Cassini Space Probe To Fall Into Saturn Friday Morning

Friday morning, around 5 am Pacific time, NASA will send the Cassini space probe falling into the planet Saturn -- until it is crushed by the pressure in the ringed planet's atmosphere. NASA is commanding Cassini to "commit suicide" before its propellant runs out and it can't be steered any more. Since Saturn has two moons which might harbor some sort of primitive life, we wanted to make sure we did not contaminate those worlds.

The planet Saturn is made mostly of gas and liquid (and its make-up is dominated by the two lightest elements in the universe, hydrogen and helium.) So you can't land ON Saturn, you can only fall INTO Saturn (like a giant ocean world.)

Cassini has been orbiting Saturn for 13 years, sending back amazing pictures and information on the planet, its complicated rings, and its 62 moons. It's made a slew of remarkable discoveries, including the presence of warm salt-water geysers on the relatively small moon called Enceladus, and lakes and rivers of liquid fuel oil on the giant moon Titan. It was launched 20 years ago (so it's had a long and fulfilling life for a spacecraft.)

Since April, it has been swooping in and out of the space between Saturn's cloudtops and its inner rings, an area we had never had the nerve to explore before. NASA estimates the spacecraft has traveled almost 5 billion miles in total and has sent back more than 450,000 picture (that's a Flickr file not even your most picture-taking relatives can compete with!)

In our image, you can see Saturn and its complex ring system, with a painting of the spacecraft above the planet's north pole, ready to make a dive.

Some of my favorite pictures in the introductory astronomy textbook I am the lead author on come from Cassini (which was the most complicated planetary explorer ever built.) On Friday morning, let's give it a thought as we wake up -- we'll miss you, Cassini!

You can see live coverage of the last days of the mission on NASA TV at: http://nasa.gov/live

You can access the image galleries and latest videos from the mission from this page (designed for the media, but which anyone can use): https://saturn.jpl.nasa.gov/mission/grand-finale/for-media/

By the way, you can access my free textbook at : http://openstax.org/details/astronomy 

Ringshine on the Night Side of Saturn

A spectacular new image of the night side of Saturn and its shining rings in sunlight was released recently by the Cassini mission. You can also see the shadow Saturn casts on its rings.

This August 2016 view shows the complex structure of the planet's rings clearly. You can see the two gaps in the rings, one wider and one narrower (in the outer part of the outer ring.) Close-ups have revealed fainter rings and moons in the gaps, so they are not really as empty as they might seem.

The sunlight reflected from the rings keeps the night side of Saturn from being as dark as it could be, much as moonshine keeps the nights on Earth from being completely dark. One scientist calculated that if you could float in the upper cloud layers of Saturn (which has no solid surface), you could read an astronomy book by ringshine.

Look at the complexity of Saturn's rings -- ringlet after ringlet can be seen in each main ring. And each ringlet consists of millions of icy chunks, all orbiting together around the equator of Saturn. The main composition of these icy chunks is water -- making the rings a significant reservoir of water for future explorers. (Although two of Saturn's moons, Enceladus and Dione, are now thought to have liquid oceans of water under their icy crusts. Perhaps we can siphon some of that instead of melting ring chunks.)

The image was taken from a distance of 870,000 miles (which seems far, but Saturn was about 850 million miles from Earth at that time, so Cassini definitely had the better view.)

An Amazing Image of Saturn's Rings for the Holidays

Carolyn Porco, the dynamic leader of the Imaging Team for the Cassini Mission (exploring Saturn and its neighborhood), has sent a remarkable holiday greeting card from Saturn and I wanted to share it with you.

What we are looking at is a view of the inner part of Saturn's complicated ring system (mainly the so-called B ring.) We are seeing about 750 miles of the white rim of the B ring, beyond which is the dark emptier space called the Cassini Division. You can see a number of thin "ringlets" in that division toward the upper part of our picture.

But what is really stunning is what the long shadows cast by the Sun reveal at the edge of the bright main ring. You can see peaks or thin mountains of ice particles, sticking up more than a mile and a half (2 km) above the plane of the rings. Compare those heights with the typical height of the ring system, which is about 10 meters (or about 30 feet) in most places.

Here is what Carolyn says about the image, "Cassini scientists believe that this is one prominent region at the outer edge of the B ring where large bodies, or moonlets, up to a kilometer or more in size, are found. It is possible that these bodies significantly affect the ring material streaming past them, and force the particles upward, in a "splashing" manner."

She continues, "This image [taken in 2009] and others like it are only possible around the time of Saturn's equinox, which occurs every half-Saturn-year, or about every 15 Earth years. The illumination geometry that accompanies equinox lowers the sun's angle to the ring plane and causes structures jutting out of the plane to cast long shadows across the rings."

The rings of Saturn are made of billions and billions (as Carl Sagan used to say) of individual chunks of ice -- mostly water ice. The particles range in size from smaller than a sand grain to larger than a modern SUV. We have known for some time that the interaction of moons and rings in the Saturn system can create a wide range of artistic patterns using the ring particles as their raw materials. But these tall icicles or icy towers are among the most spectacular we have ever seen.

Happy Holidays. May 2015 bring all of us more amazing views of our cosmic environment and increase our understanding that the differences that separate us on Earth are so trivial compared to the truly alien nature of what's out there.

[Click on the image to make it bigger and see even more detail!]

Two Major Discoveries about Our Solar System

Two major finds were announced yesterday: the first rings found around an asteroid, and the most distant object -- a dwarf planet -- ever seen in our solar system. Let me fill you in on the details.

The unexpected rings were found around the asteroid Chariklo, a 150-mile wide chunk that orbits between Saturn and Uranus. A group of South American astronomers observed Chariklo moving in front of a star, and glimpsed two thin rings around it. The rings are roughly 4 and 2 miles wide and are separated by a gap of some 5 1/2 miles. In the past, when we have found distinct thin rings like this, they have been kept thin by the presence of small "shepherding moons." The analogy is the shepherds keep a flock of sheep in a thin line, and these moons keep the particles of the rings from drifting away from their formation. So we expect that one or more such moons will eventually be found around Chariklo.

How did an asteroid get itself a set of rings -- which we have previously seen only around giant planets? Our best guess is that Chariklo was involved in some sort of cosmic accident and the rings are left-over debris from that event.

By the way, Harry Potter fans and mythology buffs will appreciate the name of the asteroid. In classical mythology, Chariklo was the wife of Chiron, the best known of the Centaurs -- creatures that were half human and half horse. In the same way, the Centaurs in our solar system are also half-breeds -- they have some characteristics of asteroids and some of comets.

The second discovery was of a dwarf planet orbiting so far away from the Earth and the Sun that it is the most distant world we have seen in our solar system. The small, frozen world, estimated to be about 250 miles wide, is called 2012 VP113, a boring provisional name that includes the date of its first sighting and letters and numbers that give a code for what its order is in the discovery of small objects that year. But because the letter code happens to be VP, the discoverers have nicknamed it Biden (but that will not be its final official name.)

I can't resist noting, in case you were not aware of this, that each vice president of the U.S. already has a direct connection to the world of astronomy, since the vice-president's residence is on the grounds of the U.S. Naval Observatory. Many vice-presidents brings guests to the observatory and even have parties there.

Little "Biden" (the world) is really, really out there, beyond what we think are the borders of the Kuiper Belt -- the zone of Pluto, other dwarf planets, and small icy chunks that make a belt outside the orbit of Neptune. The conventional wisdom has been that this belt should end at 50 times the distance between the Earth and the Sun (which we call 50 astronomical units), or about 4 1/2 billion miles from the Sun.

Biden, on the other hand, makes a looping orbit further out, and never gets closer to the Sun than 80 astronomical units or 7 1/2 billion miles. And most of the time is is even further out. (Another object found earlier, now called Sedna, is also out there.) The presence of such "far-out" members of our solar system, while not impossible, is surprising enough that astronomers are now set to pondering what got them out there.

Our image shows an artist's conception (no one has a photo of it) of what Chariklo and its two rings might look like out there, provided by the European Southern Observatory.

A Fabulous New Saturn Image

This magnificent (but artificial color) image of the ringed planet Saturn was assembled and enhanced from 60 different pictures taken with the Cassini spacecraft. You are looking up at Saturn from a perspective below its rings, at a time when the Sun happened to be directly behind Saturn. Thus we are seeing the planets and the rings back-lit, something we can never see from Earth.

The image is constructed not merely with visible light pictures, but also with some in the infra-red (commonly called heat rays). So this is not what your eyes would see if you (like Cassini) were orbiting Saturn -- this is a view only our sophisticated instruments can make available. And the details have been enhanced to show you the complex structure of Saturn's rings (made up of billions of chunks of mostly water ice, all moving around the equator of Saturn in a traffic pattern so vast it puts the Los Angeles freeway system to shame.) 

From one side to the other, Saturn's giant ring system spans 170,000 miles across.   This means that if you were to take the ring system off Saturn and put it next to the Earth, it would almost fill the space between the Earth and the Moon.

Near the top of the picture, you see the round dark shadow that Saturn casts on its own rings. Below that shadow, the greenish glow is light reflected from the rings on to the cloudtops of Saturn.

The pictures that make up this fantastic mosaic were taken from a distance of about 500,000 miles from Saturn (quite close in astronomical terms.)  Just look at all the structure you can see in the ring system, which consists of many, many ringlets.

What a beautiful holiday ornament this planet makes!  Happy Holidays.