# Do all dangerous asteroids first pass through keyholes?

Now MIT researchers have devised a framework for deciding which type of mission would be most successful in deflecting an incoming asteroid. Their decision method takes into account an asteroid’s mass and momentum, its proximity to a gravitational keyhole, and the amount of warning time that scientists have of an impending collision — all of which have degrees of uncertainty, which the researchers also factor in to identify the most successful mission for a given asteroid.

“People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth,” says Sung Wook Paek, lead author of the study and a former graduate student in MIT’s Department of Aeronautics and Astronautics. “I’m interested in preventing keyhole passage well before Earth impact. It’s like a preemptive strike, with less mess.”

Questions:

1. Do all or at least most astroids that will pose a danger to Earth pass through a keyhole first?
2. If so, why? Why would previous but fairly recent close passes to some gravitational body be important for making an asteroid pose a danger to Earth?

For more on gravitational keyholes see answers to the following (and links therein)

as well as Wikipedia's Gravitational keyhole.

• I love a question that teaches me something before there are even any answers. Nice! Feb 20, 2020 at 14:28
• @WayneConrad perhaps this question can be a "keyhole" to the world of orbital mechanics
– uhoh
Feb 20, 2020 at 15:01
• A definition of gravitational keyhole in the question would be helpful, I think ) Also an interesting question, but maybe beyond of the scope - what percent of close asteroid flybys yields to a gravitational keyhole catch and later impact event? Feb 21, 2020 at 8:19
– uhoh
Feb 21, 2020 at 9:22

Dangerous asteroids are those that can hit the Earth, and are large enough to cause substantial damage. There are currently no such known asteroids. (2020-02-21)

There are two ways an asteroid could end up as considered dangerous:

1. We discover it. There may be an asteroid bound for Earth at this moment, we just haven't seen it yet. This is fairly straight forward.
2. An asteroid passes through a gravitational keyhole.

This implies that every known asteroid would have to pass through a gravitational keyhole to become dangerous.

How does 2) turn asteroids dangerous?

Asteroid orbits are only known to some limited precision. Even when the uncertainties are small enough that we can rule out a collision when it approaches Earth, even small uncertainties will cause large differences in how close it is going to get.

That is, exactly how the asteroid is going to fly by us is "blurry", we don't really know exactly how it's going to look.

That's where the butterfly effect sets in. Even tiny differences in how close the asteroid passes by is going to have a huge effect on its future orbit.

Those future orbits may be dangerous. What if, for instance, the new orbit of the asteroid is exactly 1 year? Then the Earth and the asteroid will meet again in the same spot. And we don't want to "meet" asteroids. Same goes for 2 years, 3/2 years, etc., every simple fraction is a potential future disaster.

A gravitational keyhole is essentially that "if the first encounter turns out this way, which we don't know for certain, the future orbit will make the asteroid hit the Earth."

What those scientists are suggesting is that we can knock asteroids slightly of course, so the future fly-by is certain to not happen in a way that leads to a dangerous orbit in the future

• Could I interpret your answer to "Do all dangerous asteroids first pass through keyholes?" as a qualified "more or less yes"?
– uhoh
Mar 8, 2020 at 10:52
• I can't really accept an answer until it either takes a clear position on the question or explains why that's not possible.
– uhoh
Apr 11, 2020 at 3:59