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stereo-sun-spacecraft-nasa.jpg

OK, yesterday I promised we would get to the space news.  Clearly the real story is the earthlike planet right in our backyard (erm, relatively speaking). However it isn’t going anywhere right now so I am going to blog about it later when we have all had a moment to think about the real implications.  The space story I am looking at today is closer to home, but still takes place out there in the black: back in October of 2014, NASA lost communication with Stereo B one of two paired spacecraft which orbited the sun from the distance of Earth.

The solar observatory spacecraft allow stereoscopic viewing of the sun.  One spacecraft Stereo A was ahead of Earth on its orbit, whereas Stereo B trailed behind us.  The two observatories allow us to study coronal mass ejections and other stellar phenomena.  In 2011, the craft were 180 degrees apart from each other—allowing humankind to view the entire sun at once for the very first time (a truly remarkable milestone, when you think about it, which I heard nothing about at the time).

stereo_positions2.png

Sadly, however, in 2014, as part of an automation and attitude test, Stereo B began to spin.  Mission controllers then lost contact with the craft which (because of the nature of its work) was on the other side of the sun!  NASA has patiently waited till the orbital path of Stereo B carried it further towards Earth and has used the Deep Space Network, a networked array of radio telescopes to find the errant craft.

We are still working on figuring out what sort of shape the poor guy is in (and maybe rehabilitating the spinning observatory), however I feel the story is worth telling as a sort of reminder of the fleet of crafts we have up there, which we don’t think about very often.

The Great Basin on Saturn's Tethys  (Credit: Cassini Imaging Team)

The Great Basin on Saturn’s Tethys (Credit: Cassini Imaging Team)

It’s been too long since we headed out to space.  This is true of humankind, but it is also true of this blog…so today we are going to cast our eyes across the solar system to Tethys a mid-sized moon of Saturn. In 1684 the Italian astronomer Giovanni Domenico Cassini discovered Tethys. He initially named the moon in honor of Louis XIV, but his choice was later changed so that the moon is named for a first generation Greek titan-goddess.  The moon has been approached by several human spacecraft, most notably…Cassini, which has dropped by several times (the robot space probe is named after the astronomer—the poor fellow has not been drifting in space since the 17th century).

Giovanni Domenico Cassini

Giovanni Domenico Cassini

Of all the major moons in the solar system, Tethys has the lowest density: 0.98 g/cm3 ! This means that almost the entire moon is made of frozen water—it is essentially a huge round ice cube floating around Saturn. Tethys has two extremely prominent features—a giant crater 450 kilometers (280 miles) across (named Odysseus) and a huge ice canyon 2000 kilometers (1200 miles) long, 100 km (62 miles) wide, and 3 km (1.8 miles) deep, which stretches most of the way across the moon.  Unsurprisingly astronomers speculate that the two features are related and the massive impact which created Odysseus melted a chasm along the entire side of the planetoid.

Tethys

Although you might be inclined not to expect much activity from a ball of ice in the depths of space, Tethys seems like it may be geologically active, or, at least, it may have been once.  The area around the hemisphere is comparatively flat and free of craters—which suggests that tidal flux from Saturn causes some melting—and possibly cryovolcanoes.

Ithaca Chasma: The Great Rift on Saturn's Tethys  (Credit: Cassini Imaging Team)

Ithaca Chasma: The Great Rift on Saturn’s Tethys
(Credit: Cassini Imaging Team)

Paleontologists and sharp-eyed readers already know the name Tethys.  During the age of Pangaea (when all of the world’s continents joined to form a single land mass), the great ocean in the midst was named the Tethys Ocean. In Greek mythology, Tethys was the daughter of Gaea (the mother earth) and Uranus (the heavens).  She was regarded as the mother of all waters and was married to her brother Oceanus, the first lord of the seas.  The astronomers of the age of enlightenment who renamed the moon, could not have known it was composed mostly of water, but they chose well.

Tethys_mosaic_83d40m_Phillopolis_mid4th_century_-p2fx.2

Electromagnetic radiation exerts pressure on physical matter.  The more the radiation is reflected from the surface it strikes, the greater the pressure–so sunlight presses harder on a mirror than on, say, an ostrich with the same surface area.  I’m not going to dwell on the physics underlying this fact (although I will provide a link), but rather on the remarkable ramifications.  Contingent on the amount of radiation, this force is rather weak. However, taken in aggregate, across a large surface, light (or any form of EM radiation) can move an object.  Hence…solar sails!

IKAROS (image from JAXA)Much in the manner that wind pushes a sailboat through water, light can push an object through space. Although using such a sail for space travel was demonstrated to be feasible in the laboratory, the great national space programs—NASA, Russia, ESA, and China—have never successfully tested a solar sail in interplanetary space (despite several failed attempts). However, this year on May 21st the Japan Aerospace Exploration Agency (JAXA) successfully launched the IKAROS solar sail.  IKAROS stands for “Interplanetary Kite-craft Accelerated by Radiation Of the Sun”–an acronym which somehow is both unwieldy and an allusion to a badly botched aerospace venture (JAXA can be forgiven for the awkward name however thanks to the success of the mission). IKAROS is a square sail with a diagonal diameter of 20 meters.  It is made of polymer 7.5-micrometres thick.  A solar array is embedded in the sail to supply the craft’s power needs.  To provide attitude control, the sail also contains LCD panels with adjustable reflectivity.  Various sensors, dust counters and controls are located on different parts of the craft.

IKAROS Mission Plan (JAXA)

IKAROS deployed its sails when it was approximately 4.8 million miles from Earth (smoothly deploying a delicate lattice of sails in the grim void of outer space has been a major obstacle to this sort of mission in the past). The spacecraft is currently somewhere between Earth and Venus.  When it reaches the cloud planet, it will embark on a three year trip around the sun.

To follow up its success JAXA is planning to launce a 50 meter solar sail to the asteroid belt and Jupiter sometime late in the decade.  Other space agencies have taken note and are now playing catch-up with the Japanese.  NASA has plans for several solar sail missions in the coming years (provided poor national leadership does not botch the plans or scrub the funding). Since rocket fuel is heavy (and therefore a major sorce of missin costs), solar sailing technology has interested space agencies and space exploration enthusiasts for some time.  The Planetary Society, an international group dedicated to space exploration, has long advocated solar sails as a revolutionary step forward in space travel. In fact, the Planetary Society chartered a submarine launched Russian rocket to deploy its own solar sail into space but the mission sadly failed when the rocket malfunctioned.  Fortunately, the society has regained its old maniacal chutzpah and is launching a new solar sail mission (additionally, and even more importantly, it continues to lobby national governments for additional space funding)  

JAXA's next solar sail mission will apparently look like throwing a shuriken into an asteroid. Awesome!

In the near future, solar sails might be used for interplanetary missions or for de-orbiting old satellites and space debris (this latter task is growing in importance as humankind fills up near earth orbit with junk).  Hybrid drives which utilize solar sails and solar powered ion drives in tandem are also on the drawing board. In the farther future, who knows?  So far this is the only possible option for interstellar travel which utilizes technology humankind currently possesses (well, aside from ridiculous nuclear fission designs).  It has been proposed that giant space lasers could be used in tandem with the sun to accelerate probes to nearby stars.  Unfortunately such lofty prospects are still science fiction at present.

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