Early January 2020

Introduction

Saturn was visible, bright in the western evening sky, from our balcony for much of December. One night in particular, it was close enough to Mars to appear as one bright star. As I thought about the two planets, I realized just how much has been discovered about them in my lifetime.

Back in the fifties, I suppose I knew as much about Saturn as any other kid interested in science. I knew it was a planet, the sixth from the sun. I knew it was a so-called gas giant, second in size to Jupiter. Of course, I knew the ancients knew Saturn and I certainly knew about the rings. I had read of, but probably did not remember, that Christiaan Hugyens, an eminent Dutch natural philosopher, discovered Titan, Saturn’s largest moon, in 1655. And I was certainly ignorant of Giovanni Domenico Cassini’s discovery of four more Saturnian moons during the period 1673 to 1686. I did not know that Titan was named by John Herschel in 1847. Herschel chose names for the seven moons (his father, William Herschel, had discovered two more moons) from the Greek Titans, brothers and sisters of Cronus, the Greek Saturn.

Facts

So here is a summary of some basic information about Saturn.

• Saturn is named after the Roman god of harvest and time. Saturn is equivalent to the Greek god of time Cronus.
• Christiaan Huygens discovered Saturn’s rings in 1659. Galileo had made observations of Saturn as early as 1610 but the poor optics of his telescope could not resolve the rings. Instead, Galileo thought he was seeing a ‘triple planet’.
• The diameter of Saturn is about ten times greater than the diameter of earth.
• Saturn is an oblate spheroid. That is, the equatorial diameter is greater than the polar diameter. Saturn looks like a flattened ball.
• Saturn rotates quickly on its axis, completing one rotation in about 10.7 hours. It takes 29.5 years to orbit the sun. So there are about 1,006 days in a Saturn year.
• Saturn is tilted on its axis and to about the same extent as earth. Like the earth, Saturn experiences seasons.
• Saturn’s atmosphere consists of the gases hydrogen, helium, ice crystals, ammonia, and ammonia hydrosulfide. The last three appear as white, orange, and yellow, respectively, which combine to create Saturn’s brownish-yellow appearance.
• Storms, visible as White Spots, occur in Saturn’s upper atmosphere.

Early Probes to Saturn

Three space probes have flown by Saturn; another achieved orbit around the planet. In addition, the Hubble Space telescope has provided multiple images of the ringed planet.

Pioneer 11

The Pioneer 11 spacecraft, launched on April 6, 1973, flew by Jupiter of December 3, 1974, and by Saturn on September 1, 1979. It returned images and other data from twelve instruments on board. It was the first space craft to fly by Saturn. Here is one image from the mission.

Pioneer 11 and its sister Pioneer 10 are two of five space probes that have or will leave the solar system. In 4 million years or so, it will pass near the star Lambda Aquilla. Could, sometime in the distant future, the ghost probe be of interest to space traveling aliens? NASA prepared for that – each Pioneer has a gold-anodized aluminum plaque with information that, presumably, aliens can understand. Click here for information about the plaques.

The Voyagers

The nest two probes to visit Saturn were the twin Voyagers, 1 and 2. Voyager 2 was launched from Cape Canaveral on August 20, 1977; Voyager 1 on September 5, 1977.

The Voyagers captured images of Saturn and some of her moons. In addition, the instruments on board measured the wind speeds at the top of the planet’s atmosphere – 1,100 miles per hour at the equator, with speeds falling off towards the poles. Above 35 degrees north and south, the winds change from the easterly direction at the equator and blow both east and west.

Images of Saturn

If you want to see a video of Voyager 2’s approach to Saturn click here. Six hundred still images were used to make the video.

Voyager 2 had a slightly different camera system than her sister. The image below, in false colors, shows details of two of Saturn’s rings.

Enceladus

The two Voyagers paid attention to some of Saturn’s moons on their flybys. The image below is of Enceladus. You’ll hear more of her in a bit.

Voyagers – Still Working

Like Pioneer 10 and 11, the two Voyagers are leaving the solar system. After a successful flyby of Neptune, in 1989, NASA extended the mission so as to explore interstellar space. The two probes are currently beyond the outer edge of the heliosphere, the vast bubble-like space that is created by and surrounds the sun. The interstellar space outside the heliosphere is denser, colder, with more energetic particles than the volume within. Click here if you want to see data like current speed, mission elapsed time, distance from earth, instrument status and other data, all updated in real time. The Voyagers communicate with earth via their on-board transmitters; The Deep Space Network captures their signals. Click here for information on that.

By the way, Voyager 1 will encounter star AC +79 3888 in only 40,000 years. Rest assured – like Pioneer 10 and 11, the Voyagers contain greetings from earth. It is somewhat more elaborate than the plaque on the Pioneers. Click here for information on the Voyager’s golden disc.

Hubble Space Telescope

Although not a space probe, the Hubble telescope has provided images and data from Saturn. The space shuttle Discovery carried the Hubble into low earth orbit on April 24, 1990, and deployed it the next day. It has been serviced in orbit five times; each time it was drawn into the bay of a space shuttle and repaired and upgraded before redeployment.

The Hubble’s designer expected the telescope to last 15 years. It has been gathering images for more than 19 years now and is expected to last until 2025 or so. NASA will augment and then replace the Hubble with the James Webb Space Telescope, now scheduled for launch on March 30, 2021. Click here for information about that mission.

Cassini-Hyugens

The Cassini-Hyugens probe lifted off from Cape Canaveral the night of October 15, 1997. There were really two probes: the Cassini, a NASA endeavor, was to orbit Saturn. It carried the Huygens probe, created by the European Space Agency, designed to land on the surface of Titan. For a video of the launch click here.

Getting to Saturn is not a simple task. It took seven years, with two gravitational boosts from Venus and one the the earth-moon system. The route also included a fly by of Jupiter,

The above schematic does not capture the relative motions of the planets as they orbit the sun. For an animation that does, click here.

NASA controllers switched on some of the probes instruments at each flyby, both to test them and to gather data. For example, one set of instruments listened for lightning on Venus – none found.

The Cassini-Hyugens probe entered orbit around Saturn on July 1, 2004. On December 25, 2004, the Hyugens separated from Cassini and, with only a wake-up timer working on board, floated through space towards Titan. This phase lasted 22 days. Hyugens encountered Titan’s atmosphere on January 14. The wake-up timer revived all the instruments on board; parachutes deployed; and the probe settled though Titan’s atmosphere. The probe landed on Titan 2 hours and 30 minutes after entering the atmosphere. The Huygens transmitted data for another hour and twelve minutes after landing. Unfortunately, a software error caused 350 images to be lost – 350 were transmitted rather than the planned 700.

Titan

The Huygens probe, and subsequent observations from Cassini, showed a moon with ephemeral rivers, seasonal lakes, ice, and rain – but of methane, not water.The next image created from several images during the descent, appearr to show a dendritic river channel.

I’ve written about Titan before, in an earlier post. Warning: That post has some equations. Trust me, you can read it without worrying about the math. And it does have poetry. Check it out here.

Enceladus

Recall that one of the Voyagers acquired images of Enceladus (see above). Cassini found that Enceladus (Saturn’s sixth largest moon) was full of surprises. It is coated by clean ice, which reflects most of the incident light, and which covers liquid, salty water. In addition, jets of icy water gush out into space.

Saturn

Cassini returned stunning images of Saturn and her moons. One of Saturn’s most perplexing features is a hexagonal structure near the north poles, in which storms can be observed. For a video showing storms within the hexagon, click here.

Cassini acquired this image from behind (i.e., farther from the sun than) Saturn. Earth is visible as a pale blue dot.

The Cassini, like the two Voyagers and the Hubble, lasted longer than expected. In fact, NASA extended Cassini’s mission twice. Finally, in 2017, as its propellants were running out, it was put into a series of orbits that ended with a fiery plunge into Saturn’s upper atmosphere. Click here for more information on Cassini’s so-called Grand Finale.

The New Age of Discovery

Christopher Columbus discovered Puerto Rico on November 19, 1493, during what is known (by Europeans anyways) as the Age of Exploration. I submit the exploration of Saturn (and Mercury and Venus and Mars, etc.) show we are in a new, mostly unappreciated, age of exploration.

Some might say I’m too limited in this – these discoveries are the inevitable continuation of the scientific revolution. In his book Sapiens, Yuval Noah Harari speculates on the future of Homo sapiens. In his last chapter, he focuses on biomedical issues. He wonders if humans will bio-engineer themselves right out of existence and, if so, what will replace them.

I don’t know about that. I do know Saturn was a bright presence in the early evening sky a few weeks ago, and it was fun to look at it and think about how much has been learned about it and her sister planets.

Notes and Sources

All of the images are from NASA-sponsored websites and are thus in the public domain. To get to these sites, google Voyager 1 mission, or Cassini mission, or similar. The search results will include the NASA sites.

Harrari, Yuval Noah. Sapiens: A Brief History of Humankind. Harper Perennial, 2018 (Paperback edition). Highly recommended.

The featured image (the one above the title) was taken by Cassini two days before it burned up in Saturn’s atmosphere.