Introduction, evolving - Steve Suess, 19 April 1999.
For use by the NASA Interstellar Probe Science and Technology Definition Team
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Take a look at the Solar Sail Project the Planetary Society is going to launch in 2001.

The Interstellar Probe

- what follow is...
An Introduction to, and Rationale for The Interstellar Probe

The Mission

  • A small spacecraft will be sent out of the solar system to take data on the nearby interstellar medium and heliosphere- interstellar medium interactions. It will be sent in the direction which the solar system is moving through the local insterstellar medium - as shown in this schematic. 

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  • The Interstellar  Probes a small step towards the stars. It will leave the heliosphere at least five times faster than any preceding man-made object. 

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  • The purpose is to explore the interstellar medium - that part of our galaxy that lies between the stars - that is closest to the Sun. 

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  • To reach the interstellar medium as soon as possible, the Interstellar Probe will go in the direction that the boundary between the heliosphere and the interstellar medium is closest to the Sun. It will be propelled either by a solar sail or by nuclear-electric propulsion. 
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    Figure I.01 (click image for .5Mb jpeg)

    The Heliosphere

  • The heliosphere is that part of our galaxy which is swept by the solar wind.

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  • The schematic above shows the heliosphere, with the locations of the planets shown for scale.

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  • The heliopause in this picture marks the outer boundary of the heliosphere and it is the goal of The Interstellar Probe to go far beyond this boundary.



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    The Interstellar Probe Topics (click on it to visit that topic):

    Scientific Objectives

     

      1. Explore the nature of the interstellar medium and its implications for the origin and evolution of matter in our Galaxy and the Universe

      2. Explore the influence of the interstellar medium on the solar system, its dynamics, and its evolution

      3. Explore the impact of the solar system on the interstellar medium as an example of the interaction of a stellar system with its environment

      4. Explore the outer Solar System in search of clues to its origin, and to the nature of other planetary systems


     

    Scientific Objectives Expanded and Explained:

    1. Explore the nature of the interstellar medium and its implications for the origin and evolution of matter in our Galaxy and the Universe

  • Measure the interstellar magnetic field and the density, temperature, and ionization state of the interstellar gas and their variations over a variety of temporal and spatial scales

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  • Determine the interstellar spectra of galactic cosmic rays and their contribution to the ionization, heating, and dynamics of the interstellar medium.

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  • Determine the mass and velocity distributions and the composition of interstellar dust

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  • Measure the elemental and isotopic composition of interstellar plasma, neutral gas, unmodulated galactic cosmic rays, and their interaction products, and study their implications for galactic evolution and the origin of the solar system

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  • Measure cosmic ray electrons and positrons and study their implications for galactic gamma ray production, recent nucleosynthesis, and interstellar radio emission

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  • Determine the abundance of deuterium and He-3 in the local interstellar medium as constraints on Big Bang and galactic chemical evolution theories

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  • Observe the cosmological infrared background radiation without contamination by emission from the Zodiacal dust

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    2. Explore the influence of the interstellar medium on the solar system, its dynamics, and its evolution

  • Investigate the location and motion of the termination shock and heliopause as it responds to solar variations and interstellar pressure

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  • Investigate the structure of the termination shock, including the roles of thermal plasma, waves, pickup ions, and anomalous cosmic rays in determining the shock structure.

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  • Search for evidence of a heliospheric bow shock

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  • Determine the global properties and dynamics of the heliosphere with both in situ measurements and imaging.

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  • Determine the properties of interstellar gas and dust that penetrate into the heliosphere

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  • Study the deflection of the subsonic solar wind and interstellar plasma flows in the heliosheath, including the effects of thermal pressure gradients, magnetic fields, and the neutral interstellar gas.

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  • Determine the influence of the interstellar medium on solar wind dynamics

    • 3. Explore the impact of the solar system on the interstellar medium as an example of the interaction of a stellar system with its environment

  • Explore in situ the structure of the hydrogen wall and relate its properties to observations of similar structures and winds observed in neighboring stellar systems

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  • Determine the scale of the heliosphere and the extent of its influence on the temperature, ionization state, and energetic particle environment of the local solar neighborhood.

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  • Study the structure and dynamics of the outer heliosphere as an example of the interaction between a star and its environment.

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  • Study, in situ, the acceleration of anomalous cosmic rays and other particle species at the termination shock and elsewhere.

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    4. Explore the outer Solar System in search of clues to its origin, and to the nature of other planetary systems.

  • Determine the composition and the mass and orbital distributions of dust in the outer solar system and identify creation and destruction mechanisms

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  • Explore the structure of the Zodiacal dust cloud and its implications for infrared observations of the Galaxy, for the evolution of the solar system, and for studies of planets around other stars.

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  • Search for dust structures associated with planets, asteroids, comets, and the Kuiper Belt, and determine the probable creation mechanisms

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  • Determine the radial distribution of small Kuiper Belt objects

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  • Determine the nature of organic matter in the outer solar system and the interstellar medium

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    Phenomenology of the Heliosphere

     

    This is a  picture of how the heliosphere might look, similar to that above. There are many specialized terms that can be defined with this picture.

  • Termination shock: The solar wind streams away from the Sun supersonically, at a speed of more than 450 kms/. When it runs into the interstellar medium, the solar wind slows down by passing through a standing shock wave called the “termination shock”.

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  • Bow shock: The solar system is moving through the local interstellar medium supersonically. So, the interstellar gas also passes through a shock wave as it is diverted to flow around the heliosphere. 

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    Heliopause: The boundary between solar wind plasma and interstellar plasma is called the “heliopause”. 

    Inner heliosheath: The region between the termination shock and the heliopause. 
    Figure I.02
    (click on image for 248Kb jpeg)
  • Outer heliosheath: The region between the heliopause and the bow shock.

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  • Heliotail: The inner heliosheath downstream from the direction the heliosphere is moving through the interstellar medium.

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  • Interstellar neutrals: Interstellar matter is not fully ionized. In fact, a larger portion is neutral matter in the form of atoms, molecules, and dust. Neutral matter can pass right through the bow shock, heliopause, and termination shock. However, the atoms are mostly eventually ionized by charge exchange with solar wind ions and my photoionization from solar radiation. Interstellar dust is, of course, eventually melted, sublimated, or evaporated by solar radiation as it falls into the Sun.

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    Very little is really known about the local interstellar medium. For example;

    Figure I.03
    (click on image for 196Kb jpeg)
    		 One of the many unknowns is the strength of the magnetic field in the interstellar medium. If the field is very weak, then it will have little or no effect on the flow properties. 

    If the field is strong, it will influence how the heliosphere looks. An example of this is the shape of the termination shock and the heliopause. 

    The pictures above are both for a strong insterstellar magnetic field and the resulting termination shock is approximately spherical. This happens because the external flow speed against the heliosphere is less than the Alfven speed. 

    If the interstellar magnetic field is weak, then the termination shock can be greatly elongated in the direction of the heliotail because the speed of the flow speed against the heliosphere may be much more than the Alfven speed. This is depicted in the picture shown here. 

    Not only can the termination shock greatly elongated downstream, the inner heliosheath would be relatively thin. 
    This will be one of the measurement objectives of The Interstellar Probe. 

    Using remote sensing of:

    and direct observations of: and any other techniques that we can imagine, a great deal is already known about the LISM. However, as indicated above beside Figure I.03, there are huge holes in our knowledge. Moreover, these holes are directly related to our ability to understand the origins of planets and the solar system and the formation of our own galaxy - the Milky Way. It also means we cannot make any further progress in analyzing these problems and their relationship to the interaction between the heliosphere and the LISM without an in situ mission into the local interstellar medium. The Interstellar Probe is defined to meet objectives which will further our understanding of these processes.

       The Observing Objectives described in the next section address what the Interstellar Probe will do in more detail.

    Go to Observing Objectives
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