Cy, you ask several interesting questions.  Let’s look at them.

Because Earth spins once on its axis every 23.9344696 hours (and why it’s not exactly 24 hours is a subject for a separate post) centrifugal force causes it to bulge slightly at the equator and flatten slightly at the poles. 

The diameter of our planet measured from North Pole to South Pole is 12,714 km, but the diameter of our planet measured across its equator is 12,756 km, a difference of 42 km (26 miles).

From these numbers you can calculate that Earth is out-of-round by about 0.3%

A regulation basketball has a diameter of a little more than nine inches (~23.5 cm).  A basketball that was out-of-round by 0.3% would translate to a difference between extreme variations in diameters of less than 3/100ths of an inch (about 0.7 mm).  Such a ball would absolutely roll across a hardwood floor just fine.

Would Earth be round enough for a game of basketball? YES!

The extremes of Mt. Everest (8.8 km above sea level) and the Mariana Trench (10.9 km below sea level) translate into a change in diameter significantly less than the bulging of our equatorial diameter produced by Earth’s rotation.

I don’t have the equipment to make exact measurements, but I suspect that if you measured the little bumps on a basketball’s surface those bumps would be more significant than the scale height of Mt. Everest on the ball.  I also suspect that a scale-sized Mariana Trench on the ball would not be as deep as the seams holding the pieces of the basketball together.

In other words, the “roughness” we observe on Earth’s surface may seem huge to us as we gaze up at mountains or down into an ocean abyss, but in terms of the overall shape of our planet these imperfections are trivial. 

Our planet is at least as round and “roll-able” as basketball.  

Cy’s last question was, “Was the earth ever smooth and uniform like a basketball? If so why didn’t it stay that way?”

We’ve seen that Earth is already at least as round and smooth as a basketball.  Was it ever smoother than this?  I really doubt it. 

Depending on where you measure, Earth’s crust is only 5 to 50 km thick, and represents only a few thousandths the diameter of our planet.  Our planet’s fragile, thin crust has been constantly jostled, poked, stretched, and otherwise roughed-up by forces both internal and external since our planet formed more than four and a half billion years ago.

And for a final moment of geek…  Now that we’ve established that Earth is at least as round and smooth as a basketball, what would a basketball-sized Earth weigh?  Earth’s mean density is 5.5 grams per cubic centimeter, and a basketball has a volume of about 6,800 cc, for a total basketball-sized Earth tipping the scales at about 37 kg, or roughly 80 pounds. (Remember, a significant portion of Earth’s interior is nickel and iron.)

Good luck making a three-pointer with that. 

Maybe we should try bowling?

Why are astronauts aboard the International Space Station weightless while they are still so close to the Earth?

Great question!

The strength of Earth’s gravitational field as felt by astronauts aboard the Space Shuttle is almost exactly the same as the gravity we feel here on the ground.  So why are the astronauts weightless?

The answer goes back to Sir Isaac Newton in 1687 and a “thought experiment” he performed using just his imagination and his newly-formed Theory of Gravitation.

Isaac Newton - 1689

Newton imagined what would happen if you had a cannon located on a mountain top and aimed at the horizon.

If you fire a cannon parallel to the ground the ball moves horizontally away from the cannon due to its energy from the gunpowder.  At the same time the cannon ball also begins to accelerate vertically toward the ground because of gravity.

The higher the speed of the cannon ball, the farther it travels before it hits the ground.

If you fire the cannon ball fast enough, the ball travels so far from you that the curvature of the Earth begins to curve the ground away from the falling cannon ball.

If you fire the cannon ball at just the right speed – about 17,500 mph, the cannon ball never hits the ground because the speed of the cannon ball’s fall towards the ground is exactly cancelled by the speed of the ground curving away out from under the falling cannon ball.

Newton realized that in the vacuum of space no force would act to slow the cannon ball down, and so a cannon ball that once achieved enough velocity to put it into orbit would remain in orbit indefinitely.  It would be in perpetual free-fall around the Earth.

So even though you are “weightless,” in orbit around Earth, you’re still experiencing almost exactly the same gravity you are right now as you read this.  The difference is that as you’re orbiting the planet at roughly five miles per second, the ground curves away beneath you at exactly the same speed that you’re falling toward the ground.

“If the speed of light is the highest attainable speed, why can’t it escape a black hole?”

First, a few words about the speed of light, which is indeed the fastest speed attainable through space.  How fast is it?

The speed of light: It's not just an engineering challenge - it's the law.

The speed of light is 299,792,458 meters per second.  That works out to about 186,000 miles per second.

Trying to go at or faster than light through space requires inventing exotic new mathematics that permit real number answers to equations that involve division by zero and square roots of negative numbers. If you can figure out how to do this sort of math, a Nobel prize is yours for the asking.

The speed of light is more than just a zillion times faster than we’ve ever been able to achieve with our technology, it’s also a fundamental constraint on everything – both matter and energy – in the universe.

So if nothing is faster than light, than how can a black hole “trap” light?

Light is trapped in black holes because black holes bend space itself.

All objects with mass curve the space around them.  Objects with little mass, such the Earth and Moon, only curve space a tiny amount, while objects with the mass of stars curve space a lot more. For a really massive object, like a black hole, the curvature of space they create in their vicinity is so severe that space is wrapped completely around itself.

Here’s a way to create a model of a black hole:

Take a sheet of paper.  That’s the universe.  To keep things simple, let’s declare that this is a one-dimensional universe, in that objects within this universe all exist along a single mathematical line and they can move in one direction only – left and right along that line.  In this 1-d universe there is no such thing as moving up and down on the paper, nor can you be anywhere except on the paper.

A one-dimensional universe. Everything exists on a straight line.

To get from the left side of the paper (we’ll call that point “A”) to the right side of the paper (we’ll call that point “B”) you have to move in a straight line on the surface of the paper.

Without massive objects being present, the 1-d universe lies completely flat, and the shortest route (indeed, the only route) between A and B is along that flat straight line.  So far so good.  The shortest path between two points in flat universe is along a straight line.

But what if you introduce a massive object, like a star, into your 1-d universe?

The mass of the star bends space itself.  You, living on the paper in this simplified universe, don’t see this curvature because your line of sight can only follow the line through space.  Seen with the benefit of having extra dimensions (as you are when you hold the paper) you see a straight line traveling on a curved piece of paper. Is the line still straight?  YES.  It’s the space itself that’s curved.

The line from A to B really is straight, it just travels through curved space.

In this 1-d universe imagining a jump from A to B without following the straight line is the equivalent of imagining a science-fiction jump through “hyperspace.”

What if the object on the line of your paper is so massive that it curves space completely around on top of itself?  What if point B were inside the region where the curvature of space exceeds 360 degrees?

Then you’d have a black hole.  Traveling along a straight line from A to B (as you must in this 1-d universe) you’d encounter a place where space had wrapped around itself and once you enter this region, no matter how fast you go, even at the speed of light, you can never leave.

That’s a 1-d black hole.

Once inside a black hole, you can't ever leave no matter how fast you're going.

Now try imagining a point in space where space itself has been curved on top of itself in all dimensions – left-right, up-down, forward-backward, and time itself.

Black holes capture light (thus making them “black”) because light is trapped within a region of infinitely inward-curving space.

In a real-world Black Hole, space curves on top of itself in all dimensions.

The term “mind-bending” seems appropriate, don’t you think?

I’ve heard that all galaxies have a black hole at the center. Does our galaxy have a black hole?

You heard correctly, Carina.  Astronomers have found evidence that all galaxies in the universe, including our own Milky Way galaxy, have Supermassive Black Holes at their centers.

A black hole of the type that most folks have heard about is the remains of a large star (considerably more massive than our Sun) that has reached the end of its life and has died in a spectacular explosion known as a supernova.

Black Holes come in a variety of sizes, including "Supermassive."

If the star-corpse that survives a supernova explosion has a mass greater than several times the mass of our Sun it will collapse under its own gravity and become a black hole. These “stellar” black holes have masses ranging anywhere from roughly four times the mass of our Sun up to about two dozen times the mass of our Sun.

Astronomers have directly observed large stars in high-speed (close to 1,000 miles per second) elliptical orbits swarming around an incredibly massive yet invisible object at the center of our Milky Way Galaxy that is closely associated with extremely powerful radio emissions – the classic hallmarks of a Supermassive Black Hole.

The Supermassive Black Hole at the center of our Milky Way galaxy has a mass that astronomers estimate to be about four million times greater than the mass of our Sun, but occupies a region of space smaller than our solar system.

A Supermassive Black Hole, as massive as 4 million Suns, sits in the star-packed center of our galaxy.

In galaxies where the Supermassive Black Hole is actively devouring stars and gas astronomers have found huge jets, hundreds of thousands of light-years long, of electrically charged subatomic particles that have been focused by the black hole’s powerful magnetic fields and blasted into space at a whisker less than the speed of light.  That’s right – Supermassive Black Holes are sloppy eaters – not everything that spirals toward the maw is swallowed. Some stuff headed for oblivion in a Supermassive Black Hole instead gets shredded into plasma, irradiated and then violently ejected from the galaxy.

Centraurus A (left) and M87 (right) are examples of galaxies displaying huge jets of subatomic particles that are being violently ejected from the Supermassive Black Holes at their centers.

Fortunately for us, the Supermassive Black Hole at the center of our galaxy, nearly 30,000 light-years from us, doesn’t appear to be gobbling stars at any great speed.

Not only have astronomers discovered that all galaxies appear to have Supermassive Black Holes at their centers, they’ve also discovered that these monster black holes are essential to galaxies taking the shapes that they do and being so good at making stars, which is a necessary first step in the creation of planets where folks like you and I can live.

Supermassive Black Holes: ubiquitous, mysterious, fantastically powerful, terrifying, and yet necessary (from a respectable distance) for life.

How fast is the Earth moving relative to everything else?

Let’s break it down.

The first motion of Earth that we’re all familiar with is rotation – the movement of the Earth that gives us day and night.  Here in Salt Lake City at about 40 degrees north latitude, Earth’s daily rotation about its axis carries us along at about 760 miles per hour.

At mid-latitudes, your speed from the rotation of the Earth is better than 700 miles per hour.

Then there’s the next familiar motion of our planet, Earth’s orbit around the Sun each year.  Our distance to the Sun is roughly 93,000,000 miles, which makes the circumference of the circle (yes, I know our orbit is slightly elliptical but for the purposes of this exercise let’s keep it simple) divided by 365.25 days work out to an average speed of  67,000 mph.   That’s like covering the distance between Los Angeles  to New York City in 3 minutes.

Earth orbits the Sun at an average speed of 67,000 mph!

Next, we have the motion of our solar system, which includes us, through the Milky Way galaxy.  We’re about 27,000 light years from the center of the galaxy, and complete one “orbit” of the galactic center in about 220 million years.  That works out to a speed of about 500,000 miles per hour. How does L.A. to New York in 20 seconds sound?

Our little neighborhood of stars orbits the center of our galazy at 500,000 mph.

Our Milky Way galaxy is itself moving relative to other galaxies.  We’re actually moving toward the Andromeda Galaxy, our nearest neighboring spiral galaxy, at 200,000 mph.  In a few billion years Andromeda and the Milky Way will merge, and that will be spectacular, but that’s a subject for blogging about at another time.

Finally, our “Local Group” of galaxies (made up of our Milky Way galaxy, the Andromeda galaxy and a few nearby smaller galaxies) is moving towards the “Great Attractor” supercluster of galaxies hundreds of millions of light years from us at better than a million miles per hour.

Our Milky Way Galaxy and the rest of the Local Group of galaxies are headed for the "Great Attractor" supercluster of galaxies at better than a million miles per hour.

But there’s one more bit of brain-bending universe-in-motion information to consider.  None of the speeds described above can hold a candle to the speed at which space is itself expanding as a result of the Big Bang which began the universe 13.7 billion years ago.

Imagine you’re watching a NASCAR race.  Cars are roaring around the track at 200 mph, jostling for position, some cars gaining on the others and some cars pulling away from the others.

Now imagine that the racetrack itself is expanding in all directions 100 times faster than the fastest car on the track.

Yes, it still matters that car #1 is moving towards or away from nearby car #2, but those motions pale compared to how fast the track itself is getting bigger, and overall the distances between race cars is increasing as the track itself expands.

The most distant galaxies visible from Earth are more than 10 billion light years from us.  We can measure their speed away from us by studying the red-shifting of their light.  It turns out that these galaxies are receding from us at something like one-fourth the speed of light – not because that’s how fast they’re moving through space, but because space itself is expanding and carrying these galaxies with it away from us.

So when your toddler is tearing around the house, seemingly incapable of holding still even for a moment, you can take comfort in the knowledge that even if that adorable child should briefly and blessedly take a break, you’re still screaming through the universe at astonishing speeds, and the concept of “holding still for a minute!” really is just a parental fantasy.

There is actually no such thing as "holding still." The universe decrees it.

Any takers?

The “2012″ disaster-pic opens Friday, and some folks are actually asking, “Is the world really going to end in 2012? Will the Earth’s magnetic field really reverse?  Is there really a planet Nibiru headed toward us? Is it all tied to the Mayan calendar?”

The short answer to all these questions is “no.”

Here’s how sure I am of that. If Earth’s magnetic field reverses and compass arrows begin pointing south instead of north by December 21, 2012 then I’ll buy you a dozen really good doughnuts  - provided that there are any bakeries still open in the post-apocalyptic  world envisioned by the folks claiming that life as we know it comes to an end on December 21^st, 2012.

If Earth’s magnetic polarity reverses in 2012 then the doughnuts are on me.

Here at Clark Planetarium we settle differences of opinion with a three-step, tried-and-true problem-solving process:

First, competing opinions are required to make falsifiable predictions.   An example of a falsifiable prediction is, “Earth’s magnetic field will reverse polarity in 2012.”  An example of a non-falsifiable prediction is, “Something unusual will happen in the world in 2012.”  Get the difference? One prediction is specific and you can test it, the other is vague and impossible to test.

Next, we find a way to test the prediction.  In this case it’ll involve watching compass needles in 2012.

Finally, if the prediction is proven wrong then the person who made that prediction brings doughnuts to the next staff meeting.

Why am I willing to bet doughnuts on this?

Because while it is true that Earth’s magnetic poles have reversed polarity many times in Earth’s past, and doubtless will again many times in Earth’s future, it takes a minimum of several thousand years to accomplish a polarity reversal.

If you take this wager and the magnetic poles don’t reverse in 2012 then you owe me a dozen really good doughnuts.

I’m not only willing to wager fresh doughnuts against Earth’s magnetic field reversing in 2012, I’m also extending a “doughnut bet” challenge for all the other “the world ends in 2012!” predictions.

Specifically, I’m betting a dozen fresh, frosting-with-sprinkles doughnuts that in 2012:

#1.  No Center-of-the Galaxy Alignment. The Sun is no better-aligned with the center of the galaxy on December 21st than it has been at any time in the past several hundred years or will be any time in the next several hundred years.  On 12/21/12 the Sun will be more than six degrees (twelve times the diameter of the full moon) from the galactic center. That’s not much of an alignment.

The Sun will not align between us and the Galactic Center in 2012. Even if it did, nothing would happen.

More to the point, the center of our galaxy is two billion times farther from the Sun than the Sun is from Earth.

This is exactly like worrying about whether these two asterisks ** here in Salt Lake City ever “align” with Sydney, Australia when I move my computer monitor around. If there is anything the center of the galaxy can do to our solar system, it’s already doing it, whether it’s December 12st or the 4th of July. Galactic “alignments” are irrelevant.

#2. No mystery planet, whether it’s called “Nibiru” or by any other name, wanders through our solar system disrupting orbits and generally wreaking havoc in 2012.  How do we know this?  Because if such a large planet or Brown Dwarf star really did orbit our Sun every 3,600 years, as some imaginative folks are claiming,  then 3,600 years ago the passage of this object through the inner solar system would have been devastating and tremendously noteworthy.  There was plenty of history being recorded in 1,600 B.C.E., and the complete absence of records describing something as phenomenal as Nibiru is compelling evidence that Nibiru exists solely in the imagination. Plus, so large a planet or Brown Dwarf star would have been the #1 target of thousands of professional astronomers worldwide for the past 50 years, and the #1 target of millions of amateur astronomers for at least the past decade.

The only way to see “Nibiru” is with your imagination (and maybe Photoshop).

#3.  The Mayan Calendar is interesting, but not a big deal. The resetting of the Mayan “Long Count” (144,000 days) calendar on 12/21/2012 will have the exact same impact on human behavior that the calendar that we use has when it “resets” to January 1^st every year.  There will be parties, and then we’ll go on with their lives.

The Mayan “Long Count” Calendar resets every 144,000 days, just as our calendar “resets” every 365 days.

Any takers on the doughnut bet?

Mmmmm… science.  It not only works, it’s also delicious with a fresh cup of coffee.

And, yes, I’m planning to see the movie.  I love a good disaster flick.

The quick and uninteresting answer is, “Too many to count.”

Here’s the longer and hopefully more interesting answer:

Almost as soon as the telescope was invented astronomers began counting craters on the moon. Knowing the number of craters on the moon lets you make an estimate of how old the moon is and how often objects in the solar system get smacked by other objects in the solar system. If you’re a scientist trying to understand the origins of planets and moons, that’s important information to know.

The first thing you notice when you look at the moon through a telescope is that it is covered with craters.

When the best telescopes in the world could see features on the moon as small as a few miles in diameter the number of craters on the moon measuring at least 30 kilometers in diameter (~20 miles) was estimated to be about 100,000.

As telescopes got better astronomers began to see smaller and smaller craters, and a lot more of them.

For every crater that was 100 km or so in diameter, there were roughly a hundred craters that were about 10 km in diameter. For ever crater that was about 10 km in diameter, there were about 100 craters that were on the order of 1 km in diameter… and so on.  The harder you looked on the moon for craters, the more you saw.

For every big lunar crater, there are many more smaller craters.

Earth has about 120 impact craters visible on its surface.  Well-known among these are the Barringer Crater in Arizona, Wolf Crater in Australia and Utah’s Upheaval Dome.

Why are there so many craters on the moon, but not so many on Earth?  There are two reasons for this, and they both have to do with Earth’s atmosphere.

First, the moon lacks a thick atmosphere to burn up or at least slow down the smaller bits of rock that enter Earth’s atmosphere. Space rocks smaller than a kitchen table are slowed down by our atmosphere so much that they don’t impact the ground with enough speed to blast out a crater.

The second reason Earth has so few visible craters on its surface is because our weather slowly erodes ancient meteor craters, effectively erasing them from the surface.

Arizona’s Barringer Crater is only about 50,000 years old, and erosion hasn’t had time to do much erasing.

Arizona's Barringer Crater is a young, well-preserved impact crater.

Australia’s Wolf Creek Crater is a few hundred thousand years old, and is considerably more weathered.

The Wolf Creek Crater in Australia is about 300,000 years old and is still distinctive as an impact crater, but shows weathering.

Utah’s Upheaval Dome is well over 100 million years old, and erosion has worn it away to the point that it took considerable geological research to establish that it was in fact an impact crater.

Upheaval Dome near Utah's Canyonlands National Park

Earth is a bigger target for space rocks floating around in our solar system, and has had many more collisions than the moon. Because there is no atmosphere on the moon, there can be no weathering and erosion, so the moon’s craters are preserved for the ages. On Earth, however, a few million years is all it takes for erosion to obliterate a crater.

Without an atmosphere to protect its surface or erase ancient craters, the moon’s surface is saturated with craters, and many of them can be extremely small.  Soil samples returned by Apollo astronauts include tiny beads of glass that are the sites of even tinier craters – some about 1/10^th the width of a human hair.

If craters can be just any size on the moon, including so small you need a high-powered microscope to see, how many craters do YOU think there are?

I’ve learned that the Andromeda galaxy is the only galaxy you can see without a telescope. Where do we look in the sky to see it?

The Andromeda Galaxy is the most distant object the unaided human eye can detect.  The galaxy itself is a collection of about 300 billion stars located almost 3 million light-years from us.  That means that when you’re looking at the Andromeda Galaxy the photons that are registering on your retinas are ending a speed-of-light journey through intergalactic space that began 3 million years ago.

The Andromeda Galaxy, nearly 3 million light years from us, is the most distant object visible to the unaided human eye.

Why is it called the Andromeda Galaxy?  The galaxy carries that name because it is seen in the same general direction as the stars in the constellation of Andromeda, the chained-to-a-rock daughter of the boastful and vain queen Cassiopeia of ancient Roman mythology.

Late summer and autumn is an ideal time to find the Andromeda Galaxy.

On a clear night an hour or two after sunset, face the eastern sky and find the Great Square of Pegasus.  It will look like a baseball diamond with “home plate” being lower-most above the horizon, then proceeding upwards and counterclockwise to “first base,” “second base,” and finally “third base.”

The star that marks the “third base” position in the Great Square of Pegasus is called Alpheratz. This star is also associated with the head of the princess Andromeda.

Now that you’ve located the head of the princess, look to your left (northward) for a long narrow funnel or cornucopia-shaped pattern of stars. This is the constellation Andromeda.

In the middle of this horn, about the same distance from Alpheratz as the space between the “bases” of the Square of Pegasus, is the star Mirach.

Look straight upwards a short distance and you’ll see a somewhat dimmer yellow star, and above that your eyes will just barely perceive a dim gray smudge that won’t quite come into focus.

A modest pair of binoculars will reveal an oblong gray smudge of light, bright in the center and faint on its edges.  That’s the Andromeda Galaxy!

Now here’s the fun part.  That little gray smudge is the light of hundreds of billions of stars that are located almost 3 million light years from your eyes.  How far away is that?

Most the bright stars in the constellation Andromeda are about two hundred light years from us.  That’s many trillions of miles.

Now imagine that you’re looking through a plate of glass that’s one foot in front of your face.  On this plate of glass are microscopic bits of dust.  Those are the stars of the constellation Andromeda.  At this scale (200 light years = 1 foot) our Milky Way Galaxy, the galaxy within which our solar system is located, is about the size of a football field.

Looking through the plate of glass and far beyond the tiny specks of dust on the glass, nearly three miles away is the Andromeda Galaxy, about half-again the size of our own Milky Way Galaxy.

The stars you see with your unaided eyes as you perceive Andromeda on a crisp fall evening are, practically speaking, right in front of your nose, while the Andromeda Galaxy is far, far beyond them.

Enjoy the view – those photons from Andromeda have traveled a long way to get to you.

Fun question!

The closest star to our solar system is, of course, our Sun.  Earth orbits the Sun at an average distance of 93 million miles.

Our Sun is one of about three hundred billion stars in the Milky Way Galaxy.  Of those hundreds of billions of stars in our galaxy, the next closest star to our Sun is a little red dwarf star called Proxima Centauri, at a distance of 4.2 light-years, which works out to be roughly 25 trillion miles (40 trillion kilometers).

Proxima Centauri is a "Red Dwarf" star - small, dim and fairly cool.

What is the practical difference between 93 million miles to the Sun and 25 trillion miles to Proxima Centauri?  Numbers that large are generally impossible to grasp.  You could just as well say that it’s “a Gajillion Bazillion” miles to Proxima Centauri and most people would just shrug and say, “If you say so.”

Even though Proxima Centauri (the red star in the center of this image) is only 4.2 light-years from us, viewing it requires a telescope.

Getting your head around interstellar distances is a challenge astronomy educators are always grappling with.  One of the best ways devised to communicate the vastness of space is to mathematically shrink the universe to a much smaller scale size so that sizes and distances can be appreciated in terms that are more intuitive to us.

If aircraft designers, architects and city planners can construct cool little models of their projects so that they can get a better idea of what they’re dealing with, why can’t astronomers make little models of solar systems and galaxies?

Let’s scrunch the universe down to a manageable scale.

If Earth were shrunk down to the size of a grain of sand, then:

• One inch represents over 200,000 miles.
• The Sun is the size of a grapefruit and is 38 feet away.
• Jupiter is the size of a pea 200 feet from the grapefruit-sized Sun.
• Neptune, the most distant “classical” planet from the Sun, is the size of a BB and orbits the grapefruit-Sun at a distance of 400 yards.  That’s right – with Earth shrunk to the size of a single grain of sand this scale model solar system would still be so large that it would cover more than four square blocks of downtown Salt Lake City.

If our grapefruit-sized Sun were in the center of downtown Salt Lake City, Neptune’s orbit would encompass multiple city blocks.

• And Proxima Centauri, our Sun’s closest stellar neighbor, is the size of a cherry, and is 2,000 miles away. That’s the distance from Salt Lake City to Orlando, Florida.

A scale model of the distance between the Sun and Proxima Centauri

And in between?

Nothing.

Seriously… nothing.

Interstellar space contains roughly one hydrogen atom per cubic centimeter (about the volume of a sugar cube).  Do you have any idea how empty that is?  No?

Imagine that you have attached a bucket on the outside of your spaceship so it can scoop-up interstellar hydrogen as you travel to Proxima Centauri.

During your 25 trillion mile Sun-to-Proxima Centauri trip, the bucket on your spaceship would collect one tenth the number of atoms that are normally in the bucket when it’s just sitting empty on a shelf here on Earth.

Now you know why they call it “space.”

This week’s Cosmic Quiz winner is Mary Newland, who asked, “Every year I like to watch the Perseids meteor shower. If the meteors are continually being shed, why does the comet not cease to exist?”

That is a great question!

As a matter of fact, comets do cease to exist over time.

Let’s start with some background information on comets:

What is the life expectancy of a comet?

Comets can be thought of as large dirty snowballs. The core of a comet, known as the nucleus, is typically a several mile-wide lump of frozen water that also contains various amounts of frozen gasses such as carbon dioxide, carbon monoxide, methane, and traces of ammonia. Mixed into all these ices are small amounts of rock and dust. An assortment of tar-like hydrocarbon compounds covers the surface of the nucleus.

Nearly all comets follow huge looping orbits around the sun, spending almost all of their time in the dim, cold regions of space far beyond the outer planets. For many comets, the time between successive close approaches to the sun is measured in thousands of years.

Comets spend most of their lives far from the sun.

Even a “Short Period” comet like Halley’s Comet (above) with its 76 year orbit spends most of its time far from the sun.

A comet’s orbit will periodically carry it, briefly, to the inner solar system. In the toasty-warm inner solar system the heat from the sun causes some of the ices near the surface of the comet’s nucleus to sublimate, transforming the ices directly from a frozen solid into a gas.

The expanding ball of gas and dust forms the comet’s head, also known as the coma, which grows to become hundreds of thousands of miles in diameter.

As these ices turn themselves into gas they release into space the trapped dust and bits of rock that are embedded in the ice.

Comet Halley's 10 mile-long nucleus as imaged by the Giotto spacecraft in 1986.

Pressure from the sun’s light and magnetic field pushes the coma away from the sun, creating the comet’s tail, which itself can become tens of millions of miles long. All this from mountain-sized lump of dirty ice!

While a comet is in the inner solar system it is warmed by the sun and constantly sheds bits of dust and tiny pieces of rock. These drift through the solar system following roughly the same orbit as the comet.

Comet 73P/Schwassmann-Wachmann losing matter as it warms.

It is not uncommon for a comet’s orbit to intersect with Earth’s orbit, and our planet finds itself several times each year plowing through a region of space that’s been recently dirtied-up by a comet. For the few days during which earth passes through a comet’s orbit our world encounters an above-average number of meteors.

It’s sort of like driving your car through the countryside. There are always a few unfortunate bugs that will cross paths with your windshield, but occasionally you’ll encounter a hapless swarm of gnats. Splat!

That’s the way it is with meteor showers. The sky overhead in the wee-small hours of the morning is the “front windshield” of our planet as we zip through space at 60,000 miles per hour, and meteor showers are ill-fated collections of dust and gravel, shed by comets, whose orbits around the sun place them in front of us, where they meet their brief, fiery demise.

Astronomers estimate that a comet nucleus can survive anywhere from a few dozen to a few hundred passages around the sun before it has lost so much material that it no longer creates a coma and tail and is doomed to spend the next few billion years as an unremarkable asteroid.

Nucleus of nearly-dead Comet 9P/Tempel, imaged by the 2005 Deep Impact spacecraft.

Comet Swift-Tuttle, with an orbital period of 133 years, is the comet that is the source of next week’s Perseid Meteor Shower.  This comet’s life expectancy is thought to be only 50,000 years or so, which in astronomical terms is an eyeblink.

Now that we know that comets can indeed “cease to exist,” the really interesting question becomes, “If comets don’t last very long, then what replenishes the supply of comets to our solar system?”

That, dear reader, needs a whole ‘nuther blog post.

But I’ll give you a hint: “Oort Cloud.”