With specific regard to Apophis, IIRC the impact would be far less than one teratonne, nowhere near enough to be globally threatening and it would make more sense to evacuate people from the expected impact area.
This 99942 Apophis - Wikipedia indicates that while Apophis wouldn’t have represented a biosphere-destroying event, simply “evacuating the area” might be pretty impractical.
Nobody has ever evacuated a whole small country or the entire Atlantic shoreline before.
Not a small impact either. It would be rather devastating
Indeed, I didn’t say it would be simple, though, given notice, evacuation could be people simply walking away. The real problems would come with the feeding and re-settling of the people evacuated. Imagine that Apophis were to hit London dead centre: the UK would have to relocate over 10M people, maybe over 20M. How soon could London be rebuilt? And how quickly?
Here is a program developed by the Imperial College in London that allows you to enter the variables of density, speed, angle of impact, and distance from your location to get an idea of what the effects of an impact might be should one happen near you.
It has been around for a while and has probably been replaced by something better. But it is interesting to play with.
One problem with evacuation is it would be difficult to know where we should be evacuating from. Even if knew the exact moment of strike, roughly half the earth’s surface would be a potential target without a very accurate determination of the orbit. If you don’t know the hit time to within 12 hours, the entire surface of the earth is a potential target.
Even if we discover the asteroid/comet with a year’s lead time how soon would be have an accurate enough orbit calculated to have less than a continent we’d need to evacuate? And if it strikes in the Pacific (very likely given its size), how far inland on each coast must we evacuate to escape the tsunamis?
The chances that we would have anything like enough time to take any real steps to protect ourselves are minimal. It’s only in the last three years that NASA has been actively looking for threatening objects and we would need ten or twenty years to develop a vehicle that could do anything about one. If we were lucky we might have six months. If the object’s albedo is low enough, we wouldn’t know about it until it reached our atmosphere.
We’ve found about 10,000 significant near earth objects (that is, sizeable bodies that will cross our orbit.) That includes 90% of those that are more than a kilometer in diameter. However, NASA thinks there are about a million left, and they don’t have to be a kilometer across to do serious damage.
The Tunguska object was about 50 meters across and exploded in the upper atmosphere, but knocked down millions of trees. A 200 meter object that struck the earth would leave a crater miles across.
Something that never seems to get mentioned much is, how is that asteroid moving? Not it’s path through the Solar System, but it’s motion relative to itself?
My (naive) thought is that most of these are tumbling – not motionless, not rotating about 1 axis, but actually moving about 2 or 3 of its axes. Is this a reasonable thought, or would the gravity drag from other objects actually stop this tumbling? (Anne?)
Many of the proposed ways of deflecting an asteroid or comet supposes landing on the surface and “doing something”, but if the object is tumbling, these methods would seem to be useless.
Any comments?
J.
IIRC, the only reason the earth’s rotation is stable is the gravitational influence of the moon. So I would guess you are correct.
Why would it be harder to perturb Ceres than a small asteroid? Wouldn’t both just follow the gravitational fields? It seems that if something (such as Jupiter) could perturb one it could perturb the other.
Like dropping a marble and a bowling ball from the tower of Pisa; or dropping a feather and a hammer on the moon.
I’m pretty sure the idea is that Jupiter has done all the perturbing it’s going to in the last couple billion years. If an asteroid is going to be perturbed now, it’ll be a result of other asteroids. Since something like Ceres is so much larger than the other asteroids, it will be the other, smaller, asteroids that get the biggest perturbation in orbit.
Nope. If you have an object exerting a gravitational force on Ceres, Ceres is going to exert gravitational force on that object. They’ll exert equal forces on each other, but the acceleration that each experiences won’t be equal. If the object is much less massive than Ceres, it gets most of the acceleration and Ceres doesn’t get much.
Many asteroids do tumble, and this is indeed a complication if we want to try to change the orbit of one. Phil Plait discusses this issue in his book, Death from the Skies.
In simple English: We’re trying to change the velocity (= speed & direction of motion) just enough so that when it goes through the Earth’s orbit, the Earth isn’t in the way.
The heavier it is, the more we have to push to make the same change in speed. If I had a place to stand, I could make the necessary velocity change in a baseball-sized asteroid with my arm. I’d need to use both legs to push enough on something the size of a car. As between something the size of a house or something the size of a state the increment push required to make the same change in velocity starts getting significant.
Bottom line: deflecting a Ceres-sized object takes a lot of push. That’s either a huge force for a short time, or a smaller push for a long time. But whichever approach we took, the total push required would be enormous by the standards of current or near future human tech in asteroid pusher/puller devices.
“A place to stand” is a problem in space. If you’re on Earth pushing something, and your feet don’t slide out from under you, the force the object exerts is on you plus the Earth, so most of the acceleration will go into the object. You don’t have a planet to stand on if you’re trying to deflect an asteroid that is far away from a planet.
I once got a rather nice demonstration of this when I was a physics teaching assistant. I was wearing slippery-bottomed shoes and trying to push a heavy cart of equipment for a classroom demonstration. Instead of the cart moving forward, my shoes slipped on the floor, and I moved backward.
Of course, if you want to launch a massive object into interplanetary space, that’s going to be difficult and expensive. You’d probably have to get around that by launching a smaller object and pushing on the big object for a long time. This is part of the reason why you want lots of lead time if you want to deflect an asteroid.
Agreed. I was trying to keep it very simple and paper over all the details for BBB who’d apparently gotten lost in the trees & couldn’t find the forest.
One idea I heard (from Bill Nye, I think) on how to deflect an asteroid / comet is to wrap it completely in something very shiny. This would have the effect of turning the entire object into a solar sail, of sorts. Deflecting it in this manner would take quite a while (years? decades?) but it seems like a potential way, if we have enough warning. A question in my mind is, is the photon pressure from the sun enough to alter the course of a massive object? Or would the impulse given not be enough?
J.
That’s going to depend on the mass and density of the asteroid and how long we have to change its orbit. It’s going to work best on a low-mass, low-density asteroid. It’s not going to provide a lot of force, so you’d need a lot of lead time to change the orbit this way. Probably decades to a century or more.
In fact, Jupiter has cleared out significant gaps in the asteroid belt where asteroids would be in an orbital resonance with Jupiter.
We’re talking about objects from our own solar system. But what about objects from outside? Could they be faster? Could we see them coming? What directions would they likely come from?