redefine what it means to be a moon.
In the old days of astronomy, before Galileo, there was just the
Moon. Then scientists had to accept the clear and visible evidence
of four objects orbiting Jupiter, satellites the master saw through
a crude telescope in 1610.
Things have only grown more complex, especially of late.
With a spate of discoveries in the past three years, the number of
known moons in the solar system has jumped to 118 as of this
writing. That figure won't stand long. Astronomers expect the tally
to double, at least, in the next few years. Most won't be surprised
if it eventually triples. And that's not counting the small stuff
-- boulders the size of football stadiums and countless smaller
rocks that are surely trapped in orbit around the big outer
planets.
Meanwhile, the complexity of moon types and behavior grows with the
tally, and astronomers are struggling to sort out what it all
means.
Several interviews with moon hunters and top theorists reveal that
the definition of a moon is not clear and that there has been
almost no discussion, professional or casual, about whether there
should be any lower size limits set to separate real moons from
miniature imposters. And nobody is in a rush to do anything about
it.
Orbiting rocks and atoms
Last month, astronomers announced they'd found the smallest known
satellite, a moon just 0.62 miles (1 kilometer) in diameter
orbiting Jupiter. It is one of dozens of small moons that behave
strangely. Most go backwards compared to the orbits of the larger
Galilean moons. Some take long, elliptical paths and stray far
above and below the planet's plane of rotation. Few are
round.
Saturn, Uranus and Neptune all have similar satellites.
Other configurations are more complex. Saturn's rings contain
myriad chunks of ice and rocks that scientists don't consider
moons. Yet embedded in the rings are distinct moons. Size -- and
the ability to be noticed -- has so far played a role in being
designated a moon.
Yet some moon-like objects are so small they are invisible.
Hydrogen atoms at the outer fringes of Earth's atmosphere orbit the
planet in an arguably moon-like manner.
"We don't call those satellites," says Alan Stern of the Southwest
Research Institute (SwRI), making a firm delineation between moon
and non-moon at least at the very lower end of the size spectrum.
Dust grains, too, are out, Stern says. From there on up, however,
nobody has thought much about where to draw the line, if at all, on
what constitutes a moon.
"There is no accepted definition," Stern said.
So what would Stern call a 6-inch rock orbiting a planet? "I would
call it a 6-inch rock." But, he acknowledges, "It's technically a
satellite."
By that strict definition, there could be thousands of moons in our
solar system, or millions if you count the debris in Saturn's
rings. Nobody is proposing such an approach. Stern thinks it might
be useful to create a separate category called "minor moons" to
distinguish the small stuff from what is conventionally thought of
as a moon.
Stern's suggestion was made to this reporter and is not an official
proposal, but someone might need to propose something official
pretty soon. NASA's Cassini spacecraft, due to arrive at Saturn
next year, may force the issue.
For now, the smallest known satellite of Saturn is Pan, at 12.4
miles (20 kilometers) wide. Pan sits within Saturn's rings and was
spotted in photos taken by the Voyager spacecraft.
The Voyager survey was incomplete, says Joe Burns, a Cornell
University mathematical theorist and astronomer who is on the
Cassini imaging team. Cassini will probably find much smaller
moon-sized rocks in Saturn's rings, Burns said, and "no jury has
voted" on whether to call these imminent discoveries moons or
not.
"I think we'll have to confront that," he said.
Rich vernacular
When humans first put hunks of fabricated metal into orbit around
Earth a few decades ago, astronomers needed to differentiate them
from stuff that was already in space, so the term natural satellite
came into use. In hindsight, it might not have been an overly
inspired choice of words.
"I believe that the public is often confused when the term
'satellite' is used," said Matthew Holman, an astronomer at the
Harvard-Smithsonian Center for Astrophysics who is a co-discoverer
of three moons around Neptune. "The public normally equates
'satellite' with artificial satellite.'"
From a lay person's perspective, Moon terminology has only become
more obscure.
Astronomers have developed a loose classification system that
roughly separates moons into three types based on size and distance
from their host planet: irregular moons, regular moons, and inner
moons (sometimes called ring moons).
Irregular moons
Irregular moons are of unknown origin. They might be captured
asteroids or comets, or perhaps they're pieces of young planetary
hopefuls that didn't survive to orbit at the big table of nine,
researchers believe. Many irregulars travel in packs that indicate
they were once parts of larger objects.
The irregulars are typically small and orbit at great distances and
often on odd trajectories. Their orbits are stretched and tilted in
just about every way you can imagine. They soar high above and
below the plane in which regular moons orbit. They come close to
the planet and then zoom far away on elliptical trajectories. More
often than not their orbits are retrograde -- opposite the
direction of regular moons and of the planet's spin.
About the only thing they don't do is orbit in a plane
perpendicular to that of the planet's orbit. Research led by Burns,
the Cornell mathematician, has shown that the Sun's gravity causes
such an orbit to be unstable, so any object captured in one would
crash into the planet or escape.
Most irregulars have probably been in orbit since their host planet
was very young, which astronomers cite as a good reason to call
them moons, despite their odd and varied behavior. Nobody knows how
they got captured, however.
Burns said it is very difficult for a planet to capture a moon.
Unless something slows an incoming asteroid or alters its
trajectory -- like a highly improbable pass at just the right angle
through an atmosphere -- it will either hit the planet or fly on
past.
"If it comes from infinity, it's going to go back to infinity," he
said.
Perhaps, Burns speculates, the irregulars ran into an extended
envelope of gas that might have surrounded a giant planet shortly
after its birth 4.5 billion years ago. That would explain why
irregulars tend to be small, because a smaller object has more
surface area in relation to its mass, and is more likely to be
captured than a large object.
Scientists aren't sure how the giant planets formed, let alone what
conditions were like around them back then. So figuring out moons
will help astronomers understand planets, researchers agree.
Regular moons
While irregular moons define the outer orbital reaches of a
planet's domain in space, the next region in is dominated by
regular moons. Examples include Saturn's moon Titan and Jupiter's
Galilean satellites: Io, Europa, Ganymede and Callisto.
These classic, large and round moons tend toward entirely stable,
simple, nearly circular orbits. All move through a plane in space
that is roughly equal to the planet's equatorial plane. This
conformity leads theorists to believe the moons formed out of the
same nebula of gas and dust that built the planet.
Other so-called regular moons were probably carved by collision.
Our own Moon likely developed after a Mars-sized object slammed
into Earth. Some Uranian satellites might have been formed by an
impact that flopped the planet over on its side.
Pluto's moon Charon is thought to have been created by collision,
too, says the SwRI's Stern, who leads NASA's New Horizons mission
to study the planet and set for launch in 2006. Interestingly, the
gravitational midpoint around which those two objects revolve is
not inside Pluto, but instead out in empty space. Some researchers
prefer to call this a binary planet system (others don't consider
Pluto a planet at all).
The Pluto-Charon system is not unlike pairs of other rocks orbiting
each other out there on the fringes of the solar system, a region
known at the Kuiper Belt. The dancing Kuiper Belt Objects,
so-called KBO binaries, often involve a primary object with a
smaller satellite and further stretch usage of the word moon.
"The KBO binaries aren't rocks, they are worlds," Stern says. "In
fact, most are large enough to be rounded by gravity and therefore
mini-planets in their own right."
More than 30 asteroids are known to have moons, too.
Inner moons
Inside the orbits of the regular moons, things get crazy.
Inner moons, sometimes called ring moons because they often travel
amidst other debris, are almost surely the youngest. They're also
the most likely to disappear. Here's why:
Planets, especially the large ones, act like gravitational brooms,
gradually clearing the solar system of comets. That's bad news for
any moon close to the broom.
"It's more dangerous down there," says Kevin Zahnle of NASA's Ames
Research Center.
If you are a moon, the odds of being hit by a comet grow the closer
you are to your host planet. Zahnle imagines flies buzzing around a
hunk of raw meat. Pretend your hand is a moon, and pass it a few
feet from the meat: you might not be hit by a single fly. Pass your
hand a couple inches from the meat, and many flies might hit
it.
Making matters worse, orbital mechanics dictates that inner moons
travel more swiftly than outer moons, so collisions are more
fierce, making it less likely that a moon will survive a hit. "And
if you're smaller to begin with, it's more precarious to begin
with," Zahnle adds, because you don't have the self-gravity to
survive an encounter.
All this means that the life of an inner moon is very
unstable.
"These are the most ephemeral and are presumably young," Zahnle
said. He suspects inner moons might be well less than a billion
years old, but cautions that there are no data to support that
view. More needs to be learned about the population of comets, he
said, and how frequently they impact the giant planets.
So are these precarious little objects moons? Definitely, Zahnle
said. He adds, however, that in a ring system, the smaller rocks
are constantly running into each other, their movement governed
more by collision than by traditional orbital mechanics. Zahnle
would tend toward calling these things debris, not moons.
Outcast moons
Outside the traditional categories of satellites are two sorts of
objects that really stretch definitions.
Ahead of and behind Jupiter are two packs of asteroids -- tens of
thousands of them -- that orbit the Sun but are also
gravitationally bound to Jupiter. Astronomers call them Trojan
asteroids. They are technically satellites of Jupiter, some
astronomers say, but others, including Burns, consider them mere
companions to Jupiter.
"The one property that I think a satellite must possess to be a
satellite is an orbit that encircles its planet," Burns said.
Earth, Mars and Neptune are each known to control one or more
Trojan objects in similar fashion.
Earth has acquired a sort of enhanced Trojan, too. A rock called
2002 AA29 orbits the Sun while also carving a horseshoe-shaped path
around Earth. Every few centuries it gets close enough to Earth to
be considered more than a Trojan, becoming what astronomers now
call a quasi-satellite.
To astronomers, all this is quite clear, but there might be room
for further definition.
"I do not think a new classification [system] is needed," said
Earth's quasi-moon co-discoverer Martin Connors of Athabasca
University in Canada. "For technical purposes we may wish to define
sub-categories."
A moon by any other name ...
Nobody has found more moons than David Jewitt of the University of
Hawaii. Along with colleague Scott Sheppard, Jewitt's team has
found three dozen satellites orbiting Jupiter and has more in the
observational pipeline. Jewitt is not keen to redefine moon terms
or set any lower size limit.
"To me, they're all natural satellites," he said. "Is a small dog
not a dog because it is small?"
If there is some confusion now, Jewitt says, then that reflects the
healthy progress of discovery, and greater clarity will be the
ultimate result. Jewitt realizes, however, that from a public
perspective there can be some "pain" associated with no longer
being able to look satellites up in a textbook. "Many people are
not comfortable with a world that changes quickly."
One thing that hasn't changed, not since well before Galileo's
time: There is still only one natural satellite you can step
outside and see with your naked eye, the one we call, simply, the
Moon.
