Space and astronomy

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• Watch Try it What If? Lab What if Earth had no Moon? The Moon's pull stretches Earth's oceans and gives us most of our tides. Take the Moon away in our tide model and see what is left: the Sun still makes tides, but they are less than half as big as the Moon's.
• Watch Try it What If? Lab What if Earth had two moons? Our Moon goes round Earth in a neat, repeating loop. Add a second moon in our orbit model and watch what changes: the two moons tug on each other as well as on Earth, so their paths start to wobble.
• Watch Try it What If? Lab What if the Sun disappeared? The Sun cannot vanish, and it will keep shining for billions of years. But imagining it gone answers a real question: what keeps Earth going round? Switch off the Sun's pull in our model and Earth stops curving and travels on in a straight line.
• Watch Try it What If? Lab What if Earth rotated faster? One spin of Earth gives us one day and one night. Spin Earth twice as fast in our model and the days and nights each shrink to about six hours, with less time for the ground to warm up or cool down.
• Watch Try it What If? Lab What if Earth rotated slower? Slow Earth's spin to one turn a week in our model and a single afternoon lasts days. The ground has time to bake in the long sunshine and to chill in the long dark, so the gap between the hottest and coldest times grows far bigger.
• Watch Try it What If? Lab What if Earth stopped rotating? You might think a world that does not spin would have no day and night. But Earth also travels around the Sun. Stop the spin in our model and the Sun still crosses the sky, just once a year: six months of daylight, then six months of night.
• Watch Try it What If? Lab What if Earth’s tilt changed? Earth's axis leans 23.4 degrees, and that lean is what gives us seasons. Straighten Earth up in our sunlight model and the Sun climbs to the same height every noon, every day lasts 12 hours, and summer and winter disappear.
• Watch Try it What If? Lab What if Earth had weaker gravity? When you throw a ball, gravity curves its path back down to the ground. Halve gravity in our model and the same throw climbs twice as high, stays up twice as long and lands twice as far away.
• Watch Try it What If? Lab What if you lived on Mars? You would be made of exactly the same amount of stuff on Mars, but Mars pulls on you less than Earth does. In our model, a bathroom scale there would show only a little over a third of your Earth reading, and your jumps would go more than twice as high.
• Watch Try it What If? Lab What if Earth were farther from the Sun? Sunlight spreads out as it travels, so a planet farther from the Sun gets less of it. Move Earth out to where Mars orbits in our model, keep everything else the same, and its average temperature drops from about 15 °C to about −40 °C.
• Watch Try it What If? Lab What if a spacecraft traveled faster? A rocket's top speed depends on how fast its hot exhaust gas shoots out and on how much of the rocket is fuel. In our model, switching from a kerosene engine to a faster hydrogen engine lets the same rocket gain nearly half as much speed again.
• Watch Try it What If? Lab What if the Moon were closer? The Moon's tides depend very strongly on distance. Bring the Moon to half its distance in our model and its stretching of the oceans becomes 2 × 2 × 2 = 8 times stronger, while each trip around Earth takes only about ten days.
• Watch What If? Lab What if a comet passed close to Earth? Comets are icy leftovers from when our solar system formed. Near the Sun, they warm up and grow a glowing cloud and long tails. Dust they leave behind gives us meteor showers. Astronomers have predicted comets' returns for centuries: Halley's Comet is due back in 2061.
• Watch What If? Lab What if a day lasted 48 hours? Earth spins once about every 24 hours, and almost every living thing has a body clock tuned to that day. Imagine a day twice as long: long, bright days and long, dark nights. What would happen to your sleep, to plants and to animals?
• Watch Try it What If? Lab What if a pendulum clock went to the Moon? A pendulum clock keeps time by counting swings, and gravity sets how fast each swing goes. In our model, a 1-meter pendulum takes about 2 seconds per swing on Earth but almost 5 seconds on the Moon, so a clock set up on Earth would show only about 24 minutes for every real hour.
• Watch Try it What If? Lab What if a plane flew on Mars? A wing makes lift by pushing air down, so the thickness of the air matters a lot. In our model, a small plane at 60 meters per second makes about one and a half times its weight in lift on Earth. In Mars's thin air, the same plane makes only about 2 percent of what it needs.
• Watch Try it What If? Lab What if a rocket carried less fuel? Most of a rocket is fuel. In our model, a rocket that is 85 percent fuel gains about 5.7 kilometers per second. Cut the fuel to half the rocket and it gains only about 2.1, less than half as much speed. Find out why rockets need so much fuel, and why they drop parts on the way up.
• Watch What If? Lab What if a solar eclipse lasted all day? During a total solar eclipse, the Moon's shadow races across Earth at more than 2,400 kilometers per hour, so the Sun stays fully covered for only a few minutes in any one place. In April 2024, the longest was 4 minutes and 28 seconds. Find out why eclipses are so short, how animals react to the sudden dusk, and imagine one that never ended.
• Watch What If? Lab What if an asteroid struck the Moon? The Moon's craters are a record of space rocks landing over billions of years, and small ones still land there every day. Scientists watch for tiny flashes of light on the Moon and later find the brand-new craters. Explore what happens when a rock hits the Moon, and why the Moon keeps going around Earth as always.
• Watch What If? Lab What if auroras could be seen everywhere? Auroras glow when tiny particles from the Sun crash into the air high above Earth. Earth's magnetic field steers them toward the poles, so that is where auroras shine most. Imagine lights over the whole planet, find out what makes the colors, and see how people keep satellites safe in big solar storms.
• Watch What If? Lab What if Earth had no magnetic field? Deep inside Earth, swirling melted iron makes a magnetic field all around our planet. It turns compass needles, helps sea turtles find their way and shields our air from the solar wind. Imagine it gone, compare Earth with Mars, and find out why scientists are calm about Earth's changing field.
• Watch What If? Lab What if Earth had rings? Saturn's rings are made of billions of chunks of ice and rock, all circling the planet. Earth has no rings today, but scientists have found clues that it may once have had one. Explore what rings are, where they come from, and what a ring might do to our sky.
• Watch What If? Lab What if Earth had two suns? More than half of the stars in the sky have a partner star. Astronomers have even found planets that go around two stars at once. Imagine Earth with two suns: you might see two shadows and two sunsets. Explore what scientists have really found.
• Watch Try it What If? Lab What if Earth moved much faster around the Sun? Earth races around the Sun at about 30 kilometers every second, just the right speed for a nearly round path. Speed it up and its path stretches into a long oval. Speed it up to about 1.41 times as fast, around 42 kilometers per second, and the Sun's pull can no longer hold it: in our model, Earth flies off and never comes back.
• Watch Try it What If? Lab What if Earth slowed down in its orbit? A slower Earth does not just go round the same circle more slowly. In our model, starting Earth at 70 percent of its speed makes it fall inward on an oval that swoops closer to the Sun than Mercury's average distance, racing at over 64 kilometers per second at its closest, before swinging back out. Its year shrinks to about 197 days.
• Watch Try it What If? Lab What if Earth spun on its side like Uranus? Earth's spin axis leans 23.44 degrees, which gives us gentle seasons. Tip it all the way over to 90 degrees, like Uranus, and each pole points at the Sun in turn. In our model, a place 60 degrees north would get about four months of non-stop daylight, then about four months with no sunrise at all.
• Watch What If? Lab What if Earth were much bigger? Astronomers have found many planets bigger than Earth, called super-Earths. A simple calculation shows that an Earth twice as wide, made of the same rock, would pull twice as hard. Such a world might hold on to more air and might not grow such tall mountains.
• Watch What If? Lab What if Earth were the only planet? Imagine a solar system with just the Sun, Earth and our Moon. The bright 'wandering stars' would vanish from our night sky, the paths of asteroids and comets would change, and scientists would lose the other worlds that teach us how planets work. Explore what Earth would miss.
• Watch Try it What If? Lab What if Earth’s orbit were twice as wide? Move Earth's orbit out to twice its distance from the Sun and two things change at once: Earth has twice as far to travel, and the weaker pull lets it move only about 71 percent as fast. In our model, a year stretches from 365 days to about 1,033 days, almost three of our years.
• Watch What If? Lab What if Mars had oceans again? Mars is a cold, dusty desert today, but rovers have found dry riverbeds, deltas and an ancient lake. Its air is now so thin that liquid water can't last on the surface. Imagine a blue Mars, find out what it would need, and see a possible clue to past life that NASA's Perseverance rover has found.
• Watch What If? Lab What if meteor showers happened every night? A few shooting stars streak across any clear night sky, but during a meteor shower you might see dozens an hour. Showers happen when Earth passes through dust left by a comet, which is why they come back on the same dates every year. Imagine a shower every night, and find out what that would take.
• Watch What If? Lab What if our galaxy met another galaxy? The Andromeda galaxy is heading toward our Milky Way. Astronomers once expected a head-on meeting in about 4 billion years. A 2025 study called it a 50-50 chance in the next 10 billion years; a 2026 study says about 90 percent. Nobody is certain yet. If they meet, stars are so far apart that they would almost never bump, but the galaxies could blend into one new shape.
• Watch What If? Lab What if our Sun were a different color? A star's color tells you how hot it is: red stars are the coolest and blue stars the hottest. Our Sun is actually white, though it looks yellow through our air. NASA scientists have even worked out that plants on planets around other stars might not be green.
• Watch What If? Lab What if planets were not round? Big worlds are round because gravity pulls everything toward the center from all sides. Small asteroids don't have enough gravity to do that, so most stay lumpy. Imagine a cube-shaped planet, meet the lumpy asteroids, and compare worlds to see how big something must be before gravity rounds it off.
• Watch What If? Lab What if Pluto were still called a planet? Pluto was called our ninth planet for 76 years. Then astronomers found many similar icy worlds beyond Neptune, and in 2006 the International Astronomical Union wrote a definition of 'planet' that made Pluto a dwarf planet. What if Pluto had kept its old title? Explore how a planet is defined, the other dwarf planets, and why science sometimes changes its words.
• Watch What If? Lab What if Saturn’s rings vanished? Saturn's rings are billions of icy chunks, and some of that ice is slowly raining down onto the planet. NASA scientists estimate the rings could be gone in less than 300 million years. Explore what the rings are, how they fade, how they already seem to vanish from Earth's view, and what Saturn would be like without them.
• Watch What If? Lab What if telescopes found planets around every star? Astronomers have confirmed thousands of planets around other stars, called exoplanets: 6,451 in NASA's Exoplanet Archive in October 2026. Most were found by watching a star dim slightly as a planet passed in front. Imagine planets around every star, meet some worlds a bit like Earth, and see how a tiny dip in starlight reveals a planet.
• Watch What If? Lab What if the Moon had air? The Moon has almost no air, so its sky is always black and it has no weather. Scientists have found that the Moon once had an atmosphere, billions of years ago, but it was lost to space or became part of the ground. Explore what air would change, and why small worlds lose theirs.
• Watch What If? Lab What if the Moon had no craters? The Moon is covered with craters made by space rocks crashing into it. With no wind, rain or flowing water, the Moon keeps its craters for billions of years, while Earth wipes most of its craters away. Imagine a smooth Moon, and discover how scientists count craters to tell how old a surface is.
• Watch What If? Lab What if the Moon turned red? During a total lunar eclipse, Earth moves exactly between the Sun and the Moon, and Earth's shadow covers the full Moon. The Moon does not go black. NASA explains that sunlight passing through Earth's air loses its blue light and bends toward the Moon, so the Moon glows red or orange, lit by all of Earth's sunrises and sunsets at once.
• Watch What If? Lab What if the stars were much closer? Every star is a sun, but even the nearest one is more than four light-years away, so stars look like faint points. Bring them closer and they would shine much more brightly. In real crowded star clusters, the sky would be full of bright stars, but stars that close may make it hard for planets to form.
• Watch What If? Lab What if the Sun had no sunspots? Sunspots are darker, cooler patches on the Sun made by strong magnetic fields, and their numbers rise and fall about every 11 years. Imagine a Sun with none: what would happen to its light, to solar storms and to the auroras they bring? Once, for about 70 years, the real Sun came close.
• Watch Try it What If? Lab What if the Sun were heavier? The Sun's gravity bends Earth's path into a yearly loop. Make the Sun twice as heavy in our model and its pull gets stronger, so Earth must race around faster to stay on the same circle. The year shrinks from about 365 days to about 258.
• Watch What If? Lab What if the universe stopped expanding? Since the 1920s, astronomers have known that distant galaxies are moving apart as space stretches. In 1998 they found the stretching is speeding up, driven by something nobody understands, called dark energy. What if the expansion stopped? Our galaxy wouldn't notice, but the universe's long future would change, and scientists are still working out where it is heading.
• Watch What If? Lab What if there were a planet between Mars and Jupiter? Between Mars and Jupiter lies the asteroid belt: millions of rocky leftovers that never built a planet. NASA explains that young Jupiter's gravity stopped them. Explore the belt, Jupiter's tug, the dwarf planet Ceres, and what kind of planet could have formed there.
• Watch What If? Lab What if there were no satellites? Thousands of satellites quietly circle Earth every day. They watch the weather, help phones find where you are with GPS and carry TV signals and phone calls. Imagine a world without them, and discover how people found their way and forecast the weather before the first satellite launched in 1957.
• Watch What If? Lab What if Venus had Earth's air? Venus is about the same size as Earth, but its surface is hot enough to melt lead. Its thick carbon dioxide air traps heat in a runaway greenhouse effect, so Venus is even hotter than Mercury. Imagine swapping in Earth's air, and explore what a NASA climate model says about a cooler, cloudier Venus long ago.
• Watch What If? Lab What if we could see the far side of the Moon? The Moon spins once for every trip around Earth, so the same face always points at us. The far side isn't dark: it gets sunlight too. It has been seen only by spacecraft and astronauts, and it looks very different, crowded with craters. Find out why one face points at us, what the far side holds and how a rover explored it.