Risky Asteroid Census

[MeteorWayne]

An interesting new trend with Catalina and now PANSTARRS coming online is that many more small Near Earth Asteroids are being discovered.

For example, 17 of the 18 on the Sentry "Recently Observed Objects" list (PS -4.24 to -8.25) are smaller than 50 meters (highlighted in blue).

http://neo.jpl.nasa.gov/risk/

Also the rate of discovery of small (< 1 km) NEA's is averaging about 800 a year since 2008 and rising (Current total 7444), while the discovery rate of NEA's larger than 1 km peaked in the 2000-2002 time frame (~ 80/Yr) and have decreased ever since to only about 20/yr now (23 in the last full year; 2nd half of 2009, 1st half of 2010). We really have enumerated a large percentage of the > 1 km objects (Current total 819).

MW

http://neo.jpl.nasa.gov/stats/

 

[silylene]

In your second link, there is a plot of NEA's vs diameter:

In all likelyhood, and exponential distribution would be expected, with many more small asteroids than large ones. This means there are still a whole bunch of 300m and smaller NEA's waiting to be found.

 

[kg]

I thought that one of the definitions a planet is it has to have cleared its neighbouring region of planetesimals. How many more NEAs do the need to find for Earth to loose it status as planet?

 

[MeteorWayne]

You are misintepreting what the IAU said. It said "it's orbit", saying nothing about objects that cross a planet's orbit. If that was the case, there would be no planets at all, since asteroids cross the orbit of every object in the solar system...and the sun eats many as well

"A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Full Resolution:

RESOLUTION B5
Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,<2>,
(c) has not cleared the neighbourhood around its orbit, and
(d)is not a satellite.

(3) All other objects<3>,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

<1> The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
<2> An IAU process will be established to assign borderline objects to the dwarf planet or to another category.
<3> These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs),comets, and other small bodies.

BTW, this discussion re Pluto, properly belongs in one of the Pluto threads.... I may merge it into one.

 

[kg]

I suppose you can also point out that these asteroids pose a risk because the Earth is doing it's job as a planet which is clearing them out of the neighborhood!