For most numbered asteroids, almost nothing is known apart from a few physical parameters and orbital elements.
For many asteroids, lightcurve analysis provides estimates of pole direction and diameter ratios.
More recent determinations for several dozens of asteroids are collected at the web page of a Finnish research group in Helsinki which is running a systematic campaign to determine poles and shape models from lightcurves.
A body's dimensions are usually given as a triaxial ellipsoid, the axes of which are listed in decreasing order as
The masses of the largest asteroids 2 Pallas, and 4 Vesta can also be obtained from perturbations of Mars.
[6] While these perturbations are tiny, they can be accurately measured from radar ranging data from the Earth to spacecraft on the surface of Mars, such as the Viking landers.
Apart from a few asteroids whose densities have been investigated,[4] one has to resort to enlightened guesswork.
Krasinsky et al. gives calculations for the mean densities of C, S, and M class asteroids as 1.38, 2.71, and 5.32 g/cm3.
For irregularly shaped bodies, the surface gravity will differ appreciably with location.
On a rotating body, the apparent weight experienced by an object on the surface is reduced by the centripetal force, when one is away from the poles.
The negative sign indicates that it acts in the opposite direction to the gravitational acceleration
The simplest method which gives sensible results is to assume the asteroid behaves as a greybody in equilibrium with the incident solar radiation.
Then, its mean temperature is obtained by equating the mean incident and radiated heat power.
, the asteroid is spherical, it is on a circular orbit, and that the Sun's energy output is isotropic.
Using a greybody version of the Stefan–Boltzmann law, the radiated power (from the entire spherical surface of the asteroid) is: where
A rough estimate of the maximum temperature can be obtained by assuming that when the Sun is overhead, the surface is in thermal equilibrium with the instantaneous solar radiation.
Infra-red observations are commonly combined with albedo to measure the temperature more directly.
and the asteroid's surface temperature will change in a regular way depending on its distance from the Sun.
If the day of the relevant observations is known, the distance from the Sun on that day can be obtained from sources such as the NASA orbit calculator,[10] and corresponding temperature estimates at perihelion, aphelion, etc.
(the proportion of total incoming power reflected, taking into account all directions), while the IRAS and MSX albedo data that is available for asteroids gives only the geometric albedo
which characterises only the strength of light reflected back to the source (the Sun).
While these two albedos are correlated, the numerical factor between them depends in a very nontrivial way on the surface properties.
Actual measurements of Bond albedo are not forthcoming for most asteroids because they require measurements from high phase angles that can only be acquired by spacecraft that pass near or beyond the asteroid belt.
For want of a better alternative for most asteroids, the best that can be done is to assume that the two albedos are equal, while keeping in mind the inherent inaccuracy present in the resulting temperature values.
The typical inaccuracy in calculated temperature from this source alone is found to be about 2%.
Data from the IRAS minor planet survey[11] or the Midcourse Space Experiment (MSX) minor planet survey[12] is the usual source of the diameter.
Rotation period is usually taken from lightcurve parameters at the PDS.
[12] Astronomical albedos are usually given by either the IRAS or MSX minor planet surveys.
of a spherically symmetric body, the escape velocity is: Some other information for large numbers of asteroids can be found at the Planetary Data System Small Bodies Node.
[14] Up-to-date information on pole orientation of several dozen asteroids is provided by Doc.