I always thought the cannon explanation was helpful. If you shoot a cannon horizontally, the ball will arc in a parabola and come down to Earth. The ball experiences freefall during that period, but it’s not for very long (that’s suborbital flight).
If you increase the power, the cannon shoots so fast that although Earth is still pulling on it, it manages to miss hitting the ground. Again, the ball experiences freefall but for an extended period, since it can loop around repeatedly. That’s orbital flight. You can stay in orbit pretty much forever since the loops repeat.
Finally, if you increase the power further, the ball can escape Earth’s gravity. It’s going so fast that it doesn’t just swing around the Earth, it flies away. And once it’s some distance away, the pull from Earth gets significantly weaker, and the ball can just keep flying forever. It’s again in freefall, but this is the only case where you can say that Earth is not pulling on it.
And as for why any all of these things are freefall: well, it’s because everything in flight is being accelerated the same way. If you’re in a spaceship, the spaceship is being accelerated downward, but so is your body. Earth’s gravity accelerates everything at the same rate, so anything falling together will stay together. You feel it in your gut because even your organs are just flying in formation with the rest of your body, as compared to being squished toward the ground as is normally the case.
I think the main reason nobody’s mentioned Lagrange points is just that they aren’t all that interesting. I’m not sure why they get as much attention as they do. I mean, sure, you could put a space habitat at a Lagrange point, but you could also put a space habitat in any other orbit, too, with the same effect.
Some satellites are at L2, including the upcoming James Webb Space Telescope (fingers crossed). If I understand correctly, this allows them to be a considerable distance from Earth, while still maintaining a constant distance from Earth, with only a little adjustment needed now and then.
Huh, I certainly knew of the JWST, and I even knew that it was to orbit a LaGrange point. But somehow I assumed it was at L4 or L5, because those are the stable points. But it’s actually at L2! I’d have thought the instability would make that worthless. But according to Wikipedia it only needs 2-4 m/s of delta V for stationkeeping per year. That’s really not much.
Quite the opposite: The stability of L4 and L5 makes them worthless for satellites. Junk tends to accumulate at those points, making them unnecessarily hazardous for satellites.
I had previously assumed that L2 was chosen so as to keep the Sun permanently eclipsed behind the Earth. On doing the calculations, however, it turns out that it’s not quite close enough to make that possible. Given that, I’m not entirely sure why that spot was chosen. Maybe even a partial eclipsing is useful?
There are also some satellites at the point between Earth and the Sun, mostly for climate observation, because that way, they can constantly see the entire sunlit hemisphere.
But even if there are some specific applications for those points, for the vast majority of missions, they’re completely irrelevant. There’s no justification for the science fiction trope of “We want to put something in space. Let’s put it at a Lagrange point, because that’s in space!”.
I guess. My intuition is that it’s probably like the asteroid belt. Movies make asteroid fields look hazardous but in practice the density is incredibly low and you’d have to go out of your way to hit something. But I don’t have great intuition about L4/5.
As best I can tell, it’s so the Earth and Sun stay in the same location of the sky, so the heatshield can point in that direction, with the other side getting consistent exposure to the cold of space.
Technically, there IS no distance you can go where you will escape Earth’s gravity. If Earth were alone in space, you could put your ship a hundred million miles away and it would eventually fall back to Earth.
Free falling from a height in vacuum doesn’t ‘simulate’ weightlessness, it IS weightlessness, just as much as floating around on the ISS. The ISS is constantly ‘falling’ towards Earth. It’s just that it is going so fast horizontally that it keeps missing it. (-:
There’s also no such thing as ‘standing atill’ in space. Velocity, unlike acceleration, is always measured in reference to something. If you have zero velocity relative to Earth, then you are still going 30 km/s around the Sun. If you have zero velocity with respect to the sun, you are still going 600 km/s around the Milky Way. And so on, ad infinitem.
I’m not trying to split hairs on wording. I am pointing out that if a journalist says that Bezos experienced zero gravity, the average person thinks that he went so far away that he was beyond any practical effects of Earth’s gravitational field.
Do they? I think most people understand that you can even experience zero gravity in a jet doing parabolic arcs. Everyone had seen video of trainees floating around inside the ‘vomit comet’. You can even buy tickets for it.
The people on the ISS are not weightless because they are far away from Earth. The ISS is only about 480 kilometers up. If you stood on top of a 480 km tower mounted to the Earth, the gravity you would experience would only be about 10% less than at sea level.
It’s the falling that does it, whether on the ISS, a jet in a parabolic arc, or sitting in the tip of a giant penis falling from space.
That’s true, but the zero g part isn’t what’s distinguishing suborbital from orbital flight. The OP could just easily have asked “If the Space Shuttle is already flying up in zero gravity, why couldn’t it fly to the moon?” (plenty of movies make the same mistake). Educating them a bit on orbital mechanics feels like the right response to me. The basics aren’t all that difficult.
That wouldn’t make any difference. Generally, things at L4/5 points do not stay at exactly those points. Instead, they oscillate around those points. If you plotted their position relative to the secondary body over time, you would get long kidneybean-shaped loci with the actual Lagrange point in the middle. Which means that bodies at L4/5 don’t even go through the actual Lagrange points. That would apply to any satellite we put there, so any junk collected there is not going to be a problem. The junk is going to be doing kidney bean-shaped movement too, but each piece at a different distance from the Lagrange point.
According to Wikipedia, there are currently four craft at the Earth-Sun L1 point, which is the one between the Earth and the Sun. Three observe the Sun and/or solar wind. The other one observes the Earth but also keeps a lookout for Coronal Mass Ejections. (BTW, they also oscillate around the actual Lagrange point.)
You’re not going to run into a big ol’ asteroid at a stable Lagrange point. Space is indeed too sparse for that, even there. But you probably will run into dust.
On 2 October 1991 Hiten was temporarily captured by the Moon and then put into a looping orbit which passed through the L4 and L5 stable libration points to look for trapped dust particles. No obvious increase was found.
ETA: Ahh, sorry. That’s the Earth-Moon L4/5, not Earth-Sun.
Sounds like the place to go if you want to study interplanetary dust. But really, there may be a concentration at the Lagrange point itself, but the area of stable orbits is much larger than that. So avoiding any concentration should not be difficult. Indeed, I’d expect the opposite to be true.
