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Register a new userLow-cost precursor of an interstellar mission
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Authors
Rene Heller
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The solar photon pressure provides a viable source of thrust for spacecraft in the solar system. Theoretically it could also enable interstellar missions, but an extremely small mass per cross section area is required to overcome the solar gravity. We identify aerographite, a synthetic carbon-based foam with a density of 0.18 kg/m^3 (15,000 times more lightweight than aluminum) as a versatile material for highly efficient propulsion with sunlight. A hollow aerographite sphere with a shell thickness eps_shl = 1 mm could go interstellar upon submission to the solar radiation in interplanetary space. Upon launch at 1 AU from the Sun, an aerographite shell with eps_shl = 0.5 mm arrives at the orbit of Mars in 60 d and at Plutos orbit in 4.3 yr. Release of an aerographite hollow sphere, whose shell is 1 micrometer thick, at 0.04 AU (the closest approach of the Parker Solar Probe) results in an escape speed of nearly 6900 km/s and 185 yr of travel to the distance of our nearest star, Proxima Centauri. The infrared signature of a meter-sized aerographite sail could be observed with JWST up to 2 AU from the Sun, beyond the orbit of Mars. An aerographite hollow sphere with eps_shl = 100 micrometer and a radius of 1 m (5 m) weighs 230 mg (5.7 g) and has a 2.2 g (55 g) mass margin for interstellar escape. The payload margin is ten times the mass of the spacecraft, whereas the payload on chemical interstellar rockets is typically a thousandth of the weight of the rocket. Simplistic communication would enable studies of the interplanetary medium and a search for the suspected Planet Nine, and would serve as a precursor mission to alpha Centauri. We estimate prototype developments costs of 1 million USD, a price of 1000 USD per sail, and a total of <10 million USD including launch for a piggyback concept with an interplanetary mission.
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The exploration of interstellar space will require autonomous navigation systems that do not rely on tracking from the Earth. Here I develop a method to determine the 3D position and 3D velocity of a spacecraft in deep space using a star catalogue. As a spacecraft moves away from the Sun, the observed positions and velocities of the stars will change relative to those in a Earth-based catalogue due to parallax and aberration. By measuring just the angular distances between pairs of stars, and comparing these to the catalogue, we can infer the coordinates of the spacecraft via an iterative forward-modelling process. I perform simulations with existing star catalogues to demonstrate the method and to compute its performance. Using the 20 nearest stars and a modest angular distance measurement accuracy of 1, the position and velocity of a spacecraft light years from the Sun moving at relativistic speeds can be determined to within 3 au and 2 km/s respectively. These accuracies improve linearly with the measurement accuracy, e.g. with angles measured to 1 mas the navigation accuracy is 1000 times better. Performance can also be improved using more stars, or by including onboard measurements of the stars radial velocities, as these too are affected by the spacecrafts position and motion.
A solar sail propelled small satellite mission concept to intercept and potentially rendezvous with newly discovered Interstellar Objects (ISOs) is described. The mission concept derives from the proposal for a technology demonstration mission (TDM) for exiting the solar system at high velocity, eventually to reach the focal region of the solar gravitational lens (SGL). The ISO mission concept is to fly a solar sail inward toward a holding orbit around the Sun and when the ISO orbit is confirmed, target the sailcraft to reach an escape velocity of over 6, AU/year. This would permit rapid response to a new ISO discovery and an intercept within 10 AU from the Sun. Two new proven interplanetary technologies are utilized to enable such a mission: i) interplanetary Smallsats, such as those demonstrated by the MarCO mission, and ii) solar sails, such as demonstrated by LightSail and IKAROS missions and developed for NEA Scout and Solar Cruiser missions. Current technology work suggests such a mission could fly and reach an ISO moving through the solar system within this decade. Such a mission might enable the first encounter with an ISO to allow for imaging and spectroscopy, measurements of size and mass, potentially giving a unique information about the objects origin and composition.
Microwave propelled sails are a new class of spacecraft using photon acceleration. It is the only method of interstellar flight that has no physics issues. Laboratory demonstrations of basic features of beam-driven propulsion, flight, stability (beam-riding), and induced spin, have been completed in the last decade, primarily in the microwave. It offers much lower cost probes after a substantial investment in the launcher. Engineering issues are being addressed by other applications: fusion (microwave, millimeter and laser sources) and astronomy (large aperture antennas). There are many candidate sail materials: carbon nanotubes and microtrusses, graphene, beryllium, etc. For acceleration of a sail, what is the cost-optimum high power system? Here the cost is used to constrain design parameters to estimate system power, aperture and elements of capital and operating cost. From general relations for cost-optimal transmitter aperture and power, system cost scales with kinetic energy and inversely with sail diameter and frequency. So optimal sails will be larger, lower in mass and driven by higher frequency beams. Estimated costs include economies of scale. We present several starship point concepts. Systems based on microwave, millimeter wave and laser technologies are of equal cost at todays costs. The frequency advantage of lasers is cancelled by the high cost of both the laser and the radiating optic.
The Atmosphere-Space Interactions Monitor (ASIM) is an instrument suite on the International Space Station (ISS) for measurements of lightning, Transient Luminous Events (TLEs) and Terrestrial Gamma-ray Flashes (TGFs). Developed in the framework of the European Space Agency (ESA), it was launched April 2, 2018 on the SpaceX CRS-14 flight to the ISS. ASIM was mounted on an external platform of ESAs Columbus module eleven days later and is planned to take measurements during minimum 3 years.
Formation-flying studies to date have required continuous and minute corrections of the orbital elements and attitudes of the spacecraft.This increases the complexity, and associated risk, of controlling the formation, which often makes formation-flying studies infeasible for technological and economic reasons. Passive formation-flying is a novel space-flight concept, which offers a remedy to those problems. Spacecraft in a passive formation are allowed to drift and rotate slowly, but by using advanced metrology and statistical modelling methods, their relative positions, velocities, and orientations are determined with very high accuracy. The metrology data is used directly by the payloads to compensate for spacecraft motions in software. The normally very stringent spacecraft control requirements are thereby relaxed, which significantly reduces mission complexity and cost. Space-borne low-frequency radio astronomy has been identified as a key science application for a conceptual pathfinder mission using this novel approach. The mission, called FIRST (Formation-flying sub-Ionospheric Radio astronomy Science and Technology) Explorer, is currently under study by the European Space Agency (ESA). Its objective is to demonstrate passive formation-flying and at the same time perform unique world class science with a very high serendipity factor, by opening a new frequency window to astronomy.
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