Engineering Notes Mars Surface Exploration Caching from an Orbiting Logistics Depot

E XPLORATION missions on Earth have used the principle of caching for centuries. Caching involves prepositioning discrete quantities of supplies at specific locations along a route in order to lighten the burden or extend the range of an exploring party. Historical missions that made use of caching on Earth are the Lewis and Clark expedition in 1803–1806 [1] and the ill-fated Imperial Trans-Antarctic Expedition of 1914–1917 [2]. Caching may play an important role in the future of manned planetary exploration as well. Without it, exploration sorties are inherently limited to a specific distance from a base or landing location given by the type of transportation vehicles used, the size of the crew, the mix and quantity of consumables, operational rules, and the nature of the terrain. For manned lunar missions, for example, this distance has been estimated at 3 km for exploration on foot, 5–15 km with unpressurized rovers, and up to 30 kmwith pressurized rovers [3]. In part, exploration range is determined by the quantity of consumables that can be carried along, limiting the amount of useful exploration that can be done at distances greater than 30 km from the landing site or base. Similar issues exist for manned Mars exploration. This Note explores the concept of storing caches in a logistics depot on orbit and deliberately deploying them to the surface to extend human exploration range.Wewill useMars as the motivating context, but the idea may apply to other situations as well. We first introduce the concept and then perform initial sizing of such a depot. This Note does not explore the full tradespace of orbital depots and caching architectures. Caching in space exploration can be implemented as two different architectures: direct entry from inspace trajectory and orbital entry from a low Mars orbit. While the direct entry does not contain the Martian orbit insertion maneuver and can be more efficient in terms of required V, this Note only considers the orbital entry because of the following two reasons. First, the short time gap between deployment decision and actual landing of the supply unit provides meaningful flexibility to operate on orbit. Secondly, advanced propulsion systems such as a nuclear electric propulsion system (NEP) and solar electric propulsion can significantly reduce the cost resulting from the orbit insertion, and the efficiency gap gets very small. Let us consider an orbiting logistics depot that is composed of a cluster of supply units deployed to aMartian orbit. Each prepackaged supply unit contains supply items (fuel and consumables) that allow extending the range of humans exploring the surface of Mars. Figure 1 shows the orbiting depot concept and its concept of operations. Typically, one or more surface vehicles start exploration from a base and return to the base before all fuel and consumables (e.g., fuel, food, water, and oxygen) have been expended. This is the case of an unassisted surface route, yielding a total distanceD. In this case, the amount of fuel and consumables that can be used during a single route is determined by the capacity of the surface exploration vehicle. Now suppose that there is a logistics depot in Martian orbit and it is possible to command the depot to drop a supply unit (pod) filled with a known mix and quantity of consumables corresponding approximately to a vehicle’s capacity. If the supply unit lands at such a location that the surface exploration vehicle can reach the unit before it runs out of consumables, the vehicle’s range can effectively be doubled to 2D. This is the case of the depot-assisted surface route shown in Fig. 1 (right) along with a simulated pod landing error ellipse. In the following sections we discuss the most important design decisions for such an orbiting depot, including orbit selection, individual supply unit design, and depot operations. These calculations are meant to establish a realistic “strawman” for the concept. Studies regarding detailed design in the context of specific surface exploration campaign strategies are left for future work.