Ah, I gotcha AndrewL. The “towards the sun” speed would have to be much, much greater than earth’s orbital velocity. And that’s hard. Now I understand. Thanks!
But there’s still something bugging me. I fully understand that if there were no other forces acting besides the orbital velocity and the initial thrust of the rocket pushing itself off earth, this is very difficult. But there is another force operating here - well, several others. That being the forces of gravity of the earth and the sun (and maybe the other planets too).
Is there some way to use the gravity of the sun to slowly pull the rocket in, so it doesn’t have to expend all the fuel to either counter earth’s orbital velocity, or shoot into the sun so fast that earth’s orbital velocity doesn’t matter?
What I’m thinking of here is basically, get it up into high earth orbit, then deborit it in such a way that it makes a long, slow spiral (or other decaying orbit) into the sun. Is that possible, and how much fuel would it take above and beyond what it takes to get out of earth’s gravity well?
-Ben
The sun has a diameter of 835,000 miles, and is 93,000,000 miles away from the earth, which orbits it at a velocity of 66,500 mph. An object fired directly at the leading limb of the sun, without accelerating to cancel the earths orbital motion, would thus cross the entire diameter of the sun in 12.56 hours (835,000/66,500). Shooting your rocket towards the sun at any angle other than straight on would add a component of acceleration or deceleration from the earths orbital motion, and thus would not be allowed by your scenario.
If you don’t want your rocket to miss, the sideways motion resulting from the earths orbital requires that you’d have to get your rocket to hit the sun in less than 12.56 hours. Dividing 93,000,000 by 12.56 gives the velocity you’d need for such a “non-orbital” shot to work: 7,400,000 mph. That’s roughly 1.1% of the speed of light, and would require FAR more energy than simply canceling the earths orbital velocity of 66,500 mph.
-Obviously I left out a bunch of tiny-angled trigonometry here. Anyone who has a few hours to kill is welcome to add it in.
Why does the earth not fall into the sun? It’s because earth has momentum, and is orbiting around the sun at a speed of 30 km/s. The sun is pulling on the earth all the time, but the earth’s movement creates a centrifugal force so the two forces balance out, and the earth stays in orbit. If the earth were to slow down, centrifugal force diminishes and it will get pulled closer to the sun. However, there is nothing in space to slow down the earth - nothing big to collide with, and nothing to cause friction. So the earth stays in orbit for billions of years.
If you launch a spacecraft at a slow speed relative to the earth, it’ll still have most of earth’s orbital speed, close to 30 km/s. It will get pulled by the sun, but the spacecraft is moving fast enough that centrifugal force keeps it away from the sun. If you want the spacecraft to fall into the sun, you have to kill the orbital speed and let the gravity win. So you want to change the spacecraft’s speed from 30 km/s to zero.
Now, if you are on earth, slowing down is easy. You have plenty of stationary stuff with which to interact - you can hit a brick wall, or use friction with the ground. But in space, there are no walls to hit, or road surfaces to use for braking. The only realistic option is to use a rocket engine of some kind. But it takes a lot of fuel - slowing down from 30 km/s to zero takes as much fuel as accelerating from zero to 30 km/s, because the two are exactly the same - there really isn’t a difference between “acceleration” and “deceleration.” You are accelerating the spacecraft to 30 km/s relative to earth. Which, by the way, is faster than any space probe ever launched.
Now, there is one thing you could use - photon pressure. Photons have momentum, so it can push. If you don’t care how long it takes, you can attach a large reflective “light sail” on the spacecraft and angle it so that the reflected light is directed “forward” (in terms of your orbital motion). This results in a slight backward force, and over time it will slowly kill your orbital speed and drop you into the sun. And of course, the closer you get to the sun the more powerful the force.
To get some realistic ideas on how difficult it is to get to the sun, take a look at the Solar Probe web site. You can see it takes a lot of complex maneuvers just to get to 3 solar radii from the sun.
The earth’s orbit isn’t decaying. Why would the rocket’s orbit decay? There is no friction in space. Well, very little friction anyway. At least not enough to decay a rocket’s orbit.
If you have access to a children’s playground that has a roundabout installed, you can try this experiment:
Set the roundabout spinning fairly fast and climb aboard, walk toward the centre post and try to kick it - it is almost impossible - your foot will veer to the side. Sit at the edge of the roundabout facing inwards and toss a pebble at the centre post (don’t do this if there are kids playing there, obviously) - again, it is almost impossible to hit because the pebble acquires your own velocity in addition to the thrust you impart.
I’m not convinced that the two scenarios are similiar enough to compare. A pebble thrown on a roundabout has a tendancy to travel tangentially when released, whereas a rocket launched from the surface of the earth would have a tendancy to remain in an orbit of some kind.
You are right, gravity is not replicated in the pebble experiment, however, the point stands that relative to the person conducting the experiment, the centre post appears to be a stationary target.
A better experimental setup would be to sit on the rim of a large spinning bowl and try to roll a ball at a target fixed at the bottom of it, however, I don’t know of any such ready made setup.
Why not just send our junk to Venus? I mean it’s not like we’re going to ruin their atmosphere or anything. I would think, this would be easier then the sun.
If you really wanted to dump waste in space, the moon is far closer and easier to get to. I don’t think any environmentalist would complain about that.
But even then, it took the largest rocket in the world to send a few tons of stuff to the moon. We don’t even have that capability anymore. It makes no economic sense to dump wasete in space.
I had one of those nutty environmental professors that use to do things like spike trees, block the entrance to nuke plants and get arrested. Rush L. would call him an extreme environmental wacko nazi.
He proposed dumping such waste in deap ocean trenches in areas where they would be ‘pulled’ down by plate movement. IIRC his artical was written up in the NY times, but it might have been some scientific jornal.
Larry Niven actually went on to dismiss the idea of disposal of nuclear waste and proposed that we make currency out of it; the high toxicity and residual radioactivity in the coins would keep the money in fast circulation, boosting the economy.
On earth accelerating uses energy while slowing down can be done by friction. In space there is no friction and slowing down uses just as much energy as accelerating. They both have to be done with rockets. In fact, they are both the same thing. You are just accelerating in different directions. Cancelling out the speed of the earth needs a lot of energy.
rather than throwing a ball out the back of a truck, think of it as throwing it out the back of a supersonic jet. No matter how fast you can throw the ball is still going forward supersonically. You would need to fire it with a powerful gun just to cancel the speed of the jet so it would fall vertically.
Because Venus can be much further from the Earth than the Sun is, depending on where they are in their orbits around the Sun. You also still have the problems of decelerating so as to come into Venus’ orbit.
I think, though, that everyone is missing one damned good reason not to even worry about this: Cost. Rocket fuel is not cheap, and it takes a heck of a lot of it just to get to Earth orbit, never mind braking so as to leave the Earth’s solar orbital path to go somewhere else.
Put another way, would you suggest we wrap our garbage in gold leaf before throwing it into the trash can? That is what shooting stuff into space amounts to from a costs viewpoint.
Mort Furd: Build a garbage can that will hold nuclear waste long enough for it all to become lead, and we’ll abandon all talk of shooting it into space. For any credit, ensure that future generations, who will not share a language with us, will not be harmed by our mistakes.
Put it back where it came from - under ground. The extremely radioactive stuff has a shorter half-life, and the lesser stuff is what you’ve pretty much dug up anyways.
Dig a deep hole somewhere where plates come together in the Earth’s crust and dump it in. Dig your hole so that the plate tectonics will push the crap further down and the problem is solved.
It’ll probably cost as much as firing it off into space, but it won’t be a dangerous.
Besides all of that, what makes you think you’d actually manage a successful launch? The Greens over here in Germany routinely sabotage (or try to sabotage) rail transport of nuclear waste. They’d raise such holy hell about the launch that it almost certainly would be cancelled, and if it wasn’t some Green would try to screw it up just to prove how dangerous it is to launch a ship carrying such stuff.
As an aside, what would happen if we just dumped nuclear waste into the lava pit at Kilauea? Would radioactive gas immediately gush out? Would the lava flows that roll out and harden be much more radioactive than usual?
