Kathleen Howell is an American scientist and aerospace engineer. Her contributions to the theory of dynamical systems have been applied to spacecraft trajectory design, which led to the use of near-rectilinear halo orbit (NRHO) in various NASA space missions.
Unlike an ordinary flat orbit, an NRHO can be slightly warped. Further, it stands on end, almost perpendicular to an ordinary orbit – hence “near rectilinear”.
NASA have decided that an NRHO would be an ideal place to put the Lunar Orbital Platform-Gateway, which is a planned way station for future human flights to the Moon and eventually Mars. The plan is for the Gateway’s circuit to pass tight over the Moon’s north pole at high speed and more slowly below the south pole, because of the greater distance from the moon.
“Imagine moving your hand in circles, as if washing a window, while you walk forward. Except you’re making hand circles around the moon while walking around Earth.” – Bloomberg
Although this orbit seems to be an ordinary circuit of the moon, it’s actually part of a family of orbits, centred on an empty point, called L2 (or Lagrange Point 2). Here, around 45,000 miles beyond the far side of the Moon, the gravitational forces of the Earth and the moon are in balance with the centrifugal forces on the spacecraft.
Although we are taught in school that orbits must be around something, it is quite possible to orbit around nothing, so long as that ‘nothing’ is a Lagrange point.
“It is elegant and very rich. All the forces come together to produce an unexpected path through space” – Howell
Howell’s work build on an 18th century discovery, by Euler, who theorised that for any pair of orbiting bodies, there are 3 points in space where gravitational and centrifugal forces balance precisely. In 1772, Lagrange found two more such spots. All five are now known as Lagrange points.
In 2017 Kathleen Howell was elected to National Academy of Engineering “for contributions in dynamical systems theory and invariant manifolds culminating in optimal interplanetary trajectories and the Interplanetary Superhighway“.