Why Is Snot Green?

Why Is Snot Green?

by Glenn Murphy


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Scientific answers to mysteries kids really want to know.

Sure a lot of kids want to grow up to be astronauts, but according to scientist Glenn Murphy, even MORE kids want to know what happens to astronaut farts. (Short answer: Not good things!) And they want to know: Why don't all fish die from lightening storms? Why haven't we all been sucked into a black hole? Do animals talk? Told in a back-and-forth conversational style, Why is Snot Green? presents science just the way kids want to learn it—with lots of laughter.

Product Details

ISBN-13: 9781596435001
Publisher: Square Fish
Publication date: 04/14/2009
Pages: 240
Sales rank: 667,643
Product dimensions: 5.40(w) x 8.20(h) x 0.70(d)
Lexile: 970L (what's this?)
Age Range: 8 - 12 Years

About the Author

GLENN MURPHY gathered many of the questions he includes in Why Is Snot Green? from his years in the Explainer team at the Science Museum in London. He now lives in Raleigh, NC.

Read an Excerpt


Lost in Space

The universe can be a pretty dizzying place.

It was born in an almighty explosion of energy. It’s so massively, hugely, immensely enormous that it’s almost impossible to imagine how big it really is. Within it, there are spinning planets, burning suns, icy comets, and vast clouds of floating dust and rock. Planets, moons, and asteroids whip around each other like cosmic dance partners. Stars are born, stars die, and stars collapse into mysterious black holes in space.

But why did it turn out that way?

Where is it all headed?

Are we all alone in it?

And come on—how big could it really be?

Want to find out? Then read on …

How big is the universe?

Big. Really big. Crazy big. Billions of times bigger than the biggest thing you can imagine.

I don’t know about that—I can imagine some pretty big stuff …

OK, let’s give it a shot. Let’s imagine the size of the universe. It’s probably best to start small and work up—so let’s start with something fairly big—the Earth. The Earth is about 8,000 miles wide. If you drove a tunneling car straight through the middle,1 you’d get to the other side in about 5½ days, going nonstop at an average highway speed of 60 mph.

That doesn’t sound so far.

Right—it’s not. So let’s try a longer journey. Say, from here to the Moon. The Moon doesn’t go around us in perfect circles—it gets closer and farther away from us at different times of the month. But, on average, it’s about 240,000 miles away. It would take about 168 days to get there in a 60-mph flying space car. Even with rocket propulsion, the Apollo astronauts took about three days to get there (and it was really crowded in their spacecraft).

Similarly, the journey from Earth to the Sun is about 93 million miles, so it would take about 176 years by space car. To get right across our galaxy, the Milky Way, it would take about a million billion years (or 1,181,401,000,000,000 years to be more precise) to make the journey of 621 million billion (or 621,000,000,000,000,000) miles.

So what does that tell us?

That a space car would be cool, but at 60 mph it’d be pretty useless for getting around in space?

Errr … yes.

That, and that the galaxy is pretty huge in itself—let alone the universe. I’m running out of space to put all the zeros after the numbers here.

All right—what if you had a space car that could go at the speed of light?

Now we’re talking. The speed of light is about 670 million mph, so a car that fast could go about 6 thousand billion miles (or whole year. We call this distance a light-year, and it’s much more useful for measuring the huge distances—between stars and across galaxies—that we’ve been talking about. For example, the Milky Way is about 100,000 light-years across, so it’d take 100,000 years for our souped-up, super-fast, light-speed car to cross it. Still way too long to manage, but easier to imagine, maybe.

Go on, then—how big is the whole universe?

Well, we can only measure the universe as far as we can see it. With the best telescopes we have, that’s about 15 billion light-years (or 90 billion trillion miles—I won’t even bother trying to write that out with zeros) in every direction. So at the speed of light, it’d take at least 30 billion years to cross it. That’s about 16 billion years longer than the age of the universe itself.

Ah. So it’s big, then?

Like I said, crazy big. And that’s just the part we can see. Beyond that, we know it extends even farther because the light from the stuff we can see at the “edge” has taken 14 billion years to reach us, and the universe has expanded quite a bit since then! It might even curve back on itself, like the sea does as you sail around the globe. If that were the case, you could circle the universe and end up back where you started.

Now that would be cool.

Yes, it would. But all your friends would be billions of years older. So even if they were still around, they probably wouldn’t know what cool was any more. Bummer.

What is space made of?

Well, it’s not just “nothing.” Space is, at the very least, filled with gases spread out very, very thinly. It also bends—and possibly rips—so it must be made of something …

But space is, well, space, isn’t it? No air, no gravity, … Well, not exactly. Gravity is actually everywhere in space.

Its pull becomes weaker the farther you move away from one particular source—like a planet—but it’s still there.

And while it is true that there’s no air in space, there are other things spread around it. It’s only because the stuff is spread out so thin, and space is so big, that we can’t detect it very easily.

So what is this “stuff”?

Mostly hydrogen and interstellar dust left over from the Big Bang.

How much of it is out there?

Well, there’re billions of tons of it, but it’s spread so far and wide across the universe that you won’t find more than one atom per half a cubic inch of space in most places.2

You’ve probably been told that gases spread out to fill their containers, right? Well, if there’s nothing else in the container, then they do. In this case, the container—the universe—was empty and is now at least 180 billion trillion miles wide. Spread over this distance, even billions of tons of material can look like virtually nothing. It just depends on how hard you’re looking for it.

OK … so rather than say “there’s nothing in space,” you could say “there’s almost nothing in space” instead?

Exactly. That will not only be more accurate, but it will also freak people out. Which is always fun.

Top 10 things to do in Space

1. Float

2. Drift

3. Hover

4. Do somersaults

5. Spill some milk—and catch it again

6. Play zero-gravity football

7. Try to hit the Moon with a Frisbee

8. Draw a halo above your head with toothpaste

9. Wonder where your spaceship went

10. Panic

Why do planets bother going around the Sun?

Because the Sun’s gravity pulls planets around it, preventing them from whizzing off into space. But despite this, the planets are still gradually inching away from the Sun over time.

Yikes. That doesn’t sound good. I thought we’d just go around and around the Sun forever.

I’m afraid not. We’re getting a tiny, tiny bit farther from the Sun with each lap we do around it. The Earth gets about a half inch farther away from the Sun every year.

Why’s that?

It all has to do with how gravity works. A very clever scientist named Isaac Newton explained how gravity works over 300 years ago. If, like me, you can’t read Latin and math gives you a headache, it basically goes like this:

• Everything attracts everything else.

• The bigger the things are, the bigger the pull.

• The closer together the things are, the bigger the pull.

• The force that causes this attraction is called gravity.

Now, the Sun is by far the biggest object in the solar system, so it pulls everything else toward it. That includes planets, comets, asteroids—everything.

Hang on a minute—so why don’t the planets all just get pulled right into the Sun?

That’s because the planets all formed from chunks of stuff that were already circling the Sun to begin with. When the solar system began, these chunks clumped together to form planets and settled into regular circuits (or orbits) around the Sun. Closer to the Sun, all the icy bits got vaporized, so we ended up with the small rocky planets—Mercury, Venus, Earth, and Mars. Farther away, it was cool enough for gas to hang around, so we got the gas giant planets—Jupiter, Saturn, Uranus, and Neptune.

You forgot Pluto.

No, I didn’t. Most astronomers don’t count it as a real planet these days. There are a whole lot of small Pluto-sized objects out there beyond Neptune, and these (it has been decided) aren’t planets either.


Anyway—as I was saying—the planets have settled into moreor-less fixed orbits around the Sun. They don’t get pulled right into it because they still have some circling speed (or rather, momentum)3 left over from when they were just baby chunks of planet (or planetesimals, as they’re called). It’s like they’re excitable puppies on a long leash—they’re trying to whiz off into space but the Sun’s gravity keeps pulling them around it instead.

So why are they gradually getting away from it, then? Because the Sun is burning up its fuel and, in doing so, it’s shrinking. As it gets smaller, the strength of its pull on the planets decreases.

Doesn’t that mean we’re going to fly off into space and freeze?

Well—do you want the good news or the bad news?

The bad …

Before any of this happens, the Sun will swell up into a red giant star and frazzle the Earth anyway.

Ouch. OK—the good …

It’ll take a while, so there’s a good chance we’ll be able to hop planets (or preferably solar systems) beforehand.

Woohoo!! Better get cracking on those spaceships.

Yep. Time’s awastin’—only got about 4.5 billion years left.

Why do stars twinkle?

Because we’re looking at them through the murky veil of our atmosphere. From outside it, they look clear, steady, and bright.

You mean … stars don’t twinkle? All those nursery rhymes—they lied to me!

Well, you could see it that way. The shifting brightness and shape that we see is actually caused by churning gases in our atmosphere, which we have to look through in order to see the stars. Outside the atmosphere, the light from the stars is more constant and even, so there’s no “twinkle.” From down here, though, they do seem to twinkle. So they weren’t really lying. Whoever they are.

Fine. If they don’t twinkle, what do they do?

They burn. They burn fiercely for billions of years. Then, when they die, some can explode with enough force to sweep up 1,000 suns—leaving nothing but a vast, deadly hole in space behind them.

OK, that sounds cooler than “twinkle.” Tell me more.

Are you sitting comfortably? Good. Then let’s begin …

Once upon a time there was a cool cloud of gas. It was pretty dense, but all its gas-cloud buddies thought it was cool, and everybody knew that one day it would become a star. There it was, minding its own business, doing whatever gas clouds do, until finally it collapsed. Under the pull of its own gravity, it crunched up on itself really tightly and got hotter and hotter, starting a chain reaction and turning the cloud into a huge, dangerous nuclear reactor floating in space.

This is my kind of nursery story …

Good, now there’s a great bit coming up with giants and dwarfs in it, so be quiet.


No problem. Where was I? Ah, yes …

Well, by now the cloud had truly become a star. And it was enjoying itself immensely. It happily burned up its hydrogen gas—turning it into helium—for a few billion years, heating up a few nearby planets in the process. Life evolved on one or two planets, and the whole solar system bobbed along happily in its arm of the galaxy. Until one day the star had used up nearly all the hydrogen in its core and was forced to frazzle the nearby planets as it grew into a red giant.

As if that wasn’t enough, its core shrank some more over the next few billion years. Then it became a giant again, then it shrank a bit again, until eventually the star had had enough and decided to go out in style. So it imploded.

The rebounding explosion—a supernova—burned brighter than the entire galaxy and left behind a huge invisible hole in space (a black hole), from which nothing that fell in could ever escape.

The end.

Wow! That was excellent. Can I see one of those now, please?

Well, these supernova explosions don’t happen to all stars—they have to be massive enough to make one. In our galaxy, a new star is born and an old star dies about once a year; a supernova occurs only once every fifty years. Of course, you might spot one in another galaxy sooner if you’re patient, lucky, and have a big enough telescope.

Similarly, not all supernova explosions leave behind a black hole. Plus, you can’t see a black hole, unfortunately, as nothing can escape from it—not even light.

Boo. Some happy ending …

There’s no pleasing some people.

Who’s firing all the shooting stars?

No one is. They’re just small lumps of space dust plowing into our atmosphere and burning up. Besides—it’s often us running into them.

Come again?

It’s true. If you think about it, the Earth is going around the Sun—a round trip of about 584 million miles—once every 365 days. This means the Earth is traveling at about 67,000 miles per hour. Just imagine—you and I, and everyone else on the planet, are doing 67,000 mph right now.

Whoa. Why does it feel like I’m sitting still, then?

Because just like when you’re on a fast (but comfy) train, you don’t really notice you’re moving until you look out the window to see the world whipping by in the other direction.

That’s what happens when we see shooting stars—we see the evidence that we’re moving through space. Rather rapidly too.

Huh? How did you get all that from a little streak in the sky? Next time you’re lucky enough to spot a shooting star, try and figure out which bit of the sky it came from and keep watching that spot. Chances are you’ll see more of them—loads of shooting stars that seem to shoot up, down, to the left, and to the right of the same central point in the night sky. It’s a bit like being directly underneath a shower head and looking up—you see droplets fanning out in every direction.

Cool—but what does that mean?

It means that the whole Earth is moving at 67,000 mph, and its path crosses chunks of rock and dust also moving through space. As each chunk slams into our atmosphere, it heats up due to friction, and most chunks burn up completely way before they hit the ground. This makes the burning streak in the sky that we call a meteor, or a shooting star. Chunks that burn for longer, we call fireballs. Chunks that make it to the ground, we call meteorites.4

Got it. But if we’re always moving, and always hitting rocks and stuff, why don’t we see meteor showers all the time?

Well, we do hit (or get hit by) bits of dust and rock millions of times every day and night. But these are random individual chunks (or meteoroids), and it’s easy to miss their meteor trails in such a big sky if you don’t know where to look.

Meteor showers happen when the Earth plows through a swarm of meteoroids in space—like those left behind by passing comets as bits break off them. When this happens, you get lots of meteors at once, and you can easily spot them one after another if you’re looking in the right direction.

But how would you know where to look?

Astronomers can often predict where the showers will be in the sky because we pass through some of the same swarms every year (at the same point in the Earth’s circuit around the Sun). So, if you want to, you can find out where and when, and head out for an evening’s meteor spotting.

Go on, give it a shot!

Know your stuff: cool things to see by night

Meteor showers: Big ones happen every year around the same date. These include the Leonids (around November 17) and Perseids (around August 12).

Lunar eclipses: These happen when the Moon lies in the shadow of the Earth, causing it to change color from white to a deep red. The next few visible from North America are on June 26, 2010, December 21, 2010, and December 10, 2011.

Comets: These are a bit rarer. Although there are many visible with telescopes, the next time we see one just by looking up might not be until Halley’s Comet returns in 2061. But there’s always a chance a bright, shiny new one might turn up—as Comet Hale-Bopp did in 1997.

What are Saturn’s rings made of, and why don’t other planets have them?

The rings of Saturn are made of millions of small chunks of ice the size of tennis balls. And other planets—including Neptune, Uranus, and Jupiter—actually have rings too.

Really? They have rings too? How come you never see them in pictures?

Saturn’s rings are easier to see—they’re pretty clear to anyone looking at the planet with a half-decent telescope. In fact, Galileo spotted what he called “ears” on Saturn with his telescope way back in the seventeenth century, but he couldn’t explain them. Almost fifty years later, another physicist and astronomer, Christiaan Huygens, recognized them as rings around the planet. Saturn has been famous as “the ringed planet” ever since.

Even though they couldn’t see them directly, astronomers in the 1970s predicted that Neptune and Uranus would have rings; they didn’t get a clear picture of them until the Voyager satellite swung by them in the 1980s. On the way there, it spotted rings around Jupiter too—which was a nice surprise. So there are pictures of rings on other planets—it’s just that fewer people have seen them.

Are all those rings made of ice too?

Some are ice, some are chunks of dust or rock, and some are probably a mix of all of these.

We know that Saturn’s, Neptune’s, and Uranus’s rings are made of millions of small chunks of dirty ice—most of them smaller than a tennis ball. Jupiter’s single ring seems to be made of tiny dust particles.

How did they get there, and why doesn’t Earth have them? Small planets and moons don’t seem to have rings, so we think only big planets can have them. One idea is that they are formed when a moon gets too close to the planet it orbits. When this happens, tidal forces caused by the pull of the planet’s gravity rip the moon apart. The pieces then spread out and encircle the planet, forming a ring that shows where the moon used to orbit. The Earth and the smaller planets aren’t big enough to rip their moons apart, so they don’t make rings.

So why are Saturn’s rings the biggest and best, then?

Well, it’s true that Saturn’s rings are the most impressive. The pieces are so small, and there are so many of them, that together they look like a single big icy disk. It could be that Saturn used to have a big, icy moon that was smashed into tiny bits when it was hit by a comet or an asteroid. That’d do it.

I like that. That’s definitely what happened.

It’s as good a guess as any.

If the Earth’s a big ball, why don’t we fall off the bottom of it?

Because Earth’s gravity doesn’t make things fall down—it makes things fall toward the middle of the planet. That goes for everything on Earth—its skies, its oceans, and its people.

Oh, yeah. Of course. Er … what?

Let’s go back a bit. Isaac Newton told us how gravity works—like how and why things fall toward the ground when you drop them, and how fast you can expect them to go when you do. Agreed?

S’pose so.

OK. Well, he also told us that gravity wasn’t just about things falling toward the ground, but rather all objects falling toward—or attracting—each other. This force of attraction is stronger for larger objects, and Earth is by far the largest object on … well … Earth. So everything on Earth is held on by the Earth’s super-strong gravity. Like a big ball-shaped magnet with bits stuck all over it—not just metals, but all kinds of things, like air, water, trees, and people. That’s why none of it falls off.

OK—I get that. But magnets work by being magnetic, right? So why does gravity work just because something is big?

Good question. You’re right—there is a bit of the story missing here. So here’s the rest …

Newton’s explanation of gravity was all very clever—and we’ve since used it to figure out everything from orbiting planets to moon landings—but he still didn’t say what gravity actually is.

Where did the force come from? Why was it there at all? Newton didn’t know, couldn’t say.

So Albert Einstein bravely had a crack at these and other questions almost two hundred years later, saying (more or less):

• Space is not empty, or even flat—it’s more like a fabric with lumps and dents in it.

• The lumps and dents are distortions caused by objects (like stars and planets) in the fabric.

• Gravity is just objects rolling into dents (or around lumps) in the fabric.

Er … having trouble with this one …

OK—try to picture it like this: Imagine space as a big sheet of rubber, and the Sun as a basketball plunked in the middle. The sheet will bend around the ball, making a big dent in it, right? Now imagine rolling a few marbles or tennis balls across the sheet: Some will go straight across, but the ones that pass close to the basketball will get drawn into an arc around it as they dip into the dent. The marbles might even do a complete circle around the basketball before contacting it.

So …

So this is why planets circle (or orbit) the Sun—not just planets, but asteroids and comets too. It’s also the reason why moons orbit planets, and why satellites and space shuttles can orbit the Earth.

Basically, big things make a dent in space, and other things “roll” or “fall” toward the source of the dent. If they have enough speed and momentum, small things can circle around and around the dent forever. If they don’t have enough speed, they fall into the dent and eventually settle next to the object that made it.

So planets, comets, and asteroids roll around a gravity dent made by the Sun. The Moon rolls around one made by the Earth. Rockets and satellites get launched to the edge of the gravity dent and roll around it until it’s time to come back.

And the skies, oceans, deserts, glaciers, trees, animals, and people of Earth all sit in the depths of a dent in space created by the planet—stuck to its surface by the force of gravity.

Dents in space, eh? Weird.

You said it.

If the Earth spins around once a day, what started it spinning?

The Sun and planets were formed when bits of a huge spinning dust cloud clumped together to form solid lumps. The Earth—like all the other clumps—has kept on spinning ever since.

Let me get this straight—the whole solar system was born from a lump of spinning space dust?

Basically, yes. All stars and planets are born this way. Get enough dust in one place at one time, and gravity will start pulling things together. When enough dust accumulates, it collapses in on itself, heats up, and forms a star. If the dust was rotating beforehand, the star will rotate too and draw other lumps of dust into circuits around it. These circling dust lumps become planets, and the star becomes a sun. The sun and the dust lumps around it continue to spin long after they have formed.

But wouldn’t they slow down after a while? Most spinning things slow down and stop in the end, don’t they?

That’s true—most things we see every day do stop spinning in the end. But, when they do, they don’t just stop all by themselves—there’s a force that’s working against the spin and slowing them down. That force is friction.

Let’s say you’re spinning a coin on a table. From the second you release it, it’s constantly being slowed down as it whips against the air around it and scratches against the table below. If you spun that coin while floating in space, however, it would keep spinning forever. There would be no air (or table surface) to rub against, and so no friction to slow it down.

This is what happens with spinning suns and planets. In fact, they even speed up a bit as they are formed.

Huh? How’s that? I don’t get that at all.

As the spinning sun or planet clumps together, it gets denser but also smaller in size. When this happens, its speed of rotation increases because it’s now moving in a tighter circle. You see this happen when Olympic ice skaters do a spin: They start by spinning slowly with their arms wide apart, then pull them in to go faster. Same thing with stars, planets, and space dust.

OK—I get it. So the Earth clumped up, shrank up, and sped up until it was spinning around once a day?

Not quite. It sped up at first, but then slowed down. In fact, its spin is still slowing down now. Spinning planets and stars can be slowed down a bit by each other’s gravity. We call these effects tidal forces. In fact, the Moon has been applying a constant tidal force on the Earth (and the Earth upon the Moon too) for billions of years now, and this has slowed the spin of the Earth by quite a bit. Thanks to that, the days and nights are getting longer and longer.

Cool! So every day I get to sleep in longer and have more time to see my friends?

Well … a bit longer. It’s only getting longer by about two milliseconds (or thousandths of a second) every century at the moment. Not much of a sleep-in, really.

I can’t wait. Ha-ha!! Just think—in a few billion years I might get an extra hour in bed …

Er … OK. If you say so.

Will the Sun go out one day?

Yes, like all stars, the Sun will one day shine no more—ending its life as a ball of white ash. But by then we’ll be fried, rather than frozen.

But if the Sun goes out, won’t we all freeze?

If it just fizzled out like a sparkler on the Fourth of July, then yes, we would. But that’s not how stars die. Depending on how big they are, lots of things can happen to stars before they finally kick the bucket. As for our Sun, it’ll swell up into a monstrous red giant; barbecue Mercury, Venus, and the Earth … and eat them.

Harsh! Why would it do that?!

It doesn’t have a choice. Once most of the hydrogen gas is converted to helium, the inside of it will collapse, and the hydrogen burning on the outside will be pushed outward. As it does this, it’ll grow much bigger—big enough to swallow Mercury, Venus, and maybe Earth too.

Should we move to Mars now, then?

Not just yet. We’ll be fine for the time being. Most stars like the Sun live for about 10 billion years, and ours has only been around for about 5 billion—so it’s roughly middle-aged. It’ll be at least 4.5 billion years before it turns into a red giant, and a few billion more before it finally collapses and ends its life as a burned-out white dwarf.

Whew. That’s a relief.

On the other hand, life on Earth will have had it long before the Sun engulfs the planet.


And even if you make it to Mars, it won’t be very comfortable for anything to live there either.

Hey—no fair! Why not?

Because most of life as we know it can survive only when the temperature is just right. Some animals can survive in the desert at temperatures of over 248°F, but even they couldn’t stand it much hotter. Turn the temperature up by an average of 50° or 70°F, and you’d kill most things on the planet. Things in the sea would survive a bit longer, but even they would keel over once the oceans had boiled and evaporated. Again, all this would happen way before the Sun engulfed the planet itself.

Will it happen all at once, or will we see it coming?

It’ll take a while, and (if we’re all still here in 4.5 billion years) we’ll know when it’s about to happen—the Sun will turn red as the hydrogen inside it moves to the surface. Don’t panic and confuse this with a normal sunset, though.

Is there nothing we can do to protect ourselves?

Sunscreen, maybe. If you can find some with an SPF of 5,000.

That’s not funny.


Know your stuff: types of star

Yellow dwarf: Young and fairly small. Our Sun is a yellow dwarf star.

Red dwarf: The most common type. Relatively small, cool, and faint. Proxima Centauri is a nearby one.

Red giant: Older, bigger, and hotter. Betelgeuse, 600 light-years away from us, is one of these. It’s 20 times bigger and 14,000 times hotter than our Sun.

Supergiant: The biggest stars. When they die, they explode (supernova), and some become black holes.

White dwarf: Small and very dense. Neutron star: Incredibly small and dense. Some are less than 10 miles wide, but weigh more than the Earth.

Pulsar: A spinning neutron star that emits pulses of energy. Often mistaken for alien radio signals.

Binary star: Two stars circling each other. About half of all stars in the sky are actually pairs of stars like this.

Pop star: Loud, rich, annoying. Likes to sing. Gangsta: Tough, moody. Shines with a “bling.”

Where did the Moon come from?

Our best guess is that the Moon came from the Earth. Astronomers think that a massive asteroid hit us billions of years ago—blasting huge chunks of molten rock out of the planet and into orbit. The Moon formed from the shrapnel.

That may well be the coolest thing I have ever heard. The Moon formed because we were hit by a space missile?

Yes. Well, sort of. But it was probably more like a violent trick-shot in a game of cosmic pool. The asteroid was huge—probably about the same size as Mars—and it struck the Earth a glancing edgeways blow. The chunks of rock released in the impact were so hot that they actually vaporized. It was only later that these bits clumped up and reformed into molten—and then solid—rock.

Is this how all moons get made? By asteroids hitting planets?

Some moons are made this way, but probably not many. Asteroid impacts are fairly common—especially on bigger planets like Jupiter and Saturn because they’re bigger targets. But they have to be pretty big asteroids to throw debris out into space—most just leave a hole and a heap of rocks around it, which we see as a crater.

By big asteroids hitting planets, then?

Well, even if it is a big enough impact, more often than not any rocky debris released from an explosion is thrown right out into space and lost forever. In fact, most of the vaporized rock released from the Earth’s big impact was lost in this way—only some of it was held in orbit by the Earth’s gravity and clumped together to form the Moon.

Also, just because some debris gets captured doesn’t mean it’ll automatically shape itself into a nice, round moon. Sometimes the bits just encircle the planet, and you get rings instead. Sometimes you get moons and rings—as seen on Saturn. There, several small “shepherd” moons orbit alongside the rings, helping to hold their shape.

So where do all the other moons come from?

Some form around the same time as (or just after) their host planets, from bits drawn in by the new planet’s gravity. Others are “captured” by a planet’s gravity much later on. Big planets like Jupiter and Saturn have collected many of their moons (between them, they have more than 30!) in this way.

Could that happen here, giving us an extra moon?

Yes, it could. And it may have happened already.

Huh? We have two moons?

Kind of. About twenty years ago, astronomers spotted an object about 2 miles wide, not that far from Earth, and named it Cruithne. About ten years ago, they realized that it was still with us—sharing our orbit around the Sun—and that it is actually orbiting the Earth. But while the Moon takes just a month to do this, Cruithne takes about 770 years, and it will eventually leave us, flinging away into space forever. So you could say it’s a Near-Earth Asteroid (NEA) with temporary moon status.

But if a planet has lots of moons, don’t they all crash into each other?

Not at all. When planets have more than one moon, the moons are usually all different sizes, so they orbit at different distances. Even when they’re the same size, they can settle into a shared orbit like Saturn’s moons Janus and Epimethius and just chase each other around the planet like a pair of happy cosmic puppies.

One last thing: If the Moon came from the Earth, does that mean the Moon’s made of the same stuff?

Yes, but in different amounts. Both have a crust of solid rock floating on top of a mantle of semimolten rock.5 Both also have a core of iron and nickel in the middle. But inside, the Moon is much cooler than the Earth, so while the Earth has a solid inner core and a liquid outer core, the Moon’s outer core is still pretty solid. A bit gloppy, at best.

No cheese, then?

Sadly, no.

Do UFOs exist, and could my math teacher be an alien?

UFOs definitely exist—they’re spotted all the time. But none yet have been alien spacecraft. And your math teacher is probably just a (weird) human—no matter how alien he seems.

Hang on a minute—you said UFOs do exist?

Yes. Definitely. They’re spotted all the time, all over the world.

Aha! Got you! So there are flying saucers, then! You’re just covering it up, like all the shifty science guys on The X-Files …

No, I’m not.

Yeah, right—you would say that. You’re one of them.

Listen—UFO just means “unidentified flying object,” right? That basically means anything in the sky not clearly identified as something sensible like an airplane, glider, helicopter, balloon, or bird.

La, la, la … I’m not li-sten-ing … you’re just trying to brainwash me with your evil government conspiracy stuff … la, la, la …

So—look, stop it, I’m not brainwashing you—that means even a tennis ball or a Frisbee can be a UFO, at least temporarily (or until someone says, “Hey, wait a minute—that’s just a Frisbee!”).

Yeah, whatever. Some of them are huge, and they glow in the dark. So if they’re not spaceships, then what are they?

Some are just the result of rare atmospheric events, like sprites and ball lightning. Scientists still aren’t completely sure exactly how they are formed, because it’s not like you can study them “in the wild.” But it’s thought that they happen when lightning strips bits off nitrogen atoms in the air, leaving a glowing ball of colored plasma (or super-heated gas) behind.

Sprites form in the upper atmosphere, about 13 miles above the ground. Each one lasts less than a second, but when lots of them appear and disappear in a row, they can look like a single, fast-moving object.

Ball lightning can appear nearer to the ground, creating an eerie glowing ball (sometimes with a tail). This can hover and float about for several seconds before disappearing or discharging itself into a nearby object.

What about the UFOs that last longer than a few seconds?

Some sightings have turned out to be unusual but real military aircraft. Their existence may be denied at the time to keep them secret, but we find out later what they are (think how strange a stealth bomber would have looked thirty years ago). Others are just hoaxes, ranging from the very good (using Hollywood-style digital effects) to the very poor (the Frisbee-on-a-string-dangled-in-front-of-a-video-recorder type).

What about those alien crop circles?

A clever but proven hoax, created by two guys with planks and ropes. They even owned up to it and showed how it was done.

But what about aliens kidnapping people?

Well, many have been reported, but look at it this way: almost all of them have been in the United States, the UK, and France. These three countries make up only 6% of the world’s land mass. So either aliens are ignoring the other 94% of the world (and the entire Southern Hemisphere) … or we’re ignoring the evidence.

In the United States alone, over 5 million people have claimed they were abducted over the last fifty years. That’s 2,470 a day. You’d think maybe someone would notice all that flying-saucer traffic …

So aliens aren’t real, then?

I didn’t say that. There could well be aliens out there—it’s just that we almost certainly haven’t met any of them yet. If you’re looking for evidence, you can join the Search for Extraterrestrial Intelligence (SETI)6 project. They haven’t found any aliens yet either, but they have a better chance of finding real ones than anyone else on the Internet.

… and my math teacher isn’t one?

Probably not. I can’t say for sure, but I’m betting he’s more inhuman than nonhuman. Mine was, at least. Better do what he says, just in case …

What would happen if you farted in a space suit?

It would be the worst kind of fart ever: You couldn’t deny it, you couldn’t escape it, and the smell would stay with you all the way back to the space station.

Couldn’t you just open a flap or something and let it out?

You mean open the suit? Ummm … no. You wouldn’t want to do that.

Why’s that? Would you suffocate?

Well, no—not necessarily. In many space suits, the oxygen supply is sealed off in the helmet, so you’d still be able to breathe even if you opened a flap somewhere else.

So why not just pop open a handy bum-flap, then?

Bad idea. Trust me on this.

But why?

OK, you asked for it …

For an astronaut in space, the suit is all that stands between you and the deadly airless vacuum outside. Open a flap in your suit to let the fart out, and the air in your lungs and guts would expand, causing them to swell and rupture if it happened quickly enough. Then water in the muscles and soft tissues would boil into an expanding vapor, bloating parts of your body to up to twice their normal size. Then bubbles of nitrogen gas would form in your blood vessels, causing immense pain as they pressed on the nerves around them. And—eventually—you’d freeze.

Ooh, that’s gotta hurt. But why would that happen? And how could you boil and freeze at the same time?

This is all due to one thing: In space there’s no atmosphere around you—so there’s nothing to keep your body under pressure, and nothing to absorb and retain heat.

I don’t get it.

OK, let me explain. On Earth, the atmosphere is constantly pushing against your body. Without it, the air in your lungs and guts would push outwards unopposed, and the water in your soft tissues would start to boil. This is because there are two ways of boiling liquids: One is to heat them up, breaking the weak bonds between the molecules inside. The other is to drop the pressure around the liquid, taking away the squashing force that keeps the molecules packed tightly together. This allows them to drift apart, and the liquid turns into a gas.

The temperature of the water in your body tissues is normally kept at about 98.6°F and is happily kept liquid under the pressure of the atmosphere. But in space there is no atmosphere, so 98.6°F is enough to boil some of your body fluids.

A space suit stops this from happening by keeping the body under constant pressure. It does this by inflating on the inside (like a bike tire) and constantly pressing against the skin. But if you released this pressure—by, say, opening a handy bum-flap to let out a fart—the water in your tissues would boil and bubble. This would also make your skin and organs swell up as the blood inside them expanded.

Yuck. Nasty. OK, what about the freezing part?

This would follow the nasty lung-swelling part if you found yourself in the shade (for example, if the Earth, Moon, or your spacecraft is between you and the Sun). Without a thick atmosphere of gases to hold the heat, temperatures in shaded space reach well below -148°F. A space suit is lined with special insulating materials that stop you from losing the heat inside (produced by your body) to the outside. Open a flap in it and all the heat radiates out of your suit and into space, freezing you solid within minutes.

So, if you fart in a space suit, you’re stuck with it?

I’m afraid so. Of course, some of it will be recycled into the suit’s life-support system, so you’ll breathe a lot of it back in.


The rest will drift out when you get back to the ship and take the suit off. No doubt making you very unpopular with the other astronauts, since you can’t open a window in there either.

So no eating beans in space, then?

Not unless you want to be called Major Fartpants all the way home …

What is a black hole, and what would happen if you fell into one?

It’s a super-massive object, left behind after the death of some stars, from which nothing—not even light—can escape. If you fell into one, you’d be ripped apart, fried, or trapped forever. Possibly all three.

Black holes are dead stars?

Yes, kind of. Although not all stars end up as black holes—some just burn out. Otherwise there would be black holes everywhere.

So how do you get one?

If a star is big enough—and we’re talking at least twenty to twenty-five times bigger than the Sun here—it will end its fiery life in a massive explosion called a supernova. When this happens, the outer shell of the star gets blown apart, but its inner core collapses in on itself. For some, the core forms a small, dense lump, and the star ends its life there. For others, the core just keeps getting smaller and denser and smaller and denser. The pull of its gravity is so immense that anything within a certain distance gets sucked in and is trapped forever. Like a huge spherical invisible whirlpool in space.

Cool. But why are they invisible?

Because nothing within a certain distance of the black hole’s center (a boundary line we call the event horizon) can get out of it—not even light. It was Albert Einstein who figured out, among other things, that light is actually bent around massive objects like stars by gravity. If the object is super-massive, like a black hole, then any light rays close enough to it will spiral right around into it and never escape its grasp. So it’s not just that it fails to make light (as stars do)—you can’t even see light bouncing off it (as we can with moons and planets). So it becomes invisible, and we can only perceive it as a “gap” in space. As you might imagine, these are not easy to spot!

So how do we know they’re even there?

By looking for telltale signs of light bent around them—light that passes close enough to be affected by the black hole’s gravity, but not so close that it gets sucked into it. Once you spot that, you can confirm the black hole is there by looking for X-rays. This is because things that are being sucked into the hole heat up as they spin around it, and when they get hot enough, they start to radiate X-rays. So if you spot a weird light-bending pattern in space and it’s also emitting X-rays, there’s a good chance it’s a black hole.

So why would all that nasty stuff happen if you fell in one?

Well, you’d be ripped apart by tidal forces caused by its super-strong gravity. Basically, if you jumped in feet first, your feet and legs would be sucked in faster than your head and upper body (or vice versa if you dove in head first). So your body would be stretched out lengthwise until you snapped like a rubber band.


But let’s say you fired yourself into the black hole from a super-fast cannon. Maybe you could go fast enough to get to the middle before this happened. Even so, the chances are all those X-rays it chucks out would fry you before you made it through.

Eeek! But what if you managed to survive that somehow? What would happen then?

Actually, no one knows for sure. We know you could never escape the black hole’s gravity, so you’d never come out of it again. You could be trapped forever. But some scientists think it’s possible that black holes are “wormholes” in space: doorways to other points in the universe, or even other universes. But if you think about it, even if you made it in alive, it’s not clear how you’d escape the door on the other side, since this would be a black hole too.

Stretched, snapped, fried, and possibly trapped forever, eh? Remind me not to try that …

Absolutely. There are probably safer ways to travel!

Could comets or asteroids really blow up the Earth if they hit us—like in the movies?

They probably couldn’t blow up the whole planet, but they could give it a good whack and wipe out everything living on it. And dodging them wouldn’t be as easy as in the movies either …

What’s the difference between an asteroid and a comet, anyway?

Well, they’re basically similar: lumps of rock, dust, and ice in space. They’re debris left over from the formation of the solar system—the spare bits that didn’t get to be part of a star, planet, or moon. The main differences lie in where they come from and how they behave.

Where do they come from?

Asteroids form from small particles of dust and rock that pull together under their own gravity into lumps of various sizes. They can be anything from 30 feet to about 620 miles wide. In our solar system, most of them orbit the Sun in a huge ring or asteroid belt between Mars and Jupiter. There, Jupiter’s gravity holds them in place, keeps them spread out, and stops them from forming bigger lumps (like a moon or even a small planet). But sometimes one of them will get knocked or pulled out of orbit, and when this happens, they can end up on collision courses with planets.

Comets are formed far from the Sun and have a core (or coma) made of dust and ice surrounded by a crust of dust and rock. Most of them begin life in belts or clouds way beyond the planets. From there, they’re pulled inward by the Sun’s gravity, and they start doing huge, oval-shaped laps around the Sun that can take hundreds or even thousands of years. Comets also melt as they pass close to the Sun, venting trails of rock, dust, and water vapor—which we see as the comet’s “tail.” Some comets end up spiraling inward and crash into the Sun. When this happens, we give them one of the coolest names in astronomy: sungrazers.

Have we ever been hit by one before?

It’s not clear whether the Earth has ever been hit by a comet, but we’ve certainly been hit by a number of good-sized asteroids. These are a bit more commonplace. In fact, a decent-sized asteroid explodes in the upper atmosphere—with the force of a small nuclear weapon—more than once a year.

But thankfully, there haven’t been any really big ones for a while now. The last really big one to hit us was in 1908, when an asteroid exploded above Tunguska, Siberia. This blew with the force of 1,000 atomic bombs, leaving no crater, but flattening more than 770 square miles of forest below. And that was nothing compared to the comet or asteroid that caused the 9,500-square-mile crater in Chicxulub, Mexico. That one hit us about 65 million years ago and is thought to have killed—among many other things—practically all of the dinosaurs.7

What would happen if a big one hit us tomorrow?

Well, if the Chicxulub one is anything to go by, an asteroid about 6 miles across would punch right into the atmosphere as if it wasn’t there. The explosion on impact would vaporize the asteroid, the ground, and just about anything else within a 60-mile radius, leaving a hole in the Earth about that wide. Some of the dust and rock thrown up by the blast would fly so high, it’d go into orbit around the Moon. The rest would rain back down on the planet like fire, cooking anything or anyone not hiding deep underground and enveloping the Earth in a dark, choking cloud. This would then block out the Sun for about a year, killing all plant life and most or all of the animal life along with it. So, even if you survived the blast, you’d probably starve to death.

Couldn’t you just hide underground with a year’s supply of ramen noodles or something?

Maybe. But even if ramen noodles were that nutritious (which they aren’t), it’s unlikely you could get enough for everyone.

Couldn’t we just blow them up?

Sadly, no. Even Bruce Willis couldn’t do it. All the bombs and missiles on the planet could barely dent a good-sized asteroid or comet. But we might be able to knock one off course by hitting it sideways. Some scientists think we could even attach a kind of solar sail to an asteroid in space so that the solar wind would push it off course gradually over time.

That’s crazy!

Not as crazy as eating nothing but ramen noodles for a year.

Know your stuff: space rocks

Asteroid: Any one of the thousands of smallish (30 feet to 620 miles wide) rocky objects that orbit the Sun.

Asteroid belt: A ring of asteroids orbiting the Sun. The largest belt in our solar system lies between Jupiter and Mars.

Comet: An object made of dust, ice, and rock that melts as it nears the Sun, releasing gases seen as a long “tail” or trail across the night sky.

Meteor: A streak of light across the sky seen as a small object enters the Earth’s atmosphere and burns up. A shooting star.

Meteorite: A small chunk of rock that has made it all the way to the ground, striking the surface of a planet or moon.

Meteoroid: A chunk of rocky matter, typically smaller than an asteroid. Causes a meteor as it burns up in the atmosphere.

Meteor shower: Lots of meteors spotted at once, which all seem to come from the same point in the sky.

Micrometeorite: A tiny meteorite. Millions of these hit the Earth each day.

What did the Big Bang sound like?

Like nothing at all—because

1. there was nobody there to hear it,

2. sound can’t travel through space,

3. there was no space for it to travel through, and

4. there was no real bang anyway.

Whoa! Hold on, there. I think I get the first bit …

Right—the first bit’s easy. The Big Bang—the huge eruption of matter and energy that started the expansion of the universe—happened about 13 billion years ago. That’s a really, really long time ago.

To give you some idea of how long: Our planet, Earth, is only about 4½ billion years old (that’s 4,500,000,000 years). Life didn’t appear on it until 3 billion years ago. The first humans didn’t evolve until about 40,000 years ago. The ancient Egyptians, the Romans, and all that lot didn’t hit the scene until about 3,000 years ago. So you could say we missed the Bang by quite a bit.

OK—but if we were there, what would it have sounded like?

Like nothing. Nada. Zip. Not a peep. Total silence.

Eh? Why not?

Because for you to hear a sound, it has to travel from something (like an alarm clock, or the barrel of a gun) through something (like air or water) and into your ears, where it wobbles your eardrums. Then your brain translates that wobbling into a sound (like Brrrrrrrrrnnngg! or Bang!).

The problem with bangs in space is that even if you make one, the sound has nothing8 to travel through, so it doesn’t make it to your ears. (The same goes for exploding spaceships in science-fiction films—you wouldn’t hear those either).

So, even if you were hovering right next to the Big Bang, listening for it, you wouldn’t hear a thing.

Some scientists have pointed out that you can translate the radiation left over from the Big Bang into an audible sound using a computer. But even if you do that, it just sounds like a constant hissing sound, rather than a big impressive Kablooie or Kaboom!

But that’s ridiculous. Why call it “the Big Bang,” then?

Well, it was a big explosion, of a sort. It was an explosion of matter, energy, and space. It just didn’t go Bang!

And besides—“The Big Explosion of Matter and Energy” isn’t as catchy.

OK, fine—you couldn’t hear the Big Bang because you can’t hear bangs in space. Got it.

Well … not quite. There’s another problem. There was no space before the Big Bang either.

Er … come again? If there was no space, what was there?

Nothing. Absolutely nothing. The Big Bang didn’t just create all the stars and planets floating in space—it created the space they were floating in too. Space isn’t just nothingness—it’s like a fabric that the galaxies, stars, and planets are stitched into. The Big Bang started from a tiny point and expanded outward, creating the fabric of space as it went.

Gahhh!!! If there was nothing there, what did it explode into, then? What was there before it?

Nothing. Before the Bang, space, time, the universe … none of it existed. Of course, there may have been other universes …

I think I need to lie down.

Good idea. Just relax and think of nothing.



Text copyright © 2007 by Glenn Murphy

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Why is Snot Green? (Turtleback School & Library Binding Edition) 5 out of 5 based on 0 ratings. 7 reviews.
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