Abstract
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 ϵshl = 1 mm could go interstellar upon submission to solar radiation in interplanetary space. Upon launch at 1 AU from the Sun, an aerographite shell with ϵshl = 0.5 mm arrives at the orbit of Mars in 60 d and at Pluto’s orbit in 4.3 yr. Release of an aerographite hollow sphere, whose shell is 1 μm thick, at 0.04 AU (the closest approach of the Parker Solar Probe) results in an escape speed of nearly 6900 km s−1 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, whose shell is 100 μm thick, of 1 m (5 m) radius weighs 230 mg (5.7 g) and has a 2.2 g (55 g) mass margin to allow 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. Using 1 g (10 g) of this margin (e.g., for miniature communication technology with Earth), it would reach the orbit of Pluto 4.7 yr (2.8 yr) after interplanetary launch at 1 AU. 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 α 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.
Highlights
The discovery of a roughly Earth-mass planet candidate in the habitable zone (Kasting et al 1993) around our nearest stellar neighbor Proxima Centauri (Proxima Cen; Anglada-Escudé et al 2016) and recent evidence of a Neptunemass planet candidate (Damasso et al 2020; Kervella et al 2020; Benedict & McArthur 2020) motivates a reconsideration of the possibility of interstellar travel
Our concept involves a hollow sphere, approximately one meter in diameter, made of aerographite, which is first brought to space (LEO, translunar orbit, or interplanetary space) by a conventional rocket and released to the solar photon pressure for acceleration to interstellar speed
Stable orientation can be achieved for moderately conical shapes and only reduce the cross section surface area per given mass by a few percent compared to a hollow sphere (Hu et al 2014)
Summary
The discovery of a roughly Earth-mass planet candidate in the habitable zone (Kasting et al 1993) around our nearest stellar neighbor Proxima Centauri (Proxima Cen; Anglada-Escudé et al 2016) and recent evidence of a Neptunemass planet candidate (Damasso et al 2020; Kervella et al 2020; Benedict & McArthur 2020) motivates a reconsideration of the possibility of interstellar travel. 1 http://breakthroughinitiatives.org using laser technology to accelerate highly reflective light sails to interstellar speeds. The polyimide sheet had an areal mass density of about 10 g m−2, resulting in a total sail mass of 2 kg While this setup allowed IKAROS to gain about 400 m s−1 of speed from the Sun within almost three years of operation, it is impossible for the solar photon pressure to accelerate such a sail to interstellar speeds (Heller & Hippke 2017). The limited structural integrity of a graphene monolayer requires additional material thereby further increasing σ and complicating the experimental realization All of this ruins the beautiful theory of a pure graphene sail. Our concept involves a hollow sphere, approximately one meter in diameter, made of aerographite (our “solar sail”), which is first brought to space (LEO, translunar orbit, or interplanetary space) by a conventional rocket and released to the solar photon pressure for acceleration to interstellar speed
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