A ballistic missile on a suborbital trajectory and a spacecraft on an orbital trajectory are both following paths governed by the same equations.

The specific parameters differ — the energy levels, the altitudes, the time scales — but the mathematical framework is the same.

An engineer or mathematician who understood missile trajectories deeply was also, by the nature of the mathematics, well-positioned to work on spacecraft trajectories.

Mary Ross was working on missile-related problems during the period when the American missile program was one of the highest national security priorities in the country.

The specific nature of that work is not fully documented, for the reasons already described.