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Solar System Galactic Frontier

this web site in Belarusian language

this web site in Swedish language

invited talk (15 min.) at the Fall Meeting, AGU, December 2012

Energetic Neutral Atom Imaging: The Next Step  –  ENA imaging

Energetic neutral atom imaging of space plasma (review) (pdf)
by Mike Gruntman in Review of Scientific Instruments, 1997

Solar System Galactic Frontier


Voyager 1 and Voyager 2 spacecraft reached and crossed the termination shock in December 2004 (at 94 AU from the Sun) and September 2007 (at 84 AU), respectively. (1 AU is one astronomical unit, the mean distance between the Sun and Earth.) Voyager 1 crossed the heliopause and plunged into the surrounding interstellar medium (Local Interstellar Medium, or LISM) in the summer of 2012 at the heliocentric distance 121 AU. Voyager 2 crossed the heliopause and reached LISM in November 2018. The figure shows positions of Voyagers in the early 2000s.

The Voyager 1 spacecraft leaves the solar system with the escape velocity about 17.1 km/sec, or 3.6 AU/year. (Voyager 2 is slower with the velocity 3.1 AU/yr.) Both spacecraft will be turned off around 2025 because of gradual reduction in power producing capacity of their power source, radioisotope thermoelectric generators (RTGs).

Are the Voyager spacecraft properly instrumented to explore the galactic frontier of the solar system? No. Voyager 1 and 2 spacecraft were designed primarily for study of giant planets during flybys. Their instruments do measure however some very important physical properties of the region of the interaction between the solar wind and the interstellar medium, the heliospheric interface. The detailed in-situ study of the processes at and beyond the Solar system galactic (interstellar) frontier will be performed in the future by the Interstellar Probe. This mission has been considered for the last 40+ years. (See Instrumentation for interstellar exploration and references therein.) Depending on programmatic priorities, NASA will lauch the Interstellar Probe during the next 10–20 years. The mission goal is to reach the "pristine," "unperturbed" (by Sun's presence) interstellar medium (300–500 AU from the sun) surrounding the solar system and explore it in-situ.

In reality, the neutral solar wind would "contaminate" (disturb) the surrounding interstellar medium for several hundreds astronomical units aroujnd the Sun, as was first discovered in 1982. So, the interstellar medium is affected by the Sun at large distances.

Voyagers have answered some important questions about the physics and processes at the heliospheric boundary and established ground thruth facts by in-situ measurmeents. The Interstellar Boundary Explorer (IBEX), launched in 2008, mapped the entire galactic frontier (interstellar boundary) of the Solar system in fluxes of energetic neutral atoms (ENA). It also discovered a new still not fully understood and explained feature, called the "ribbon."

ENA imaging have emerged as a new power experimental technique to remotely probe distant space plasmas (see a tutorial and a review article). Actually, an instrument, somewhat primitive, to detect ENAs originatibng at the interstellar boundary and reaching the Earth was built in ... 1980s (!) but was never flown.

An article in the Journal of Geophysical Research (JGR) leading to the IBEX mission described the concept of remote mapping of the heliospheric inrterface region. NASA program officers specifically pointed to a role of this "scientific publication in JGR" in the debrief on the mission win in 2003.

A complementary to ENA imaging technique has been advanced for several years that will map the interstellar boundaty in estreme ultraviolet (EUV). Major instrumental features have been successfully demionstrated and the concept of imaging ("heliopause mapping") formulated. The experimental approach will also allow one to map, remotely, the three-dimensional flow patter of the solar wind (see also another publication).

Then the Interstellar Probe will penetrate the surrounding galactic medium and performes in-situ and remote observations there.

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