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Chapter 11 TECHNOLOGIES for HUMAN EXPLORATION

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Chapter 11 TECHNOLOGIES for HUMAN EXPLORATION Chapter 11 TECHNOLOGIES FOR HUMAN EXPLORATION Scott Lowther lays out the possibilities for Mars mission heavy lift boosters. Pathfinder Chief Scientist Matt Golombek explains his mission’s results. 502 MAR 98-051 MARSSAT: ASSURED COMMUNICATION WITH MARS Copyright © 1992, 1997, 1998 by Thomas Gangale, 430 Pinewood Drive, San Rafael, California 94903. E-mail: [email protected]. The Martian Time Web Site: www.jps.net/gangale/mars/calendar.htm. Thomas Gangale In the past, robotic missions to Mars have accepted the inevitable communications blackout that occurs when Mars is in solar conjunction. This interruption, which lasts several weeks, would seem to be unacceptable during a human Mars mission. This paper proposes a relay satellite as a means of maintaining vital communications links during conjunction, and explores candidate orbits for such a spacecraft. The basic approach to system design is to minimize size, weight, and power of spaceborne elements of the communications system, since it is more economical to compensate with large, heavy, and power-consuming elements on Earth. Ideally, it is the Earth-to-Mars link which should drive the overall system design, with the Earth-to-relay and Mars-to-relay links impacting system design as little as possible. This ideal is approached by minimizing the length of the link between the relay spacecraft and Mars. An orbit whose period is one Martian year, but whose eccentricity and inclination both differ from that of Mars, assures communications between Earth and Mars during conjunction while minimizing the length of the link between the communications satellite and the Mars mission. 1.0 STATEMENT OF NEED At the end of April of this year, the Deep Space Network (DSN) lost contact with the Mars Global Surveyor spacecraft in orbit around Mars. An entire month passed before communications were reestablished with the vehicle. This hiatus was not caused by any hardware or software failure; rather, it was an inevitable consequence of planetary orbital mechanics. During this period in May 1998, Mars passed behind the Sun as seen from Earth, a planetary configuration known as solar conjunction. Fortunately, links were re-established at the end of May, and Mars Global Surveyor is continuing its mission. During the nearly three Mars years over which the Viking 1 Lander operated, there were three such lengthy communication blackouts due to solar conjunctions. This Mars-Sun-Earth alignment occurs at 780-day intervals on the average, varying from 766 to 803 days due principally to the eccentricity of Mars’s orbit. Various mission profiles have been proposed for human expeditions to Mars. Among these is the conjunction class mission, which utilizes minimum-energy Hohmann trajectories both to and from Mars. However, the use of Hohmann transfers on both the outbound and inbound legs of the mission requires roughly a 500-sol layover on Mars to await the proper planetary configuration for the return flight. As can be seen in Figure 1, solar conjunction occurs near the midpoint of this 500-sol stay on Mars. Although the use of the conjunction class scenario on initial human Mars missions is an issue yet to be decided, it is likely that this type of mission profile will be flown at some point in a human Mars exploration program, not only because it is the most propellant-efficient profile, but also because it maximizes stay time on Mars while minimizing travel time to and from Mars with respect to other mission profiles using the same class of propulsion systems. 503 Figure 1 Until the Tracking and Data Relay Satellite System was completed in the 1980s, human missions historically endured short-duration interruptions in communications. These blackouts would last for a few minutes either while passing between ground stations or during reentry. There were communications losses of as much as an hour during the Apollo program when vehicles passed behind the Moon. But it is hard to imagine that a communications outage on the order of one month will be tolerated on a human Mars mission. Now, the specific duration of the interruption during a solar conjunction depends on several factors, such as the amount of link margin designed into the communications system, as well as the minimum data rate that is acceptable from a mission standpoint. Still, regardless of how much robustness is designed into the communications links, the minimum blackout period will always be on the order of weeks, not the few minutes or hours that have been experienced on past human space missions. Communications interruption by the Sun will become even more of a problem as human Mars operations build up to permanent bases. The conjunction blackout will of course hold true for a Mars base as well as for a conjunction-class mission, but furthermore, such a base will also have to contend with oppositions, which are, from the Martian point of view, inferior conjunctions of Earth, i.e., when Earth passes in front of the Sun as seen from Mars. During oppositions, Earth will be able to receive signals from Mars, but Earth’s transmissions to Mars will be drowned out by the Sun’s radio noise. 504 Some means of assuring uninterrupted communications between Earth and Mars will have to be included in human Mars program planning. To satisfy this need, I propose a relay satellite concept which I call MARSSAT. 2.0 PRELIMINARY SYSTEM TRADE-OFFS Figure 2 Figure 2 depicts the connectivity for a Mars communications system designed to circumvent the solar conjunction blackout, including Mars surface stations, low Mars orbit vehicles, a constellation of communications relays in Mars orbit, MARSSAT, and the DSN. The basic considerations in sizing a satellite communications system are embodied in the link budget, which includes the following parameters: On a conceptual level, spacecraft weight (and therefore cost) is driven by the size of the antenna and the power requirements of the transmitter and receiver. These in turn are driven by the range over which the link is required to operate. Exact numbers for the size, weight, and power of specific mission elements are a subject for detailed system engineering, and thus beyond the scope of this presentation. However, the basic approach is to minimize size, weight, and power of spaceborne elements of the communications system, since it is more economical to compensate with large, heavy, and power-consuming elements on Earth. 505 Ideally, it is the Earth-to-Mars link which should drive the overall system design, with the Earth-to-MARSSAT and Mars-to-MARSSAT links impacting system design as little as possible, since these alternative links represent additional costs. Assuming that the range of the Earth-to-MARSSAT link will be on the same order as that of the Earth-to-Mars link, the MARSSAT communications equipment need only be comparable to the equipment on the near- Mars elements. The stressing case, however, will be the Mars-to-MARSSAT link, for size, weight, and power will be at a premium on all spaceborne mission elements. It is for this link that the system must be optimized, since in this case, there are no large ground stations to figure into the link budget. To achieve minimum system impact, the maximum range over which the Mars-to-MARSSAT link must operate should therefore be as short as possible. At the same time, however, a minimum angular separation between the MARSSAT spacecraft and Mars, as seen from Earth during solar conjunction, must be maintained in order to reduce the impact of solar noise on the links. In general terms, this identifies the trade space to be investigated in the system engineering process. For Mars Global Surveyor, mission planners and telemetry engineers defined the solar communications outage as occurring when the Sun-Earth-Mars angle was within seven degrees, and they planned for a loss of signal from 30 April to 26 May during the 1998 conjunction. It was also noted that a quiescent sun could have reduced this angle to five degrees. Several factors could reduce this minimum solar separation angle for human missions. The link throughput requirements might be limited to voice communication and only that telemetry necessary to the safety of the crew, while science data taken during the conjunction period could be recorded in situ and transmitted to Earth after the conjunction. Also, the use of a laser communications system might offer advantages over a conventional radio system. In my preliminary analysis, required minimum solar separation angles between two and three degrees were assumed. These may be unrealistically small angles from the mission operations perspective, but they provided me with some stressing cases for evaluating orbit stability. This leads to my next point. Another consideration in the location of the relay spacecraft is the stability of its position relative to Mars over the design life of the satellite, since the more stable the orbit, the less fuel must be expended to maintain the spacecraft’s position — yet another factor affecting design weight. In this paper, the eight-Mars-year (fifteen-Earth-year) cycle over which the relative positions of Earth and Mars more or less repeat is defined as the mission life of the MARSSAT vehicle. Thus the spacecraft would be required to provide communications through seven solar conjunctions. 3.0 ORBIT SIMULATION AND EVALUATION CRITERIA 3.1 MARSSAT Simulation To investigate candidate orbits, I developed a simulation which provides two split-screen graphic displays. Display 1 (Figure 6) consists of the familiar solar system “overhead” view from above (north of) the ecliptic, and the in-the-ecliptic view along the vector of the vernal equinox. Display 2 (Figure 7) provides two views edge-on to the ecliptic plane.
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