A comparison of methods for the Mars Sample Return mission

Abstract

This paper analyzes the three major approaches to the Mars Sample Return mission; direct return using propellants transported from Earth, Mars orbit or interplanetary rendezvous with all propellants transported from Earth, and direct return from the Martian surface using in-situ propellants. It is found that the direct return with terrestrial propellant fails on the basis of cost, while the orbital rendezvous approaches fail on the basis of risk. In contrast, the approach employing direct return utilizing indigenous propellants appears to be attractive on both a cost and risk basis. In addition, the in-situ propellant technology is found to offer maximum benefits for follow-on missions, including robotic Mars hopper science missions and human exploration missions. Introduction: Three Approaches to the Mars Sample Return Mission The Holy Grail of the robotic Mars exploration program is the Mars Sample Return (MSR) mission. In contrast to the limited capability offered by investigations performed on Mars, a sample returned to Earth could be subjected to thousands of different types of tests an investigations. Thus for example, while the results of the Viking life detection experiments are still regarded by some as contradictory and ambiguous, the return of such samples to terrestrial labs would have enabled a battery of tests and examinations that would have left no doubt in interpretation of results. For these reasons, among others, NASA's solar system exploration branch has penciled in a Mars Sample Return Mission (MSR) mission for 2005. There are fundamentally three ways this might be done: The Brute Force (BF) method, the Orbital Rendezvous (OR) method, and the InSitu Propellant Production (ISPP) method. All three of these approaches have been the subject of considerable study for some time. The Brute Force Approach The first, and conceptually the simplest of the MSR mission strategies, is the method. In this case, a launch vehicle in the class of a Titan IV is used to deliver to the surface of Mars a very large payload consisting of a Mars Ascent Vehicle (MAV), massing perhaps 500 kg, completely fueled for an ascent from Mars and flight back to Earth. The lander also has on board a robotic rover which is dispatched to wander about under human operator control and collect samples of geologic interest. The samples are then loaded aboard a capsule on the rocket vehicle. When the launch window from Mars back to Earth opens up, about 1.5 years after arrival, the MAV ascends and flies back to Earth. Upon approach to Earth, the capsule separates from the rest of the vehicle and performs a high-speed re-entry, much in the manner of an Apollo manned capsule. Depending upon design, the capsule may be decelerated by a parachute or simply use a crushable material like balsa wood or styrofoam to cushion the landing shock, when it hits the targeted desert landing area. This Brute Force mission is pretty simple conceptually, but the problem with it is that is likely to be very expensive, as robotic explorations missions go. The Titan IV needed costs NASA $400 million, and the large lander needed to carry to fully fueled ascent vehicle is also likely to be very costly. Thus, while studied numerous times in the past as the baseline for the MSR mission, the brute force approach has always led to cost estimates that have made the mission a non-starter. A possible method to reduce the cost of the Brute Force approach is to use a Russian Proton as the launch vehicle. There is a technical problem with this, as the large payload required by the BF mission requires a large aeroshield, that may not be able to fit within the 3. 6 meter diameter Proton fairing. (the Titan IV fairing has a inner diameter of 4.6 meters). Moreover, recent history has demonstrated a willingness of Russian authori t ies offering transportat ion for interplanetary missions to engage in bait and switch tactics that may make the prospective savings resulting from Proton use quite illusory. Therefore, in an effort to reduce costs, several other methods have also been studied.