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Chapter 9. ENA Experiment That Never Flew

GAS Experiment in in the "heroic" ENA era (pp. 201-203)

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Our instrument never made it to space. Sensors GAS-2, GAS-3, and control unit GAS-E had been built by the early 1990s. MPAe could have provided the GAS-1 sensor whenever needed. The space mission Relikt-2 (Fig. 9.5), finally rescheduled for launch in 1993, never took off the launch pad as the entire country disintegrated and the space program nose-dived.

The GAS experiment was the first comprehensive dedicated space experiment of the "heroic" ENA era with the first-generation neutral-atom instruments to detect the neutral solar wind and heliospheric ENAs from the interstellar boundary of the solar system. I was glad that we briefly described this work at the First COSPAR [8] Colloquium "Physics of the Outer Heliosphere" in 1989 (Chapter 10). If it were not for this talk and associated publication [9] in the colloquium proceedings, the experiment would have been forgotten by now.

It is interesting that nobody has ever definitely measured the neutral solar wind fluxes and characterized their properties to this day. An instrument on the IMAGE mission observed enhancements in a response of a wide field-of-view sensor that might have been caused by ENAs in the solar wind. It could not unambiguously attribute the signal and determine the NSW properties in detail. [10]

Launched in late 2008, Interstellar Boundary Explorer did detect heliospheric ENAs from the interstellar boundary of the solar system and probed its properties. [11] NASA specifically selected and launched this Earth-orbiting mission to discover and remotely map the solar system frontier. Our early work at IKI began this pursuit in the 1970s and contributed to the development of the concept of such a mission and ENA instrumentation.

The road to IBEX was long. Heliosphere imaging in ENA fluxes was first included as part of a proposal for a larger Interstellar Pathfinder space mission that emerged from an initiative [12] started in 1998 (Chapter 10). The first proposal combined ENA imaging of the interstellar boundary with detailed measurements of the isotopic and elemental composition of the pickup ions in the solar wind. A team led by Principal Investigator George Gloeckler twice proposed Interstellar Pathfinder as Medium Explorers [13] in 1998 and 2001 (Fig. 9.6), without success. Then in 2003 NASA selected a Small Explorer (SMEX) mission, Interstellar Boundary Explorer, Principal Investigator Dave McComas, which focused on detecting heliospheric ENAs.

The Moscow Physical-Technical Institute stood out from other institutions of higher learning by its unique system of education and exceptional quality. In the late 1930s, several leading Soviet scientists published a letter [3] in Pravda (Fig. 1.4). The daily newspaper Pravda, the main organ of the Central Committee of the Communist Party of the Soviet Union, CPSU, was the most authoritative official publication in the country.

The road to IBEX was long. Heliosphere imaging in ENA fluxes was first included as part of a proposal for a larger Interstellar Pathfinder space mission that emerged from an initiative12 started in 1998 (Chapter 10). The first proposal combined ENA imaging of the interstellar boundary with detailed measurements of the isotopic and elemental composition of the pickup ions in the solar wind. A team led by Principal Investigator George Gloeckler twice proposed Interstellar Pathfinder as Medium Explorers13 in 1998 and 2001 (Fig. 9.6), without success. Then in 2003 NASA selected a Small Explorer (SMEX) mission, Interstellar Boundary Explorer, Principal Investigator Dave McComas, which focused on detecting heliospheric ENAs.

The debrief of the successful IBEX proposal by the NASA program officers specifically mentioned that "the boundary between the solar wind and the local interstellar medium was one of the last unexplored regions of the heliosphere," that "[i]n the scientific merit, the proposal received very, very high marks" and there was "high probability of major ground-breaking science from this mission." They specifically pointed out that the proposal risk was "mitigated ... by the results of detailed modeling presented in the proposal and in the scientific publication in JGR." [14] This was the reference to our article in the Journal of Geophysical Research published in 2001 [15], with its origins going all the way back to the late 1970s and my 1979 List.

Fig. 9.5. A mockup of the microwave instrument panel, dated 1993, of the Relikt-2 spacecraft in the IKI museum. In the second half of the 1980s, the institute planned to install sensors of the GAS instrument on this spacecraft. Photograph (2018) courtesy of Mike Gruntman.

Fig. 9.6. Space mission proposals leading to the Interstellar Boundary Explorer, IBEX, launched in 2008. The mission would successfully map the interstellar boundary of the solar system (Fig. 3.8). Left: Interstellar Pathfinder. A Mission to the Inner Edge of the Interstellar Medium (Principal Investigator George Gloeckler), 1998. Middle: Interstellar Pathfinder. A Mission to Explore the Inner Edge of the Interstellar Medium (PI George Gloeckler), 2001. Right: Interstellar Boundary Explorer. Imaging the Edge of our Solar System and Beyond, (PI David McComas), 2003. From collection of Mike Gruntman.

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