why is there gravity on earth but not in space
This question was asked by Joseph from Plymouth, UK. P
Although gravity has always existed on Earth, it wasn t until the 17th century that we began to understand exactly what it is. A famous physicist called Isaac Newton realised that there must be a reason why objects always fall downwards. The legend tells that he realised this after an apple fell onto his head while he was sitting under a tree; although this is probably untrue it s a funny story. Isaac Newton worked out that this reason is a force called gravity. Any object that has mass (it takes up space and has density) has gravity, which means that it pulls other objects towards itself. This gravitational pull becomes stronger the closer to the object you are, and the bigger the object is. So although people have gravity it s so slight that we don t notice it. The stars, planets and moons have a much more noticeable effect on each other. Stars have a lot of gravity, so they pull planets towards them; at the same time the planets own gravity is pulling the star towards them, just not as strongly. This is why planets orbit stars, and moons orbit planets. How much an object weighs depends how much gravity is acting upon it, so you would weigh different amounts on different planets and moons. If you weighed 100kg on Earth you would only weigh 38kg on Mars and 16. 5kg on the Moon. You d weigh 236kg on Jupiter! This is also why astronauts bounce on the Moon instead of walking or floating; there is gravity but it s a lot less than we have on Earth. Because gravity causes every object to pull other objects towards it, there s gravity in space as well.
But because there s a lot of distance between the objects in space, unless you re close to one of them the gravity is very weak. Scientists call this microgravity. You might think that this is why orbiting astronauts float, but it isn t. Astronauts aboard the International Space Station (ISS) are only 400km above the Earth s surface; at that height gravity is still about 90% as strong as it is on the surface. Astronauts and objects on the ISS float because they re actually falling towards the Earth s surface, pulled downwards by gravity. They orbit instead of plummeting towards the surface because of the speed the ISS is going. Isaac Newton worked this out too. He explained that when you fire a cannon, the ball travels horizontally for a short way before falling to the ground; if you fire it at a faster speed the ball travels further before falling. Newton reasoned that because the Earth is spherical, there must be a speed at which the ball would keep travelling horizontally and falling towards the surface but never actually hit it. He was right the ISS is constantly falling towards the Earth but because it s also travelling around it at 27,600 kph, it never hits. Astronauts and space tourists may rhapsodize about feeling weightless during spaceflight, but don't be fooled by the somewhat misleading term zero-gravity. Every object in space still feels the gravitational pull from other objects, including space travelers who imagine themselves free of Earth's gravitational shackles. Earth's gravity affects everything at or near the planet's surface. We feel the force of gravity on Earth through our mass, and that force also translates into a downward pull of 9. 8 meters per second squared (32 ft/s^2).
That's why astronauts need powerful machines such as the space shuttle's main engines and twin boosters or the Russian Soyuz rockets to travel beyond Earth's immediate gravitational tug. How to stay Gravity represents the mutual attraction between two objects, and the strength of that pull depends on both mass and the distance between the objects. Greater mass leads to a greater gravitational pull, as anyone who has fought to lose a few pounds knows firsthand. By contrast, greater distance leads to rapidly diminishing gravitational pull. But w here the space station roams, some 220 miles (354 km) up, the force of gravity is still about 90 percent what it is here on the surface. Earth's gravity is still pulling down on astronauts in orbit. A spacecraft or space station can counter Earth's downward pull by creating enough horizontal speed so that it continually slides sideways as it simultaneously falls toward the planet, creating an orbit. For instance, the space shuttle typically travels at a blistering 17,000 to 18,000 mph around the Earth to stay aloft. That continuous free fall around the planet gives astronauts the impression of being weightless. Huge objects with enormous mass can make their gravitational effects felt across much greater distances. The moon maintains a free-falling orbit around the Earth, and the Earth itself remains in orbit around the massive sun. Our sun contains over 99 percent of all mass in the solar system, which explains why its gravitational pull has managed to snag eight planets and a host of other objects.
Jupiter, the largest planet in our solar system, has also flexed its gravitational muscle across vast distances of space by pulling in space rocks and other debris which might otherwise threaten Earth. That has allowed Earth observers to also witness several spectacular impacts on the gas giant, such one that recently the size of the Pacific Ocean. Even asteroids and other smaller space rocks exert weak gravitational pull. And on the flipside, some scientists have proposed using the mere mass of spacecraft to act as that gently tug threatening space rocks out of Earth's path. Albert Einstein proposed another way to think about gravity in space. Consider if the 3-D universe was a flat, 2-D sheet. Each object in space acts like a ball that weighs down on the space-time fabric and creates a bulging pocket similar to a shallow depression in the ground. That curvature of space-time has an inward falling effect on the paths of other objects, and particularly on smaller passing objects. It's like having a sheet stretched between two people, and watching a marble roll down into the bulge created by a large ball sitting on the sheet. More massive objects such as black holes create bigger bulging pockets of space-time, while tiny objects such as a spacefaring human would barely represent a dent. So gravity may be everywhere in space, but that won't stop astronauts or others from describing the wonderful feeling of weightlessness. Sometimes the illusion of human experience speaks volumes more than strict scientific fact.
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