How many earth-orbit-crosssing asteroids are known to exist?

Question

They scare the $%#@ out of me!

Answer 1

There are 4617 Near-Earth Asteroids cataloged so far (and about 60 Near-Earth Comets). Some have very well-characterised orbits because they've been followed over many years, others we only know about because we saw them fleetingly for a few days (sometimes even only a few hours like 2004 FU162). Of the 4617 cataloged, 2262 are in "Apollo" type orbits, 390 in "Aten" type orbits and 1965 in "Amor" type orbits. (These numbers are from the Minor Planet Center, the JPL NEO Program has slightly different numbers). Apollo-type orbits are mostly further from the Sun than Earth but cross Earth's orbit (at least in a 2-dimensional sense when viewed from "above" or "below" the plane of Earth's orbit). Aten-type orbits are mostly closer to the Sun than Earth but also cross Earth's orbit in the 2-d sense. Amor-type orbits are fully outside Earth's orbit but get close-ish. None of the cataloged objects is on a known collision course with the Earth.

About 800 of the cataloged objects are estimated to be larger than 1 km (the biggest are about 30 km) in diameter. The numbers are a bit rubbery for many reasons. Size is only inferred from brightness, which itself is not often well known. If an absolute magnitude of 17.75 translates to 1 km diameter then there are just over 700 bigger than 1 km, if one says anything less than (but not equal to) absolute magnitude 18 is 1 km then there are just over 800 and if you allow equal to 18 (most catalogs go to 1 decimal place in magnitude) the number is about 850. Several studies have estimated that there are about 1000 to 1100 objects out there of 1 km or larger. Thus we have found about 70% of them.

What's the big deal about 1 km? It is just a number and also represents a size that current tracking surveys (and many, many amateurs) can track regularly. The other thing is that at about that size an asteroid impact is predicted to release so much energy that the effects will be global and not confined to a city, country or continent. There are expected to be many more smaller objects but the surveys have a harder time tracking them, they are fainter and may only be able to be followed when they are close to us.

Perhaps best known is (99942) Apophis (previously 2004 MN4), an object about 300 m in diameter. It is predicted with very high certainty to pass Earth within about 36,000 km on April 13, 2029. At one time its orbit was uncertain enough that there seemed to be about a 2% chance it would hit the Earth on that date. Further observations, including highly accurate radar tracking have now ruled that out. However, what still can't be ruled out with certainty is the possibility that during the pass in 2029 it will pass through a small region of space near the Earth at just the right time to cause its orbit to shift in just the right way to bring it back for a collision in 2036. We now know that this scenario is unlikely (around 1 in 45,000) but we will have to wait for late 2012 or early 2013 for the next chance to get more data.

Answer 2

NEAR EARTH OBJECTS (NEOs)

In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer (0.6 miles) wide. At this size and larger, an impacting NEO would cause catastrophic local damage and significant to severe global consequences.

Approximately 800 of these NEOs have been detected. According to the most widely accepted estimates, there are still 200 more that have not been found yet. The United States, European Union and other nations are currently scanning for NEOs in an effort called Spaceguard.

Currently efforts are under way to use an existing telescope in Australia to cover the ~30% of the sky that is not currently surveyed.

NEAR EARTH ASTEROIDS

Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earth's orbit. Some NEAs' orbits intersect Earth's so they pose a collision danger.

Roughly 1000 near-Earth asteroids are known, ranging in size up to ~32 kilometers (1036 Ganymed). Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometer in diameter at up to 2,000.

NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated by orbital decay and accretion by the Sun, collisions with the inner planets, or by being ejected from the solar system by near misses with the planets.

Such processes should have eliminated them all long ago but they are resupplied on a regular basis by orbital migration of objects from the asteroid belt.

Answer 3

The total of known Near Earth Objects is increasing all the time.

NASA has a congressional mandate to catalog all Near Earth Objects greater than 1 km in diameter. (although smaller NEOs are still dangerous, they are much more difficult to detect)

As of now, there are at least 800 NEOs that have been catalogued... and at least another 200 still to be found.

But don't let it get you down... the odds of one of them hitting us are still very slim... but it is better to be prepared than to just let something preventable happen.

There are many plans in the works to prevent a catastrophic event.... Check out the SpaceGuard and Asteroid Deflection links, below.

Answer 4

There are 213 known asteroids whose orbits cross Earth's. See the source for a list.

Answer 5

it is not Jupiter's orbit that impacts asteroids, yet Jupiter itself. The orbit itself is basically an digital ellipse that charts Jupiter's path around the solar. And after 4.5 billion years Jupiter fairly does not impression the asteroid belt as much because it used to. maximum of this is astonishing.