Tag Archives: planet

[January 15, 1963] Venus’ true face (Scientific Results of Mariner 2)

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

by Gideon Marcus

Remember five years ago, when Explorer 1 was launched?  At first, the big news was that America had answered Sputnik and joined the Space Age, but it soon became clear that the flight had larger significance.  For Explorer discovered the giant bands of hellish radiation that girdled the Earth, particles trapped by the Earth’s magnetic field.  Until 1957, these “Van Allen Belts” had been virtually unsuspected.  With one flight, our conception of the universe had drastically changed.

It’s happened again.

Mariner 2 is humanity’s first successful mission to another planet, and the scientific harvest is absolutely enormous.  Moreover, thanks to recent changes in policy, the initial results of this harvest were released unprecedentedly quickly (scientists are now reporting upon submission and acceptance of papers rather than publication).  Just one month since the probe’s encounter with Venus, the flood of information has been almost too much to parse; nevertheless, I think I’ve gotten the broad strokes:

Getting there is half the fun

Before I talk about Mariner’s encounter with Venus, it’s important to discuss what the spacecraft discovered on the way there.  After all, it was a 185 million mile trip, most of it in interplanetary space charted but once before by Pioneer 5.  And boy, did Mariner learn a lot!

For instance, it has finally been confirmed that the sun does blow a steady stream of charged particles in a gale known as the “Solar Wind.”  The particles get trapped in Earth’s magnetic field and cause, among other things, our beautiful aurorae. 

Mariner also measured the interplanetary magnetic field, which is really the sun’s magnetic field.  It varies with the 27-day solar rotation, and if we had more data, I suspect the overall map of the field would look like a spiral. 

Why is all this important?  Well, aside from giving us an idea of the kind of “space weather” future probes and astronauts will have to deal with, these observations of the sun’s effect on space give us a window as to what’s going on inside the sun to generate these effects. 

One last bit: along the way, Mariner measured the density of “cosmic dust,” little physical particles in space.  It appears that there’s a lot of it around the Earth, perhaps trapped by our magnetic field, and not a lot in space.  It may be that the solar wind sweeps the realm between the planets clean.

Unattractive planet

Given how magnetically busy the Earth is, and since Jupiter fairly crackles on the radio band thanks to its (likely) magnetic dynamo, one would expect Venus to impact its local space environment.  Nope.  In fact, Mariner 2 flew past the second planet without detecting a trace of Venusian magnetic field, nor any concentration of space dust around the planet.  Now, it’s possible that Venus has a weak field, or that its field is so oddly shaped that Mariner just hit a low patch, but the simplest explanation is usually the right one — Venus has no magnetic field.

Taking her temperature

Right up until December 14, some scientists (and many writers!) had held out hope that the thick clouds of Venus hid a reasonably hospitable surface, potentially teeming with life.  Earth-based sensors had indicated that the Venus was unbearably hot, but such could be explained by an unusually active Venusian ionosophere.  But as Mariner 2 turned its microwave and infrared radiometers across the face of Venus, it was clear that the edges of the planet were cooler than the center.  This is what one would expect from a hot surface, cooler atmosphere; the reverse would be expected of the “hot ionosphere” model.

So how hot is Venus?  At least 400 degrees Kelvin (260 degrees Fahrenheit), and probably a lot more.  There’s no way there is any liquid water under that hellish greenhouse of carbon dioxide.  Moreover, it’s not any nicer at night time.  There appears to be no real difference in temperature between the illuminated and dark halves of Venus, probably for the same reason the Earth’s oceans run a fairly consistent temperature – Venus’ atmosphere is thick enough for efficient distribution of warmth. 

Amtor dispelled

Mariner 2 and terrestrial radar have determined that the Venusian day incredibly long (~250 days, backward with respect to the other planets), but the Venusian winds blow across the planet far faster than the planet rotates; clouds have been seen racing around the disk of Venus in just 4-5 days.  Recent radar observations indicate that Venus’s surface is smoother than that of the Earth or the Moon. 

This, then, is our new picture of Venus.  It is a truly hellish place, more worthy of its less common moniker, Luciferos — a bleak, half-lit world scoured by hurricane-strength sandstorms hot enough to melt lead.  Bradbury’s All Summer in a Day, not to mention Burroughs’ “Venus” series’, will need some serious revision. 

Details, details

One of the nice things about sending a probe far from Earth is it allows for more accurate measurement of basic units – like the distance of the Earth and Venus from the sun.  This will help in future expeditions, manned and unmanned.  Another bit of bounty from Mariner’s flight is a refinement of the mass of Venus.  It is 81.485% that of Earth – one of the few ways Venus remains “Earth’s Twin.”

What’s next?

Opportunities to explore Venus occur every 19 months, when the second and third planets of the solar system are aligned in their orbits for easy travel.  Mariner 2 was so successful in its mission that NASA has canceled plans for a repeat flight in 1964.  Rather, the space agency will focus on Mars that year and follow up with Venus later, perhaps 1965. 

One reason to launch a new probe to Venus sooner rather than later is, despite the wealth of information passed back by Mariner 2, we did not get a single photograph of the planet.  That’s because the spacecraft was too small to carry the transmitting equipment required to send back pictures from so far away.  But by ’65, the new Centaur booster stage will have replaced the weaker Agena, which will allow a beefier payload. 

In the meantime, telemetry is worth a thousand pictures.  For now, let us revel in this scientific bonanza. Venus may not be a great place to live, but visiting has paid off tremendously.

(that’s rolls of data, not paper towels)

[P.S. If you registered for WorldCon this year, please consider nominating Galactic Journey for the “Best Fanzine” Hugo.  Check your mail for instructions…]

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

[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, twice as far from the sun as Uranus! 

After a comparatively short search to find Planet 8, the Frenchman Le Verrier discovered it in1846, 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…

[August 27, 1962] Bound for Lucifer (the flight of Mariner 2)

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

by Gideon Marcus

If familiarity breeds contempt, then enigma must breed fascination.  So it has been with the planet Venus.  “Earth’s twin” in size and density, the second planet out from the sun is, in fact, the closest planet to us.  Yet, thanks to its shroud of clouds, very little can be determined of its nature.  At least, such was the state when I wrote my first article on the planet just three years ago.

Things are changing.

Opened eyes improve vision of Venus

Until recently, humanity was limited to examining the universe in the narrow band of light frequencies discernible to the eye.  That’s actually a tiny portion of the electromagnetic (EM) spectrum, which ranges from super-high frequency gamma rays, down through X-Rays, microwaves, and ultraviolet light, passes quickly through the visual light spectrum, and then to the lower-frequency infrared and radio waves.

In the last decade, we have developed ways of probing many of these EM bands from the Earth’s surface, and they have begun to reveal Venus’ true nature.  For instance, measuring microwave emissions from the planet, we find that the dark side simmers at a whopping 650 degrees Kelvin (710 degrees Fahrenheit).  Radio wave measurements seem to confirm this figure. 

The atmospheric pressure at “sea level” is some 50 times greater than on Earth.  It is not certain what components make up the Venusian atmosphere, but likely gases are Carbon Dioxide, Nitrogen, and water, in order of amount.  This combination is what causes the planet to swelter so – the air creates a greenhouse effect, trapping heat like a blanket.  The surface of Venus is probably like an oven, extremely dry (despite the potential for water vapor in high clouds), dimly lit by a blurry yellow sun, largely windless, and extremely inhospitable.  So much for the jungle-covered Amtor of Edgar Rice Burroughs.

Using radar, scientists have learned that Venus is more reflective than the moon (presumably the surface, or whatever the waves are bouncing off of, is smoother).  It has also been determined that Venus, if it rotates at all, does so extremely slowly.  A Venusian day may well be as long as its year: 225 days.  Scientists have used radar observations to confirm the greenhouse atmospheric model over others that had been advanced in the absence of data.  Radar also has given us a better idea exactly how far away the planet is from us, a critical piece of information for plotting the course of investigating spacecraft.  Which brings us to…

Let the onslaught begin

Every 19 months, the Earth and Venus are as favorably aligned in their orbits as they can get; that is the opportunity to send the heaviest spacecraft (i.e. with the most experiments) to investigate.  The first chance of the Space Age to send a probe to Venus took place in summer of 1959 – too soon for either superpower to loft a probe.  The United States did send up Pioneer 5 to the orbit of Venus in March 1960 to test long distance communications, however. 

The next alignment took place in February 1961.  No American probe was ready, but the Soviet http://galacticjourney.org/tag/venera-1/Venera 1 almost made it to Venus before mysteriously going silent. 

19 months have elapsed again, and this time, both major participants in the Space Race are ready.  Just a few days ago, the Soviets launched another Venera.  It failed to depart Earth’s orbit and will likely decay in a few days, but I can’t imagine it will be their only attempt.  Last month, America’s first try, Mariner 1, veered off course and had to be destroyed after only five minutes in flight.

Of course, I wouldn’t be talking about this if I didn’t have good news.  This morning, a new Mariner rose to the heavens atop an Atlas Agena rocket, and this one is safely on a course for the Planet of Love.

It’s a little probe, really a close cousin to the Ranger probes that have had such ill luck with the moon.  NASA had hoped to send a larger spacecraft, but the new Centaur second stage booster isn’t ready yet.  So the Agena-propelled Mariner carries just 40 pounds of equipment.  There’s no camera onboard, for Mariner lacks the cargo to carry a strong enough transmitter to send pictures. 

But there are several experiments that will be just as valuable.  For instance, there is a pair of radiometers that will tell us, once and for all, just how warm Venus really is.  There are a series of particle counters that will measure radiation both on the way to and in the vicinity of the planet.  This kind of exploration of interplanetary space has only been done once before, and it tells us volumes about the sun and how it affects us.  We will also learn about the fields of electrical force surrounding Venus.

To that end, Mariner 2 also carries a magnetometer, designed to tell us the strength and disposition of Venus’ magnetic field.  I’ve got a personal stake in this little experiment as two good friends, Chuck Sonett and Paul Coleman, are vital members of the team that built it.  These fine fellows worked in the private sector on Pioneer 5, and now NASA has seduced them onto the government payroll.  A win for the United States, I’d say!

So stay tuned.  Mariner will reach Venus in December, and if the probe still be active come then, you can bet there will be a bonanza of scientific results – and you’ll be able to read all about it at Galactic Journey!

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

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!

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

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.

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

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…

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

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.

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

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.

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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