Planetary Society Executive Director Lou Friedman on solar sails IKAROS and LightSail-1

Episode Summary

Executive Summary: This Planetary Radio episode centers on two solar sail efforts: Japan’s Ikaros mission, an ambitious hybrid sail-plus-solar-power technology test heading toward Venus, and the Planetary Society’s LightSail 1, a lighter pure solar sail still in development. The show also covers Voyager 2’s software glitch, Jupiter’s changing cloud bands, and a skywatch/space trivia segment with Bruce Betts.

Main Topics: Voyager 2 software anomaly and recovery: Emily Lakdawalla reports that Voyager 2 began returning nonsense data, likely due to a flipped bit in its flight data system. Engineers confirmed the spacecraft still responds to commands and can send engineering data, and are working to identify and correct the corrupted software bit. Japan’s Ikaros solar sail mission: Lou Friedman explains that Ikaros is a Japanese technology demonstration piggybacking on a Venus mission. It combines solar sail propulsion with embedded thin-film solar cells and attitude control via variable albedo, aiming to become the first true solar sail flight. Planetary Society’s LightSail 1 progress: Friedman outlines LightSail 1’s design, launch timeline uncertainty, and upcoming critical design review. The project is smaller and lighter than Ikaros, emphasizing a pure solar sail approach and very high acceleration for its mass. Solar sailing challenges and mission architecture: The discussion compares sailing in Earth orbit versus deep space, emphasizing the difficulty of launching from Earth’s gravity well, deploying large sails, and maintaining control of sail orientation and acceleration. Jupiter’s changing appearance: Lakdawalla notes that Jupiter’s south equatorial belt has faded from red to white, making the planet resemble its Voyager-era appearance. The change is said to recur every 3 to 15 years. Night sky, history, and trivia with Bruce Betts: Betts gives observing tips for Venus, Mars, Saturn, Jupiter, and the Moon, revisits Apollo 10 history, explains the origin of the term asteroid, and runs a trivia contest about Pioneer 1 and the meaning of Ikaros.

Key Arguments: Voyager 2 is likely fixable because it still responds normally to commands and can transmit engineering telemetry, suggesting the problem is software formatting rather than a hardware failure. A single flipped bit in spacecraft software can create major functional errors, especially if it occurs in a critical section of code. Ikaros represents a serious and innovative hybrid propulsion-energy test: it uses the sail both for propulsion and for embedded solar power generation. The Japanese mission’s deployment near Earth is strategically chosen for communications and imaging, not because the sail is intended for Earth orbit use. LightSail 1 is designed as a purer solar sail experiment than Ikaros, with much lower mass and higher expected acceleration. Solar sailing is conceptually proven, but practical control and deployment remain the main engineering hurdles. Access to space is the major bottleneck for sail missions because deployment and control are far harder in Earth orbit than in deep space. Jupiter’s belts change periodically, so the current disappearance of one band is unusual to casual observers but not unprecedented. Data Points: Voyager 2 distance from Earth: 13 light hours - Lakdawalla describes how far Voyager 2 is while troubleshooting its data corruption Jupiter belt cycle: every 3 to 15 years - Frequency with which Jupiter’s south equatorial belt can fade and reappear Ikaros sail size: 20 meters by 20 meters - Lou Friedman describes the Japanese sail’s dimensions Ikaros spacecraft mass: 350 kilograms - Friedman compares Ikaros to the Planetary Society’s earlier and later sail concepts Cosmos 1 mass: 100 kilograms - Friedman references the earlier Planetary Society solar sail mission LightSail mass: 5 kilograms - Friedman says LightSail is dramatically lighter than Ikaros LightSail sail deployment area: 5.5 by 5.5 meters - Friedman gives the deployed sail dimensions for LightSail 1 LightSail sail area: 32 square meters - Calculated sail area Friedman uses to compare acceleration LightSail spacecraft mass: 4.5 kilograms - Used to explain the high acceleration of the craft Apollo 10 lunar altitude: 50,000 feet - Betts cites how close Apollo 10 came to the lunar surface Planetary Society membership: nearly 100,000 members - Promotional segment for the Society and Planetary Radio Launch review timing: a couple of weeks - Friedman says LightSail is approaching critical design review soon Launch window uncertainty: late 2010 to 2011 - Friedman says LightSail launch may slip depending on vehicle availability

Pivotal Quotes: "They think that there is a flipped bit somewhere." — Emily Lakdawalla: Explaining the likely cause of Voyager 2’s data corruption "sails are still the only practical technology humans have come up with that could get a spacecraft to other stars." — Lou Friedman: Friedman’s broader argument for the importance of solar sailing "Everybody knows the theory is correct, but they don't know whether you can practically control it." — Lou Friedman: Summing up the main technical challenge for solar sail missions

Implications: The episode frames solar sailing as a near-term frontier with multiple competing demonstrations. If Ikaros succeeds, it could validate hybrid sail technology; if LightSail succeeds, it strengthens pure sail propulsion. Voyager 2’s recoverability also shows the longevity—and fragility—of deep-space missions.