The Interstellar Heliopause Probe: Heliospheric Boundary Explorer Mission to the Interstellar Medium

After the exciting in situ observations of the termination shock and the entry of the Voyager 1 spacecraft into the heliosheath (see Fig. 1), there is a growing awareness of the significance of the physics of the outer heliosphere. Its understanding helps to clarify the structure of our immediate interstellar neighborhood (e.g. Breitschwerdt and de Avillez 2006), contributes to the clarification of fundamental astrophysical processes like the acceleration of charged particles at a stellar wind termination shock (e.g. Florinski and Zank 2006) and beyond, and also sheds light on the question to what extent interstellar-terrestrial relations are important for the environment of and on the Earth (Frisch and Slavin; Kayser-Threde 2004; Scherer et al. 2006). A summary of open issues in heliospheric physics is given in Wimmer-Schweingruber (2007) in these proceedings.

Schematic view of the heliosphere depicting termination shock, heliopause, hydrogen wall, and bow shock

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In order to explore the boundary region of the heliosphere, it is necessary to send a spacecraft to perform advanced in situ measurements particularly in the heliosheath, i.e. the region between the solar wind termination shock, and the heliopause, as well as in the (very) local interstellar medium. The interstellar heliopause probe (IHP) will provide the first comprehensive measurements of key parameters of the local interstellar environment such as its composition, state, and magnetic field. Together with an accurate determination of the state of the heliospheric plasma across the heliosphere, these quantities are crucial to our understanding how the heliosphere, and, much more generally, astrospheres, are formed and how they react to varying interstellar environments.

Remarkably, the better we understand the physical processes at work on our Sun, the more we view our sun as a typical stellar object. The processes that give rise to our solar wind are clearly at work at other stars, possibly without exception. We are beginning to understand not only how the Sun heats its corona and powers the solar wind, but how these processes relate quite generally to stellar coronae and stellar winds. The heliosphere inflated by our solar wind is the direct analog to astrospheres inflated by the stellar winds of other stars. Current theoretical and modeling understanding of the heliosphere and its boundaries and interfaces is severely hampered by a lack of high-quality in situ measurements. The only current data come from the ageing Voyager spacecraft, both launched in the 1970s. Their power is likely to last another 10 years which will allow them to reach a heliocentric distance not exceeding 130–140 AU. Comparison with Fig. 2 shows the likeliness that these spacecraft will not manage to measure the outer boundary regions of the heliosphere, the heliopause, hydrogen wall, and bow shock and will not be able to determine in situ the properties of the local interstellar medium at all. To explore these regions is the primary science goal of IHP/HEX.

Radial profiles of protons (left) and neutral hydrogen (right) in the upwind (red), the crosswind (black), and the downwind (blue) direction. The vertical dashed lines indicate the main discovery region of IHP, the vertical solid line denotes the maximum distance that the Voyager spacecraft will be able to investigate before their power runs out

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IHP addresses the three core science goals summarized in Table 1.

IHP/HEX currently baselines the use of solar sailing technology to reach the outer solar system in 25 years. The spacecraft itself is very light, the sail structure module as well. Launch mass is on the order of 500–550 kg, required to reach Earth escape orbit. This is a modest requirement and allows us to use a modest launch vehicle. It can be shown that an initial positive hyperbolic excess velocity, C3, does not significantly reduce the interplanetary transfer time out to 200 AU and consequently, all calculations presented here have assumed an insertion into interplanetary orbit with essentially zero excess hyperbolic escape velocity and optimization of the orbit was performed from there. Nevertheless, orbit and mission duration optimization with a finite C3 is possible, but has not yet been studied by us.

Since the solar radiation pressure declines as 1/r ² with distance from the Sun, the propulsive capabilities of a solar sail are more and more limited as proceeding into the outer regions of the solar system. Consequently, continuous outward spiraling to reach the outer solar system is not practicable. However, a sailcraft may gain an enormous amount of orbital energy when first approaching the Sun before proceeding to the outer solar system. By performing such a ‘Solar Photonic Assist’ the transfer orbit about the Sun can turn hyperbolic, allowing reasonable trip times to the outer planets and targets beyond without applying gravity assists. The trajectory design foresees an extended time in the inner solar system prior to reaching hyperbolic energy after the second solar photonic assist. This time spent in the inner solar system which can reach 4–7 years is balanced by the higher escape speed which can be achieved by this strategy.

The trajectory shown in Fig. 3 is optimized for minimum flight time to 200 AU. With the heliocentric escape velocity for this trajectory it is possible to reach 200 AU in a flight time of about 25 years. The transfer time can be reduced by improving the overall mass-to-area ratio, thus increasing the sailcraft acceleration. Here, an acceleration of 1 mm/s² at 1 AU was used. This value is based on a 2 μm polyimid foil (see Sect. 4). Such foils are currently available on roll widths of 1.5–2 m and improvements towards a 1 μm foils in sufficiently large size to assemble a sail are highly probable. Having a second perihelion passage enhances the overall mission robustness because it allows to perform orbit corrections after the initial perihelion passage.

Solar sailing cruise orbit. First aphelion at 1.05 AU is reached after about 150 days, first perihelion at 0.51 AU after ∼220 days, second aphelion at 5.76 AU after 3.8 years, second perihelion at 0.25 AU after 6.6 years. Sail is jettisoned after 6.7 years, after which the full science mission begins

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4.1 Proposed Payload Elements

In order to achieve the scientific objectives of the IHP, the payload includes a magnetometer, plasma analyzer and plasma wave experiment, an energetic particle detector, and a neutral atom detector. The key resource requirements and characteristics of the instruments are summarized in Table 2.

IHP/HEX will require one spacecraft to reach 200 AU within 25 years. We have baselined a solar sailcraft (4) which needs to withstand the tremendous thermal stresses associated with the mission profile (see Sect. 3) and deliver science in the outer heliosphere. The sail structure is composed of three major elements, the deployable booms, a central Deployment Module, and the sail film segments. The four supporting CFRP booms are unrolled from the central Deployment Module, and the four folded, triangular sail film segments are released from sail containers which are part of the central Deployment Module. The deployable booms, developed by the DLR Institute of Structural Mechanics, combine high strength and stiffness with extremely low density and can be stored within a very tight volume. The booms consist of two question-mark-shaped laminated shells which are bonded at the edges to form a tubular shape. They can be pressed flat around a central hub for storage, and uncoil from the central hub during deployment. Once free of the Deployment Module, the booms resume their original tubular shape with high bending stiffness. The concept has successfully been demonstrated in a ground test campaign. In stowed configuration all four booms are co-coiled on a central hub. A mechanism allows to simultaneously deploying all booms in a controllable way. Besides the compartment housing the coiled booms and the associated deployment mechanisms the Deployment Module has four individual sail containers which house the folded sails for the launch phase prior to deployment in space. The baseline for the sail material is a thin (1–2 μm thick) Polyimid film coated on the front side with Aluminium and a back-side Cr coating.

Science configuration of the IHP spacecraft

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We envisage the spacecraft to employ slow spin stabilization for both its configurations (sail, science). The spin mode will enable the design of a simpler and lighter GNC system for the sail and will enable scanning of the space environment for the science campaigns. The torque and angular momentum needs of the science platform will be less when the sail is ejected. The biggest challenge is mass and compactness. Development of a sailcraft attitude control system (ACS) needs to be carefully planned because of the large inertia of the sail. A two-degrees-of-freedom gimbaled 20 m long central mast is foreseen to hold the IHP platform at roughly this distance from the sail. This configuration allows us to move the center of gravity relative to the center of (photon) pressure, thus allowing solar-sail navigation. The science mode employs spin stabilization as well. Miniature Indium (In) FEPPp in thrusters were selected as the lowest mass option. In FEEP emitters, unlike most ion engines, ions are directly extracted from the liquid phase. The thruster can accelerate a large number of different liquid metals: Indium is usually selected for its high atomic weight, low ionization potential, relatively low melting point (m.p. = 156°C), and good wetting capabilities on the emitter substrate.

5.1 Estimated Overall Resources (Mass and Power)

The mission places a high demand on the power subsystem which needs to supply the spacecraft with on-board power of a solar range of 0.25  to 200 AU. First analyses in power consumption have shown that the power subsystem requirements are 120 avg./145 W peak (EOL min) and 150/190 W (Design/BOL). BOL peak power requirements are entirely driven by the deployment of the solar sails. Following previous trade-offs with other propulsion/power systems, RPSs are the most promising option as they also have the necessary flight heritage. However using RPSs is a very sensitive and challenging issue due to limited availability, long lead times and political issues related to RPS purchase. Specific power (W/kg) is the key factor in RPSs. The key assumption in designing the power subsystem is the 8 W/kg specific power figure (2nd generation RPS) and a degradation rate of 1 W/year. Thermal power must be radiated, hence the large fins and the heat shield between the RPS and the spacecraft. The Multi-Mission RPS (MMRTG) under development for Prometheus is based on the GPHS module, with a desired power output of 100 W estimated from a wide ranging mission analysis. The goal of the MMRTG program is to provide sufficient power for many missions while reducing the carried Plutonium. For the heliopause mission two RPSs will be required, placed opposite to the highly integrated payload suite. More details on the resource allocation can be found in (Kayser-Threde 2004).