Abstract

Project Ares Acquire is an unmanned Mars sample return mission that utilizes propellant manufactured in situ from the Martian atmosphere for the return trip. In order to meet the 200 million dollar budgetary constraint for this mission, Project Ares Acquire will rely primarily on existing or near term technology and hardware for the construction of its components. For the purposes of this study, four mission alternatives are considered:

1. Seed Hydrogen - Methane Produced (SHMP), in which liquid hydrogen is imported from Earth and processed through a Sabatier reactor with Martian carbon dioxide to produce methane and oxygen propellant.
2. Water-Sabatier (WATS) which uses the Water Vapor Adsorption Reactor (WAVAR) , a concept study developed at the University of Washington, to extract water vapor from the Martian atmosphere. The water is then electrolyzed and the hydrogen processed through a Sabatier reactor with carbon dioxide to produce methane-oxygen propellant.
3. Methane Transported-Oxygen Produced (MTOP) in which all the necessary fuel (liquid methane) is imported from Earth while only the oxidizer (liquid oxygen) is produced on Mars, and
4. the more traditional Terrestrial Propellant (TERP) option is included as a baseline. Here, all propellant for the Mars retro-burn, and for the return trip, is imported from Earth.

Ares Acquire launches during the March 2001 launch opportunity on top of a Delta II 7925 launch vehicle into low Earth orbit (LEO). The spacecraft consists of a Mars Landing Vehicle (MLV) with a Mars Transfer Cruise Stage (MTCS), a Mars Ascent Vehicle (MAV), and a Sample Return Capsule (SRC) with an Earth Transfer Cruise Stage (ETCS). After achieving LEO, the PAM-D third stage provides the energy required to inject the Ares Acquire spacecraft onto a 6 month conjunction class trajectory to Mars. The MTCS, based on JPL's pathfinder cruise stage, provides control during the transfer to Mars.

Surface operations begin with deployment of the solar arrays and charging of the battery systems.
Atmospheric samples are taken after exhaust gases are fully vented and before the propellant production is started. The rover is deployed to pick up samples of the Martian surface outside the exhaust plume contamination area. Sample are selected for acquisition by controllers on the Earth in near-real time. Additionally, a core drilling device attached to a Remote Manipulator Arm (RMA) will take a sample of the Martian crust at the landing site. A basic meteorological package will collect data throughout the surface stay.

The sample acquisition portion of the surface operations will be completed within ten days, and the sample acquisition device (planetary rover) will begin an extended autonomous mission. Once all sample cylinders are stowed in the sample containment canister and the containment canister is locked in the SRC, the propellant production from the Martian atmosphere is started to ensure enough propellant is produced during the rest of the approximately 570 day stay on Mars to give the MAV a DV of 6100 m/s (6300 m/s for the TERP MAV) for the return trip to Earth.

After the surface stay, the MAV launches from the MLV using the indigenously manufactured propellant and begins the return to Earth.

The SRC and the Earth Transfer Cruise Stage (ETCS) are separated from the MAV after it reaches burnout. The spacecraft assumes entry attitude 30 minutes before Earth atmospheric entry. Aerobraking reduces the SRC entry speed at Earth to approximately 50 m/s and a parachute is deployed at an altitude of 12 km. The parachute reduces the speed of the SRC to 10.6 m/s where it is recovered via an aerosnatch maneuver at 7 km.

The SHMP option presents problems in the transporting of liquid hydrogen from Earth to Mars and storing it on-the planet until propellant production is complete, yet has the advantage of a more mature propellant production technology as compared to MTOP. The MTOP scenario has advantages in that it avoids the thermal difficulties associated with the long-term storage of the hydrogen feedstock. The zirconia electrolyzers, however, represent less mature technology. The transported methane increases the Earth launch and Mars landing mass.

Scenario mass and power comparison

                           TERP         SHMP         MTOP       WATS   
Earth Launch Mass  (kg)    1700         830          800        730    
Mars Landed Mass   (kg)    1140         520          490        430    
Average Daiy Power (W)     50           148          227        290    

The WATS, while incorporating a proven technology (Sabatier reactors) with a new concept (WAVAR) , is advantageous in that it is a completely ISRU mission with no propellant or feedstock imported from Earth. It does, however, require the largest power plant of the four mission scenarios. The TERP mission is designed to fulfill the sample requirements, but does not demonstrate the feasibility of ISRU technology. The mass estimate for the TERP is almost twice that of the ISRU options, exceeding the capability of the Delta II launch vehicle.

Over the course of this study, two trends became obvious. First, ISRU missions offer dramatic decreases in Earth launch and Mars landing mass over the more traditional terrestrial propellant options. All three ISRU scenarios have Earth launch masses in th 700-800 kg range. The terrestrial propellant option has an Earth launch mass close to 1500 kg, well beyond the capability of the Delta II launch vehicle. Second, considerable research and development still needs to be focused on the propellant production plants to ensure reliable, autonomous operation. ISRU is the imperative for the future exploration of Mars, but the propellant production technology needs to move into the prototype stage in order to identify the technological hurdles and to begin more detailed mission planning.