Tag: Orbital Mechanics

Why don’t these “thousands” of satellites fall back to Earth, if the likes of the ISS has to use booster rockets to stay up there?

It all depends on how thick the atmosphere is where the satellite is orbiting.

The atmosphere doesn’t just “end” at a certain height. It just gets thinner and thinner.

In low Earth orbit, where the ISS is orbiting, there is still some atmosphere. It’s very, very thin, but it’s there. At the top of Mount Everest, just 5 and a half miles above sea level, you need oxygen to survive a long duration. At the International Space Station, 250 miles above sea level, you need a space suit.

Atmosphere, however, is still there – but very thin. Over time the ISS needs a boost to stay in orbit because it slows down due to atmospheric drag.

Most objects orbiting the Earth will succumb to falling eventually, but the higher they are, the longer they’ll stay. Some, way out in geostationary orbit 26,199 miles high, atmosphere is virtually non-existent, so those satellites will probably stay up there a few million or billion years.

How come an orange inside the ISS just floats in the air? We know gravity keeps the Moon in orbit. Why is there no gravity on the ISS?

This is an image of Newton’s Canon. Given a high enough elevation, shooting a canon-ball at increasing velocity will cause it to travel further and further until it just goes into orbit, falling around the earth.

That is what’s happening inside the ISS. It’s called free-fall – the same as Newton’s canon-ball.

The ISS is falling toward the earth. The only reason it doesn’t crash to the ground as it falls is because it’s also moving horizontally so fast that as it falls, the earth curves away and they just keep missing, again and again, around and around. That’s called an orbit.

This is why we don’t say the astronauts on the ISS are weightless. We say they are in microgravity.

As an example, let’s say you get into an elevator at the top of the tallest building in the world, and the elevator drops in free fall. Fun! You would fall at the same speed as the elevator and to an observer you’d appear to be weightless. In fact, if you held an orange, it would appear to float out of your hand.

That would end tragically, however, when the elevator hit the ground. Not fun anymore!

That tragic end doesn’t happen to the ISS because the ground just curves away under them and they keep missing Earth! Inside, however, the oranges float around and the astronauts float because the ISS and everything inside it is falling at exactly the same rate.

Therefore there is gravity up there – almost the same gravity that we experience on the ground. If the spaceship could stop moving forward and just hover somehow (it can’t) then everybody would be able to walk around just like we do here on Earth. It’s only because the ISS is in free-fall around the earth that the people and things inside appear to float.

Elon Musk’s Automobile to be put in Orbit around Mars


This rather large SpaceX rocket is called the Falcon Heavy. Inside is the automobile below, which SpaceX hopes to launch in orbit around Mars with a flight schedule February 6, 2018 (assuming it doesn’t blow up on the launchpad).

You just can’t make this stuff up. The automobile, which belongs to the founder of SpaceX Elon Musk, is a Tesla Roadster. If successful, the automobile could stay in orbit around Mars for a few billion years. The car will just be let go to float on it’s own ’round and ’round Mars.

I guess that’s in case you ever need to catch a ride to orbit Mars!

Why don’t we see the hidden face of the moon? Is this because the moon rotation is perfectly synchronized with Earth?

You are correct. The moon is synchronized with Earth, although not perfectly. There is a wobble which I’ll show you below.

The “face on the moon” means that some people see what looks like a face on the side of the moon we can see. It’s not on the far side, but the side we see.

Here is a picture of the moon with the “face” outlined that people call the “face on the moon.” It is not meant to be comical (though it looks funny), but outlines the moon features people imagine look like a face.

More below the picture.

Now, here’s a comparison of the near and far side of the moon taken by a satellite we placed in orbit around the moon. On the left is the familiar side of the moon we see facing Earth. On the right is the side we never see from Earth because it faces away from us. This shows the moon is tidally locked with Earth so that one side always faces us. More below.

And finally, here’s an animation that shows that the moon isn’t perfectly synchronized with Earth, as you put it (scientifically it’s called “tidally locked”), but wobbles quite a bit from full moon to full moon. Due to this wobble, sometimes you can get a peak of what’s on the far side. You’ll also notice the moon gets larger and smaller in the animation. This variation happens because the moons orbit is slightly elliptical around us, so sometimes it’s a little closer and looks bigger, and sometimes a little further and looks smaller. A full moon when the moon is closest is called a supermoon.

moon-wobble

If an intelligent species preceded us on Earth and wanted to leave a record of themselves in a time capsule, then one of the Lagrange points would be a good place for it. Is anyone looking for one there?

Great question! You asked specifically if one of the Lagrange points could be a good place to hide a time capsule from the past. The answer is “no” because, you see, an orbit in a Lagrange point is inherently unstable and would eventually either fall to Earth or fall to the Sun.

According to space.com, “L1, L2 and L3 are all unstable points with precarious equilibrium. If a spacecraft at L3 drifted toward or away from Earth, it would fall irreversibly toward the sun or Earth, “like a barely balanced cart atop a steep hill,” according to astronomer Neil DeGrasse Tyson. Spacecraft must make slight adjustments to maintain their orbits.”

Lagrange_points