PDA

View Full Version : Asteroids that come to earth



Tster2005
2011-Dec-10, 03:02 PM
Hi,

I wonder if anyone can help me? I'm just trying to find out, on average, if an Asteroid (a non life threating one) was to free itself (involved in a collision) from our nearest Asteroid belt, how long would it take to impact on the earth?

I thank you for your help in advance.

Lee

antoniseb
2011-Dec-10, 03:41 PM
If there was a collision between two large asteroids in the inner part of the Asteroid belt between Mars and Jupiter, and one large chunk of debris was on a direct flight to Earth, it would take about a year to get here. It's hard to be more precise because there are so many loose parameters. You could contrive an implausible situation that would get it here in a few months if the exit velocity from the collision was unnaturally high.

All that being said, the likelihood of such an event is so close to zero that we can't measure it.

Tster2005
2011-Dec-10, 03:46 PM
@antoniseb

Thank you for your prompt reply. It's much appreciated.

Romanus
2011-Dec-10, 06:39 PM
To add to what Antonise wrote: Realistically, thousands to millions of years, depending on size, proximity to any other perturbing bodies, nearby orbital resonances, etc. Note that the prominence of asteroid "families" is due to the fact that most asteroid collisions are low-velocity events, so fragments are slow to disperse far from their parent body's orbit.

Jeff Root
2011-Dec-10, 07:42 PM
It isn't at all clear what you mean by "our nearest
Asteroid belt". Asteroids are pretty much all over
the place. It is just that there are more in some
places than in others. The asteroids nearest Earth
are called "near-Earth asteroids", or more generally,
"near-Earth objects" (NEAs or NEOs). Their orbits
come very close to or cross Earth's orbit.

Asteroids whose orbits are entirely or almost entirely
inside Earth's orbit can be especially difficult to see
because they are in the daylit sky most of the time.
Those are the asteroids which typically are first seen
only when then they make a close pass by Earth.

I think the most likely scenario for a chunk of asteroid
from the Main Belt (between Mars and Jupiter) to reach
Earth quickly would be if the asteroid were struck by
another one orbiting in almost the opposite direction.

-- Jeff, in Minneapolis

JustAFriend
2011-Dec-10, 08:57 PM
We just had one the size of an aircraft carrier come between the Earth and the Moon a few weeks ago....

http://www.reuters.com/article/2011/11/04/us-space-asteroid-idUSTRE7A36FN20111104

They are all over the system, not just in the belt.

PraedSt
2011-Dec-10, 09:26 PM
I'm just trying to find out, on average, if an Asteroid (a non life threating one) was to free itself (involved in a collision) from our nearest Asteroid belt, how long would it take to impact on the earth?
Working out an average time-to-impact is too difficult. An asteroid knocked out of the Belt would be orbiting the Sun, just as we are. It could hit us in the first time round, or on the millionth time round, i.e. it could take a year or a million years.

PraedSt
2011-Dec-12, 10:09 PM
Thought about this some more. You can't get an average time, but you can get some figures if you add a lot of constraints.

If an asteroid got dislodged from the belt and crashed into Earth on the first pass, you can work out the longest time this would take by assuming it would follow an elliptical Solar orbit, and that it would hit Earth at the asteroid's periapsis.

The asteroid belt is 3 AU from the Sun. We're 1 AU from the Sun. So the semi-major axis of the asteroid's orbit is 2AU. This is twice Earth's semi-major axis, so the asteroid's orbital period is sqrt(23) or 2.8 times that of Earth. The longest time while still colliding on the first pass would mean colliding at the furthest end of the asteroid's orbit, which is half of the full orbit, so the time taken is 2.8/2 or 17 months.

Now if it doesn't collide on the first pass, but will definitely collide on another pass in the future, then I think the time to collision will be some multiple of 2.8 (perhaps, I'm not sure about this).

Jens
2011-Dec-13, 12:01 AM
I wonder if anyone can help me? I'm just trying to find out, on average, if an Asteroid (a non life threating one) was to free itself (involved in a collision) from our nearest Asteroid belt, how long would it take to impact on the earth?


I'm not really sure, but just from your wording it seems that you might have some misconceptions. And asteroid is not trapped in a belt, so rather than saying "free itself from" it would make more sense to say that its orbit would change. Chances are most likely that when two asteroids come close to colliding, they will simply settle into orbits that are slightly higher or slightly lower. They wouldn't spiral into earth or even into the sun, because the velocity of the earth is much higher than that of the asteroids in the main asteroid belt. They would have to lose a lot of velocity to be able to settle into an orbit near the earth. Ceres (one of the big asteroids) has an orbital velocity of about 17 km/second, whereas the earth travels at like 30 km/second. So in order for Ceres to get into an orbit near the earth, it would somehow have to lose 13 km/second, so that's like 47,000 km/hour. That's a lot of velocity to lose for something that size.

Jeff Root
2011-Dec-13, 09:04 AM
Ceres is probably the very least likely of all asteroids to collide
with Earth. However, any rock in an orbit close to that of Ceres
will need the same change in velocity as Ceres would.

Asteroids actually are trapped in belts to a significant extent.
Nearby planets-- especially Jupiter-- herd the asteroids into
the belts and keep them there via regular gravitational pulls
every synodic year. Since the asteroids tend to have rather
elliptical orbits, the herding effect is more pronounced on the
asteroids' orbital periods than it is on their distances from the
Sun. That is, Jupiter nudges their orbital periods away from
certain values, even though their distances from the Sun vary
a fair amount. The gaps between the belts (actually, gaps
between orbital periods) are called "Kirkwood gaps" for the
astronomer who pointed out the shortage of asteroids with
certain orbital periods in the main asteroid belt, and explained
the gaps as due to Jupiter's gravitational influence. Asteroids
straying into those gaps are pulled out, one way or the other,
over the course of many orbits.

-- Jeff, in Minneapolis

HypersonicMan
2011-Dec-21, 06:46 PM
Asteroids actually are trapped in belts to a significant extent.
Nearby planets-- especially Jupiter-- herd the asteroids into
the belts and keep them there via regular gravitational pulls
every synodic year. Since the asteroids tend to have rather
elliptical orbits, the herding effect is more pronounced on the
asteroids' orbital periods than it is on their distances from the
Sun. That is, Jupiter nudges their orbital periods away from
certain values, even though their distances from the Sun vary
a fair amount. The gaps between the belts (actually, gaps
between orbital periods) are called "Kirkwood gaps" for the
astronomer who pointed out the shortage of asteroids with
certain orbital periods in the main asteroid belt, and explained
the gaps as due to Jupiter's gravitational influence. Asteroids
straying into those gaps are pulled out, one way or the other,
over the course of many orbits.


This is close, but not quite correct. Jupiter doesn't "herd" asteroids away from the gaps. Instead, the Kirkwood gaps are like a sieve. Asteroids that are in them are removed, and asteroids that are not are left undisturbed. Jupiter's gravitational influence produces regions of instability at low-integer orbital period ratios (for instance, an asteroid that orbits the Sun 2 times for every time Jupiter orbits 1 time). An asteroid that finds itself in a orbital period resonance with Jupiter will have its eccentricity raised over time until it begins to cross the orbits of the major planets. It then leaves the main belt to become a Near Earth Asteroid (or if it crosses Jupiter's orbit, is tossed out of the solar system entirely, or rarely, hits Jupiter), and eventually impacts either the Sun or the inner planets. The gaps that occur at low-integer orbital period ratios with Jupiter are called the Kirkwood gaps.

Most Main Belt asteroids that become Near Earth Asteroids (NEAs) do not do so directly due to collisions. Instead, asteroids are constantly drifting in their orbits due to the Yarkovsky effect, which is a force imparted on them by thermal radiation. For instance, a spinning asteroid has a day side that is heated by the Sun, and a night side where it cools off into space by radiating thermal photons. Because of thermal lag, the "evening side" is warmer than the "morning side," which means that thermal photons are radiating asymmetrically with respect to the orbital direction, and a very very tiny force is imparted. Large asteroids are not effected by this as strongly as small ones, so collisions work to increase the population of small fragments that drift large distances over time.

Small asteroids drifting due to the Yarkovsky force will eventually reach a region of instability (a Kirkwood gap, or other type of resonance) and have their orbital eccentricity increased until they escape the belt and come under the influence of the major planets, when they become NEAs. Some NEAs eventually impact a planet, and if one impacts the Earth and survives the passage through the atmosphere, it can become a meteorite. We can measure how long this process takes, from the time a fragment is ejected due to a collision until the time it reaches the ground to become a meteorite, by measuring how long the surface of the meteorite was exposed to cosmic rays in space.

The reference here gives a summary of some of that work: http://adsabs.harvard.edu/abs/2003TrGeo...1..347H

In short, iron meteorites take longer to reach the earth than stony ones. Typically, stony meteorites (like ordinary chondrites) take on average a few tens of millions of years to go from collision to Earth impact. Iron meteorites take longer, more like 200 million years or more. This is probably due to both the kinds of dynamical pathways available to these different types of objects as well as how long the fragments can survive the impact environment in the Main Belt. Irons are stronger, so they can survive against impact disruption (which resets the cosmic ray exposure clock) than stones.

Typically impacts are far less likely once an asteroid becomes an NEA, so the bulk of those tens of millions of years are likely taken up by the slow drifting by Yarkovsky. Gravitational dynamics simulations of NEAs suggest that objects don't last more than a few million years as NEAs.

So, to answer the OPs question, based on data from meteorites, the average time it takes for an asteroid to reach the Earth after it is formed in a collision is between a few tens of millions of years to a few hundred million years, depending on the details.

PraedSt
2011-Dec-21, 08:15 PM
So, to answer the OPs question, based on data from meteorites, the average time it takes for an asteroid to reach the Earth after it is formed in a collision is between a few tens of millions of years to a few hundred million years, depending on the details.Using actual data instead of pie-in-the-sky theorizing? You...you...heretic! :)