# How well can current physics predict the movement of rocks in space?

I have a question about the precision of predicting the movement of objects in space (which I believe fall under physics and not space exploration). While I suspect the main limitations here are measurement and mapping, I do not know whether there are also theoretical objections to the following scenario:

With present-day technology, I set up a mass driver on a small asteroid. This has the ability to throw rocks in any direction. I know the composition (density) of the entire asteroid to the same detail that I would on Earth. I follow up to buy a 100x100 km plot of land in Sahara. Will I be able to steer the asteroid with sufficient accuracy to hit the 100x100 km plot of land? To avoid course corrections arbitrarily close to Earth, let's say I am not allowed to run the mass driver any closer than the orbit of Mars. I have not specified the size of the asteroid, but it should be big enough not to burn up in the atmosphere (presumably it is made of very valuable material) and small enough not to cause a disaster when impacting Earth.

If this is not possible, is it due to:

1. insufficient knowledge about other atmospheric bodies that will impact the trajectory of this object?

2. insufficient calculating capacity to know how all known bodies will affect the path of this object?

3. impossibility of running a mass driver with sufficient accuracy with present-day technology?

4. uncertainty in weather patterns, atmospheric density or the path/orbital speed of the Earth?

5. some other factor?

Your window from Mars to your 100kmx100km box in the Sahara is a damn tight one.

While I agree with Codename 47 above in general, it is not as easy as simply calculating trajectories for spacecraft. The target windows they are hitting are on planetary scales, you are trying to thread a needle.

At our current technology, we can't even predict with high certainty where satellites with known trajectories will come down on the planet.

Your best bet would be to strap a set of rockets on the asteroid and shoot it into high earth orbit. From there it could be moved into low earth orbit, and then brought down to your box.

Interestingly enough the Pentagon has looked at a Rod from God weapon system for years. (too expensive) but there is nothing technologically difficult about your proposal once you are in orbit.

• "At our current technology, we can't even predict with high certainty where satellites with known trajectories will come down on the planet." Really? How come? I answered from a theoretical physics point of view, but do you have any examples or explanations for this? It sounds quite interesting. Apr 23, 2019 at 15:27
• Skylab, Jules Verne ATV, European Space Agency's GOCE, UARS, Cosmos 954 are just some examples. I think there was a Chinese lab that fell to earth uncontrollably and unpredictably as well lately...
– Rick
Apr 23, 2019 at 19:23
• Thank you for the examples. My thought is that an object which flies towards Earth at a more steep angle (and higher velocity) than a satellite would be easier to predict. Apr 24, 2019 at 8:32
• It probably would, but a shot from Mars to Earth has a LOT of variables. It would be safer to bring it into orbit and then drop it. You have more control that way.
– Rick
Apr 24, 2019 at 10:26

Predicting the trajectory of an object in space within the precision stated in your scenario should be very possible. We have extremely accurate models of the influence of the planets and moons of the solar system, which allowed us to e.g. launch the Voyager probes on very specific trajectories with only a few, small corrections along the way.

The main problem occurs after you enter the atmosphere, since this is more turbulent. However, since noone is riding inside your asteroid, you don't need to slow it down artificially before impact, so the high speed should let it be relatively unperturbed by atmospheric interruptions. And again, the likelihood of it striking another atmospheric object is extremely small (I am pretty sure no plane has ever crashed because of it being hit by a meteor).

As such, there should be no theoretical limitation to your scenario, only technological ones. Space trajectories is one of the earliest concepts to be studied by physics, and we have gotten quite good at it over the centuries.