Want to get to the outer Solar System quickly? Try this on for size: Two and a half years to reach the heliopause, six and a half years to get to the Sun’s inner gravitational focus (550 AU), with arrival at the inner Oort Cloud in no more than thirty years. A spacecraft meeting those targets is moving at 403 kilometers per second, roughly twenty times as fast as anything we’ve put into space before. Such a mission could perform useful astrophysical observations enroute, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin.
The combined Oort Cloud explorer/gravity focus probe grows out of work by Gregory Matloff and Roman Kezerashvili (CUNY), Italian physicist Claudio Maccone and Les Johnson (NASA MSFC). Matloff, of course, has been studying solar sail technologies for decades, looking at missions that could reach velocities in the range of 0.003c-0.004c, with metallic sails that, parachute-like, pull a payload attached to diamond-strength cables. The cables (and the sail itself) can be wound around the payload enroute to provide cosmic-ray shielding and, in the case of true interstellar missions, redeployed upon arrival at a destination star.
That’s a familiar sail concept, but it’s one with a problem: Such designs do not scale well. In fact, as you increase the size of the spacecraft, the proportion of its mass that is devoted to cable rises rapidly with payload. Because of his long-standing interest in ‘generation’ ships, capable of carrying human colonies on millennia-long journeys to the stars, Matloff has a natural interest in refining the sail so it can be used in ever more ambitious missions. It’s natural to turn to the idea of inflatable beryllium sails, hollow-body sails that feature sail surfaces just tens of nanometers in thickness, with the body inflated by a low-pressure gas like hydrogen. Unlike the ‘parachute’ concept, the payload would be mounted on the space-facing surface — the inflatable sail is a ‘pusher’ model.
The concept goes back to Joerg Strobl, who first published it in a 1989 paper for the Journal of the British Interplanetary Society. And it’s a design that seems to scale well if properly deployed. The team studied two configurations, one a generation ship with inflated sail radius of 541.5 kilometers, a payload of 107 kg, and a separation between the sail faces of one kilometer. A second is a near-term extrasolar probe with sail radius of 937 meters, a 30 kg payload and a 1.8 meter separation. The numbers show how well the concept adjusts to different missions:
From the point of view of kinematics, mechanical stress, and thermal effects, the hollow-body solar photon sail scales well. Both conﬁgurations had a spacecraft areal mass density of 6.52 × 10−5 kg/m2, a peak internal gas pressure of 1.98 × 10−4 Pa, and a peak perihelion temperature of 1412 K. If fully inﬂated at the 0.05 AU perihelion of an initially parabolic solar orbit, both had a peak radiation-pressure acceleration of 36.4 m/s2 and exited the solar system at 0.00264c after an acceleration duration less than one day.
The new paper looks hard at the issues these designs face, including problems with the proposed 0.05 AU close pass by the Sun and the effects of solar radiation on sail materials and the hydrogen fill gas. The result is a modification of the near-term concept discussed above, with perihelion adjusted to 0.1 AU. The greater distance lowers the sail temperature considerably and reduces the need to replace hydrogen fill gas that will have diffused through the sail walls at higher temperatures. Even so, the team calculates that the gas must be replenished some fifty times during this solar acceleration process. The challenge is manageable:
To err on the side of caution, it is assumed here that a hydrogen reserve of 100 times the required ﬁll gas mass is carried aboard the spacecraft. This amounts to only 170 grams of hydrogen. If hydrogen ﬁll gas is dissociated from water as required, no more than about one kilogram of water is required. Even water-storage and dissociation equipment will not add more that a few kilograms to the payload and have a very small effect on spacecraft performance.
Also manageable is the constant ionization of beryllium sail atoms during the acceleration period, the result of solar ultraviolet radiation. The surface of the sail will lose electrons and become positively charged. And because the tensile strength of beryllium degrades with temperature, the sail could burst from electrostatic pressure at the earlier 0.05 AU perihelion. Increasing the perihelion distance lowers the electrostatic pressure dramatically and makes the mission feasible.
Can a beryllium sail of this description be launched from the surface, or does it demand space manufacture? We don’t know the answer to that yet, or to the question of whether beryllium is indeed the best material for the sail walls. It seems clear that an inflatable sail will be more massive than other designs despite its advantages in scalability, and it’s also more likely to experience significant damage from micro-meteorites. Plenty of questions remain as we work out the various sail designs and rigging arrangements that may make a fast mission to the Oort Cloud a reality in, the paper suggests, the post-2040 time frame.
The paper is Matloff, Kezerashvili, Maccone and Johnson, “The beryllium hollow-body solar sail: exploration of the Sun’s gravitational focus and the inner Oort Cloud,” now available online.