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#### MeteorWayne

##### Guest

Here are the top 6 risk asteroid impacts as of Aug 1 2010:

Object............. VI years ...# of VI's... Impact odds...Vinf...H....size(km)...PS cumulative..PS max..TS

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101955 1999 RQ36 .............2169-2199... 8... ..7.1e-04 ...6.36 ...20.7 ...0.560... -1.12 ...-1.52 ...n/a

(No Perm #) 2007 VK184 ......2048-2057 ...4 ...3.4e-04... 15.63 ...22.0... 0.130 ...-1.82... -1.83 ...1

99942 Apophis (2004 MN4) ...2036-2103.... 6 ....7.4e-06 ...5.87... 19.7 ...0.270 ...-2.97 ...-3.08... 0

(No Perm #) 1994 WR12 .....2054-2106... 121... 9.1e-05 ...9.84 ...22.4 ...0.110... -2.99 ...-3.92... 0

(No Perm #)1979 XB ..........2056-2086..... 2 ....3.0e-07... 24.59 ...18.5... 0.687... -3.05... -3.09 ...0

(No Perm #) 2010 AR85 ......2015-2106 ...35 ...1.9e-08 ...32.84 ...17.4 ...1.100... -3.11... -3.37 ...0

For those new to the asteroid game, let me explain the data listed (this is all from the JPL Sentry system, more on that later; the other source is NEODyS, and for well studied objects, the numbers are very close)

Object description

Any new asteroid is given a discovery designation (I'll use Apophis as an example since it has all 3) such as 2004 MN4

This consists of the year of discovery (2004) a letter for the half month of discovery (M) and a letter and numbers for objects discovered during the half month (N4).

For objects well observed over multiple oppositions, a permanent number (like 99942) is assigned.

After that suggested names (such as Apophis) may be submitted to the IAU.

Virtual Impactors:

For each object, the observations (with their error bars) are used to calculate orbits that match the observations. In the beginning there are thousands of orbits that can fit the observations. As more data comes in, the number (or really the distribution) of the possible orbits is reduced. Orbits that could possibly impact earth are called virtual impactors. The years during which those impacts can occur is the next entry, then the number of VI's.

Impact odds:

Next is the cumulative possibility of impact. For example, the highest risk, for 1999 RQ36 is

Impact Probability: 7.1e-04

0.071000000% chance of Earth impact

or

1 in 1,410 chance

or

99.92900000% chance the asteroid will miss the Earth

Vinf:

This is the approach speed o the asteroid to earth before earth's gravity affects it. The earth provides a constant acceleration of 11.2 km/sec, so this is added to the Vinf according to the formula Vimpact=SQRT(Vinf^2 + 11.2^2).

So the lowest possible impact velocity is 11.2 km/sec (for an impossible object with zero starting speed relative to earth), for Apophis (5.87 Vinf) the impact velocity is 12.6 km/s, for 2010 AR85 impact speed is 34.7 km/s, and the fastest possible for an object orbiting the sun is ~ 72 km/sec for an object orbiting the sun retrograde such as the parent comet of the Leonids or Halley's comet (Orionid and eta Aquariid meteor showers).

H and diameter are related; H is the absolute magnitude (standard distance and illumination) and the diameter is an estimate in most cases based on assumptions of the albedo (reflectivity) of the object. In some cases the size is firmed up by direct visual or radar observations, but in most cases it's a best guess.

PS is the Paletmo Scale, cumulative is all VI's, max is the highest likelyhood individual impact.

TS is the Torino Scale, only defined for impacts within the next century

Palermo Scale: http://neo.jpl.nasa.gov/risk/doc/palermo.html

Torino Scale: http://neo.jpl.nasa.gov/torino_scale.html