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Solar Sail NEA Missions Proposed

If we chose to launch an interstellar probe using near-term technologies, it would almost surely be a solar sail, unfurled on a close approach to the Sun and flung outward at speeds that could get it to Alpha Centauri in perhaps 1000 years. That’s the thinking of Gregory Matloff, author of Deep Space Probes: To the Outer Solar System and Beyond (Springer, 2005) and a leading proponent of interstellar studies. Later technologies might also use sails, driven by laser or microwave or even particle beams. But as we take incremental steps toward an interstellar future, solar sails have to be developed first.

On that score, it’s interesting to note the contributions of the Deutschen Zentrum für Luft- und Raumfahrt (DLR), Germany’s aerospace research center and space agency. An early consolidation of sail studies, for example, came from a joint NASA/DLR feasibility study conducted in 1996-97. The working group recommended a demonstration mission that could help develop the basic principles of sail fabrication, packaging, storage, deployment, and control.
Solar Sail Competition A Possibility

A solar sail competition to drive research? It’s a great idea, and one that has been explored in the past. Indeed, a whole variety of groups have looked into the possibility, from France’s Union pour la Promotion de la Propulsion Photonique (U3P) to Russia’s Space Regatta Consortium and the Aero-Club de France. And official rules for the Luna Cup were approved by the International Astronautical Federation at the World Space Congress in August of 1992, outlining a solar sail race to the Moon.

Now I’m looking at a NASA announcement passed along by James Benford that outlines prize competitions to be conducted under the agency’s Centennial Challenges umbrella. To quote from the document, “By making awards based on actual achievements instead of proposals, Centennial Challenges seeks novel and lower-cost solutions to engineering obstacles in civil space and aeronautics from new sources of innovation in industry, academia, and the public.”

The challenge possibilities are outlined in a NASA Request for Comments (RFC) document that explores competitions in a number of areas, ranging from low-cost space suits to lunar night power sources. And the one that has caught Benford’s eye involves solar sails. Here’s the relevant information:

The Station-Keeping Solar Sail Challenge is designed to promote the development of solar sail technology and the commercial services that may result from the ability to operate in novel orbits such as artificial Lagrange points.

The Station-Keeping Solar Sail Challenge has two prizes. To win Prize One and the $2,500,000 purse, a Team must be the first to deploy a solar Sailcraft, demonstrate a resultant trajectory acceleration change of at least .05 millimeters per second squared, and fly along a trajectory that will pass through a defined target located at the first Sun-Earth Lagrange point (L1). To win Prize Two and the $2,500,000 purse, a Team must enter a defined region above or below the ecliptic plane at L1 and remain there for 90 consecutive days.

Laser Beaming to Boost Solar Sails

Solar sails are ideal for long missions within the Solar System, but their manifest advantages (no fuel onboard!) are not unalloyed. A major issue is the time needed to escape Earth orbit. Working the numbers on this, Gregory Benford noted that if a sail used the momentum from solar photons alone, unassisted by any other propulsive force, it would require time scales on the order of years to escape from Earth’s gravity. And that’s with sail deployment from an altitude of 800 kilometers, beyond the reach of decelerating air drag.

What we can do to get that sail on its way faster is the subject of Benford’s new paper in Acta Astronautica, written in collaboration with Paul Nissenson. One possibility is to coat the sail with a material that sublimes; when heated, the material vaporizes and is ejected, adding to the momentum transfer of photons (Benford and his brother James at Microwave Sciences have done groundbreaking work on the nature of such sublimation, also called desorption).

But the paper goes further; its authors examine the idea of decreasing sail escape time by using a high-power ground or space-based photon generator — a beamer — that would increase the photon density well beyond solar power alone. Their work on the orbital dynamics of such a beamer/sail combination assumes that an orbiting beamer (ORB) would be deployed behind the sail in the same initial circular orbit. When the beamer illuminates the sail, a resonance between the two is established, with sail and beamer returning to their original positions after a certain number of orbits, where the sail is boosted once again.

The paper is “Reducing solar sail escape times from Earth orbit using beamed energy,” in Acta Astronautica Vol. 58, Issue 4 (February 2006), pp. 175-184.
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