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Mission Technology

There are two ideas for how the Interstellar Probe might get to 250 AU quickly enough to make the mission worth while. These are:

  • Use a solar sail, which would be driven by solar photon pressure.

  • Use a nuclear-electric propulsion (NEP) system in which, e.g., Xenon ions would be accelerated by an electric field powered with a nuclear reactor.

  • The solar sail technology has been selected for further study, in preference to NEP due to concerns that the particles and fields instruments would be unable to operate in the vicinity of the argon or xenon ions being expelled from the thruster. 


    Further discussion: 

    Technology Requirements

  • Assume the use of solar sail propulsion
  • Low-mass/power optimized instrumentation
  • Light-weight, low-cost spacecraft


    Measurement Strategy

  • Measure, in situ, the properties and composition of interstellar plasma and neutrals, low energy cosmic rays, and interstellar dust
  • Determine the structure and dynamics of the heliosphere with in situ measurements and global imaging
  • Map the infrared emission of the zodiacal dust cloud, measure in situ the distribution of interplanetary dust, and determine the radial distribution of small Kuiper Belt objects
  • Mission Description

  • Send a spacecraft to at least 200 AU in 15 years or less, using solar-sail propulsion.
  • Deploy the sail shortly after launch and use it to swing in to ~0.25 AU, where photon pressure can accelerate the spacecraft towards the upwind direction
  • After jettisoning the sail at ~10 AU, coast to >200 AU, exploring the Kuiper Belt, the boundaries of the heliosphere, and the properties of the nearby interstellar medium.
  • Further Explanation

       In the view discussed in the Introduction, solar wind flows outward to the termination shock, surrounded at somewhat greater distance by the heliopause.  The bubble of solar wind shields the inner heliosphere from the plasma, energetic particles, dust, and fields of the interstellar medium; to observe these directly it will be necessary to get outside the heliopause. Several recent estimates place the distance to the termination shock at ~80 to 90 AU, with the heliopause at ~120 to 150 AU. 

       The Interstellar Probe Mission would be designed to cross the solar wind termination shock and heliopause and make a significant penetration into nearby interstellar space in the direction the heliosphere is moving through the local interstellar medium (LISM). The principal scientific objectives of the mission would be to (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, and (4) Explore the outer solar system in search of clues to its origin, and to the nature of other planetary systems. I comprehensive set of observing objectives has been prepared which meets these general scientific objective. 

       To achieve these broad, interdisciplinary objectives, the strawman scientific payload includes an advanced set of miniaturized, low-power instruments specifically designed to make comprehensive, in situ studies of the plasma, energetic particles, fields, and dust in the outer heliosphere and nearby interstellar medium. Included would be studies of solar wind, neutral species, and pickup ions in the interplanetary medium, in situ studies of particle acceleration at the termination shock, which is a model for other astrophysical shocks, studies of the penetration of interstellar gas and dust into the heliosphere, and exploration of the hydrogen wall apparently responsible for low frequency radio emissions.  Infrared and in situ studies of the zodiacal dust cloud would have important implications for the evolution of the solar system, for studies of planets around other stars, and for studies of the cosmic infrared background radiation. The large-scale structure and dynamics of the heliosphere would be studied in situ and with energetic-neutral-particle imaging.  The interaction of our heliosphere with the interstellar medium is a model for similar interactions occurring around other stellar systems. 

       Once beyond the heliopause Interstellar Probe would make direct measurements of properties of interstellar gas, dust, and the interstellar magnetic field, and of low-energy cosmic rays unaffected by the processes of “solar modulation” that shield the inner heliosphere. Direct measurements would be made of the composition of interstellar dust, and of the elemental and isotopic composition of the ionized and neutral components of the interstellar gas and low-energy particle components, including key isotopes such as 2H, 3He, 13C, and heavier species.  The possibility of identifying organic matter in the outer solar system and interstellar medium is under investigation.

    To accomplish its objectives Interstellar Probe should acquire data out to a distance of at least 200 AU, with a goal of continuing on to ~400 AU. Spacecraft velocities of ~10 to 20 AU/year (3 to 6 times that of Voyager 1) can be achieved if solar sail propulsion is developed, with potential benefits for many other missions.  Interstellar Probe can serve as the first step in a more ambitious program to explore the outer solar system and interstellar space. 


    Propulsion Technologies - Solar Sails

    The Solar Sail Mission might be called:

    (click here for a 129Kb jpeg of the Great Republic)
    Figure MT.01

    There are various ideas for sail technology, depending on how the sail is deployed and how its shape is maintained against solar photon pressure.

  • The most robust sail technology is to have the sail supported by a system of guy wires and struts. This technique will be used on the planned "GEOSTORM" spacecraft which will be put out in front of the Earth to detect coronal mass ejections and shocks from the Sun before they strike the Earth.



    Figure MT.02

  • Another concept that has been developed is a "gyro-sail" which uses a combination of extendable arms and centrifugal force from rotation of the spacecraft. This structure is also robust.



    Figure MT.03

  • A centrifugally deployed and maintained sail is being proposed for the Interstellar Probe. The reason for this is that a much larger sail is required for Interstellar Probe than for GEOSTORM. The resulting support structure would be far too massive if a design like that used in GEOSTORM were employed. The centrifugally deployed sail has a relatively negligible mass in support structure.



    Figure MT.04

  • A centrifugally deployed sail has already been test-deployed on a module returning to the Earth from the Russian Mir Observatory. The photograph below shows that the sail section separations were extremely large in order to avoid adjacent panels from interfering with each others deployment.



    Figure MT.05

    Propulsion Technologies - Nuclear Electric Propulsion

    Nuclear Electric Propulsion offers a viable alternative to solar sails in spite of concerns about effect on particles & fields measurements.

  • This image shows a concept for a NEP mission in which a nuclear reactor is combined with large thermal radiators along the spine of the spacecraft and thrusters on side-extensions. It could reduce the impact on particles and fields measurements if the exhaust plume cleared the instrument package and here the instrument package is placed at the front of the spacecraft.

  • An alternative is that the thrusters work normal to the spacecraft spar between the payload and the reactor. The radiators would have to be turned by 90 degrees but this option would also reduce interference between the thruster plume and payload for at least some portions of the mission.



    Figure MT.06

    Present Activities (Spring 1999)

       Presently, studies are being carried out on costs and technical feasibility of the solar sail mission. Both sail and NEP technologies will be carried along in basic mission planning, but the baseline mission is presently planned using a solar sail.

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