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Rediscovering the Solar System

by Les Johnson

Water geysers on Saturn's Moon

Water geysers on Saturn's moon, Enceladus, as photographed by the Cassini spacecraft. (Image courtesy of NASA.)

The Solar System isn’t what it used to be. No, the Solar System hasn’t changed all that much, but our understanding of it certainly has. Dramatically. In the past fifty years, we’ve learned that Mars has water; Jupiter’s moon Europa has lots of water – an ocean, in fact; Neptune had a big blue spot, much like Jupiter’s red one… and then it didn’t; and Pluto, planet or not, isn’t alone out there. We’ve also learned, and only in just the last couple of years, that there are planets around other stars, forming stellar systems all their own. No, this isn’t the Solar System I learned about in school. It’s much more interesting.

When talking about the Solar System, we usually begin with the Sun. After all, it is the largest object around and firmly anchors the rest of the objects in the Solar System in orbit around it. Instead, we’ll begin our tour at the very edge and work our way inward.

The Oort Cloud

Comet Hyakytake

Comet Hyakutake. (Image courtesy of NASA.)

First we encounter the Oort Cloud, thought to begin at about 1 light year from the inner Solar System. From here, traveling at the speed of light, it will take us one year to reach the Sun. It is in the Oort Cloud that we find a repository of comets and small bodies that orbit in the outermost region of the Sun’s gravitational influence. These are the leftover building blocks of the Solar System, hurtling through space at temperatures nearing absolute zero. As our sun orbits the center of the Milky Way galaxy, it has occasionally passed close enough to other stars for these passing visitors to exert their own gravitation influence on the denizens of the Oort Cloud, causing some of them to come careening into the inner Solar System where we see them as the long period comets. It is estimated that there may be several trillion objects measuring about a mile in diameter or larger in this Cloud.

It is likely that in the 4.5 billion year history of the Solar System, we’ve come close enough to other star systems for us to share members of our Oort Cloud with theirs. Yes, some of the objects now circling our star might actually have originated elsewhere – from around a star now long-gone on its own trek around the Milky Way.

The Heliopause

Sun's influence in deep space

This artist concept shows the various structures making up the outermost regions of the Sun's influence in deep space and the relative locations of the Voyager spacecraft within them. (Image courtesy of NASA.)

We next encounter a region of space where the outward solar wind pressure – a more or less continuous gust of high-speed atoms and electrons shooting into the Solar System at over one million miles per hour -- is ‘balanced’ by the inward radiation pressure from all of the other stars in the galaxy combined. This region is called the heliopause and would immediately be detectable by anyone traversing it on the way to the inner Solar System. In the vacuum of deep interstellar space, there are about 1,000,000 atoms per cubic meter. By comparison, in the room surrounding you as you read this article, there about 10,000,000,000,000,000,000,000,000 atoms per cubic meter. When a ship enters the heliopause, the number of detectable atoms per cubic meter begins to increase. The amount of matter will still be small, when compared to that here on Earth, but it will be a significant increase above that found in the interstellar void.

The Kuiper Belt and Scattered Disk (including Pluto)

Moving inward, we next encounter the Scattered Disk and the Kuiper Belt. It is here that we find hundreds, if not thousands, of dwarf planets – the reason that poor Pluto was demoted from being known as a full-fledged planet to mere dwarf planet status. Objects here come in all shapes and sizes, from small rocks to larger objects, some larger than Pluto, that orbit the Sun in highly elliptical orbits. (Elliptical orbits are those shaped like eggs, rather then circular.) These objects orbit the Sun between 30 and 100 times the Earth-to-Sun distance.

Now, let’s talk about Pluto. When NASA’s New Horizon’s mission was launched in 2006, it was billed as the first spacecraft to visit the last unexplored planet in the Solar System. Ironically, that same year, Pluto lost its status as a planet and was instead determined to be one of many Scattered Disk and Kuiper Belt Objects and was thus recategorized as a dwarf planet. How is Pluto different than the other planets? Since I’m frequently asked this question, I’ve made a list. It is important to note that no single item on the list would (perhaps) be enough to warrant reclassification of Pluto from its status as a full-fledged planet. Taken together, however, the case becomes compelling:

  • Pluto’s orbit around the Sun is highly elliptical, more so than any other planet.
  • This elliptical orbit causes Pluto to sometimes be closer to the Sun than the planet Neptune.
  • Pluto’s orbit around the Sun rises significantly out of the ecliptic plane. The ecliptic plane contains the orbits of all of the other planets. Put another way -- the orbits of the planets around the Sun lie mostly in the same plane. One planet doesn’t orbit the Sun’s equator and another its poles. Rather, all of the planets orbit the Sun roughly around its equator. All of the planets, except, of course, Pluto…
  • It has not cleared the neighborhood near its orbit of other material and debris objects.

Pluto is not the most massive object in the Kuiper Belt. That distinction, for now, belongs to the dwarf planet Eris, discovered in 2005. Eris, and its moon Dysnomia circle the Sun at a distance of about 96 times the Earth-to-Sun distance. Eris is at least 25 percent more massive than Pluto.



Neptune photographed up close and personal by the Voyager spacecraft. (Image courtesy of NASA.)

Moving inward, we encounter the blue jewel of the Solar System – Neptune, the outermost of the gas giant planets. Prior to 1989 we didn’t know much about this giant blue ball of hydrogen and helium. In that year, the plucky Voyager spacecraft flew by Neptune on its 74,000-year journey to the stars. The pictures and other scientific data it sent back provided the information that now fills our textbooks. To the surprise of many, Neptune’s gaseous atmosphere was not blah, featureless and colorless, as might be expected for a planet so far from the warming light from its parent star. Rather, it bedazzled with a brilliant blue color and sported storms racing across its upper atmosphere, some with wind speeds in excess of 1200 miles per hour. One of these storms, see by Voyager 2, is known as the Great Dark Spot and was determined to be larger than the entire Earth. Mysteriously, when the Hubble Space Telescope looked at Saturn in 1994, the Great Dark Spot was nowhere to be seen! Jupiter’s Great Red Spot seems to be a constant feature of the planet and has been seen for centuries.

Did I mention that 11 of Neptune’s 13 moons were undiscovered before the Space Age? The Voyager spacecraft alone discovered 6 of them. And, like the planet Saturn, Neptune has rings.



Uranus as seen by Voyager. (Image courtesy of NASA.)

Uranus, discovered in 1781, is another planet about which we knew very little until the Space Age. Like Neptune, it is a gas giant composed mostly of hydrogen and helium gas. And is it ever a strange place. Unlike every other planet in the Solar System, its axis of rotation (the way it spins) is not aligned with the ecliptic. In addition, the north rotational poles of most planets point up and out of the ecliptic, with their equators being roughly aligned with it (the plane of the ecliptic). But not Uranus! Instead, for some as-yet unknown reason, it orbits the Sun with one of its poles pointing at the Sun and its equator perpendicular to the ecliptic. (Imagine a top spinning sideways on a table instead of on its tip.) This is very strange and there is not a widely accepted theory to explain it.

Uranus also has rings, and ten of its fifteen moons were not known to exist until Voyager 2 flew past in 1986. The pictures Voyager sent home were of a rather bland appearing world with not much going on. After careful analysis of its data, we now know there are fierce winds blowing through its upper atmosphere.



This spectacular image of Saturn was taken by the Cassini spacecraft as it orbited the planet. (Image courtesy of NASA.)

Next to the Earth, Saturn is arguably the most beautiful planet in the Solar System. I fondly recall the first time I used a telescope to look at Saturn. I was about 12 years old and using a rickety old department store refractor telescope. The image of the planet and its majestic rings nearly knocked the breath out of me. I could not believe I was looking at a real planet and not a painting. The view was so exciting that I ran into the house and forced my entire family to come outside and share the experience. The magic of its beauty has not gone away as we’ve learned more about the second largest planet in our Solar System.

Until the 1970s, we thought Saturn was unique in having rings. We now know that it is not alone in that regard, but it does, by far, have the most majestic ring system of any of the gas giants. NASA’s Pioneer spacecraft visited Saturn in 1979. Next came the Voyager spacecraft as they flew by on their way toward the outermost planets. A quick spacecraft “flying by” will tell you a lot about a planet and its moons, but not as much as a spacecraft sent to orbit it and gather scientific data for years, rather than months. NASA sent the Cassini orbiter, launched in 1997, which entered Saturn’s orbit in 2004 -- where it operates still. Saturn has 62 moons; only 9 of them were known to exist before 1960.

What have we learned about Saturn just in the last fifteen years? A great deal. In fact, we’ve learned so much that several books could be written about Saturn and its moons. Here are some highlights:

Saturn’s moon, Titan, has lakes, rivers, mountains, sand dunes, clouds and even snow! Though before you get your winter gear out for a quick camping trip, you should note that the average temperature on Titan is -290 degrees Fahrenheit and that those lakes and rivers are made of methane and ethane, not water. The Huygens Probe, carried to Saturn with the Cassini orbiter, took this image on the moon’s surface. (Image courtesy of NASA.)


The moon Enceladus is covered with snow (yes, the kind of snow made from water) and thus reflects nearly all of the light falling on it – making it very, very cold. (Image courtesy of NASA.)

Star Wars fans should just love the moon Mimas for its distinctly Death Star shape. Until our spacecraft visited, Mimas was visible in our telescopes as just a small dot in a mosaic of moons circling Saturn. (Image courtesy of NASA.)


Another glorious picture of Saturn taken by the Cassini orbiter. In this view, the Sun is in eclipse directly behind the planet. (Image courtesy of NASA)

Saturn’s rings are mostly composed of millions of pieces of water ice and small rocks. The rings have a diameter of approximately 155,000 miles but, to just about everyone’s surprise, they are less than one mile thick. And we still don’t know how the rings formed…



Jupiter, the largest planet in the Solar System, could contain more than a few Earths. Shown here, to compare their relative sizes, is an image of the Earth -- on the same scale -- with an image of Jupiter. Note the relative size of Jupiter’s Great Red Spot, a massive hurricane moving through the Jovian atmosphere, as compared to the size of the Earth. (Image courtesy of NASA.)

Jupiter is easily visible to the unaided eye and has been observed by our ancestors ever since they began looking into the skies. The best way to introduce Jupiter is to talk about its size: It is much smaller than the Sun, having only one one-thousandth of its mass but 2.5 times the mass of all the other planets in the Solar System combined. It has at least 66 moons, and one of them, Ganymede, is larger than the planet Mercury! Several spacecraft have visited the planet, with the Galileo orbiter making detailed observations of the planet and its moons from 1995 until 2003. Only 16 of its 66 moons were known to exist before the year 1975.

Jupiter has some of the most dramatic moons in the Solar System, and we were blissfully unaware of their being so interesting until we actually sent robotic probes there to check them out.

First, there’s Europa. Only slightly smaller than our own Moon, Europa is covered in what appears to be a salt-water ocean and ice. Though it is very cold, the water on Europa is not completely frozen as might be expected. It appears there is liquid water under the surface kept warm by the heating of the moon from tidal forces resulting from its movement around Jupiter.

Hmmm. A moon almost as large as a planet that has liquid water and an external supply of energy (the tidal heating). This almost sounds like a place that might harbor life. Prior to the Space Age, Europa was a smudge in a telescope’ s picture of Jupiter. (Image courtesy of NASA.)

Io, another of Jupiter’s moons, has hundreds of active volcanoes and is the only other body in the Solar System known to have such. Prior to the Space Age, Io was simply another smudge. (Image courtesy of NASA.)

The Asteroid Belt


Vesta, the second largest object in the Asteroid Belt, as photographed by NASA's Dawn spacecraft. (Image courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Banish from your thoughts that the asteroid belt is filled with rocks and boulders careening hither and yon, waiting to strike a helpless starship trying to escape the evil empire. Yes, within this belt of rocks that circle the Sun between the orbits of Jupiter and Mars you will find over a million asteroids measuring at least a half mile in diameter. But don’t forget that space is big. Actually, space is really, really big and the chance of randomly hitting an asteroid as you move through the Asteroid Belt is very small. Ceres, the largest object in the Asteroid Belt, is now considered, like Pluto, to be a dwarf planet. With a diameter of over 500 miles, it is the only dwarf planet in the inner Solar System. Vesta, the second largest object in the Asteroid Belt is a mystery no more thanks to NASA’s Dawn spacecraft which took this photograph in 2011.

Now let's look at a planet that has water, an axial tilt giving it seasons, a day almost 24 hours long, and geographic features such as mountains, valleys, plains and (dry) lake beds. Earth? No, Mars…



Mars as seen by NASA's Viking Orbiter in 1976. This image was created from over 100 images of Mars taken by the Viking Orbiters. (Image courtesy of NASA.)

Mars has been the fascination of those believing in extraterrestrial life since before it was fashionable to believe such life might exist elsewhere to begin with. Its blood red color led our ancestors to associate it with war. Much later, astronomers imagined seeing canals on its surface – which was perceived as evidence of an advanced civilization modifying their world as we humans were modifying our own. Unfortunately, even the best telescopes were unable to see Mars as the dry, desolate place we now know it to be – thanks to the many spacecraft we’ve sent into its orbit and the rovers we’ve sent to wander its surface. Earthbound telescopes were able to determine that Mars does have frozen water and carbon dioxide at its poles. While we haven’t yet sent people there, we now know a great deal about Mars and its potential for harboring life.

Until Mariner 4 flew by Mars in 1965, some astronomers thought there were lakes of liquid water on its surface. Mariner, along with subsequent space missions, instead revealed a planet mostly devoid of surface water but with evidence that it was once abundant there. Landers and rovers have inspected the planet’s surface and found water ice in many places, including right under the feet of the Phoenix, which landed on Mars in 2008. The water frozen at the planet’s south pole, if thawed, could cover the entire planet in about 33 feet of water.

Mars tracks

This is my personal favorite photograph taken on the surface of Mars. Shown are the tracks made in the Martian dirt made by a rover from Earth. We made those tracks on another world! (Image courtesy of NASA.)

Mars has the highest known mountain the Solar System, Olympus Mons, and a canyon, similar in features to The Grand Canyon, that is as long as the European continent.

Was there life on Mars? Is there life on Mars? Will there ever again be life there? These questions may remain unanswered until we actually send people there to explore.



“Earthrise” as seen from Apollo 8, the first human mission to orbit the Moon in 1968. While in orbit, Command Module Pilot Jim Lovell said, “the vast loneliness is awe-inspiring and it makes you realize just what you have back there on Earth.” (Image courtesy of NASA.)

Earth, with its icy poles, huge oceans, and richly varied continents, remains the only place in the universe known to harbor life. It may come as a surprise to many how much we’ve learned about our own planet in the last fifty years.

First of all, before Sputnik, we didn’t even have a picture of our own planet showing it to be the round ball in space we long knew it to be. The Apollo astronauts took the iconic image (seen in the figure above) of our planet as they circled the Moon, viewing their home world come above the lunar horizon. The image, popularly known as “Earthrise,” subsequently became famous and is one of the most viewed images ever taken from space.

Did you know that prior to the launch of Explorer 1 in 1958, we didn’t have confirmation that Earth possessed radiation belts like those seen around other planets in the Solar System? These radiation belts form part of the Earth’s magnetosphere, which protects the planet from much of the many highly energetic radiation coming from space that would otherwise irradiate, with potentially disastrous effect, the life that exists on the surface beneath it. The strength of the Earth’s magnetosphere is second only to that of Jupiter.


Astronauts in the International Space Station photographed this volcano erupting in the Kuril Islands, near Japan, as they flew over in 2009. (Image courtesy of NASA)

Only in the last 50 years, and mostly due to space satellites, have we been able to continuously study the Earth’s climate; the global water cycle; our oceans, much of what is in them, and how they drive the climate; land surface features; global vegetation patterns; the effects of air and water pollution, etc. We’ve learned far more about our home in the last hundred years than in all the centuries prior combined.



Two images of Venus: The one on the right is how Venus looks to the naked eye. The one on the left shows the surface of the planet as seen by the Magellan spacecraft that used its radar to peer though the dense clouds that otherwise hide it from view. (Image courtesy of NASA/JPL/RPIC/DLR.)

Next we’ll visit a world that many view as the planet most similar to Earth: Venus. Yes, it is similar to Earth in size (95 percent of Earth’s diameter), mass (82 percent of Earth’s mass), and distance from the Sun (73 percent of Earth’s distance). But it is there that the similarities end. How do clouds of sulfuric acid sound? What about an atmospheric pressure at the surface that is 92 times greater than the atmosphere which surrounds you now? (Venus’ surface pressure is about the same as being under 3000 feet of water!) Better yet, a surface temperature of 860 degrees will greet us should we ever decide to send a human crew there to explore. Current theories are that Venus at one time had liquid water on its surface – until it experienced a runaway greenhouse effect that evaporated it all, leaving the planet’s surface a desert. The thick atmosphere and the dense gases within it prevent us from seeing the planet’s surface from space. The entire planet is under perpetual cloud cover.

In the 1970s, the Soviet Union sent landers that actually touched the surface of Venus and survived long enough to send back pictures. (What? You don’t understand why engineers have difficulty engineering spacecraft to survive true acid rain at high pressures and in intense heat? I guess sometimes rocket scientists make it look too easy!) The Russian Venera Program successfully landed a series of spacecraft on the planet, taking the only visible light photographs of its surface.


The surface of Venus as seen by the Soviet Union's Venera Lander. (Image courtesy of the USSR/NASA National Space Science Data Center)

To better understand the terrain on Venus, NASA sent the Magellan spacecraft there in the late 1980s. Magellan used high-powered radar to map over 98 percent of the planet’s surface during its mission life. Stitching together the many strips of radar data produced a stunning portrait of the planet’s surface and terrain, allowing armchair explorers to mentally fly down close to the surface, past mountains and valleys, all within the confines of their own homes.


The surface features of Venus, as mapped by the Magellan spacecraft's radar. (Image courtesy of NASA.)



Mercury, as photographed by the Messenger spacecraft. (Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.)

Just when you thought that things couldn’t possibly get worse than Venus, along comes poor Mercury. Barely a third the size of Earth and orbiting the Sun at just less than half the Earth-to-Sun distance, this rocky planet has been baked by the Sun and bombarded with meteorites for billions of years. When you look at a picture of Mercury, at first glance, and if you are not careful, you might think that you are looking at the Moon. Both bodies are covered with impact craters that haven’t changed much in the millennia after which they formed – unlike on Earth, there is no atmosphere on either to cause "weathering." Mercury might have once had an atmosphere, but, if so, then the fierce closeness of the Sun long-ago would have boiled it away into space.

The sunward side of the planet can reach temperatures of nearly 800o F and the side facing deep space as cold as -315o F.

Sol – our sun


Sol, our star. (Image courtesy of NASA)

How often we take for granted the giant fusion reactor that lights up our day. Being 93 million miles away lures us into a false sense of security in our ignorance of the hellish ball of plasma that keeps the planets in their orbits and provides us with heat and light. If the Sun were to "wink out" and go away, we wouldn’t know it happened until about 8 minutes later when the light stopped shining and the (lack of) pull from its gravity sent the Earth careening into deep space. (It takes about 8 minutes for light, traveling at 186,000 miles per second, to reach us from the Sun. It is presumed that the gravitational effects are also limited to the speed of light.)

Powered by the same process that man has harnessed for hydrogen bombs, the Sun has a diameter of over 865,000 miles – about 109 Earths would fit side-by-side across the Sun’s equator. To put it another, equally awe-inspiring way, over one million Earths would fit inside the Sun.

The mass of the Sun alone accounts for over 99 percent of the total mass in the Solar System. Its fusion power source is slowly turning its abundant hydrogen into helium at a rate of over 881,000,000,000 pounds every second! Some of this gas is flung outward into space in the form of Coronal Mass Ejections (CME’s) or solar flares. These are in addition to the ever-present solar wind. Burning over 800 billion pounds of hydrogen per second and streaming the solar wind won’t make the Sun go out anytime soon. At the current rates of hydrogen-to-helium conversion, our Sun should still be shining for another several billion years.

The Solar System – One of Many

The tour of our Solar System is over. But the journey and discovery is just beginning. We are poised to learn details of other stellar systems now that we know they exist. Thanks to more powerful ground and space-based telescopes, new astronomical techniques, and the results of the Kepler spacecraft, scientists have documented the existence of at least 750 other planetary systems similar to our own. Our observations of them to-date are reminiscent of the observations of the planets in our Solar System made prior to the invention of the telescope. For now, most of these extrasolar planets are known only by their relative size and the distance at which they orbit their parent star.

I suspect in the next fifty years, someone at Baen will write an article like this – but it will be filled with fantastic details of these now-mysterious other worlds circling other stars.

Baen author and anthology editor Les Johnson is also the Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama. Johnson and Jack McDevitt are the editors of Going Interstellar, a collection of science and science fiction stories on interstellar travel using known technology.

Copyright © 2012 by Les Johnson

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