The dynamic method is a procedure for the determination of the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

es of asteroids. The procedure gets its name from its use of the Newtonian laws of the ''dynamics,'' or motion, of asteroids as they move around the Solar System. The procedure works by taking multiple position measurements to determine the gravitational deflection caused when two or more asteroids move past each other. The method relies on the fact that the large number of known asteroids means they will occasionally move past one another at very close distances. If at least one of the two interacting bodies is large enough, its gravitational influence on the other can reveal its mass. The accuracy of the determined mass is limited by the precision and timing of the appropriate

astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. His ...

observations being made to determine the gravitational deflection caused by a given interaction. Because the method relies on detecting the amount of gravitational deflection induced during an interaction, the procedure works best for objects which will produce a large deflection in their interactions with other objects. This means that the procedure works best for large objects, but it can also be effectively applied to objects which have repeated close interactions with each other such as when the two objects are in

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...

with one another. Regardless of the mass of the interacting objects, the amount of deflection will be greater if the objects approach nearer to each other and it will also be greater if the objects pass slowly, allowing more time for gravity to perturb the orbits of the two objects. For large enough asteroids this distance can be as large as ~0.1 AU, for less massive asteroids the conditions of the interaction would need to be correspondingly better.

Mathematical analysis

The simplest way to describe the deflection of the asteroids is in the case where one object is significantly more massive than the other. In this case the equations of motion are the same as for that of Rutherford scattering between oppositely charged objects (so that the force is attractive rather than repulsive). When rewritten in the more familiar notation used in celestial mechanics deflection angle can be related to the eccentricity of the hyperbolic orbit of the smaller object relative to the larger one by the following formula: :

\sin \left( \frac \right) = \frac

Here

\Theta

is the angle between the

asymptote In analytic geometry, an asymptote () of a curve is a line such that the distance between the curve and the line approaches zero as one or both of the ''x'' or ''y'' coordinates tends to infinity. In projective geometry and related context ...

s of the

hyperbolic orbit In astrodynamics or celestial mechanics, a hyperbolic trajectory or hyperbolic orbit is the trajectory of any object around a central body with more than enough speed to escape the central object's gravitational pull. The name derives from the fa ...

of the small object relative to the large one, and

\epsilon

is the eccentricity of this orbit (which must be greater than 1 for a hyperbolic orbit). A more sophisticated description using

matrices Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

can be achieved by separating the observed objects position on the sky as a function of time into the sum of two components: one which is a result of the relative motion of the objects themselves, and the other the motion induced by the gravitational influence of the two bodies. The relative contributions of the two terms in the best fit of this equation onto the actual observations of the objects yields the objects masses.

References

{{Asteroids Asteroids Astrometry Celestial mechanics