Meteor Soft Landing ``` Name: Les Status: other Grade: 12+ Location: WA Country: USA Date: Winter 2011-2012 ``` Question: Given the surface speed of the earth (daily rotation, yearly orbit, etc.) in relation to everything, is it possible for a meteor or asteroid at just the right path and speed to make a "soft" landing on the surface? Replies: Les I guess it is possible that a meteor or asteroid would have just the right course and speed to make a soft landing on Earth, but it is highly unlikely. Most meteors and asteroids travel and really high speeds. You see them burn in the atmosphere nightly and very brief streaks of light. Sincere regards, Mike Stewart Les, It would not be possible without risking burning up in the atmosphere. To make a soft landing, the meteor would also have to be able to make a soft launch. Barring energy loss to the atmosphere, energy conservation requires that such a landing work the same in both directions. Such a landing could be no slower than the speed needed to launch the meteor back to where it came from. With energy loss due to the atmosphere, slowing from this necessary speed to a soft landing would probably burn the meteor up. An Apollo spaceship landing without open parachutes is better than a meteor could do. Dr. Ken Mellendorf Physics Instructor Illinois Central College It is difficult to make a certain answer about any possibility, but given the number of factors that would have to occur at just the correct moment and at just the right “amount”, a ‘soft’ landing would have to be put in the “highly unlikely” category. It is hard to imagine how one would put the brakes on, or keep the meteor / asteroid in a safe orbital trajectory for it to slow down by making several orbits etc. Vince Calder Les, The answer depends on how much latitude you are willing to give this problem. For example, the Apollo astronauts had “soft landings” on earth after approaching from space. In this case, the atmosphere slowed the space capsule through friction on the capsule’s heat shield. Also, parachutes opened once the capsule was slowed to a certain speed to allow the astronauts to land safely. My feeling though is that allowing for atmospheric drag would be “cheating” and would not answer the spirit of your question. So first, let us throw the atmosphere out. Consider Earth to be a bare rock in space. Further, let us ignore for the moment outside influences like the moon, the sun, and so on. In this case, we have a rotating Earth and a meteor coming at any angle and at any speed. It does not matter how earth is moving in space in this case, as we can always shift our reference frame to earth and change the meteor’s trajectory accordingly. Now we can use a trick to make some conclusions. Imagine that we are standing on the surface of Earth and we are strong enough to throw a meteor of any size in any direction we wish such that it leaves Earth’s surface and travels away in space. It turns out that we can use Newton’s laws of motion to describe how the meteor would travel away from Earth and into space. In this case, the laws of motion are symmetric in time. In other words, if we were to film the meteor as it was thrown into space, then run the film backwards, Newton’s laws would still describe the meteor’s motion from space and back into our hands. The converse is true as well. Any meteor, coming from any direction at any speed, would have to hit Earth’s surface just so it would stop moving exactly at the time it touches the surface. If you now run time backwards, the meteor would have to rise from the surface of Earth in just the right fashion that it would end up traveling back away from Earth in the opposite direction from which it originally came. Backwards or forwards in time, Newton’s laws would apply here and have to be consistent. Call the case where the meteor comes from space “forward time” and the case where the meteor leaves Earth “reverse time.” Now, assume we are standing on Earth next to a stationary meteor (as a side note, we should properly call it a “meteorite” as it is a meteor on Earth). In reverse time, somehow, the meteor would have to “jump” from Earth in a direction such that it ended up in space because when we viewed it in forward time it has to satisfy Newton’s laws. Further, we cannot throw the meteor from the ground as we assumed the meteor hit the ground (in forward time), at the very least, with little force. In reverse time, we have to apply the same force in the opposite direction to get the meteor to travel to space. In our experience, we know that rocks do not spontaneously jump from Earth’s surface and travel to space. Even if we allow for some force to throw a rock in reverse time, we cannot consider the force as a soft landing if the meteor ends up in space. At minimum, we have to throw the rock faster than the escape velocity of Earth (about 11 kilometers/second). We can use the rotation of Earth to improve this situation, but a rock will still not spontaneously jump into space. If the rotation of Earth is such that rocks do launch into space, it means the rotational speed is so great that Earth would disintegrate. Thus, it seems there are no conditions under these assumptions in which a meteor can approach earth in just the right way to softly land on Earth. Kyle J. Bunch Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs