by Bob McDonald
Your eyes do this all the time when you see moving objects, such as a ball that is bouncing toward you. Your eyes move up and down with the ball so that you can keep it in focus and make the catch. If your eyes didn’t move, the ball would be blurry… and you would get hit in the head.
When it comes to telescopes, size matters. The bigger the mirror, the farther into the universe the telescope can see. While telescopes on the ground are becoming gigantic, there is also a bigger one heading to space called the James Webb Space Telescope. It has a mirror made of eighteen segments that fold up so that the whole thing will fit into the nose cone of a rocket. When it reaches space, the telescope will unfold like a flower in space to make one single mirror that’s 6.5 meters across—more than twice the size of Hubble. Out in space, with no air to look through, the Webb telescope will see even farther into the depths of the universe than we ever have before.
YOU TRY IT! The Twinkle Test
Not all stars twinkle the same. Some twinkle more than others, and some are more colorful depending on where they are in the sky.
WHAT YOU NEED
A clear night when you can see the stars, even in a city
Your own eyes
A pair of binoculars (optional)
WHAT TO DO
Find a place where you can see the stars all the way from straight overhead down to the horizon. Watch the stars straight above you and try not to blink (this works better with a pair of binoculars). Notice how much they twinkle.
Now find a star that is really low in the sky close to the horizon. Watch it carefully for a few moments and see if there is a difference in how much it twinkles compared to the star overhead.
The star near the horizon should twinkle more because you’re looking through more air in that direction. The more air you look through, the more it obstructs and moves the starlight.
You might also find that the stars close to the horizon are more colorful. Air stops some colors of light more than others. Red light goes through air better than blue, which can give stars a pink color. That is also why the sun turns red at sunset—blue light is being stopped while the red gets to your eyes. It also makes it more beautiful, don’t you think?
29 What Are Shooting Stars?
Despite what their name might say, shooting stars are not really stars. They’re actually tiny bits of sand, pebbles, rocks, and boulders that fall from space and shine like stars for a second or so as they burn up in our atmosphere.
Space is not as empty as it looks. Our solar system started out as a nebula, an enormous cloud of gas and dust. Most of the cloud became the sun and the planets that we know today, but there were a lot of little bits left over that are still floating around. That’s what the Earth runs into as we journey around the sun every year.
Let’s not forget that our planet is speeding along through space at one hundred thousand kilometers per hour as we circle the sun every year. We don’t feel that motion because we’re carried along with the Earth and the ride is super smooth. The same thing happens in an elevator. Once the doors close and it gets going, it’s hard to tell you’re moving between floors because it runs at a constant speed. You only feel the motion when that speed changes or the elevator stops.
Every second, we travel thirty kilometers through space. Think of a place that is thirty kilometers away from you right now. (You may have to look at a map.) Then count, one-two. That’s how long it would take to get there if you could move as fast as the Earth moves through space.
Our speedy planet plows through all that dust and dirt that lies in our path, like a car running into raindrops when driving through a storm. And in the same way that rain hits a car’s windshield, the dust and dirt floating around in space hit our planetary windshield: the Earth’s atmosphere.
The particles are not just lying still out there. They’re also moving extremely quickly, so they hit the atmosphere traveling many times faster than bullets. At that speed, friction from the air heats them to high temperatures, and, for a second or two, they shine as bright as stars while they burn up and shoot across the sky. That’s where they get the name “shooting stars.”
Shooting stars are properly called meteors, and most of them are the size of a grain of sand. They’re so small that they burn up completely in the air and never reach the ground. There are plenty of them, too—the Earth picks up about one hundred tons of space dust every day.
Some meteors are a little bigger, the size of your fist or a loaf of bread. Let’s call them space rocks. Space rocks can make it all the way to the ground, and when they do, we have a new name for them: meteorites. A good way to remember the difference between meteors and meteorites is that meteors don’t reach the ground, but meteorites, might hit you on the head… though if one hits you on the head, it would be a meteorwrong!
Then there’s the really big debris. Some of the objects floating in space are the size of houses, others as big as a mountain. These are asteroids and comets. It doesn’t happen very often, but if one of those big ones were to hit, it would be a problem, as the dinosaurs found out sixty-six million years ago.
Thankfully, we can still enjoy close-up views of some of these space projectiles without the risk of going the way of the dinosaurs. The best time to see meteors is during meteor showers. One of the best meteor showers happens every year during the second week of August. It’s called the Perseid meteor shower, and it occurs every summer when the Earth plows through the same path of dirt and ice in space that was left behind by a comet.
Comets are big boulders the size of mountains. They’re different from asteroids because they also contain a large amount of ice. When comets get close to the sun, the ice warms up and turns into a gas that, along with a lot of dust, blows off the comet, forming a beautiful tail. Comet tails can be millions of kilometers long. The dust they give off stays in space, just like the dust cloud left behind by a big truck driving along a dirt road.
The dust that forms the Perseid meteor shower trails from a comet called Swift-Tuttle, which crosses the Earth’s orbit around the sun. Every August, we pass through that leftover comet tail and get a light show of meteors.
Catching a glimpse of this celestial show is an activity best done with a group of friends. Check newspapers or the NASA website to find out what time in August is best for seeing the shower. (It is usually around the twelfth or fifteenth.)
Pick a clear night in that time when you can see the stars, then head to a place as far away from city light as possible. You will have to stay up till around ten p.m., when it is dark. Lay back on the ground or sit in a lawn chair facing east and look up. Try not to blink too much because meteors are fast and easy to miss. Be patient. Once every minute or so, you will see a quick flash in the sky, a streak of light that lasts less than a second. Once in a while, you might see one that has a glowing head and long tail behind it. Those are the bigger meteors, the ones the size of pebbles. You might even catch the odd satellite passing overhead as a bonus.
When you see how fast those meteors are moving, remember, it’s not just their motion that causes the brilliant streaks across the sky. It is also you, zooming through the cosmos on your very fast spaceship: Earth.
YOU TRY IT! Moon Meteors
The Earth is not the only planet that gets hit by meteors. All planets do, as does the moon. Look at the face of the moon and you will see it is covered in round craters of all sizes. Those craters were made by meteors large and small that have been hitting the moon for billions of years.
You can make your own moonscape of craters.
WHAT YOU NEED
A large bowl
Flour
A glass of water
WHAT TO DO
Pour the flour into the bowl to a depth of about five centimeters. Shake the bowl from side to side to make the surface of the flour nice and smooth.
Hold the glass of water about a meter above the bowl and let just a few drops fall into the flour. You will see the drops will make little crate
rs in the flour as they strike the surface. Notice how some of the flour gets blown out of the crater, forming a circle of debris around it?
Experiment with large and small drops until the whole surface of the flour is covered.
You have just made a model of the moon! Now compare your model to a close-up picture of the moon and see if there are any similarities.
30 Why Do Comets Have Tails?
Comets are sometimes called dirty snowballs because they’re made of a lot of dust and dirt mixed in with ice. They’re dark, almost as black as charcoal. They’re also extremely old. They’re leftover pieces from the huge cloud of gas and dust that formed our solar system—the extra bits that did not become planets. That makes comets time capsules, because they haven’t changed for billions of years. They allow us to look back to a time before the Earth was even born.
So why are comets usually a surprise when they show up? Because most of the time they’re invisible. We don’t see them unless they have tails, and comets only have tails if they come close to the sun and warm up. Most comets hang out beyond Pluto, where it’s cold and dark and they stay frozen. If they don’t get warm, no tail. And no tail means the comet is too small to be seen in our night sky.
Comets cross the orbits of the planets, which means there is a good chance of a collision. In 1994, astronomers from around the world trained their telescopes to see, for the very first time, a comet running into a planet. And it wasn’t just one collision. The comet, called Shoemaker-Levy 9, started out in one piece, but when it got close to Jupiter, the giant planet’s powerful gravity tore the comet into pieces, forming a long train of comet pieces that plunged into the planet’s atmosphere at speeds of more than two hundred thousand kilometers per hour. Gigantic explosions sent enormous balls of fire above the planet, and dark scars marked the face of Jupiter for weeks afterward. It was a spectacular event but also a little scary because if a comet can hit Jupiter, it means a comet could hit us. And we know Earth has been hit by comets many times in the past.
In 1908, a comet struck northern Russia. Fortunately, there were no people living in the area, but the impact flattened trees in the surrounding forest for hundreds of kilometers in all directions. If such a powerful event happened today near a city, it would be one thousand times more devastating than the first atomic bomb dropped on Hiroshima.
No one knows exactly how many comets are out there, but there are certainly more comets in space than there are people on Earth. Several robots have been sent to take pictures of comets. One robot, called Stardust, flew right through a comet tail and picked up pieces of the dust that were brought back to Earth for analysis. Another robot, Deep Impact, launched a heavy slug that smashed into a comet to see what was inside. As expected, the inside of the comet was made of a lot of water, ice, and dust, as well as some carbon compounds. And another robot, named Rosetta, followed a comet all the way around the sun and sent a lander down onto the surface of the comet itself. From these robotic adventures, we’ve learned that most comets are about five to ten kilometers across, which is the size of a small city.
If you could ride a comet, you’d get a tour of the entire solar system, passing by tiny, icy Pluto, followed by the giant blue-green planets Neptune and Uranus. You’d continue on past Saturn, with its magnificent rings. Lastly, you’d pass mighty Jupiter, the king of the planets. By that time, you’d start to feel warmer as you’d be closer to the sun. Then things would get really interesting. An amazing transformation would take place as your comet begins to warm up. Gases would erupt from below the surface in great geysers, shooting ice crystals into the sky. Snow would fall up! And those snow geysers would be seen from the Earth as a gorgeous comet tail. The funny thing is that pictures of comets make it seem like they’re flying through space headfirst. While that’s sometimes true, they can also fly tailfirst, meaning the tail sometimes wags the comet rather than the other way around.
A comet tail is like a flag that always blows in the direction of the wind. In space, there is a kind of wind of electrified particles that blows out from the sun in all directions called the solar wind. This is the force that always keeps the comet tail pointing away from the sun no matter what direction the comet is moving. As the comet comes in from the edge of the solar system and heads almost directly toward the sun, the tail is streaming out behind. But when it swings around the sun, the tail swings, too, always pointing away until the comet is flung back out into deep space tailfirst.
If you ever happen to see a comet, you are fortunate, because they are among the most beautiful of all celestial objects to cross our skies.
31 How Much Junk Is in Space?
We humans are messy creatures. We leave garbage all over the Earth and we also leave it in space. More than seven thousand satellites are in orbit around the Earth, but most of them are not working.
Satellites are machines and, like all machines, they wear out. That means there are a lot of dead satellites in space. In 2009, a dead Russian satellite called Cosmos 2251 collided with an American communications satellite called Iridium 33. When these two objects hit, their two speeds together added up to more than forty-two thousand kilometers per hour. Needless to say, both satellites were completely blown to bits, and those bits added to the growing problem of space junk.
Dead satellites are only part of the space junk, though. Every time a satellite is launched into space, it rides on the back of a rocket, so there are used rocket boosters scattered in orbit. But there are also about a million tiny bits of debris—burned-out boosters, nuts and bolts, even flecks of paint—circling the planet, all contributing to the increasing problem of garbage in space. In short, space is littered with potential hazards. The more objects we send into space, the greater the chances of them colliding with other objects, making more pieces of junk and more chances of collisions, which also create more junk… If this keeps up, there will be so much debris in space, no one will be able to go up there without getting hit.
All these pieces of space debris are scattered over an extremely wide area, but each one of them is moving incredibly fast—up to twenty-eight thousand kilometers per hour, which is faster than a bullet. A collision with a piece as small as your finger can have the explosive force of a grenade and could completely destroy a multimillion-dollar satellite, or worse, threaten the lives of astronauts in space.
Satellites are launched just about every week, but there’s rarely a plan to dispose of them when they come to the end of their lives. Fortunately, some of them take care of themselves.
Objects in low Earth orbit, including space stations, tend to take care of themselves because they’re not entirely out of the Earth’s atmosphere. Even at three to four hundred kilometers above the Earth, there is still a tiny amount of atmosphere that objects fly through. That atmosphere drags on the satellites, causing them to slow down over time, so their orbits naturally decay until they plunge back into the thicker atmosphere below and burn up from air friction. In fact, sometimes what looks like a shooting star is actually a piece of space junk falling back to Earth. Even the International Space Station, four hundred kilometers above the surface of the planet, experiences a tiny bit of atmospheric drag, so the station has to be boosted back up on a regular basis. One day, it, too, will meet a fiery end in our atmosphere.
For the most part, smaller satellites will burn to nothing on reentry, so we don’t have to worry about them too much. It’s only when larger things—like space stations—burn up that some fragments of debris survive all the way to the ground. Of course, no one wants to be hit by a piece of space junk falling from the sky, but the problem comes when we try to predict exactly when and where the debris will fall.
A lot of space junk is moving so fast that it skips across the top of the atmosphere like a stone skipping across a pond. And like a stone, how far the pieces go and when they stop depends on a lot of factors: the shape of the stone, the angle it hits the water, and any waves that could get in the way. Some stones dig in imme
diately, while others seem to go on forever before coming to a stop and dropping straight down.
Dead satellites are odd-shaped and often tumble end over end, so how they will behave in the air is uncertain. The final impact point cannot be predicted with any accuracy until the last few orbits, and even then there is uncertainty.
That was what happened in 1979 with the fall of Skylab, the first American space station, the Russian space station Salyut 7 in 1991, and more recently, the Chinese space station Tiangong-1 in 2018. Scientists tried to reassure the public that there was little to fear by pointing out that the odds of being hit by debris falling from the sky are extremely low because very little of it actually reaches the ground. Also, most of the planet is ocean, so it is more likely that the pieces would fall in water than anywhere on land.
Tiangong-1 fell in the Pacific Ocean, but both Skylab and Salyut 7 overshot their predicted impact points. Bits of the American station fell down in Australia, and parts of the Russian station came down in Argentina. Thankfully, no one was hurt in these cases, but it shows how difficult it is to pin down the final resting place of an uncontrolled large object.
The best-case scenario was the intentional de-orbit of the Russian space station Mir in 2001. The 140-ton complex was the largest object to reenter the Earth’s atmosphere, and it was controlled from the ground. Using a small rocket, the operators on the ground drove Mir toward the Pacific Ocean where the pieces fell harmlessly into the water. It makes you wonder how the International Space Station, by far the largest object ever to fly in space, will be brought down when its mission is over, sometime after 2024.
