How come asteroids don’t falls straight down, due to Earth’s gravity? They should be pulled through space straight towards Earth for millions of miles, right? But I see videos of them streaking across the sky horizontally.
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When you fire a bullet, it moves very fast sideways but does slowly curve towards Earth. If your bullet was fast enough, it wouldn’t curve fast enough to hit Earth, and would sail off into space. That speed is between 7,000 and 12,000 m/s.
Farther from Earth, its gravity gets weaker, and this speed gets lower and lower. At some point, our gravity is hardly there at all.
Combine that with the fact that these things are flying around at 50,000 m/s or even more and it makes complete sense that they don’t fall to Earth.
They are pulled towards Earth, but that doesn’t mean they’re *moving* straight down. The pull is acceleration (*change* in velocity), the second is velocity itself. For a more elementary example: imagine you’re running forward, attached to a long elastic band. You can still be moving forward (forward velocity) even once the band begins to slow you down (backward acceleration). Or imagine rolling a ball across a sloped surface: gravity always pulls downward, but the ball follows a curved trajectory on the surface that depends on how it was rolled.
An meteor ( comes in moving very, *very* fast. So their velocities aren’t usually straight down. They actually will usually be somewhat close to straight-down – they’re not moving as horizontally as it looks like they are – but there’s no perspective for you to tell the up-and-down part of their motion from so far away. So you just see the side-to-side part, which is usually not zero. To see this in action with a *lot* of meteors, you can look at [a meteor shower](https://en.wikipedia.org/wiki/Radiant_(meteor_shower)#/media/File:Geminid%C3%A1k_meteorraj_maximuma_2007-ben.jpg), which will appear to “radiate” from a single point in the sky. What you’re seeing there is the direction the meteors are coming in from, and the random side-to-side motions around that direction carried by each individual meteor.
Same reason the moon doesn’t get pulled straight down. They are in orbit (around the sun, like most things in the solar system). “Orbit” just means that they are moving fast enough in a circle (actually an ellipse most of the time) that they won’t fall.
Edit: also the things you see in the sky streaking are not asteroids, they are meteors, and they generally burn up in the atmosphere long before they could reach the surface of the earth
The Earth is really small (comparatively). Meteoroid, comets etc. are moving really vast (relative to the Earth). Which means the chance of them actually hitting the Earth straight on is incredibly small. Mostly likely they will hit at an angle, or fly past.
While the Earth’s gravity will pull them in often it doesn’t have enough time to pull them straight down. Think about throwing a javelin horizontally; while the Earth’s gravity will pull it down it sill may end up sticking into the ground at an angle because gravity doesn’t have enough time to get it going fast enough downwards for its horizontal speed to be negligible.
Earth’s gravity accelerates an object in one direction (towards the ground), but it doesn’t decelerate that object in other directions. If an object is traveling horizontally across the Earth, it won’t stop until something other than gravity (usually the ground or air resistance) stops it.
Asteroids are traveling at speeds measured in miles per second before Earth’s gravity starts affecting them. Earth’s gravity will give them additional speed towards the Earth, but that’s not usually enough to change the trajectory so that it hits the ground. The asteroid is still traveling fast enough that it will pass the Earth.
An object in motion tends to stay in motion (same speed and direction) unless acted upon by an outside force. So unless your asteroid was headed directly at Earth from either the front or the back of the Earth’s path or unless it is stationary and the Earth runs into it, then it is headed into the influence of Earth’s gravity at an angle. The asteroid’s momentum wants to keep it on its original path and the Earth’s gravity wants to pull it toward the Earth. Specifically the Earth’s center of gravity. This bends the path of the asteroid toward the Earth more and more as it gets closer since gravity is distance dependent but the angle won’t be pulled to 90 degrees in all probability.
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