult 4.1
Looking at future occultations, there’s an intriguing possibility to hunt for the ashen light on the evening of October 10th, 2029, when then Moon occults a 57% illuminated Venus against dark skies for observers along the U.S. West Coast. Incidentally, a dawn occultation provides a better circumstance than a dusk one, as Venus always reemerges from the Moon’s dark limb when it’s waning. It enters the same when waxing, perhaps allowing for observer bias.
A simulation of the 2029 event. Image credit: Stellarium
Can’t wait for December? The Moon also occults the bright star Aldebaran on October 29th for Europe and North America on November 26th near Full phase… the good folks at the Virtual Telescope will carry the October event live.
The occultation footprint for the 2029 event. Image credit: Occult 4.1
For now, the ashen light of Venus remains an intriguing mystery. Perhaps, an airborne observation could extend the appearance of Venus during an occultation, or maybe the recently announced Discovery-class mission to Venus could observe the night side of the planet for an Earthly glow… if nothing else, it’s simply amazing to watch the two brightest objects in the nighttime sky come together.
This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Credit: NASA
Exploring the Solar System is like peeling an onion. With every layer removed, one finds fresh mysteries to ponder over, each one more confounding than the last. And this is certainly the case when it comes to Jupiter’s system of moons, particularly its four largest – Io, Europa, Ganymede and Callisto. Known as the Galilean Moons, in honor of their founder, these moons possess enough natural wonders to keep scientists busy for centuries.
As Jupiter’s innermost moon, it is also the fourth-largest moon in the Solar System, has the highest density of any known moon, and is the driest known object in the Solar System. It is also one of only four known bodies that experiences active volcanism and – with over 400 active volcanoes – it is the most geologically active body in the Solar System.
One of the 42 dishes in the Allen Telescope Array that searches for signals from space. Credit: Seth Shostak / SETI Institute.
“We either caught something shortly after an event like two planets crashing together or alien intelligence,” said Dr. Gerald Harp, senior scientist at the SETI Institute in Mountain View, California, referring to the baffling light variations seen by the Kepler Observatory in the star KIC 8462852 .
And he and a team from the Institute are working hard at this moment to determine which of the two it is.
Gerald Harp of the SETI Institute is involved in gathering and studying data from the mysterious Kepler star. Credit: SETI Institute
Beginning last Friday (Oct. 16), the Institute’s Allen Telescope Array (ATA) was taken off its normal survey schedule and instead focused on KIC 8462852, one of the 150,000-plus stars studied by NASA’s Kepler Mission to detect Earth-sized exoplanets orbiting distant stars.. The array of 42 dishes comprises a fully automated system that can run day and night, alerting staff whenever an unusual or interesting signal has been detected.
A swarm of comets has been proposed to explain the erratic and non-repeating light variations seen in the star located nearly 1,500 light years from Earth in the constellation Cygnus the Swan. But no one really seems satisfied with the explanation, and the chances that we’d catch a huge event like a comet breakup or planetary collision in the short time the star has been under observation seems unlikely. Collisions also generate dust. Warmed by the star, that dust would glow in infrared light, but none beyond what’s expected has been detected.
The Allen Telescope Array (ATA) is a “Large Number of Small Dishes” (LNSD) array designed to be highly effective for simultaneous surveys undertaken for SETI projects (Search for Extraterrestrial Intelligence) at centimeter wavelengths. Credit: Seth Shostak / SETI Institute
The ATA picks up radio frequencies in the microwave range from 1-10 gigahertz. For comparison, your kitchen microwave oven produces microwaves at around 2 gigahertz. Although Harp couldn’t reveal the team’s results yet — that will come soon when a paper is submitted in few weeks in a science journal — he did share the excitement of a the hunt in a phone interview Tuesday.
The array normally looks for a very narrow wave or specific frequency when hunting for potential “ET” signals. But not this time.
“This is a special target,” said Harp. “We’re using the scope to look at transmissions that would produce excess power over a range of wavelengths.” Perhaps from a potential alien power source? Maybe. Harp believes the star’s peculiar, a-periodic light variations seen by Kepler are “probably natural and definitely worth looking at” but considers an intelligent source a possibility, however remote.
This artist concept illustrates how two large, planet-sized objects could collide to create clumps of material in orbit around a star. They’d also create a lot of dust, which would glow in infrared light, something not seen around the Kepler star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)
During our conversation, he emphasized how special the light variations from the star were, adding how the “big gob” of material orbiting KIC (stands for Kepler Input Catalog) 8462852 is unusual in that it’s “clumped”. “We expect it to spread into a ring,” he said.
AAVSO chart of KIC 8462852. Click to enlarge or go to the website to make your own customized version. Credit: AAVSO
Meanwhile, the American Association of Variable Star Observers (AAVSO) published an Alert Notice this week requesting amateurs and professional astronomers around the world to immediately begin observing KIC 8462852 now through the end of the current observing season. To locate the star, you can either use the charts provided in our previous story or go to the AAVSO site and type in KIC 8462852 in the “Pick a Star” box to create a chart of your own.
I’m a variable star observer, so naturally I thought of variables with irregular fluctuations in light when I first heard about this stellar mystery. Time to talk to an expert. According to Elizabeth Waagen, senior technical assistant for science operations at the AAVSO, KIC 8462852 is different.
“Based on the information so far, it doesn’t seem to fit the criteria for an irregular variable,” said Waagen in a phone interview this morning. “It’s doesn’t add up.”
She encouraged an open mind. “It’s a big puzzle, so we sent out the notice,” referring to the alert described above.
All quite exciting, and I’m as eager as you to see the published results on the signals, which Harp said would appear or link from the SETI website soon. Stay tuned …
