For the majority of numbered asteroids, almost nothing is known apart from a few physical parameters and orbital elements and some physical characteristics are often only estimated. The physical data is determined by making certain standard assumptions.
Contents
Dimensions
Data from the IRAS minor planet survey or the Midcourse Space Experiment (MSX) minor planet survey (available at the Planetary Data System Small Bodies Node (PDS)) is the usual source of the diameter.
For many asteroids, lightcurve analysis provides estimates of pole direction and diameter ratios. Pre-1995 estimates collected by Per Magnusson are tabulated in the PDS, with the most reliable data being the syntheses labeled in the data tables as "Synth". 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.
These data can be used to obtain a better estimate of dimensions. A body's dimensions are usually given as a tri-axial ellipsoid, the axes of which are listed in decreasing order as a×b×c. If we have the diameter ratios μ = a/b, ν = b/c from lightcurves, and an IRAS mean diameter d, one sets the geometric mean of the diameters
Mass
Barring detailed mass determinations, the mass M can be estimated from the diameter and (assumed) density values ρ worked out as below.
Such estimates can be indicated as approximate by use of a tilde "~". Besides these "guesstimates", masses can be obtained for the larger asteroids by solving for the perturbations they cause in each other's orbits, or when the asteroid has an orbiting companion of known orbital radius. The masses of the largest asterois 1 Ceres, 2 Pallas, and 4 Vesta can also be obtained from perturbations of Mars. 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.
Density
Apart from a few asteroids whose densities have been investigated, one has to resort to enlightened guesswork. See Carry for a summary.
For many asteroids a value of ρ~2 g/cm3 has been assumed.
However, density depends on the asteroid's spectral type. 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. (Here "C" included Tholen classes C, D, P, T, B, G, and F, while "S" included Tholen classes S, K, Q, V, R, A, and E). Assuming these values (rather than the present ~2 g/cm3) is a better guess.
Spherical body
For a spherical body, the gravitational acceleration at the surface (g), is given by
Where G = 6.6742×10−11 m3s−2kg−1 is the gravitational constant, M is the mass of the body, and r its radius.
Irregular body
For irregularly shaped bodies, the surface gravity will differ appreciably with location. The above formula then is only an approximation, as the calculations become more involved. The value of g at surface points closer to the center of mass is usually somewhat greater than at surface points farther out.
Centripetal force
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 centripetall acceleration experienced at a latitude θ is
where T is the rotation period in seconds, r is the equatorial radius, and θ is the latitude. Its magnitude is maximized when one is at the equator, and sinθ=1. The negative sign indicates that it acts in the opposite direction to the gravitational acceleration g.
The effective acceleration is
Close binaries
If the body in question is a member of a close binary with components of comparable mass, the effect of the second body may also be non-negligible.
Escape velocity
For surface gravity g and radius r of a spherically symmetric body, the escape velocity is:
Rotation period
Rotation period is usually taken from lightcurve parameters at the PDS.
Spectral class
Spectral class is usually taken from the Tholen classification at the PDS.
Absolute magnitude
Absolute magnitude is usually given by the IRAS minor planet survey or the MSX minor planet survey (available at the PDS).
Albedo
Usually given by the IRAS minor planet survey or the MSX minor planet survey (available at the PDS). These are geometric albedos. If there is no IRAS/MSX data a rough average of 0.1 can be used.
Mean
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 then obtained by equating the mean incident and radiated heat power. The total incident power is:
where
Using a greybody version of the Stefan-Boltzmann law, the radiated power (from the entire spherical surface of the asteroid) is:
where
The standard value of
While this method gives a fairly good estimate of the average surface temperature, the local temperature varies greatly, as is typical for bodies without atmospheres.
Maximum
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. This gives average "sub-solar" temperature of
where
At perihelion, the radiation is maximised, and
where
Temperature measurements and regular temperature variations
Infra-red observations are commonly combined with albedo to measure the temperature more directly. For example, L.F.Lim et al. [Icarus, Vo. 173, 385 (2005)] does this for 29 asteroids. However, it should be pointed out that these are measurements for a particular observing day, and that the asteroid's surface temperature will change in a regular way depending on its distance from the Sun. From the Stefan-Boltzmann calculation above,
where
Albedo inaccuracy problem
There is a snag when using these expressions to estimate the temperature of a particular asteroid. The calculation requires the Bond albedo A (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 p 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 the majority of asteroids because they require measurements from high phase angles that can only be acquired by spacecraft that pass near or beyond the asteroid belt. Some complicated modelling of surface and thermal properties can lead to estimates of the Bond albedo given the geometric one, but this far is beyond the scope of a quick estimate for these articles. It can be obtained for some asteroids from scientific publications.
For want of a better alternative for most asteroids, the best that can be done here is to assume that these two albedos are equal, but keep in mind that there is an inherent inaccuracy in the resulting temperature values.
How large is this inaccuracy?
A glance at the examples in this table shows that for bodies in the asteroid albedo range, the typical difference between Bond and geometric albedo is 20% or less, with either quantity capable of being larger. Since the calculated temperature varies as (1-A)1/4, the dependence is fairly weak for typical asteroid A≈p values of 0.05−0.3.
The typical inaccuracy in calculated temperature from this source alone is then found to be about 2%. This translates to an uncertainty of about ±5 K for maximum temperatures.
Other common data
Some other information for large numbers of asteroids can be found at the Planetary Data System Small Bodies Node. Up-to-date information on pole orientation of several dozen asteroids is provided by Doc. Mikko Kaasalainen, and can be used to determine axial tilt.
Another source of useful information is NASA's orbit calculator.