Editing 2002 AA29 (section)

== Physical nature ==
=== Brightness and size ===
Relatively little is known about {{mp|2002 AA|29}} itself. With a size of about {{convert|20|-|100|m|ft|sigfig=1}} it is very small, on account of which it is seen from the Earth as a small point even with large [[telescope]]s, and can only be observed using highly sensitive [[Charge-coupled device|CCD]] cameras. At the time of its closest approach in January 2003 it had an [[apparent magnitude]] of about 20.4.<ref name="MPEC2003-A17"/>

So far nothing concrete is known about the composition of {{mp|2002 AA|29}}. Because of its nearness to the Sun, it cannot however consist of [[Volatile (astrogeology)|volatile substances]] such as [[ice|water ice]], since these would [[evaporation|evaporate]] or [[Sublimation (chemistry)|sublime]]; one can clearly observe this happening to a [[comet]] as this forms the visible tail. Presumably it will have a dark, [[carbon]]-bearing or somewhat lighter [[silicate minerals|silicate]]-rich surface; in the former case the [[albedo]] would be around 0.05, in the latter somewhat higher at 0.15 to 0.25. It is due to this uncertainty that the figures for its diameter cover such a wide range.

A further uncertainty arises from [[radar astronomy|radar]] echo measurements at the [[Arecibo Observatory|Arecibo]] [[Radio Telescope]], which could only pick up an unexpectedly weak radar echo, implying that {{mp|2002 AA|29}} is either smaller than estimated or reflects radio waves only weakly. In the former case it would have to have an unusually high albedo.<ref name="Icarus2">Steven J. Ostro, Jon D. Giorgini, Lance A. M. Benner, Alice A. Hine, Michael C. Nolan, Jean-Luc Margot, Paul W. Chodas, Christian Veillet: [http://echo.jpl.nasa.gov/asteroids/ostro+2003_aa29_icarus.pdf ''Radar detection of Asteroid 2002 AA29''] in: ''[[Icarus (journal)|Icarus]].'' Elsevier, San Diego 166.2003, 12, p271–275. {{ISSN|0019-1035}} (online on the Icarus Server: {{doi|10.1016/j.icarus.2003.09.001}})</ref> This would be evidence in support of the speculation that it, or at least the material of which it is composed, is different from most other asteroids so far discovered on near-Earth orbits, or represents a fragment thrown off by the collision of a medium-sized asteroid with the Earth or the Moon.<ref name="lpsc2004" />

=== Rotational period ===
Using radar echo measurements at the Arecibo radio telescope the rotational period of {{mp|2002 AA|29}} could be determined. In this radar astronomy procedure [[radio waves]] of known [[wavelength]] are emitted from a radio telescope aimed at an asteroid. There they are reflected, and because of the [[Doppler effect]] the part of the surface that is moving towards the observer (because of the asteroid's rotation) shortens the wavelength of the reflected waves, whilst the other part which is turning away from the observer lengthens the reflected wavelength. As a result, the wavelength of the reflected waves is "smeared out". The extent of the wavelength smearing and the diameter of the asteroid allow the rotational period to be narrowed down. 33 minutes is thus calculated as the upper limit of the rotational period for {{mp|2002 AA|29}}; it probably rotates more quickly. This rapid rotation together with the small diameter and therefore low mass leads to some interesting conclusions:

* The asteroid rotates so quickly that the [[centrifugal force]] on its surface exceeds its gravitational pull. It is therefore under [[tension (physics)|tension]] and so cannot be composed of an agglomeration of loosely bound debris or of fragments circling each other – as is supposed for several other asteroids and for example has been determined for the asteroid [[69230 Hermes|(69230) Hermes]]. Instead the body must be made of a single relatively strong block of rock or of pieces baked together. However, its tensile strength is probably considerably lower than terrestrial [[rock (geology)|rock]] and the asteroid also very [[porous]].<ref name="Icarus2"/>
* {{mp|2002 AA|29}} cannot possibly have been built up from individual small pieces, as these would be thrown apart by the rapid rotation. Therefore, it must be a fragment blown off in the collision of two heavenly bodies. [[J. Richard Gott]] and [[Edward Belbruno]] from [[Princeton University]] have speculated that {{mp|2002 AA|29}} might have formed together with Earth and [[Theia (planet)|Theia]], the postulated planet that, according to the [[giant impact hypothesis]], collided with Earth in its early history.<ref name="NewScientist20040814"/>