Abstract
Spacecraft and natural objects orbiting an active comet are perturbed by gas drag from the coma. These gases expand radially at about 0.5 km/s, much faster than orbital velocities that are on the order of meters per second. The coma has complex gas distributions and is difficult to model. Accelerations from gas drag can be on the same order of gravity and are currently poorly understood. Semi-analytical solutions for the evolution of the Keplerian orbital elements of a spacecraft orbiting a comet using simplified drag and coma models are derived using a Fourier series expansion in the argument of latitude. It is found that the mean element evolution is only dependent on the zeroth- and first-order terms of the Fourier series expansion. For an arbitrary, inverse-square, radial, perturbing force, there are no frozen orbits; however, the argument of pericenter has a stable equilibrium and an unstable equilibrium and the angular momentum vector of the orbit is constant. Furthermore, the radius of the orbit at two specific angles relative to the ascending node is preserved. The evolution of the orbit is governed by the argument of pericenter, resulting in orientations that raise and lower the radius of pericenter and implying safe and unsafe orbit orientations for spacecraft operations.
Highlights
Comets are valuable targets for science and exploration missions
Difficulty in accurately modeling the drag forces resulting from the coma resulted in significant operational costs and effort for Rosetta, where uncertainty in the coma drag required multiple sequences to be developed in tandem. (Bielsa and Herfort 2014; Vallat et al 2017) Both EPOXI at 103P/ Hartley 2 (A’Hearn et al 2011; Kelley et al 2013) and Rosetta at CG (Rotundi et al 2015; Davidsson et al 2015; Agarwal et al 2016) observed gravitationally bound grains orbiting the nucleus
This paper presented the derivation of differential equations for the orbit-averaged Keplerian orbital elements under the perturbation of drag about an active comet
Summary
Comets are valuable targets for science and exploration missions. It is widely accepted that comets are among the most chemically pristine remnants of the early Solar system, allowing glimpses into the chemical and physical properties of the solar nebula at the time and location of their formation. Comets contain vast amounts of easy-to-access water ice that can be leveraged in future manned and robotic mission architectures through in situ resource utilization (ISRU). In order to access the scientific and economic benefits of comets, one must be able to conduct precise operations near and on their surfaces. Several comet missions have been flown in the past, though all but one have been flyby missions where the spacecraft was only in the vicinity of the comet for several hours and was never gravitationally bound, e.g., Giotto, Deep Space 1, Deep Impact, EPOXI, Stardust,
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