Titan Explorer: the Next Step in the Exploration of a Mysterious World
Total Page：16
File Type：pdf, Size：1020Kb
Titan Explorer: The Next Step in the Exploration of a Mysterious World Principal Investigator: Dr. Joel S. Levine, NASA Langley Research Center Study Manager: Mr. Henry S. Wright, NASA Langley Research Center Cover Image: Image Credit: NASA/JPL/Space Science Institute Image Caption: This natural color image shows Titan's upper atmosphere -- an active place where methane molecules are being broken apart by solar ultraviolet light and the byproducts combine to form compounds like ethane and acetylene. The haze preferentially scatters blue and ultraviolet wavelengths of light, making its complex layered structure more easily visible at the shorter wavelengths used in this image. Inset Image 1: Image Credit: NASA/JPL/University of Arizona Image Caption: The visual and infrared mapping spectrometer instrument onboard Cassini has found an unusual bright, red spot on Titan. This dramatic color (but not true color) image was taken during the April 16, 2005, encounter with Titan. North is to the right. In the center it shows the dark lanes of the "H"-shaped feature discovered from Earth and first seen by Cassini July 2004 shortly after it arrived in the Saturn system Inset Image 2: Image Credit: NASA/JPL/ESA/University of Arizona Image Caption: This image was returned yesterday, January 14, 2005, by the European Space Agency's Huygens probe during its successful descent to land on Titan. This is the colored view, following processing to add reflection spectra data, and gives a better indication of the actual color of the surface. TITAN EXPLORER Titan Explorer: The Next Step in the Exploration of a Mysterious World Final Report for NASA Vision Mission Study per NRA-03-OSS-01 Submitted by: Principal Investigator: Dr. Joel S. Levine, NASA Langley Research Center Study Lead: Henry S. Wright, NASA Langley Research Center Date Submitted: June 10, 2005 - 1 - TITAN EXPLORER This page intentionally left blank - 2 - TITAN EXPLORER 0. Front Matter 0.1 Executive Summary Our knowledge and understanding of Titan, Saturn’s largest moon, have increased significantly as a result of measurements obtained from the Cassini spacecraft following its orbital insertion around Saturn on June 30, 2004 and even more recently with the measurements obtained during the descent of the Huygens probe through the atmosphere and onto the surface of Titan on January 14, 2005. The Titan Explorer Mission discussed in this report is the next step in the exploration of this mysterious world. The Titan Explorer Mission consists of a Titan Orbiter and a Titan Airship that traverses the atmosphere of Titan and can land on its surface. One of the fundamental questions in all of science concerns the origin and evolution of life and the occurrence of life beyond Earth. In the search for life in the Solar System, Titan holds a very unique position. Titan (radius: 2575 km) is slightly larger than Mercury (radius: 2439 km) and smaller than Mars (radius: 3393 km). Like the terrestrial planets, Titan has a solid surface and a density that suggests it is composed of a mixture of rock and ice in almost equal amounts. Titan may provide the details to explain how life formed on Earth very early in its history, shortly after the Earth formed 4.6 billion years ago. The evolution of the Earth’s atmosphere and plate tectonics have erased any early record of the primitive pre-biological Earth (the Earth’s geological record begins with the oldest rocks on our planet, dated to be about 3.5 billion years old, about a billion years after the Earth formed). The appearance on Earth of the first biological or living system and the subsequent evolution of biological systems, were preceded by the process of prebiotic chemistry or “chemical evolution.” Chemical evolution is the formation of the complex organic compounds, the precursors of living system. It is generally believed, that on Earth, chemical evolution occurred very soon after the Earth and its atmosphere formed. It is further believed that the gases in the early atmosphere, including nitrogen, methane, water vapor, molecular hydrogen, etc. were the “raw” materials that chemically formed the complex organic molecules, the precursors for the first living system. The successful entry and descent of the Titan Huygens probe through the atmosphere and landing on the surface of Titan on January 14, 2005, provided new information about the composition and structure of the atmosphere and the nature and characteristics of the surface of Titan. Titan’s atmosphere may hold answers to chemical evolution on the early Earth (references 1, 2, and 3). Titan is surrounded by a thick, opaque orange-colored atmosphere with a surface pressure of 1.5 bars-about 50% greater than the Earth’s atmosphere. The stability of methane in Titan’s atmosphere is puzzling, since the atmospheric lifetime of methane is controlled by its destruction by solar ultraviolet radiation, which is short on cosmic timescales (about 107 years). Hence, atmospheric methane on Titan appears to be buffered or re-supplied by a possible surface reservoir. The cloud and haze are sufficiently thick that ultraviolet radiation cannot penetrate to the troposphere. Photochemical and chemical reactions initiated by methane (and nitrogen) leads to the production of numerous hydrocarbons of increasing molecular complexity, beginning with ethane, hydrogen cyanide, etc., and leading to complex organic compounds such as purines, pyrimidines, and aldehydes, believed to be the chemical precursors of the first living systems on Earth (references 1, 2, and 3). The Titan Explorer mission focuses on the following scientific questions and required instrumentation to answer these questions: 1. What is the chemical composition of the atmosphere, including the trace gases? 2. What is the isotopic ratio of the gases in the atmosphere? 3. What pre-biological chemistry is occurring in the atmosphere/surface of Titan today and what is its relevance to the origin of life on Earth? 4. What is the nature, origin, and composition of the clouds and haze layers? - 3 - TITAN EXPLORER 5. What is the nature and composition of the surface? 6. Are there oceans of liquid hydrocarbons on the surface of Titan? 7. What is the nature of the meteorology and dynamics of the atmosphere? 8. What processes control the meteorology and circulation of the atmosphere? 9. What is the nature of the hydrocarbon “hydrological cycle” on Titan? 10. What are the rates of escape of atomic and molecular hydrogen from the upper atmosphere of Titan and what impact does this escape have on atmospheric chemistry? 11. How does the atmosphere of Titan interact with the solar wind and Saturn itself? 12. How have the atmosphere and surface of Titan evolved over its history? To answer these questions, instruments on the Titan Orbiter include: 1. Solar occultation spectrometer to measure atmospheriuc composition and isotopic ratios. 2. Radar mapper to measure the nature of the surface. 3. Magnetometer to search both a planetary dipole field and surface magnetism. 4. Ultraviolet spectrometer to measure the escape of gases from Titan’s upper atmosphere. 5. Visual and infrared mapping spectrometer to measure cloud and haze layers and the nature of the surface. Instruments on the Titan Airship include: 1. Imager to measure cloud and haze layers and the nature of the surface. 2. Mass spectrometer to measure atmospheric composition and isotopic ratios. 3. Haze and cloud particle detector to measure aerosol abundance and characterization. 4. Spectrometer to determine the nature and composition of the surface. 5. Sun-seeking spectrometer to measure the opacity of the atmosphere. To answer the science questions, a blend of legacy and developmental instrumentation was considered for the mission. To ensure mission fidelity, mass, power, volume, data rates, stability, and other accommodation requirements for each instrument was factored into the mission implementation. Table 1 provides the details and key attributes of the Titan Explorer Mission. Table 1: Titan Explorer Mission Architecture Parameter Value Launch date April 23, 2018 Launch vehicle Delta IV-Heavy Departure C3 12 km2/sec2 Earth gravity assist date February 15, 2020 Interplanetary propulsion type Solar Electric Propulsion (5 Next Generation NSTAR engines) Solar Electric Propulsion Module separation date, solar July 30, 2021; 5.2 AU distance Titan Orbiter – orbit insertion strategy Aerocapture Titan Airship – entry strategy Direct, ballistic Titan airship separation date March 10, 2024 Titan arrival date (airship and orbiter) March 17, 2024 Airship operational lifetime 4 months after entry Orbiter operational lifetime 40 months after Titan Orbit Insertion - 4 - TITAN EXPLORER An end to end assessment of the systems, their performance, and overall integration has been performed, including a validation of the launch vehicle to provide the requisite launch mass capability for the injection C3. Table 2 summarizes the mass for the entire launch stack. The analysis also considered heritage-derived contingencies to characterize the maximum expected mass of the systems in addition to the current best estimate (CBE), or mass estimates without contingency. All margins were set at the launch vehicle level rather than allocating the margins down to the flight systems. For a study at this stage of maturity, it is immaterial if the margins are at the flight system level or at the launch vehicle as long as the margins are of sufficiently large value. The Titan Explorer mission study had a mass margin target of greater than 25%. There are two distinct ways of computing mass margin; method 1) a method used in recent Science Mission Directorate competed missions (Mars Scout, Discovery, New Frontiers), and method 2) a method used in the JPL Design Principles and also in the NASA Langley Research Center Spacecraft Design Guide. Margins using both methods have been provided for completeness. Table 2: Titan Explorer Mission Mass Summary Element CBE Mass Contingency Max.