Satellite Imagination: Pioneer’s Near Miss

In 1961, UCLA grad student Michael Minovitch (image here) figured out the gravity assist maneuver for space travel. The young mathematician, working for the Jet Propulsion Laboratory (JPL), crunched some numbers. And the numbers showed that if a rocket was aimed carefully at a planet at just the right time, its course could be bent to arrive at a second planet with a savings in both fuel and time.

A few years later, another grad student spending his summer at JPL, Gary Flandro (image here), was tasked to design some exploratory missions to the outer planets: Jupiter, Saturn, Uranus, and Neptune. He noticed that all four of them would align in the late 70′s to permit one space probe to visit two, three, or even all four of them. And better yet, it could get to Neptune in something like twelve years, as opposed to forty or more if it flew there unassisted. It seemed too good to be true. But NASA had to get hopping. These alignments only occurred every two centuries. Could they put a mission together in a decade?

In those days, NASA had just gotten its feet wet with single-planet missions. Mariner 2 flew by Venus in 1962 and Mariner 4 visited Mars in passing three years later. But eventually the muckety mucks came around and the Grand Tour was rolled out for congressional approval in 1971. The following year, its budget was sliced, and scientists cut back from four to two planets, and from four to two probes.

On the bright side, Dr Flandro’s idea was successfully employed as the 10th Mariner, aimed first at Venus, and later gave us our first close-up views of the planet Mercury.

So the JPL scientists poured their efforts into Voyager space probes to each visit Jupiter and Saturn. They secretly kept their options open for Uranus, Neptune, and Pluto. But they didn’t tell Congress.

To guide this “stealth” mission to the way-out planets, Voyager would have to survive a fairly close pass to Saturn’s rings. Potential problem: Saturn is almost a billion miles away. And what if Saturn’s neighborhood were messy with ring debris and other gunk? The JPL Voyager folks needed a sacrificial lamb, a test project to clear the road, as it were. They found one in Pioneer 11. (Artist’s depiction below.)

Pioneers 10 and 11 were not JPL projects. These explorers were managed by the Ames Research Center. Northern California. The Jet Propulsion Laboratory is Pasadena. SoCal. Can you smell rivalry?

I recommend Mark Wolverton’s The Depths of Space: The Story of the Pioneer Planetary Probes. Excellent book. Tells a great story. The eleventh Pioneer was the crowning achievement of that series. And its Silicon Valley handlers weren’t excited about sacrificing their probe for the sexy, souped-up Voyagers.

The Pioneer project began as inner solar system explorers–built with economy, and aimed at studying the sun and the space between the inner planets. They scored big time to put together NASA’s first mission beyond the asteroid belt. Unlike what you see in Star Wars and Star Trek, navigating the sun’s asteroid belt isn’t evasive maneuvers. But still, people weren’t sure about things like small particles, meteors, and such. Maybe the intrepid Pioneers couldn’t make it to Jupiter.

But they did.

After a successful Pioneer 10, Ames Research Center decided to pull a Gary Flandro. They stole a page from JPL and targeted their Pioneer 11 mission 43,000 miles above the cloud tops of Jupiter, bending its trajectory over the orbit plane of the planets (right) to aim at Saturn which then, was on the opposite side of the sun from target number one.

I can imagine the JPL crew was a little pee-oh’ed at getting beat to a second planet. They had designed the Grand Tour mission, and the resulting Voyagers which weren’t due to launch until 1977. Here this little upstart 600-lb weakling was going to get a first look at Saturn. And it relied on the JPL imagination of Gary Flandro, the guy who originally imagined multiple-planet missions.

But Pioneer 11 has to survive an extra five year jaunt. And whatever mess is to be found near Saturn’s rings.

If you’ve been following this series, by now you’re probably wondering where the satellites enter the show. Well, look at the pair there on your left.

In the 1960′s these two satellites were imaged from earthbound telescopes. But astronomers lacked the imagination to think of two bodies sharing nearly the same orbit. None of the observations worked out, so that expected one moon inside the orbit of Mimas was a big question mark.

One year before Pioneer 11 hit up the neighborhood, astronomers Stephen M. Larson and John W. Fountain suggested that two satellites were a better fit for the observation profile. Maybe Pioneer would settle it. Little did they know …

The danger 20,000 miles above Saturn’s clouds turned out not to be ring particles and small debris. Much closer to the intrepid space probe was Epimetheus, one of those two Larson-Fountain satellites. Closer than California and Cape Canaveral. Twenty-five hundred miles close, which, on Earth doesn’t seem all that close. But remember that Pioneer 11 traveled over a billion miles to miss a moon by so little. It’s like nailing a golf hole-in-one … from two or three miles away with a bank shot. But a fifty-mile wide satellite nearly wiped out the little Pioneer–they didn’t see it coming.

Voyager nailed down the two moons–Epimetheus indeed has a partner, Janus (imaged left, from Cassini). And they have an interesting relationship, despite not quite sharing the same orbit. They almost share an orbit. The inner satellite slowly gains on the outer, taking four years to catch up. Then something cool happens. The two moons switch orbits–as the inner one closes in on the outer, it moves to the outside, and the outer gets bumped closer to Saturn, thus starting another four-year chase. Lather, rinse, repeat.

Pioneer and Voyager opened our eyes to new satellites, close in to their father planets. Shepherd moons. Orbit-sharing. Crazy misses by a cosmic fraction. 1979 ushered in a new era of satellite imagination.

 

Satellite Imagination: Honoring a Wife

Pluto has been demoted from planethood, but before that determination and after, it has been carefully studied. As much as a point of light on a photographic plate can be studied. Clyde Tombaugh really picked a needle out of a celestial haystack:

Would you catch that without the arrows? Neither would I.

Seth Barnes Nicholson, discoverer of four satellites of Jupiter, studied that faint dot after Clyde Tombaugh’s discovery. His analysis suggested that faraway Pluto had about the mass of the Earth. A respectable planet, to be sure.

As the years passed, astronomers were able to discern a bit more about that white spot. Gerard Kuiper, another satellite discoverer, whittled Pluto’s size estimate down to about a tenth that of Earth–about the size of Mars. No idea how he did that. Other astronomers saw enough of a shift in the light to determine that Pluto spun on its axis giving it a day of about 153 hours. How they got that from a small white dot on a photographic plate, I have no idea on that either.

Two things happened by the mid-1970′s. First, NASA was looking at targeting Pluto for one of its Grand Tour missions to the outer planets. Second, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii determined that Pluto’s surface was largely bright methane ice. And because a bright faraway object was likely smaller than a dark body at the same distance, Pluto’s size estimate was further reduced to about one-hundredth of the Earth. A bit smaller than the moon.

Enter James Christy. In 1978, the astronomer, a double star specialist, was looking at photographic plates snapped by the 61-inch telescope at the US Naval Observatory in Flagstaff, Arizona (USNOF). He found a bulge in the grainy image of planet Pluto. Since his specialty was observing distant stars revolve around each other, his best judgment was that Pluto had a moon. It orbited once every 6.39 days, the same as what astronomers thought Pluto’s day was.

Do you see the satellite in the discovery plate? It’s the lump appearing on different sides of the planet:

Christy checked some of the photographic plates in storage at the USNOF. Way back in 1965, someone marked an image, “Pluto image elongated.” But astronomers just assumed that it was a smudge or some error, human or instrument. Guess that might teach some people to jump to conclusions …

Most scientists agreed with Christy. But a few thought it might be a big mountain on the planet. After all, a lump is a lump, right? All skepticism was silenced in 1985, when the tilt of orbits permitted astronomers to verify, through changing light levels, that Pluto actually had a moon. And more, very rough maps were produced. By the time the Hubble Space Telescope was in orbit, there was no doubt, as you can see from this 1994 image:

No mountain, that.

Once scientists had verified a body in orbit around Pluto, it’s an easy matter to “weigh” both planet and satellite. Alas, Pluto came out on the short end again. The final determination was that it would take five-hundred Plutos to balance the scales with Earth. That new satellite was pretty hefty, relatively speaking. Twelfth largest in the solar system.

Let’s get to the name. Astronomers who discover things often get to name the object. It must pass muster with the IAU (International Astronomical Union). But as long as one keeps to the conventions, it’s likely to get approved. Christy’s colleagues at the USNOF were pushing for Persephone, the consort of the Roman god Pluto. But the discoverer wanted to honor his wife Charlene, familiarly known as Char. As it turns out, Charon is the name of the ferry operator in the Greek myths about the underworld. Greek pronunciation, however, is the hard “k.” Charon was approved. Though Mrs Christy is honored by many English-speaking astronomers who have picked up the soft “sh” sound. I pronounce it “sh.” I can appreciate honoring a wife.

I will likely not ever be a satellite discoverer. Asteroid, possibly. The list of asteroid names is ample. My wife’s is not on the list, though my daughter’s name is attached to minor planet number 51599. I don’t know how James Christy thought of his wife. If I were to indulge my imagination, my wife would be my star around which I spin. Maybe my wife and daughter a double star, and I would be a planet revolving around both. It doesn’t take much imagination to ponder that. I think you need more to see a lump on a black spot and think “satellite” rather than “mountain” or “mistake.”

 

Satellite Imagination: Before Kuiper’s Belt

As we read in the last edition of this series, Seth Nicholson’s satellite discoveries spanned nearly four decades. Let’s dial the clock back a bit from the discovery of his last, Jupiter XII, and take stock of the situation just after WWII.

Jupiter had eleven known satellites–the most number for the largest planet. And that seemed fitting. Saturn had nine. Uranus four. Mars two. Earth one. And distant Neptune one. Total of twenty-nine. Until 1900, these satellites had been discovered by telescopic observation. Jupiter’s latest six had been detected using photography–comparing film negatives and noting the movement of a small black dot in the vicinity of the planet.

Pluto was found in the same way in 1930. Clyde Tombaugh, country-boy astronomer from Kansas, not even a college degree in his resume, caught slight movement almost four billion miles away in Arizona.

By mid-20th century detecting a planet’s satellite was fraught with one or two major problems. A closer orbiter would be washed out in the glare of nearby planet. More commonly, the threshhold of size was shrinking. The moons of Mars and the smaller moons of Jupiter were the size of large cities. The smallest moon detected in the 17th century, by contrast, is Tethys, about 700 miles across. (Tethys, by the way, masses more than all of the solar system’s known smaller moons combined.)

Enter Dutch-born astronomer Gerard Kuiper. Before he had a whole belt of icy bodies named after him, he made two significant satellite discoveries.

After his schooling in the Netherlands, he came to California’s Lick Observatory in 1933. After a two-year sojourn in Cambridge at the Harvard College Observatory, he settled in at the University of Chicago in 1937, where he was noted for his early work in planetary astronomy.

He turned his attention to atmospheres, detecting carbon dioxide on Mars in 1944.

You may recall the other famous Dutch satellite astronomer, Christiaan Huygens. Professor Kuiper followed in three-century-old footsteps and detected methane on Saturn’s moon Titan, the first definitive non-planet atmosphere detected in the solar system.

Did I mention Dr Kuiper was gifted with extraordinarily sharp eyesight? Perhaps it inspired his imagination to look at the night sky and see objects four times fainter than the average skywatcher. His vision, however above average, was of no help for his satellite discoveries. While Seth Nicholson was finding small moons of Jupiter, Kuiper found previously unknown objects in orbit around the sun’s distant ice giants, billions of miles away, and far too faint to be found by a sharp-eyed observer.

Meet Miranda, his 1948 find:

Kuiper, of course, didn’t see it this way. The image above is from NASA’s Voyager 2, taken from 90,000 miles away. The earthbound astronomer is well over a billion miles distant. Far enough away that the planet Uranus washes out the view–at least for the average amateur telescope.

Meet Nereid, his 1949 discovery:

Again, Kuiper didn’t see it so close. This was also imaged by Voyager 2 during its Neptune encounter, but from a distance of a few million miles. The astronomer of the 1940′s was more accustomed to this family portrait:

Arrows mark the two moons of Neptune in this photograph: Triton near the planet, and Nereid the very faint dot.

Two very different satellites discovered in consecutive years. One orbits her planet in a circular way, just outside a ring system. The other orbits in a long loop that takes her from one to six million miles.

Humans will return to Miranda, if only by means of a future Uranus orbiter. Miranda’s irregular landscape begs many questions. Was it blown apart in a collision and reassembled in a random way? Does it harbor ice geysers and volcanoes like another small moon? Curious and imaginative minds will want to know.

Nereid, sorry to say, will likely be forgotten. Future exploration of the Neptune region will focus on the planet, it’s large moon Triton, and its clumpy ring system. Nereid is a bit out of the way, and will be left to future imaginations pondering the dark, lonely bodies of the outer solar system.

This website’s survey of solar system natural satellites now covers everything up to the Space Age. As future posts will show, the dispatching of robot explorers will put a familiar face on most of the natural satellites discovered before the first artificial ones were sent aloft. Stay with us for those journeys of imagination.

 

Satellite Imagination: Nicholson’s Quartet

Detecting natural satellites of Jupiter is darned difficult. The planet is bright. The satellites are small and dim. Most of Jupiter’s moons orbit in irregular paths, nudged by the sun. Some even orbit backwards. If you blinked, you might lose one. In fact, one moon was detected in 1975, only to be lost, then rediscovered in 2000.

One early twentieth century astronomer was up to the task, though. With big telescopes gathering a few reflected photons on photographic plates, Jupiter’s family of satellites grew and surpassed Saturn’s nine, thanks to the efforts of an Illinois-born astronomer working in California.

In 1914, a grad student found the ninth moon of Jupiter. While observing the newly discovered Jupiter VIII from Lick Observatory, Seth Barnes Nicholson found his PhD thesis topic: Jupiter IX, now known as Sinope. Over the course four decades, he added three to his tally, tying Galileo, Cassini, and Herschel for the most discoveries up to that time–four. Here are the others, all found at the Mount Wilson Observatory (image above and left), where Dr Nicholson spent his entire career:

Jupiter X, or Lysithea, in July 1938

Jupiter XI, or Carme, also in July 1938.

Jupiter XII, or Ananke, in September 1951.

Nicholson was more than an observer of dim, irregular satellites. He developed tools for assessing surface temperatures of solar system bodies (his moons as well as planets and the sun) and distant stars.

These seven outer satellites of Jupiter–Nicholson’s four, plus the three found from the first decade of the 1900′s are all thought to be captured from the nearby asteroid belt. Or they could be comets. Or they could be fragments from a few larger bodies that broke up. Since Jupiter’s outer moons move in similar orbits this last theory seems to fit observations pretty well. Don’t think clumps of moons, though. These bodies orbit millions of miles from Jupiter. A close association is more like them sharing a fairly wide pathway far out from the planet. Imagine Jupiter sitting on the thirty-yard line of a high school football stadium, its north pole pointed up, but at an angle, perhaps toward a near bank of lights. The satellite Carme and associated moons (discovered in the last ten years) would be at different places circling the field mainly on the surrounding track. Don’t imagine a handful of closely-bunched runners.

Nobody really knows for sure about any of this. No human-made spacecraft has ever flown anywhere near them. They aren’t big priorities for NASA or other nations’ space programs. It might be that the first robots or humans visiting these satellites will be lonely wanderers themselves.

In my imagination, I see a twenty-third or twenty-fourth century ship pushed by the pulse of ions. Looking around, Jupiter would be far away, only about the size of our moon as seen from Earth. Light from the sun would be only about four percent Earth levels. An unenhanced view would find us moving up next to a dark gray body with just a hint of red. Much of it might be water ice mixed in with rock. But don’t think of icebergs sticking out from soil–from Earth it seems these bodies are fairly homogeneous in surface. The ice will be dirty, laced with carbon dioxide and maybe ammonia, and studded with boulders and dust and particles of sizes in between.

As the explorer gets closer, the body–let’s say it’s Ananke–will loom large, dwarfing the living compartment and the ion engine. But even compared to asteroids, it would be small–only twenty miles across. Centuries after Seth Nicholson noted moving specks in the Jupiter neighborhood and plotted orbits, someone would finally see the moon, and not just in her or his imagination. It’s not likely there would be a landing. Maybe grappling hooks if the ship were sturdy. More likely a few small robots would tumble down to the surface. Dig around a bit. Send an optical feed back to the main ship. Taste the dust of the surface and clarify the makeup of the impurities.

It might be that the same mix of trace compounds and elements was detected at last month’s stop–ratios of cyanide, carbon monoxide, or silicate crystals. So it will be confirmed that Ananke and about one or two dozen mile-sized satellites were once a single asteroid captured by Jupiter. Then pulverized into fragments. If space explorers of the 2300′s feel the need to put down roots, Ananke would be an interesting place. There’s likely enough ice to turn into water for drinking and other industrial uses–more than on the moon, that’s for sure. Splitting water molecules into hydrogen for fuel and oxygen for breathing will be child’s play in future centuries. The sun being so far away, is only four percent as efficient as a power source as it would be near the Earth. On second thought, these intrepid explorers may have other places in their imagination, closer to the life-giving energy of our central star. After a few days, the ship will pull out its stakes, leave behind the robot probes, and move on to the next target.

It’s a lonely existence ten to twenty million miles out from Jupiter. No gazing back at Earth either–our home world will be lost in the glare of the sun–not unlike the way Jupiter’s reflected light washes out viewing some of its moons.

 

Satellite Imagination: Unnamed Moons

Galileo discovered the first satellites (except for our Moon) in the solar system. In the opener of the “satellite imagination” series, you get a piece of the story connected with that. You might think that moons half a billion miles away have little effect on the Earth. Speaking astrologically and gravitationally, you would be right. Yet those moons orbiting Jupiter shook the relationship between science and religion. It has remained in a sort of tension ever since. Occasionally it explodes like volcanoes on Io. Other times, one beholds a certain serenity, but who knows what scientist or religious person is really thinking under that crust of ice.

You might think there was a rush to name the satellites of other planets. But professional astronomy was, for the most part, apathetic. Roman numerals were enough for Jupiter’s satellites. American astronomers making discoveries didn’t really push the issue. In fact, some of them had more important things to consider: comets, star clusters, and galaxies. VI, VII, and VIII weren’t the most imaginative designations, but they were sufficient until the Space Age.

By the final decades of the nineteenth century, the baton of planetary astronomy had passed to American shores, at least as far as the discovery of natural satellites was concerned. Americans built big telescopes, and they started putting them on mountains. The Lick Observatory began operations in 1888, and six years later they scored a big find: Jupiter’s first moon in almost three centuries.

California’s Lick Observatory, above in 1900.

 

Satellite Imagination: Photographers At Work

Phoebe was the first of Saturn’s moons encountered by the Cassini space probe. In 2004, Earthlings got delicious images of the moon from just a few thousand miles away. Before Cassini, our view of this moon for the past 105 years had pretty much been a point of light on a photographic plate.

DeLisle Stewart was a Harvard astronomer taking images in Peru the summer of 1898. Several months later, in the other hemisphere, William Henry Pickering pored over the photographic plates and found movement in the outer reaches of the Saturn neighborhood. It was way out, as Phoebe orbits about four times as far out as Iapetus. Phoebe is about three times dimmer than the least-bright moon yet known–Uranus’ Umbriel.

Pickering and Stewart’s effort marked the beginning of the photographic age in satellite discoveries. Why take pictures? Two advantages make it an improvement over the human eye.

First, an astronomer can leave the lens open for long periods of time and gather more light than the unaided human eye can take in. A very dim moon will continue to send reflected photons onto a plate. Finding a moon means looking for a moving dim light.

One technique is not to develop the images, but compare negative plates, looking for a black spot that moves against the background stars. What could it be? Maybe a comet. Likely an asteroid. But if you’re probing the neighborhood of a planet, a satellite is the imaginative choice.

By the turn of the century, the known planets and their moon counts were as follows:

Earth-1, Mars-2, Jupiter-5, Saturn-9, Uranus-4, Neptune-1.

The largest planet was in second place in the satellite department. What gives with that? The first six satellites discovered in the 20th century were to be found in the Jupiter environs, seven total before the dawn of the space age. We’ll take a look at those moons in the next edition of Satellite Imagination.

 

Satellite Imagination: End of the Age of the Eye

In 1892, we can note the last human eye discovery of a solar system satellite. The eye belonged to Edward Emerson Barnard who, working at the Lick Observatory in California, found a fifth satellite of Jupiter. Hereafter, every subsequent satellite discovery was made by the comparison of photographic or other images. Barnard, while heralding the end of a sentimental astronomical age, actually had one foot in the new.

It’s interesting that Barnard worked as a photographer’s assistant from age nine, and was a pioneer in the developing use of photographic imagery for astronomy. He was also a crackerjack observing scientist, having discovered over a dozen comets. In addition, he contributed a great deal by optical assessments of distant stars and nbeula far beyond the solar system.

So almost three centuries after Galileo’s momentous revelation of “Medicean Stars,” the king of planets now had a fifth moon, now the 21st known to human astronomers.

Meet Amalthea, in two images from Galileo probe, not the astronomer:

The Galileo probe of the 90′s didn’t uncover very much about this inner satellite. The fifth discovered orbiter of Jupiter is also, fittingly enough, the fifth largest of Jupiter’s moons. But Amalthea is still significantly smaller than next-largest Europa, measuring a potato-shaped 160 by 100 by 90 miles. At the close of the 19th century, that made it the third-smallest known satellite, larger only than the two city-sized moons of Mars.

And the name? Amalthea wasn’t official until 1975, but it seemed appropriate that a close moon be named after the goat shepherdess that nursed the infant god Zeus (Greek mythology). Some variations have Amalthea as the goat nursing the future king of gods with her own milk. At any rate, Amalthea is a light and fluffy moon, about the same weight as a 160 mile-long ice cube. It might be all ice. Or it could be a loose conglomeration of rock rubble with some empty spaces inside.

Imaginative photographers will pick up the mantle of discovery from here. In the next installment, we’ll see how Jupiter netted a bevy of new moons, but every other outer planet added known companions in the 1900′s, even Pluto.

 

Satellite Imagination: The Galileo Code and the New World(s)

 

 

Author Jonathan Swift (right) had quite an imagination. In his novel Gulliver’s Travels he notes two Martian moons, “the innermost is distant from the center of the primary exactly three of his diameters, and the outermost five.”

There are two notable things about this. First, the writer wasn’t too far off; the real values are actually two and four. More amazingly, Swift wrote his book about a century and a half before American astronomer Asaph Hall recorded the actual discoveries in 1877.

How on Earth did Swift “predict” Phobos and Deimos 151 years before they were first sighted by human beings? One conspiracy theorist is sure the author was born not in Ireland but on the planet Mars. But an intriguing idea comes to us through an error of the great astronomer Johannes Kepler.

Kepler discovered no satellites, but he was the recipient of a mysterious communication from Galileo in 1610:

s m a i s m r m i l m e p o e t a l e u m i b u n e n u g t t a u i r a s.

Was Galileo concerned the Inquisition was monitoring his communication? Or was he one smart guy trying to give another smart guy a little puzzle? This is an anagram. Cover up the rest of this post if you want to take a stab at it. Keep in mind that in Latin the letter “u” may also be used for “v” and “i” may be a “j.”

Kepler unscrambled it and read: Salue umbistineum geminatum Martia proles, which in English is translated, “Hail, twin companionship, children of Mars.”

Okay.

The problem with anagram communication is that the long ones have more than one solution. Galileo’s opponents may have been kept in the dark on astronomical discoveries, but he also misled the great Kepler, for the puzzle alludes to Saturn, not Mars: Altissimum planetam tergeminum observavi. In other words, “I have observed the most distant planet to have a triple form.” That would be Saturn and its rings.

Fast forward a century: was Swift an amateur astronomer familiar with Kepler’s misinterpretation? We don’t know. I prefer to credit the novelist’s great imagination instead.

As for Professor Hall (left) of the US Naval Observatory, he was better with numbers than letters. He didn’t decipher Galileo’s code; he launched his astronomy career crunching numbers for the Harvard College Observatory in the 1850’s. By the mid-1870’s, he had a wonderful telescope (below, right) at his disposal in Washington DC. The USNO’s 26-inch refractor was the largest of its kind in the world. Viewing when he could as the summer fog occasionally wafted in from the Potomac, he observed and verified two moons of Mars in mid-August 1877.

The discoveries made Hall a sensation both in scientific circles and in the public eye. An American using an American-crafted telescope! Even the British were impressed and the Royal Astronomical Society awarded Hall its Gold Medal.

In the days of Galileo, Cassini, and even Herschel, naming satellites was not such a big deal. Hall himself seemed a little reticent as you can read from an early 1878 paper:

Since there is but little need of names for these satellites, I have delayed making a selection, but to avoid confusion I have chosen the following names: Deimus (sic) for the outer satellite, Phobus (sic) for the inner satellite.

These names were suggested by Mr. Madan of Eton, England. Theyoccur in Book XV of the Iliad, line 119, where Ares is preparing to descend to the Earth to avenge the death of his son.  –Bryant translates as follows:

“He spoke, and summoned Fear and Flight to yoke His steeds, and put his glorious armor on.”

From Earth, Phobos and Deimos (Greek for fear and flight/terror) register with magnitudes of 11.4 and 12.5 respectively, a shade brighter than Saturn’s innermost moons Enceladus and Mimas. And a lot closer to Earth. Why weren’t these satellites picked up by the great Herschel? Did he have some sort of bias against the Red Planet?

Mars’ moons are hard to detect for the same reason you can’t see 4^th magnitude Ganymede with the naked eye: the brightness of the nearby planet. And the Martian satellites orbit pretty close to their primary. From the ground on Mars, the Spirit rover caught both moons in the Martian evening sky on 30 August 2005:

As described by the NASA-JPL site:

On Mars, Phobos would be easily visible to the naked eye at night, but would be only about one-third as large as the full Moon appears from Earth. Astronauts staring at Phobos from the surface of Mars would notice its oblong, potato-like shape and that it moves quickly against the background stars. Phobos takes only 7 hours, 39 minutes to complete one orbit of Mars. That is so fast, relative to the 24-hour-and-39-minute sol on Mars (the length of time it takes for Mars to complete one rotation), that Phobos rises in the west and sets in the east. Earth’s moon, by comparison, rises in the east and sets in the west. The smaller martian moon, Deimos, takes 30 hours, 12 minutes to complete one orbit of Mars. That orbital period is longer than a martian sol, and so Deimos rises, like most solar system moons, in the east and sets in the west.

Let’s zoom in for some close-ups. First Phobos, courtesy of Mars Reconaissance Orbiter:

Then smaller Deimos, from the Viking Orbiter image archives:

 

 

Satellite Imagination: Mathematics And A Missing Planet

It would be the coolest thing to discover a planet, don’t you think? A moon wouldn’t be too bad either. For the inner eight planets of the solar system, as of the early twenty-first century, humankind has pretty much uncovered everything down to about thirty miles wide. Any subsequent real estate anybody picks out will be no larger than the size of a city. And probably a lot smaller.

The nineteenth was a heady century for solar system discoveries. It was also the last in which new satellites were found by direct observation. Astronomy was being conquered on two fronts: by the emerging art of photography and especially mathematics.

William Herschel, that one-time church musician and bandleader, built the most powerful telescope in the world. He claimed to have found other moons of Saturn in the early 1800′s, but these were never verified. Nobody had a telescope as good as his. It would be left to another generation to probe the outer reaches of the solar system. They did so by eventually matching and exceeding Herschel’s optics. They were helped by another academic discipline mathematics.

Curious things were happening in the Uranian neighborhood. The planet seemed to be a bit off from where it should be, slowing down and speeding up at odd times. Astronomers combed through early records of Uranus-sightings–the ones where people didn’t realize it was a planet. Mathematicians set to work on a notion that another large planet was out beyond Uranus. Newton’s gravity equations were put to the task. But decades passed with no sighting. Either the math was off, or the planet was super faint, or Uranus was just a strange duck orbiting the sun at 1.7 billion miles.

In these years John Herschel (left) took up his father’s mantle. John recorded what he thought was a star on July 14^th in 1830. Just his luck he was unable to tie together emerging Newtonian models for Uranus’ peculiar orbit with his dad’s keen eye. It would have been a great coup for the Herschel family: father and son planet discoverers. For that summer “star” was indeed the planet later to be known as Neptune.

In the 1840’s John Couch Adams was sure another planet was out there. His numbers led him to believe. But he could not convince his arrogant countrymen at the Royal Observatory in London to take him seriously. James Challis did attempt to pin down a trans-Uranian planet in the summer of ‘46. He actually viewed Neptune twice in August, but he lacked the imagination (or perhaps the diligence) to claim a discovery for England.

It was left to a French mathematician, Urbain Le Verrier, (above, right) to tip the balance. Once Sir George Airy saw Le Verrier’s published paper on the mathematics of a possible eighth planet, he took his compatriot Adams more seriously. However Le Verrier also told astronomer Johann Galle (left) in Berlin. Galle had no problem with letting mathematical analysis guide his imagination. On his very first night of observing, only one degree (twice the span of a full moon) from Le Verrier’s predicted position, he found a small blue orb. Adams’ estimate was off by 12 degrees (a little wider than a fist at arm’s length).

Once Neptune was nailed down, other observers had no problem with some new satellites. For the British, Neptune was lost. Astronomer William Lassell (left) managed to salvage some homeland pride with his discovery of Neptune’s largest moon, Triton, seventeen days after Galle first spotted the planet. I suppose it made him the toast of Britain. Appropriate for the guy who made his fortune brewing and selling beer so he could devote his time to his real love: astronomy.

That small image at the top right of the post is an earthbound telescope view of Neptune with Triton from the Xanadu Observatory web page. It’s a little bit better than what you might see in an 1840′s telescope, but it gives a good idea.

Two years later, while the American father and son team of William Cranch Bond (left) and George Phillips Bond spotted Hyperion tumbling between Titan and Iapetus, Lassell found the same moon from his vantage point in Britain.

The brewer wasn’t finished, as he probed to the depths of fifteenth magnitude to discover two Uranian moons, tying the record of four satellite discoveries set by Galileo and repeated by Cassini and William Herschel.

Courtesy of modern space probe close-ups, here are these four satellite discoveries, none of which, by the way, I’ve ever seen through a telescope.

Triton (1846) of Neptune, magnitude 13.6:

Hyperion (1848) of Saturn, 14.2:

Ariel (1851):

… and Umbriel (1851, below) both of Uranus, 14.4 and 15.3 respectively.

What about you?

These finds left astronomers with a curious 1-4-8-4-1 symmetry of known satellites among the solar system’s five largest planets. Earth: 1 satellite, Jupiter 4, Saturn 8, Uranus 4, and Neptune 1. Would the world’s astronomers summon the imagination to look further and break the pattern? You’ll have to read about it in the next installments as the Americans take up the task, looking near planetary glare to ferret out more orbiting bodies.

 

Satellite Imagination: The Herschel Years

After Giovanni Cassini first espied Tethys and Dione in 1684, satellite discoveries dried up for more than a century. Astronomers didn’t think to look for new planets either. Lack of imagination? Perhaps so, because many famous astronomers viewed them. They just didn’t know what they were seeing.

John Flamsteed (bust, left), Britain’s first Royal Astronomer, indeed sighted Uranus in 1690. He called it “34 Tauri” and continued with his famous cataloguing work.

The great Galileo had a close encounter with Neptune. In 1612, two days after Christmas, he actually noted Neptune as he sketched the moons of Jupiter. Just the man’s luck that Neptune had, that very day, begun its retrograde motion and was stationary in the sky. Galileo did note Neptune’s movement in some drawings a little more than a year later. But the inspiration didn’t dawn that he was seeing a new planet. The moons of Jupiter got him into enough trouble, I guess.

We can forgive Galileo and Flamsteed for missing 30,000-mile wide planets. There’s a lot of space, even along the ecliptic to hunt for things. Once you get the planet, sleuthing the moons is one step easier. Though the moons are always much smaller and dimmer than the primary, the search is greatly narrowed.

At any rate, it took a church musician to further draw back the curtain of the unknown from the outer solar system. William Herschel (right) was an immigrant from Hanover, Germany and had a comfortable life in Bath, England. He wrote music, directed bands and orchestras, and gave concerts with his brothers and sister. In mid-life he adopted astronomy as a hobby and began building his own telescopes.

And in March 1781, he began tracking a green orb (below) moving slowly against background stars in Taurus. After consulting with professional colleagues, it very soon became clear that Herschel had found a new planet. It changed his life.

King George III gave him an annual salary, with the understanding Herschel would make himself available for sky parties at Windsor Castle. (I guess they needed a distraction from those upstart American revolutionaries.) The new “King’s Astronomer” suggested the name “Georgium Sidus” for the little green orb.

The French didn’t approve. They called the new planet “Herschel,” and were unaware one of their own made no less than a dozen sightings of this planet in the 1750’s and 60’s.

German astronomer Johann Bode suggested Uranus as a name. Two things seemed to work for its acceptance. That a new planet was named for a Roman god kept a consistency with Saturn, Jupiter, and the others. Uranus was also god of the sky, and at twice the distance of Saturn from the sun the little green body was pretty “deep” in the sky.

Herschel continued to refine his telescope-building under royal patronage. He didn’t neglect his green orb discovery, and six years after it, identified two moons in orbit. Funny thing about those moons: Herschel’s telescopes were, for half a century, the only instruments in the world powerful enough to view them. The amateur had advanced to the head of world class.

Saturn’s edge-on rings in 1789 gave Herschel a chance to test the limits of his monster 49-inch reflector. First night of viewing: Enceladus. Then a month later: Mimas. Within a decade, Herschel had tied the records of Galileo and Cassini, throwing a planet into the mix if you want to consider a tiebreaker.

Here are Herschel’s 18th century moons: Titania (magnitude 14.0) and Oberon (14.2) of Uranus, both discovered in 1787, Enceladus (11.7) and Mimas (12.9) of Saturn, viewed in 1789. You’ll notice Saturn’s moons are brighter than Titania and Oberon by one and two magnitude factors. They are as hard to spot because of the nearby glare of the planet Saturn in an Earthbound telescope. Myself, I have yet to see them.

The Voyager 2 and Cassini space probes have given us great views of these four moons, far beyond the imaginative and intrepid Herschel. Check them out, first Titania:

Then Oberon:

Enceladus, of geyser fame:

And Mimas, with a giant crater named … Herschel:

It’s worth noting that the difference in magnitude between Jupiter’s bright Ganymede (4.6) and these 18^th century moons is on par with the difference between the full moon and Venus.

My imagination snags on my commonality with Herschel. Hey! I’m a church musician who loves astronomy, too. Unlike Herschel, there’s no way I’d convince my sister to patiently take notes while I observed the heavens. Also unlikely I’ll be building telescopes with the technology of the mid- to late-21^st century–fifty years ahead of my time as Herschel was.

Still, following in the footsteps of a great observer like William Herschel is exciting. No other branch of science is as receptive of the contributions of amateurs as astronomy is. It should give us all a sense of pride in our accomplishment to be standing with astronomers of centuries past, probing the unknowns of space, possibly with our own hand-made instruments and hand-drawn charts at our side.

And besides, if there are no planets or moons in the solar system left for me to discover, I guess I could write an oboe concerto.

 

Satellite Imagination: Meet The Louisians

When I was in college my friend Mark, an astrophysics grad student, took me to the Mees Observatory, our university’s in-state research telescope. Among other things, we viewed Giovanni Cassini’s Louisian Stars—named for King Louis XIV, the astronomer’s patron at the Paris Observatory.

Clockwise from upper left they are Iapetus, Rhea, Tethys, and Dione. Being mostly ice balls about half the size of Earth’s moon, it takes a bit of imagination to tell them apart. Iapetus is the “yin-yang” moon, half bright as snow, half dark as coal. (see right) Nobody is quite sure why or how. Then there’s that equatorial ridge that shapes the moon like a walnut.

Rhea is slightly larger than Iapetus and may have a ring. That would be a solar system first: a moon with a ring. Otherwise, it’s a rather pedestrian cratered orb, much like astronomers expected in the outer solar system before the Voyager probes. Looks like Earth’s moon in close-up, but don’t be fooled. There’s a lot of ice down there.

Tethys has a few big features, that big impact basin on the terminator above and a long canyon that seems to be a crack that nearly split the moon in two or more.

Dione has those wispy markings. Turns out they’re cliffs.

You can’t see this kind of detail from Earth, by the way. From the home planet, these moons look very much like faint stars. We know otherwise because they revolve around the giant planet Saturn.

I’m not sure I would find the young Cassini as interesting a skywatching partner as my friend Mark or even Galileo. Cassini got his start in life with an interest in astrology. (I didn’t misspell that.) But that didn’t stop him from scoring many successes in the world of science: tying Galileo with four moons discovered, co-discovering Jupiter’s Great Red Spot, and deducing the different rotation speeds in Jupiter’s banded atmosphere. Later in life he repudiated the topic that now rubs shoulders with celebrity gossip in our print media. Maybe Cassini isn’t such a bad guy after all. They did name that NASA probe after him.

By the end of my college days, I had observed the Louisian Stars to add to my views of Titan and the Galilean satellites. That put me at the end of the 1600′s, so to speak. The aided human eye was able to discern solar system moons down to about magnitude 11 by the end of that first century of telescope-making.

The solar system’s 17^th century moons number nine. The first four belong to Jupiter, the others orbit Saturn. Binoculars are good for seeing the moons of Jupiter. You’ll need a small telescope for Titan, and maybe something a little bit kicked up from there for the “Louisian Stars,” though nothing as grand as a research scope.

If you have that slightly kicked-up small telescope, Titan will be easy to find. Test your imagination on subsequent nights and track it in its 15-day orbit. As for the others, Iapetus will be a test of patience–it orbits beyond Titan every 79 days. The other three “Louisian Stars” will have noticeable movement in a few hours’ observing, as their orbits track full in two to four-and-a-half days.

Summing up the 17th century satellite discoveries, one has the Jovian moons discovered by Galileo in 1610 with their best visual magnitude in parentheses: Ganymede (4.6), Callisto (5.6), Io (5.0), and Europa (5.3)

Then we have Titan (8.3), first seen in 1655 by Huygens.

And Cassini’s Louisian Stars: Iapetus (11.1) discovered in 1671, Rhea (9.7) spotted a year later, then Tethys (10.2) and Dione (10.4) found in 1684.

Just a note on those magnitudes. Other things being equal, a human being with good eyesight can see down to about 6.5 in dark skies. Glare from planets can wash out satellites, especially when they orbit close. Also note the magnitude system is a logarithmic one. Iapetus at 11.1 isn’t twice as dim as Callisto at 5.6. It’s a hundred times fainter, as seen from an Earthling’s telescope.

How about some imaginative close-ups? There are courtesy of the Cassini spacecraft:

Something for your 3D glasses:

Here’s Ithaca Chasma on Tethys, below. Pretty rugged terrain, wouldn’t you say?

This close-up of Dione makes me think of our own moon, a little bit. But notice the great cliff, plus the small ones in the image:

Now you’ve met the Louisians.

 

Satellite Imagination: The Dutch Connection

With my daughter’s telescope, I can barely pick out Titan. It moves a lot more slowly in its orbit than Jupiter’s moons. We certainly can’t detect that pretty orange atmosphere. It’s just a point of light. In the image to the left, just a small smudge.

If I use my imagination, we’re chumming with Christiaan Huygens, discoverer of Titan (1655), about two generations after Galileo, and a few hundred miles north in the Netherlands.

Peering through the scope, Brittany notices that band of ice girdling the planet. “Those are the rings, right Dad?”

Huygens figured those out, too. Galileo thought of them as ears. My daughter laughs at that. “What else can we see, Dad?” she asks. And it’s off for another adventure.

Titan was left unnamed for the first two centuries of its Earthling acquaintance. Luna Saturni was enough for the Dutch astronomer. When other moons were discovered, Titan was given a roman numeral, first II and then IV. For the early 1800′s, it was known as Saturn VI. The numbering convention was I for the innermost moon, and everything else lining up in order. Not very imaginative. And a pain when astronomers kept discovering moons closer in and re-numbering the whole lot.

John Herschel, son of Uranus discoverer William Herschel, began to suggest names for the moons of the outer planets in the mid 19th century. Being the brightest and presumably the largest of Saturn’s moons, it got the name “Titan” not only for its size, but for the association of the Titans with the god Saturn.

After another century, another Dutch-born astronomer, Gerard Kuiper (left), probed Titan with spectroscopic instruments and determined the moon had an atmosphere containing methane.

Until the Voyager probes arrived in 1980 and 1981, Titan was thought to be the largest moon in the solar system. As it happens, the highest level of haze in the atmosphere rises a bit more than a hundred miles above the surface. For both mass and size, Ganymede is slightly larger than its colder, orange rival.

Titan would be a wondrous adventure for any human beings fortunate enough to get there. The Huygens probe, (a replica of which is on the right) built by the European Space Agency, was a master stroke in the exploration of space.

Piggybacking on the NASA Cassini probe, Huygens completed an amazing mission parachuting through an atmosphere and beaming back amazing images of another world.

Scientists are already applying their imaginations for a follow-up mission to Titan. Tibor Balint illustrates one intriguing idea, a hot air balloon:

The idea is to float a probe a few miles above the surface, safely above any low mountains or hills. With a heat source, no need to worry about a limited supply of ballast; just turn the burner on or off depending on something automatic like radar or air pressure. A radioactive source would last for months, maybe years. Because of the great distance to Titan, the probe would need to be almost 100% automated. If Earthling observers saw a mountain coming up, it would take over an hour to send a radio message like “Look out!” By then, our intrepid balloon would be toast, though at a frigid 290F below zero.

Here’s a Cassini image to wrap up this edition of satellite imagination. What a beautiful color for a moon. What wonders must exist under those clouds.

 

Satellite Imagination: The Galilean Moons of Jupiter

Like many astronomers, amateur and otherwise, I would love to go visiting planets and moons. For me, it’s especially the moons. Unfortunately, that’s not likely to happen. My wife Anita would never let me strap in for a space mission, present or future. Probably born a century or two too early to boot. Darn.

This past year, I’ve written a series of articles for the local astronomy club‘s newsletter under the heading of “Satellite Imagination.” The above paragraph is how I opened my first essay. I aimed at blending three aspects in my writing: the amateur observation of the satellites of other planets, the stories of their discovery, and my own experiences of viewing them.

For this web page, I can supplement my original pieces with pretty pictures, like the Galilean moons above. (Io, Europa, Ganymede, and Callisto, left to right.) I can also adjust the writing style for those among you who aren’t active astronomers, and maybe inspire a few of you to have a look yourself. Pull out a pair of binoculars or dust off that telescope in your den.

Four planets are easily visible to the unaided human eye from Earth. Add Mercury if you have a clear horizon. Two other planets, Uranus and Neptune, can be seen in small telescopes or really good binoculars if you know where to look.

But ever since I was a boy, I was fascinated not so much by the planets as their moons. I wrote lists of moons and learned the names of the thirty-one that were known in the 1960’s. Much dimmer than planets, fifteen to twenty of these are possible to view with a basic knowledge and a good telescope.

When I pull out the binoculars and aim at Jupiter, the planet doesn’t always hold my interest. True, Jupiter is handsome, and even limited magnification can resolve a disk with bands and a red spot. A day lasts ten hours there, so even in a few hours of viewing, you can see that Great Red Spot as it experiences a two-and-a-half-hour morning, then an equally short afternoon.

With that simple pair of binoculars, I can imagine myself back at the dawn of the seventeenth century, sitting alongside Galileo and his simple hand-made telescope. The great scientist had no sooner aimed his optics at Jupiter than he discerned four moving “stars.” To curry favor with the Grand Duke of Tuscany, Cosimo de Medici, he later named these points of light the Medicean Stars.

What was remarkable is that they stayed close to Jupiter, moving from side to side. After a few days, Galileo realized they all orbited that small disk of a planet. The Earth-centered universe was shattered by a few nights of observation that began not with the intent to be heretical, but to satisfy a curiosity.

In the late 1500′s, the Polish astronomer Copernicus quietly suggested the sun, not the Earth, was the center of the universe. With these four wanderers centered on Jupiter, it seemed that the sun wasn’t holding a monopoly on centrality either.

Along with Galileo, I ponder the systems within systems of the Copernican model. Not only does the Earth travel around the sun, but these star-like specks circle the orb of Jupiter. If I stay out a few hours, the movement is noticeable. If I come back the next night, the inner three will scramble themselves.

Reflecting on these mobile points of light I’m soon dreaming about other things: the volcanoes of Io, the subsurface oceans of Europa, the strange icescapes of Ganymede and Callisto. This is what modern astronomy brings us: astronomy opens the door to geology as we come to view extraterrestrial landscapes. Some scientists dream of biology coming to the fore as we probe the ice crusts of these strange new little world. We may have to wait on biology, but pondering life off Earth is a sign of a grand imagination.

As I’m looking up with Galileo, he with his handmade telescope and I with my binoculars, I can imagine dark skies over an Italian night and the unknown frontier out there.

Look over there on the right. That’s the view we see. Bright Jupiter is a glare in the night sky. Its moons are actually bright enough to be seen from Earth, but only if they weren’t so close to the planet. They are as bright as the dimmer stars in the Little Dipper.

Now, if I were on Mars, I’d be a little closer. Maybe I’d see them through the visor of my pressure suit. Wouldn’t that be an adventure!

…

Then Anita calls me from the kitchen door. Back from outer space. Back to the present day. Goodbye, Galileo.