Let us consider the orbital problem in which a particle is subject to the force (per unit mass)
![](//static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20180308104520432-0399:S025292110007305X:S025292110007305X_inline01.gif?pub-status=live)
The force consists of the Newtonian two body attraction term and a drag part which is linear in both components of velocity (radial vr and transverse vt). Depending on a physical interpretation of the parameters μ and α, as well as on the particular choice of the dimensionless constant γ, the model (1) can match various two body problems with dissipation. They include the classical Poynting-Robertson effect (Robertson, 1936) with γ = 1, Poynting’s (1903) version with γ = ½ and the two body drag cases recently studied by Mittleman and Jezewski (1982) and by Mavraganis and Michalakis (1994) under the name of Danby’s drag.