Monday, January 30, 2012

A Realistic Interstellar Explorer

1.0 Introduction

"For more than 20 years, an “Interstellar Precursor Mission” has been discussed as a high priority for our understanding (1) the interstellar medium and its implications for the origin and evolution of matter in the Galaxy, (2) the structure of the heliosphere and its interaction with the interstellar environment, and (3) fundamental astrophysical processes that can be sampled in situ.

The chief difficulty with actually carrying out such a mission is the need for reaching significant penetration into the interstellar medium (~1000 Astronomical Units (AU1)) within the working lifetime of the initiators (50 years). During the last two years there has been renewed interest in actually sending a probe to another star system - a "grand challenge" for NASA - and the idea of a precursor mission has been renewed as a beginning step in a roadmap to achieve this goal.

Ongoing studies now being carried out by the Jet Propulsion Laboratory (JPL) in conjunction with a NASA-selected Interstellar Probe Science and Technology Definition Team (IPSTDT) are focused on the use of solar sails to achieve a more modest goal of ~400 AU in 20 years with a requirement of reaching at least 200 AU. This approach, as well as all of those previously considered, obtains significant solar-system escape speeds by “dropping” the probe into the Sun and then executing a DV maneuver at perihelion. In the solar sail approach, the sail is used first to remove the angular speed of the Earth and probe about the Sun; the sail is then maneuvered face-on into the Sun at 0.1 to 0.25 AU (set by sail thermal heating constraints), and solar radiation pressure accelerates the probe away from the Sun until the sail is jettisoned (at ~5 AU in the ongoing studies). The previous approaches, and that adopted for study here, initially send the probe out to Jupiter, using that planet's gravity to remove the probe angular momentum. The probe is then allowed to fall much closer to the Sun - in this case to 4 solar radii (RS 1) - where a large propulsive DV maneuver is required over a very short time - taken here to be ~15 minutes to minimize gravity losses. At this time all acceleration is over and the probe continues on a high-energy ballistic escape trajectory from the solar system.

In the scenario studied in our Phase I work, the use of a carbon-carbon thermal shield and an exotic propulsion system capable of delivering high specific impulse (Isp) at high thrust is enabling. For the NASA/JPL case, the manufacture, deployment, and management of long-term low-thrust maneuvering with a solar sail is the enabling propulsive technology. Both implementations require low-mass, highly integrated spacecraft in order to make use of moderate (Delta-class) expendable launch vehicles (ELVs). The further from the Sun that operation is required, the more stressing are the reliability requirements for the spacecraft itself. Our Phase I concept is baselined to operate at least 2.5 times as far from the Sun as the goal for the JPL study (1000 AU versus 400 AU) and contains system-architecture elements required as next-roadmap step after the sail mission - our probe design is largely independent of the propulsion means, whether those studied previously by us, solar sails, or others. In the Phase I work we have completed an initial scoping of the system requirements and have identified system drivers for actually implementing such a mission.

In this Phase I study we began the task of looking seriously at the spacecraft mechanical, propulsion, and thermal constraints involved in escaping the solar system at high speed by executing a DV maneuver close to the Sun. In particular, to fully realize the potential of this scenario, the required DV maneuver of ~10 to 15 km/s in the thermal environment of ~4 RS (from the center of the Sun) remains challenging. Two possible techniques for achieving high thrust levels near the Sun are: (1) using solar heating of gas propellant, and (2) using a scaled-down Orion (nuclear external combustion) approach. We investigated architectures that, combined with miniaturized avionics and miniaturized instruments, enable such a mission to be launched on a vehicle with characteristics not exceeding those of a Delta III. This systems approach for such an Interstellar Explorer has not been previously used to address all of these relevant engineering questions but is required to lead to (1) a probe concept that can be implemented following a successful Solar Probe mission (concluding around 2010), and (2) system components and approaches for autonomous operation of other deep-space probes within the solar system during the 2010 to 2050 time frame."

This paper is an excellent 30 minute read. The design of a Sun diver mission, used to greatly accelerate a spacecraft on an escape trajectory from the Solar System, is clearly very difficult. There are lots of challenges, including the question of how to protect the spacecraft from exposure to the environment within several radii of the Sun. Communication is another issue, as is weight savings. A spacecraft of only 50, 75, or 100 kg is required so that propulsion demands are not excessive. Even still, a change in velocity of 10 to 15 km/sec is required in a 15 minute interval. This is massive by normal standards, and will result in a 50 km/sec or greater escape velocity from the Sun. Burning within the gravity well of the Sun, after diving into the Sun from either Jupiter or the Earth, allows for a great amplification of the delta v rocket burn. Various methods could achieve the needed thrust, but not chemical rockets. Solar sails have always been an attractive alternative, as are ion propulsion systems, and nuclear pulse (orion type) rockets. In this paper, a method of collecting solar radiation (extremely powerful at this range) to heat a gas such as ammonia. Hydrogen would work better, but does not store well on long (ca 5 years) missions particularly when the heat on approach to the Sun is taken into account. This would be an ideal propulsion method because it can achieve the high ISP of ion propulsion, but also have higher thrust. Ion engines are not likely to burn quickly enough to work in only 15 minutes. In the end, only the solar-thermal option appears to work in a sun-diver mission. Perhaps this could be augmented with the use of solar sails in a hybrid mission.

Phase I Final Report - NASA Institute for Advanced Concepts: A Realistic Interstellar Explorer

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