by Bob McDonald
Ganymede and Callisto look more like our moon, except they’re made of both rocks and ice mixed together. Ganymede has huge circular areas that look like impacts from large objects, and patterns in the ice suggest it is floating on an ocean of liquid water. Callisto has more craters on it than any other moon, which suggests it hasn’t changed much in a very long time.
You can see the moons of Jupiter with a good pair of binoculars or a small telescope.
If you can’t get one, try attending a star party hosted by an astronomical society. The parties are usually free, and you can look through their high-powered telescopes to see the weird and wonderful world beyond our atmosphere. And when you do, give a shout-out to Galileo for making it all happen!
YOU TRY IT! Look Out Below
You can do the same experiment Galileo did four hundred years ago. And you don’t need to climb a big tower to do it.
WHAT YOU NEED
A large, heavy book
A flat sheet of paper
WHAT TO DO
Hold the book in one hand and the paper in the other, with both of them lying flat.
Drop both objects at the same time and watch to see which one hits the ground first.
The book should win the race to the ground.
Galileo said that all objects should fall at the same speed, so does that mean Galileo was wrong about gravity? Not really. When you drop the piece of paper, it falls slowly because it is very light and it is slowed down by the air. If there were no air, though, both the paper and the book would fall at the same speed. Don’t believe me? Let’s try the experiment one more time.
Repeat the experiment, but this time, lay the paper flat on top of the book and drop both of them together. What happens? The paper falls at the same speed as the book because the book pushes the air out of the way.
Try the experiment one more time, and this time crush the paper into a ball as small as possible, then drop it beside the book. You will see that both objects do fall to the floor at the same speed because the balled-up paper does not catch as much air. So when you take air out of the equation, you can see that Galileo was right. Gravity acts on all objects the same way, whether it is a book and a piece of paper, the moons of Jupiter, or the planets going around the sun.
13 What Do Satellites Do?
A satellite is any object that orbits a planet. The Earth used to have only one satellite: the moon. But since the invention of rockets, we’ve sent more than four thousand satellites up into space to orbit our planet, and hundreds more are launched each year.
The thousands of satellites orbiting Earth every day are watching the weather, relaying television programs around the world, monitoring the environment… Even your phone talks to satellites. The GPS in our phones relies on a whole fleet of satellites that send out signals from space that your phone captures to find your location anywhere on the planet. When you hit the map function, your whereabouts are accurate because your phone is communicating with satellites orbiting over your head.
Canada was the fourth country in the world (after Russia in 1957 and the United States a year later) to launch a satellite. Our first was Alouette 1, which studied the upper atmosphere of the Earth, far above where airplanes can fly. Alouette 1 is no longer working, but it’s still up there.
There are so many satellites circling the Earth that you can spot one yourself. Start looking just after it gets dark—that’s when the sun is still shining on the satellites, making them look like moving stars. You have to be patient, though. Don’t be fooled by airplanes. If the moving dot has a blinking light on it, that’s a plane. Look for something that appears to be a moving star—it probably won’t be very bright—drifting slowly and smoothly across the sky. If you spot a really bright one, that could be the International Space Station.
Satellites orbit the Earth in a variety of ways, depending on their function. Some fly low, others go high; many go around the equator of the Earth while others loop over the North and South Poles. They may carry cameras to look down on our planet or telescopes to peer out to the edge of the universe.
When a satellite carries people, as in the case of the International Space Station, it orbits around the middle of the Earth at an angle across the equator, arcing high over North America and Russia, then down toward South America and Australia. This orbit is called low Earth orbit (LEO). It’s the easiest orbit to reach because it’s not very high—roughly four hundred kilometers up, which is barely above the Earth’s atmosphere—and it offers the best views for astronauts as they pass over most large cities in the world.
Satellites need to travel extremely fast to stay in orbit—an unimaginable thirty thousand kilometers per hour. That’s eight kilometers every second, which is faster than a speeding bullet. If you want your satellite to reach a higher orbit or to travel to the moon or Mars, it has to go even faster. That’s why we need rockets to reach space—they’re the only machines that can fly that fast.
To help rockets get up to that speed, most launch toward the east. Why? To save fuel! When a rocket launches to the east, it gets an extra boost from the Earth’s spin. At the equator, the ground is moving along at 1,600 kilometers per hour around the center of the planet. That’s free energy the rocket can take advantage of. If it launches in the opposite direction, west, it has to burn extra fuel to counteract the spin of the Earth.
If you want to see the entire surface of the Earth, not just the part around the middle, you have to send your satellite into a polar orbit, which means it will pass over both the North and South Poles. It takes a satellite about an hour and a half to circle the Earth once, so each time it comes around, the Earth has turned a little bit underneath it. Over the course of an entire day, the satellite will see the entire surface of the planet.
It’s hard to believe, but it’s even possible to make a satellite stay still in the sky and not seem to move at all. To do that, it has to be in a very special orbit called geostationary (GEO). This orbit occurs when a satellite is sent really, really high—thirty-six thousand kilometers above the Earth’s equator. At this orbit, the satellite will take twenty-four hours to circle the Earth. Of course, we know that twenty-four hours is also the time it takes the Earth to rotate once. So if the satellite is taking the same time to circle the Earth as the Earth takes to turn itself, the satellite will remain over one spot as both it and the Earth rotate at the same speed. It’s like turning your body around with your hand held out in front of your face—your body and hand are turning at the same rate, so your hand is always in front of your nose. This is why satellite dishes that you see in backyards or on top of buildings always point to the same place in the sky. From the ground, it looks like the satellite is still, but in fact it’s whizzing through space at thousands of kilometers per hour.
And if you like having your picture taken, look up on a clear day and wave. Some satellites up there may be spying on you. Spy satellites are used by the military so that one country can find out what another country is doing. Are they building rockets or weapons? The spy-satellite images will reveal the answers.
Some spy satellites want to see without being seen, so they are placed in an HEO—highly elliptical orbit—which is both high and low. The orbit is football-shaped, with one end way out in space, the other close to the Earth. When the satellite approaches the highest point of the orbit, it slows down and hovers over one spot for a while as it watches what’s happening on the ground. If it sees something interesting and needs a closer look, it swoops down to the lowest point in its orbit. It whizzes by, secretly snaps close-up pictures, then retreats into deep space before anyone notices.
If you want to see a working satellite, turn on the Weather Channel or go to a weather website. There, you’ll see images of clouds and storms as they appear from space. Weather forecasters depend on satellite images to see where storms are developing and how fast they’re moving. That’s how they can tell you when bad weather is likely to reach your area. Hundreds of satellites orbit the
Earth every day, sending back pictures of the weather so that we know what to wear when we step outside.
Satellites can do a lot more than just help us predict the weather, though. They can also tell us a lot about environmental problems.
The hole in the ozone layer is an example of this. The ozone layer is a level in our atmosphere that protects us from harmful radiation from the sun. You can’t see the holes in it with your eyes, but thanks to satellites, we can see that this natural sunscreen is disappearing over the North and South Poles. People didn’t even know about the ozone holes before the 1970s. Now, thanks to satellites, we can watch them grow and shrink every day.
The same is true of El Niño, a huge blob of warm water that moves back and forth across the Pacific Ocean. When El Niño comes to North America, it disturbs weather patterns along the West Coast. It’s only from the high perspective of space that we can see the shape of these huge systems in the oceans and the atmosphere.
Satellites also play a very important role in helping us understand changes on the Earth’s surface that may be the result of climate change. Looking at satellite images taken over the decades, you can see recession in the tree cover in the Brazilian rainforest, the declining amount of snow that’s on the Earth, and how the ice in the Arctic Ocean is gradually disappearing. Satellites allow us to learn about these important changes and see our world in incredible ways, but it’s up to us to make sure we put that knowledge to good use.
14 How Do Telescopes Work?
Telescopes are big magnifying glasses that make faraway things look closer. They come in different sizes, from small instruments you can hold in your hand to giant structures housed in huge domes on mountaintops.
The first telescope was invented by a Dutch lens maker named Hans Lippershey. His day job was making reading glasses. But one day, so the story goes, he was checking the clarity of two lenses by holding them up to a window. For whatever reason, he held one lens in front of the other and looked through both. To his surprise, he saw a magnified image of a church steeple across town. Oddly, the lenses also turned the steeple upside down, but Hans realized that the two lenses working together could bring distant objects closer.
So the telescope was born. Early models were long tubes with a lens at either end like the ones pirates use in the movies. They were used mostly on ships to spot the flags of other ships in the distance to tell if they were friend or foe, or to see lighthouses along the shore.
Later, the Italian astronomer Galileo pointed a telescope at the moon and was amazed to see mountains and valleys on its surface. Then he used it to look at Jupiter and saw four little moons orbiting around it. He even saw the rings of Saturn. Telescopes, then, were pivotal to the creation of modern astronomy.
We’ve come a long way since those early models. Today, there are two types. Some use the old design, with glass lenses mounted in a long tube. One drawback of those types of telescopes is that they can’t be too big because glass is heavy and actually sags under its own weight. When that happens, the lens loses its shape, so the light passing through it is distorted and the image becomes blurry. The largest telescope of this type has a lens at the front that is one hundred centimeters, or one meter, across.
In astronomy, bigger is better. The bigger the eye on the sky, the farther into space we can see. So another way to capture more light is to use a curved mirror. (Some mirrors that you can hold in your hand have two sides, one flat and one curved—the curved side makes your face look bigger.) A mirror telescope—also known as a reflecting telescope—can be made much larger than one using a lens because the light bounces off the shiny surface instead of going through it. That means that the back side of the mirror can be supported by strong beams and girders that hold the mirror’s shape and make the telescope look more like a bridge than a scientific instrument.
ON THE DRAWING BOARD
A number of new, super-giant telescopes are under construction around the world. As each one is completed, it will be the largest in the world, only to be overtaken by the one after it, which will be even larger. How large do you think they can get?
GIANT MAGELLAN TELESCOPE
This is actually seven telescopes all mounted on one frame, resembling a giant flower. Each mirror is eight meters across, providing a total span of 24.5 meters. It will operate from a mountaintop in Chile’s Atacama Desert, one of the driest places on Earth, providing many clear nights throughout the year.
EXTREMELY LARGE TELESCOPE
This telescope will have a mirror thirty-nine meters across, made up of almost eight hundred segments, each 1.4 meters wide and only fifty-five millimeters thick. The giant instrument will rest in a dome the size of a football stadium. It will also be located in Chile.
Reflecting telescopes are the largest in the world. Some have mirrors that weigh many tons and span more than ten meters. A larger mirror captures more light, which allows them to see farther out into the universe. But even reflecting telescopes run into a problem when they get really big. Glass is heavy, so as the mirror grows, so does the structure that supports it.
The Hale Telescope in California, which was the largest in the world for many years, has a five-meter mirror that weighs thirteen tons. All of that weight requires a 481-ton structure of giant beams and tubes to support it. Not only that, but all of that weight must be perfectly balanced so the telescope can swing around with the precision of a fine watch to point at any part of the sky.
Today, telescopes are more than twice as big as the Hale. Scientists and engineers get around the weight problem by making the mirrors out of many smaller, thinner, and lighter sections that fit together like a jigsaw puzzle. In the future, these giants on the ground will grow to enormous sizes, with mirrors spanning more than thirty meters across.
Of course, we don’t just want to get a quick glimpse of our universe—we want to be able to preserve it in pictures. When big telescopes were first built, astronomers took photographs of the stars and planets using cameras that were operated by hand. That meant spending long, cold nights on top of mountains, where telescopes had the best views, and sometimes, on the really big instruments, climbing up to the top of the structure to special cages where the cameras were located, and riding in the telescope itself to take the pictures.
Today, most telescopes are fitted with digital cameras and other instruments that can be operated by computer from a warm, comfortable office anywhere in the world. Usually, the only people up on the mountain are technicians who make sure the instruments are working properly. In many cases, astronomers don’t even visit the telescopes they are using. But thanks to the amazing views of the universe our telescopes provide, astronomers can still visit the stars!
SPACE PLACES
Here are some places you can visit to see big telescopes where astronomers study the universe:
DOMINION ASTROPHYSICAL OBSERVATORY
Located in Victoria, British Columbia, this was the world’s largest operating telescope just over one hundred years ago. You can still visit it today for star parties on Saturday nights, where you watch the big telescope in action and look through the smaller ones owned by members of the Royal Astronomical Society of Canada.
DOMINION RADIO ASTROPHYSICAL OBSERVATORY
Stars give off light, which we see with our eyes, but they also give off invisible radio waves, especially when they explode or when galaxies collide with each other. Also located in British Columbia, this observatory features a giant radio dish that detects signals from exploding stars and some of those other, more violent events in the universe.
DAVID DUNLAP OBSERVATORY
This big telescope, just north of Toronto, Ontario, is open to the public on weekends. It is the largest telescope in Canada operated entirely by volunteers from the Royal Astronomical Society, and it’s only an hour from Canada’s largest city.
JASPER DARK SKY FESTIVAL
Every year, hundreds of sky watchers gather in Jasper, Alberta, for the largest star party in Canad
a. It features outdoor festivities, special presentations, storytelling, and many, many telescopes to look through under the wonderfully dark skies of Jasper National Park.
MOUNT WILSON OBSERVATORY
This is the very telescope, located in California, that Edwin Hubble used to discover the expanding universe.
PALOMAR OBSERVATORY
For many years, this was the world’s largest telescope, with a mirror 5.1 meters (two hundred inches) across. It’s still the largest telescope in North America. The gigantic structure weighs 480 tons, and the whole thing rests under a huge dome that’s forty-one meters (135 feet) tall.
MAUNA KEA OBSERVATORIES
On top of this extinct volcano, whose peak is a dizzying altitude of 4,200 meters above sea level in Hawaii, is the largest gathering of telescopes in the world. More than a dozen instruments, operated by many different countries, take advantage of the high-mountain perch above the clouds to get the clearest skies anywhere on the planet.
LOWELL OBSERVATORY
This telescope in Arizona was built by astronomer Percival Lowell, who believed that there were canals on Mars, possibly built by a Martian civilization. Neither canals nor Martians were ever found on Mars, but Lowell’s idea spawned the notion of invasions from Mars that have become so popular in science fiction movies. This historic observatory, with its wooden dome, is named after him, and it’s also the place where Pluto was discovered.
KITT PEAK NATIONAL OBSERVATORY
On top of this Arizona mountain are telescopes that look at the stars as well as the world’s largest and most unusual-looking solar telescope, used to study the sun. It is one of the few places where astronomy happens during the day! Here, astronomers get a close-up view of the violent face of our home star.