Expert: Courtney Seligman - 6/15/2009

Question
How are the weights and sizes of planets like uranus, mercury and earth and asteroids determined?

Answer
Size Estimates:

(1) TYPICALLY BEST -- observations of how large the object appears (in a telescope image) are combined with distance measurements, to yield an actual size.

(2) EVEN BETTER WHEN POSSIBLE -- If the object happens to pass in front of a distant star, it will cast a shadow on the Earth which is the same size and shape as the object (as seen from the Earth). By measuring how long it takes the shadow to pass the observer, the size of the object can be calculated. This is very accurate, but depends on happenstance occultations, which are rare.

(3) OK -- The brightness or faintness of the object is measured, and compared to a calculation of how bright or faint an object of a given size would be, at the object's location. This provides an upper and lower limit for size, based on upper and lower estimates of how reflective the object is (or not). Analyzing the spectrum of reflected light can give estimates of the reflectivity, but the sizes aren't as accurate as (1) or (2).

(4) BETTER THAN OK -- The infrared radiation of the object is measured, as well as the visible light reflected by it. The infrared radiation is proportional to that fraction of the light absorbed by the object, while the visible light is proportional to that fraction not absorbed. Between the two, the reflectivity can be more accurately determined, yielding a more accurate value than for (3).

Mass estimates:

(1) BEST -- Orbital motions of satellites (even asteroids often have satellites) can be combined with Kepler's Third Law to determine an accurate mass.

(2) BETTER -- If a spacecraft goes by the object, tracking how the spacecraft's motion is affected by the object's gravity can yield extremely accurate mass values.

(3) OK -- Given the size of an object, a range of mass estimates can be made, depending upon guesstimates of the materials it is made of. These guesses are based on typical characteristics for objects of similar size found in that region of space, and comparison of their reflection properties with those of the object being studied. For instance, some asteroids reflect light like metallic meteorites, and are probably made of dense metals, which would make them heavy in comparison to their size. Other asteroids reflect light like carbonaceous meteorites, which are of relatively low density, which would make them light in comparison to their size.

Other methods of measuring/estimating sizes and masses can be used in special circumstances, but the vast majority are determined by one of the techniques listed above.