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Science / Space Race

[January 2, 1966] God of Time (The Planet Saturn)

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by Gideon Marcus

Out at the edges of the known universe, a stately sentinel makes its rounds.  Not brighter than its companions, it nevertheless impresses with its constancy, its deliberate pace.

To the ancient Greeks, the planet Saturn was one of the seven "wanderers" that included the Sun, Moon, Mercury, Venus, Mars and Jupiter.  Set in the outermost crystal sphere but one, that last being occupied by the stars, the farthest planet known before the invention of the telescope was named after the King of the Titans — possibly, because his association with time (the Greek name for Saturn is "Kronos" as in "chronometer" and "chronology") matched the long period of the planet's orbit.

For millennia, nothing else was known of this world — or even that it was a world.  Then, in the early 17th Century, Galileo eagerly turned his telescope up at Saturn and was surprised to not see a smooth disk or a crescent as with the other planets.  Instead, the planet seemed to have large bulges on either side.  Stranger still, when he observed Saturn later on, the bulges had vanished!

Telescopes got better, and observers (starting with Huygens in 1655) came to realize that Saturn has not bulges but (unique among the planets, at least so far as we can currently resolve) a system of rings.  There appear to be three: A diffuse inner one, an outer one, and a bright wide central one.  They are definitely not solid; one can see stars through them at times.  Also, different ends of the rings orbit at different rates, which is more evidence that they are composed of lots of little bodies.  The clincher is that the rings lie within what's called the Roche Limit, the area near a planet where its tidal forces are too great for a solid body of appreciable size to exist.  Indeed, someday in the distant future our own Moon may spiral in too close to the Earth and become a lovely ring.

As for why the rings seem to disappear, Saturn is tilted 28 degrees with respect to the plane of the solar system in which all of the planets circle the Sun, and thus at times, the rings are edge on to us and sometimes flared to full splendor.  At the former times, they virtually disappear.

Saturn has a host of Moons, all named after Titans (including one called Titan).  Thanks to them and Newton's laws, we have a very good idea as to the planet's mass.  Saturn is 95 times as massive as the Earth.  Measuring its disk, we know it is 72,000 miles wide at its equator.  Combine those two factors together and we find that Saturn is the least dense of all the planets, with a density less than that of water!  This has led to some silly books exclaiming that, were there an ocean large enough, Saturn would float on it.

If there were an ocean large enough to float Saturn, it would collapse into a hot sun under its own weight!

The low density and the fast day (10 hours and 38 minutes) makes for an oval-shaped planet, its most distinctive feature along with the rings.  The three rings seem to have big gaps between them, perhaps having been cleared out through gravitational resonance with one of Saturn's moons or another.

For a long time, this is all we knew about Saturn.  Aside from the rings, it was a rather dull planet compared to vibrant Jupiter, glowing Venus, or crimson Mars.  With the advent of the spectrograph and the radio telescope, we've learned a bit more.  We know from density models that Saturn, like Jupiter, is composed almost entirely of hydrogen and helium.  It thus earns the name "gas giant."  Spectrographs tell us that there is methane and ammonia in Saturn's atmosphere, though there is less of the latter, probably because it has snowed out of the colder air.  Beneath the cloud layers, the hydrogen is believed to be condensed into a vast ocean with an icelike core deeper down.

The moon Titan is particularly exciting.  Not only is it bigger than our Moon (a distinction shared with only three other moons in the solar system) but it is the only moon known to have an atmosphere of its own!  The astronomer Kuiper, in 1944, determined that it is composed at least in part of methane.  This distinctive property is the reason Titan has been one of the more popular settings for science fiction stories.

From the most recent journal articles, I have learned that infrared observations suggest that Saturn's rings are made of water ice.  This makes sense.  There is a boundary in our solar system beyond which water ceases to be volatile and instead becomes a common building material.  Most of the outer moons are probably icy, too.

Interestingly, whereas Jupiter blazes like a beacon in the radio frequencies, Saturn's staticy contributions to the airwaves remain faint and sporadic, if indeed they come from Saturn at all.  More observation will be required to clinch whether or not Saturn broadcasts to us via the shortwave bands.

It is likely that we can only learn so much from terrestrial telescopes.  Eventually, we'll have to go to Saturn and get more data first hand.  While I do not know of any planned missions to the outer planets, it is not hard to conceive of one launched by our powerful Atlas Centaur rocket or perhaps a more powerful Titan/Saturn Centaur combination. 

I have also heard from a friend at Jet Propulsion Laboratories in Pasadena that a clever grad student there may have come up with a way to explore the farther planets on the cheap.  Essentially, a probe can get a two for one deal (or perhaps as much as a four for one deal given the right circumstances!) by using the gravity of the first visited planet to accelerate it and direct it toward the next.  If this theory can be perfected, we could see a combined Jupiter/Saturn probe within the next five years.  By the end of the next decade, we may well be able to launch one that visits all of the four gas giants in turn.

Something to look forward to!

With this, the planetary series of articles is complete!  Be sure to check out all the others in the series and learn what we knew about each of the nine planets of the solar system before they were visited by spacecraft.  You can also read about what Mariner 2 taught us about Venus and the new Mars revealed by Mariner 4!




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Science / Space Race

[December 1, 1964] Planet Four or Bust! (What we know about Mars)

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by Gideon Marcus

Mars or Bust

This week, humanity embarked on its most ambitious voyage to date.  Its destination: Mars.

I use the term "humanity" advisedly, for this effort is a global one.  On November 28, 1964, the United States launched Mariner 4 from Cape Kennedy.  And just yesterday, the Soviet Union's Zond hurtled into space.  Both are bound for the Red Planet, due to arrive next summer. 

That both missions commenced so close to each other was not a coincidence.  Every two years, Earth and Mars are situated in their orbits such that a minimum of energy can be used to get from one planet to the other.  This favorable positioning applies equally to democracies and communist states.

Mariner 4 and Zond are not the first Martian probes: identical Mariner 3 was lost a few weeks ago, and Zond's predecessor, Mars 1, failed a couple of months before it could reach its target.  Let us hope these new spacecraft have more luck.  So far so good!

It is possible that these two probes will revolutionize our understanding of Mars, just as Mariner 2 changed our view of Venus forever.  It is, therefore, appropriate that I summarize our knowledge of the planet on the eve of collecting this bonanza of new information.

Another Earth?

Mars has been known to us since ancient times.  Because it wanders through the constellations throughout the year, it was classified as a "planet" (literally Greek for wanderer).  When it is in the sky, it is one of the brightest objects in the sky, with a distinct reddish tinge, which is why it has been associated with the bloody enterprise of war.

Until the invention of the telescope, all we knew about the fourth planet from the Sun was its orbital parameters: its year is 687 days, its path around the sun very circular, and its average distance from the Sun is around 141,600,000 miles. 

Even under magnification, Mars can be a stubborn target; at its nearest, about 35 million mies away, the planet measures just 25 seconds of arc from limb to limb (compared to the Moon, which subtends 1860).  Still, early telescopes were good enough to resolve light red expanses, darker expanses (believed to be seas), and bright polar caps.  Said caps waxed and waned with the Martian seasons, brought on by the planet's very Earthlike tilt of 25 degrees.  Because the Martian surface was visible, unlike those of Venus or Mercury, the day was calculated to be just over 24 hours long.  Indeed, Mars appeared to be a world much like Earth.

Mars for the Martians

In 1877, our understanding of the planet broadened.  Astronomer Asaph Hall discovered two tiny moons, named Phobos and Deimos, and we were able to deduce the mass of Mars — about 10.7% that of Earth.  Combined with the planet's diameter of 4200 miles, that meant Mars' density was about four times that of water.  This is only two thirds that of the Earth, which suggests that the planet is poorer in heavy metals, and/or that, because the planet is less massive overall, its layers are not so tightly bound together with gravity.  From Mars' measured mass and diameter, we learned that the surface gravity is 38% that of Earth; sprinting and jumping should be much easier there.  Flying…well, more on that in a moment.

1877 was also the year that Mars came into our public consciousness in a huge way — all because of a silly mistranslation.  Giovanni Schiaparelli turned his 'scope to Mars and saws something remarkable: dozens of fine straight streaks crisscrossing the planet that seemed to link up the dark patches (which were, if not oceans, at least areas of vegetation suggesting the existence of water).  He called them canali, which is Italian for "channels".  But to English ears, it came out as "canals", which strongly connotes construction by intelligent beings.

Well, you can see what an uproar that would make.  Very soon, folks like Wells and Burroughs were writing tales of Martian aliens.  And not just aliens — civilizations beyond those found on Earth.  The thinking went that the planets' ages corresponded to their distance from the Sun.  Hence, Mercury was a primordial hunk of magma.  Venus, shrouded in clouds, was probably a steamy jungle planet on which Mesozoic monsters roamed.  And beyond the Earth, Mars was a cold, ancient world, its verdant plains dessicated to red deserts.  To avoid catastrophe, the Martians built planet-spanning canals to bring water to their cities.  Being so advanced, it was obvious that they had mastered space travel, and had either visited us or were on the verge of doing so.

Even the more practical-minded scientists were hungry for evidence of life, even primitive stuff, existing off of the Earth.  Mars seemed like the prime location for extraterrestrial creatures to be found.  For one thing, the planet clearly had an atmosphere, wrapping the planet's edges in a haze and producing a marked twilight. 

Originally thought to be a touch thinner than Earth's, more recent measurement of the polarization of Martian light (the vibration angles of light reflected off the atmosphere) suggested that the surface air pressure was about 8% that of Earth.  That was too thin for easy breathing, but not too thin for life.  If there was enough oxygen in the mix, perhaps a person could survive there. 

Mars Today

Such was our understanding of the planet perhaps a decade ago.  Recently, ground-based science has made some amazing discoveries, and it may well be that Mariner and Zond don't so much revolutionize as simply enhance our understanding of the planet.

I just read a paper that says the Martian atmosphere is about a quarter as dense at the surface that thought.  This isn't just bad for breathing — it means NASA scientists have to rethink all the gliders and parachutes they were planning for their Voyager missions scheduled for the next decade.  Observations by spectroscope have found no traces of oxygen and scarcely more water vapor.  The planet's thin atmosphere is mostly made up of nitrogen and carbon dioxide.  The ice in the polar caps may well be mostly "dry".

Because of the lack of water and the thin air, erosion is probably much less of a factor on Mars.  In a recent science article in Analog, George Harper says that the planet's surface may be riddled with meteorite craters that never got worn away.  Close up, Mars may end up looking more like the Moon than the Earth!

And those canals? Telescopic advances in the late 40s made it possible to examine Mars in closer detail than ever before. The weight of astronomical opinion now disfavors the existence of canals.

Still, old dreams die hard.  I imagine we will cling to our visions of Martian life and even civilizations long after such notions are proven unworkable.  To kill such fancies, it'll take a blow as serious as that delivered by Mariner 2, which told us that it's hot enough to melt lead on the surface of Venus.

We'll find out, one way or another, in July 1965!

[Come join us at Portal 55, Galactic Journey's real-time lounge! Talk about your favorite SFF, chat with the Traveler and co., relax, sit a spell…]




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Science / Space Race

[November 10, 1962] Across the Ocean of the Night (the planet Neptune)

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[if you’re new to the Journey, read this to see what we’re all about!]


by Gideon Marcus

In the last planetary article, I discussed the discovery and nature of the seventh planet, Uranus.  It was the first sizable member of the solar system to be found since ancient times.  And yet, its very discovery sowed the seeds for the quick locating of the next planet out from the sun. 

Shortly after William Herschel spotted Uranus and deduced what it was, other astronomers realized that the green planet wasn't following a regular path around the sun.  Some invisible thing was tugging at it, causing it to deviate from its orbit.  Doing a little math, it was determined that this object must be a large planet, 30 times farther from the sun than the Earth, half again as far from the sun as Uranus! 

After a comparatively short search to find Planet 8, a Frenchman named Le Verrier discovered it in 1846, in a very neat application of orbital mathematics and organized observation, the likes of which may never again be repeated.  The English wanted to name the planet "Oceanus," but since the French found it, they chose the name: Neptune – Roman God of the sea and brother to Jupiter.

A wobble in Uranus' orbit led to the discovery of Neptune.  And, in fact, Neptune itself has a little wobble that led people, around the turn of the century, to believe one or two planets lay beyond the eighth planet.  Those planets were sought for and one little world was ultimately found in 1930 as a result, but whether Pluto is actually the cause of the wobble remains an open question.  An exciting 1957 article suggests that Pluto was once a moon of Neptune, ejected early in the planet's life, which could explain the smaller world's eccentric orbit.

So what is Neptune like?  In some ways, Uranus and Neptune are of a piece, both midway in size between the terrestrial worlds and the true giants, Jupiter and Saturn.  Uranus shows up as a small pale green disk in a telescope; Neptune is a blue circle half that size.  Neptune is a little more massive than Uranus, but also a little smaller.  Where Uranus has five moons.  Neptune appears to have just two.  Backwards-rotating Triton has been known almost since Neptune's finding, but little Nereid was just discovered in 1949.  Thanks to these moons, we know that Neptune has 17.26 masses of the Earth (compared to a little over 14 for Uranus).  Moons also tell us that Uranus is tipped over on its side; Neptune's axial tilt is 27%, very similar to the 23.5% of Earth's. 

The four terrestrial or rocky worlds are composed mostly of dense matter like metals and silicates, and the gas giants are made mostly of hydrogen and helium (like the sun).  Uranus and Neptune seem to be halfway planets, around 20% heavy stuff and 80% middling stuff, like methane, ammonia, and water – all of which, at the frigid temperatures of the outer solar system, should be in liquid or solid form. 

There is likely gaseous hydrogen and helium making up a tiny fraction of the planet's mass.  Observations in 1950 suggested Neptune has twice the density of Uranus, which would mean the atmosphere of hydrogen and helium would be thin, indeed, over a slush of methane, ice, and pressure-metallized ammonia, which in turn covers a solid core of something, about twice the mass of the Earth.

How do we know what's in Neptune's atmosphere?  A spectrograph takes visible light and separates into its components, like a prism.  Every element has a distinctive pattern when it is run through a spectrograph.  Scientists try to recreate the patterns that they see in a controlled environment.  For instance, the patterns seen in Neptune's diffracted light are most closely approximated by a mix of three parts helium to one part hydrogen at a temperature no greater than 78 degrees Kelvin (-351 degrees Fahrenheit).  Spectral "fingerprints" associated with methane have also been found.  This, then must be the general nature of Neptune's visible layer of air.

The spectroscope also tells us, based on the shifting wavelengths of light from the planet's edges (the Doppler Effect), that Neptune's day is 15.8 hours long.  That rapid spin bulges the planet like an egg, though to nowhere near the extent of, say, less-dense Saturn. 

And… that's it!  This is the entire sum of knowledge we have about the huge frigid sentinel near the edge of our solar system.  The blue orb is too far away for any surface features to be discerned, and no radio output has been detected.  Until we send a probe past Neptune, I'm afraid we will learn precious little more about the eighth planet.

Then again, at the rate our Space Race is going, Mariner 19 could be in the offing as early as the next decade…

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Science / Space Race

[May 9, 1962] The Chilly Frontier (Uranus, the Seventh Planet)

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by Gideon Marcus

Every so often, serendipity chooses what I write about.  Last month, the Traveler family Journeyed to the Seventh Planet in film.  Then, the Good Doctor wrote about the giant planet in his science fact article in The Magazine of Fantasy and Science Fiction.  And now, in this month's Galaxy, Willy Ley tells of the origin of the the names of our celestial neighbors, Uranus included. 

And there's a 7th Planet-sized gap in my series on the planets of the solar system.  Who am I to fight fate?


Uranus from a telescope

How much could we know about a world that is twice as far away from us as Saturn?  The answer is at once "more than you'd think" and "less than we'd like."

Uranus is a small green disc when viewed through a telescope.  In fact, the planet is technically visible with the naked eye, but it is so small that it is no surprise that it wasn't discovered until 1781.  Over the course of several late-Winter nights, a German expatriate living in England named William Herschel saw the fuzzy circle of Uranus slowly travel among the fixed tableau of the stars.  He thought he'd found a comet.  But its orbit and characteristics made it apparent that it was, in fact, the first new planet discovered in thousands of years.

Herschel tried to name the planet after his King, George III, just as Galileo had tried to name the Jovian moons he had discovered after his sovereigns, the De Medici family.  Others tried to name the planet after Herschel, himself!  In the end, a name of classical derivation won out – and what more fitting name than the father of Saturn, who was, himself, father of Jupiter, who was father of Mercury, Venus, and Mars?

Uranus hugs the ecliptic, the plane of the solar system, more closely than any of the other planets.  Using older observations of Uranus from before the object was recognized as a planet, astronomers quickly determined the new planet's year: 84 years.  We are fortunate that Uranus has moons (five of them, the latest discovered just 14 years ago), for we are able to determine the mass of the planet from the length of time it takes for the moons to orbit their parent.  There are 15 Earths of mass in the planet, the least of the four giant planets.  Nevertheless, you could fit 60 Earths inside Uranus.  That makes it the second-smallest in volume (Neptune has a volume of 40 Earths). 

You can tell how long the day of a planet is using a spectroscope, which breaks up light into its component wavelengths.  The waves of light coming from the side of a planet rotating toward us are compressed and made bluer.  The side going away reflects redder light.  This is the Doppler Effect – the same phenomenon that makes train whistles seem to rise and fall as the locomotives approach and recede. Uranus' day is just under 11 hours long.  This is slightly longer than Saturn's, and shorter than Neptune's.

So in terms of raw physical characteristics, Uranus is kind of a middlin' gas giant.  But there is one feature that makes it absolutely unique among the planets.  Thanks again to the trek of Uranus' moons, we know that the planet is tipped way over on its side with respect to the ecliptic – a whopping 98 degrees!  Compare that to Earth's slightly wobbled 23 degrees.  As you may know, this tilt is responsible for our planet's seasons; imagine what kind of severe seasons Uranus must have!  The Poles of the seventh planet are in perpetual sunlight for 21 years, in darkness for the same amount of time. 


Exploring the Planets, 1958, Roy A. Gallant

An observer on the surface of Uranus, if such a thing exists, probably wouldn't be able to tell the difference.  There is a 3000 mile thick atmosphere that we know contains methane, thanks again to the spectroscope.  Below that is an ocean of increasingly slushy hydrogen some 6000 miles thick.  By the time you get to solid ground, whatever that be made of, you can be sure that no light penetrates.  As at the bottom of terrestrial oceans, the surface of Uranus must be seasonless.

Now, while the edge of Uranus' atmosphere is a chilly 300 degrees below zero (Fahrenheit), it is certain that things heat up as one goes deeper into the pressure cooker of the planet's gaseous envelope.  It is even possible that an ocean of water floats at some level of the giant's composition, though we'll never know until we go there.


Exploring the Planets, 1958, Roy A. Gallant

The last bit we know about Uranus is a piece of negative information.  Over the last decade or so, we have turned the giant dishes pf radio telescopes toward the heavens and discovered all sorts of staticy emanations, some associated with things we can see, and some appearing to radiate from nowhere.  Jupiter, it turns out, is a chatty subject on the radio.  Uranus, however, is not. 

By the way, my favorite aspect of Uranus is the naming of its moons.  They are (closest in to farthest out) Miranda, Ariel, Umbriel, Titania, and Oberon.  Unlike Jove's mistresses that orbit Jupiter and the elder Titans that circle Saturn, Uranus' moons are named after the literary creations of Shakespeare and Pope.  The most ancient of Gods is thus attended by some of humanity's more recent fairies.


Uranus from a telescope

There you have it: virtually the entire sum of knowledge we have about the 7th planet.  Not a whole lot for nearly 200 years of observation.  However, I suspect that, with powerful rockets like the Saturn at our disposal, it won't be long before Uranus gets a new moon, one with a NASA sticker (or perhaps, a Sickle and Hammer) on the side.  Then we'll truly learn about this mysterious, grand, tipped-over world.


Classics Illustrated. Illustrated by Torres, Angelo, Kirby, Jack, and Glanzman, Sam. To the Stars!

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Science / Space Race

[Dec. 10, 1961] By Jove! (Jupiter, the fifth planet)

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by Gideon Marcus

An alien cataloging our solar system for an Encyclopedia Galactica might summarize our home in this brief sentence:

"Solitary yellow dwarf, unremarkable, with a single planet of note; also, a few objects of orbiting debris."

That may strike you as an affront given the attachment you have to one of those pieces of debris (the Earth), but from a big-picture perspective, it's quite accurate.  Of all the masses whirling around the sun, the planet Jupiter is by far the biggest.  It is, quite simply, the King of Planets.

As we stand on the precipice of planetary exploration, it is a good time to summarize what we know about this giant world, especially in light of recent discoveries made by ground telescopes.  Thus, here is the fourth in my series on the planets: Jupiter.

Let's start with the name.  Why did the ancient Romans choose to identify Jupiter with the King of Gods?  Certainly not based on its mass – how could they know that?  Jupiter is, however, one of the brightest objects in the sky.  It also is visible more often than the slightly brighter Venus.  Because it so dominates the night, it is not surprising that it got an imposing name.

For thousands of years, very little was known about Jupiter beyond its brightness and its motion among the stars – the latter being what flagged it as something that wasn't a star.  Then, around 1600, a fellow named Galileo built himself a telescope and aimed it at the planet. 

What a surprise that was!  Suddenly, this bright point of light was a disc.  More than that, it was blemished: alternating bands of light and dark defaced its face.  The biggest surprise?  An indisputable quartet of smaller bodies orbited the planet.  This was solid evidence that Earth was not the center of the universe.  So shocking was this discovery that it shook the very foundations of the Catholic Church, and Galileo's findings were suppressed.

Luckily for us, Rome was not the center of the universe, either.  The last three and a half centuries have seen hundreds of astronomers turning their 'scopes eagerly at the King of Planets, scribbling down their discoveries.  Here are a few:

Based on the motion of Jupiter's moons, we know its mass to be about 320 times that of the Earth.

Jupiter seems to be made mostly of hydrogen, like the sun, but without the size needed to fuse the stuff, like a star. 

There is a giant red spot on Jupiter that has been in existence at least since we started looking at the planet.  It appears to rotate, and not in a uniform manner.  It is probably some kind of atmospheric phenomenon.

The planet seems to have a day of about 10 days (based on the movement of what appear to be its clouds).

Jupiter has an axial tilt of just 3 degrees.  It essentially has no seasons in its twelve year trek around the sun.

Measuring the timing of the eclipses caused by the four "Galileian Moons" led to the first determinations of the speed of light.

Eight moons beyond those found by Galileo orbit the planet, though none of them are as big.  The latest was found just a decade ago, in 1951. 

There are stable points one sixth of the way ahead and behind Jupiter in its orbit (actually, such points of stability exist with all planets as part of their gravitational dance with the sun, but Jupiter's are the most pronounced as it is the biggest planet).  Inhabiting these points are swarms of asteroids that drifted in there over time and were trapped.  These bodies are named after heroes of the Trojan War, one point bearing Greeks and the other, Trojans.  As a result, these areas of gravitational stability are called the "Trojan Points." 

We still do not know if Jupiter has a solid surface.  Are there oceans of methane or ammonia floating around inside that huge volume, enough to swallow 1300 Earths?  Or does the hydrogen simply get denser and denser until it becomes liquid?  Is there a dense core in the middle?

Until we are able to send probes to Jupiter (and scientists are just starting to dream up missions involving the new, giant Saturn rocket), many of the planet's mysteries will remain unsolved.  But not all of them…

In the last ten years or so, a brand new way of looking at Jupiter has been developed.  Light comes in a wide range of wavelengths, only a very small spectrum of which can be detected by the human eye.  Radio waves are actually a form of light, just with wavelengths much longer than we can see.  Not only can radio be used to communicate over long distances, but sensitive receivers can tell a lot about the universe.  It turns out all sorts of celestial objects emit radio waves. 

Jupiter is one of those sources.  After this discovery, in 1955, astronomers began tracking the planet's sporadic clicks and hisses.  It is a hard target because of all of the local interference, from the sun, our ionosphere, and man-made radio sources.  Still, scientists have managed to learn that Jupiter has an ionosphere, too, as well as a strong magnetic field with broad "Van Allen Belts."  It also appears to be the only planet that broadcasts on the radio band.

Using radio, we will be able to learn much about King Jove long before the first spacecraft probes it (perhaps by 1970 or so).  It's always good to remember that Space Age research can be done from home as well as in the black beyond.  While I am as guilty as the next fellow of focusing on satellite spectaculars, the bulk of astronomy is done with sounding rockets and ground-based telescopes – not to mention the inglorious drudgery of calculations and report-writing, universal to every science. 

So just you wait, Jupiter. By hook or by crook, we'll soon figure out what makes you tick.  And click.  And hiss.

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Science / Space Race

[May 31, 1961] First from the sun (The planet, Mercury)

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For many of us, the motivation for reading science fiction is the opportunity to explore worlds beyond our own.  Only in fantasy can we fly to faraway planets and see the unusual sights they afford us.  But, as I try to convey in this column, science can also reveal places every bit as interesting as the those that are the fruits of imagination. 

For instance, there are eight planets besides the Earth whirling around the sun, each of them a wildly different orb from ours and each other.  Moreover, while we are still on the eve of a new era of observation, utilizing space probes like the recently failed Venera and the ambiguously targeted Pioneer 5, yet the progress of technology has revolutionized even ground-based observation.  Our conception of the planets has evolved significantly in the last half-century (to say nothing of a full century ago).  It boggles the mind to imagine what we might know in another fifty years.

Let me show you these worlds, as we know them today, and as we used to know them.  I've written about Venus, and I've written about Pluto.  Today is Mercury's turn.

Mercury was known to the oldest civilizations.  It was named after the swiftest of the Roman gods because, being the closest planet to the sun, it completes its trip around the star in the shortest time.  A hundred years ago, we knew very little about this little world, in large part because it is usually lost in the sun's glare; from our vantage, Mercury never strays far from its parent star.  We knew its period (year): 88 days.  We had a rotation (day): slightly longer than that of the Earth.  The latter was a guess – it seemed that some vague features could be resolved on Mercury's tiny disk, and since they did not move much from day to day, it was thought that Mercury's day must be similar to ours. 

We knew that Mercury has no moon.  This actually makes it harder to determine the size and mass of the planet; luckily, Mercury is occasionally visited by Encke's comet, on which it exerts a measurable pull.  From that and optical observations, it was guessed that Mercury was just over 3000 miles across, about fifteen times less voluminous than the Earth.  This made it by far the smallest planet in our solar system.  We knew nothing of the planet's tilt, and there was speculation that, if the seasons were severe enough, that life might survive at one of Mercury's poles.  The relative dimness of the planet, even taking into account its size, suggested that it didn't have much of an atmosphere – at least, not a reflective one. 

And that's it!  Not a big scientific haul for a planet that was closer to Earth, on average, than Mars.  Even the early science fictioneers had little to say about the planet: Edgar Rice Burroughs' Martians knew that Mercury, which they called "Rasoom," was inhabited by an advanced race, but nothing more.

Now we move to the present day…and we still don't know a whole lot about Mercury!  We do now know that Mercury must be airless or nearly so.  It would be hard for a planet so small to hold onto the energetic gases that make up an atmosphere, particularly a superheated one.  Additionally, whenever Mercury has crossed the disc of the sun, in an event known as a transit, observers have spotted no telltale halo that would betray the existence of air.  The romantic notion that life could exist on the planet seems forever excluded even from the realm of science fiction, though it should be noted that some mid-century polarimetry observations (measuring how sunlight scatters off of things) suggest that there is some Mercurian atmosphere. 

We still don't know much about the surface of the planet, but it is assumed that it mimics that of Earth's moon.  It has a similar color, and the difference in the density of light reflecting depends on Mercury's phase (both of the planets closer to the sun than the Earth exhibit phases, of course – from new, to crescent, gibbous, then full, and back again); this suggests that the planet's surface is rough.  Imagine giant Mercurian craters, jagged mountains, deep canyons, all more outsized than we generally conceive thanks to Mercury's light gravity.

And that 24-hour Mercurian day?  Well, there is another rotation scheme that fits the evidence even better.  If Mercury doesn't rotate at all, presenting one face to the sun at all times, as the moon does to the Earth, this also is consistent with its unvarying surface features over the span of several days.  In fact, given Mercury's proximity to the great gravitational pull of the sun, it is likely that Mercury is "tidally locked". 

Thus, one side of the planet is forever being broiled with terrific intensity, hot enough to melt lead!  Then you have the back side that never sees the sun.  It may well be the coldest place in the solar system – even more frigid than faraway Pluto.  Imagine an eternally dark landscape so cold that there could be lakes of hydrogen.  The dimmest of shadows would be cast by the rugged Mercurian mountainscape in the meager Venus-light.  Talk about bleakly exotic!

And at Mercury's ring of unchanging twilight, perhaps there is a temperate zone where life could yet flourish, especially if there is, though evidence be against it, a measurable atmosphere on the smallest of our solar system's worlds.  I suppose there may yet be stories to write about the first planet from the sun…

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Science / Space Race

[June 4, 1960] The Solar Frontier (Pluto: 9th "planet"?)

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Pluto is big news right now; no wonder since this year is the 30th anniversary of its discovery.  But what do we really know about this enigmatic ninth "planet?" (quotes used advisedly, more on this later.)

Not much.  We know that it is an average of forty Astronomical Units from the Sun; that is to say it orbits forty times farther from the Sun than does the Earth.  At this distance, its surface temperature must be a balmy -380 degrees Fahrenheit, cold enough to freeze almost all gasses.  We know that it reflects the sun's light ,displaying the feeble brightness of a 14th magnitude star–about 1600 times fainter than the faintest star that can be seen with the naked eye.  We have some guesses about its mass… which is how the body was found in the first place.  That remarkable story is worth review.

After the discovery of the 8th planet Neptune by measuring the wiggle it caused gravitationally on 7th planet Uranus' orbit, there was the fervent hope that finding further, unexplained wiggles in those outer planets' orbits would betray a 9th planet.  Famed astronomer and Mars enthusiast, Percival Lowell, spent the last years of his life trying to find it.  As it turns out, he did spot Pluto and even snapped pictures of it, but he took it for a star at the time, so slow is the planet's movement at the far end of the Solar System (similarly heartbreaking stories abound regarding early sightings of Uranus and Neptune.) In fact, the existence of Pluto was not confirmed until Clyde Tombaugh definitively found it, right around my 11th birthday, on February 18, 1930.

But is Pluto the planet Lowell was looking for?  "Planet X?"  There were doubts as soon as Tombaugh made his announcement.  For instance, per Lowell's calculations, for Pluto to have the effect it did on the orbits of the outer planets, it would have to have a mass seven times that of Earth (if, indeed, the effect is genuine–we haven't even mapped Neptune through a complete orbit yet, so the non-Neptune-caused Uranus wiggle is our only source of data).  Yet Pluto is so tiny, optically, that for it to have a mass that high, it would need to be a fragment of a dead, collapsed star.  In fact, early on, that's just what was opined by some–that Pluto was a piece of an old White Dwarf.

Well, soberer heads did the math and determined that, based on its size (computed from its brightness at its distance) and its confirmed effect on Uranus, Pluto couldn't have a mass of more than that of the Earth, and probably somewhere between .5 and 1 Earth masses, depending on who you ask. 

So, Pluto is not Planet X, which may still be floating out there.  One astronomer suggests that there is a big planet nearly twice as far from the Sun as Pluto perturbing Neptune's orbit.

Then the next question is: Is Pluto even a planet at all?  This is probably a good question to settle before everyone gets so comfortable with the idea that there are nine planets in the Solar System that they become stubbornly resistant to any change in that perception.

As early as 1936, a Raymond Littleton proposed that because of Pluto's unexpected tiny-ness and its strange orbit (it is tilted nearly 17 degrees to the plane in which all the other planets orbit, and the orbit is much less circular than that of the other planets–almost halfway to a comet's orbit), it is actually probably some rogue moon of Neptune that somehow got separated from the giant planet, perhaps via some primordial impact when the Solar System was formed.  Further evidence in favor of this hypothesis is the fact that Neptune's moon, Triton, orbits backwards, and at a weird angle.  Gerard Kuiper recently endorsed this origin story for Pluto.

My views tend to be more "Uniformitarian" than "Catastrophic," so I hold to the hypothesis of Dr. Frederick Leonard of UCLA.  In August 1930, just six months after Pluto's discovery, he suggested that Pluto might not be unique: "Is it not likely that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined to still be detected?"  Compare this to Ceres, the body discovered in 1801–it was once thought to be a planet, but it turned out to be the first of a new class of worlds, the asteroids. 

Is there a slew of Pluto-like objects in the outer solar system?  Only time, and more observation, will tell.

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Science / Space Race

Loveliest of Bodies (Venus, 7-25-1959)

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from Timothy Gleason

Ishtar, Aphrodite, Venus—whatever you call it, the brightest of star-like objects in the sky has mesmerized humans for the entirety of recorded history, and likely beyond.  It was among the first subjects of telescopic study, and you can bet it will be the first planetary target for space probes.

It is astonishing that, given all of the interest the Planet of Love has engendered, we know so little about our nearest planetary neighbor.  The 2nd planet from the sun, Venus is an orb of mystery–she has been most reluctant to divulge her secrets. 

Galileo saw that Venus went through phases, like the moon, and deduced that Venus was bright with reflected sunlight.  Through transits of the sun (which happen twice a century—I hope I live to see the next ones in 2004 and 2012), we have learned that Venus has a thick atmosphere.  More recently, it has been determined that Venus’ atmosphere contains several hundred times the concentration of carbon dioxide as ours.  Measuring the Doppler shift of the planet’s edges, it appears that the planet rotates very very slowly, perhaps taking months to complete a rotation. 

From the planet’s period and effect on other planets, we know that its mass is actually quite close to that of the Earth, just a touch smaller.  It almost certainly has a rocky surface.  For these reasons, many have called Venus “Earth’s twin.”

Being closer to the sun, it is likely that Venus is much hotter than the Earth, but it is difficult to say how much hotter.  It used to be thought that the surface of Venus might be a global, steaming ocean, but it is more widely believed now that there is no liquid water to be found, and perhaps precious little in the atmosphere.


From Exploring the Planets (c) 1958


From Exploring the Planets (c) 1958

But that’s all we know!  The clouds are too thick to see through, so we can’t see the surface as we can with Mars.  Perhaps in the near future, microwave and radio astronomy will be keys to mapping Venus, but for now, it’s just a silvery pearl.

Now, every year or so, when the alignment of planets is right, we have an opportunity to send a probe to Venus on a low-energy orbit that accommodates the largest payload for any given booster.  The next one will be in November.  Sadly, though NASA did have plans to launch a Venus mission this year, it has been pushed back due to development teething troubles.  A test satellite with prototype equipment will be sent into orbit next month; if it does well, it bodes well for a Venus probe, though such won’t launch until the latter half of next year.

It may well be that the Soviets beat us to the punch.  Look for a launch later this year (but note that their opportunities are slightly different from ours as they launch from a different part of the globe).

Next up—a review of the latest IF!

(Confused?  Click here for an explanation as to what's really going on)


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