Targeting the geysers on Enceladus by viffing descent through the icy plumes

摘要

The ice plumes emanating from the geyser vents at Enceladus' south pole offer an enticing route for direct access to acquire pristine astrobiological samples. Unlike traditional landing scenarios where the location of the landing target is known a priori, we propose that one or more penetrators explore the plume environment via lateral maneuvers during the descent to Enceladus' surface to infer the loca-tion of promising landing sites by autonomously measuring the plume ice/vapour concentration at multiple locations within the plume. This offers the prospect of targeting the vent sources of the plumes for direct access to subsurface material prior to its ejection. We exam-ine four types of "viffing" (vector-in-forward flight) descent profiles and the impulsive velocity increment (△V) required for the lateral maneuvers: (i) a ballistic descent trajectory with a minimal △V cost represents the reference trajectory that implements no lateral search maneuvers; (ⅱ) a planar descent with a decaying lateral oscillation which has an intermediate △V cost and a small search area; (ⅲ) a nested series of decreasing sized box trajectories offering intermediate △V costs of varying degrees depending on the specific parameters; and, (ⅳ) a quasi-spiral trajectory of decreasing radius which imposes the highest △V cost but with the largest lateral search area. Com-putations and simulations determined that the proposed viffing maneuvers are feasible and permit accurate targeting of Enceladian gey-ser vents at modest cost in △V. We also briefly assessed the problem of mapping the Enceladian plume during the descent, which will ultimately be necessary for viffing.