That's just stupid. Why send them all that way just to put them on a rock, in orbit around a planet, when you could leave them in a free solar orbit?
Not only that, but put them on opposite sides of the sun FFS, so you don't have to wait half an orbit (15 years, in Saturn's case) to get the other half of a measurement.
kurtswendson•2mo ago
Thank you for saying a carefully researched paper is "just stupid" - this gets at a fundamental design choice.
Station-keeping problem: Free-flying telescopes need constant propellant for position/attitude control. Over 30 years at Saturn, this is mission-ending. Surface installations sit on bedrock - zero propellant, infinite operational lifetime.
Baseline precision: Interferometry requires sub-wavelength distance knowledge (nanometers at 600nm). The paper uses active laser ranging between surface installations to achieve this. Free-flying spacecraft would need the same laser ranging system PLUS propellant PLUS attitude control. Surface mounting eliminates two of those three requirements.
Opposite sides of the sun doesn't work:
Solar orbit puts them in Earth's orbital plane, so Earth is rarely aligned for both simultaneously
The Sun itself blocks line of sight for large portions of the orbit
Solar radiation pressure and gravitational perturbations constantly change the baseline unpredictably
You lose continuous observation capability
The "half orbit wait" critique misunderstands the design: The Rhea-Iapetus baseline changes by <1% over the 79-day period (Section 2.3). Laser ranging tracks this continuously in real-time with nanometer precision. You're observing continuously with a precisely known geometry, not waiting for anything. The slow, predictable variation is actually advantageous - it provides multiple baseline orientations for the same target.
Power budget: 20 MW transmission power requires megawatt-class nuclear reactors. Much easier to mount on a surface than on free-flying spacecraft.
The surface installation trades mission complexity (landing, drilling, deployment) for operational simplicity (no propellant, stable mounting, unlimited lifetime). For a 30+ year mission, that's the right trade.
perilunar•2mo ago
kurtswendson•2mo ago
Here's why surface mounting beats free-flying spacecraft:
Station-keeping problem: Free-flying telescopes need constant propellant for position/attitude control. Over 30 years at Saturn, this is mission-ending. Surface installations sit on bedrock - zero propellant, infinite operational lifetime. Baseline precision: Interferometry requires sub-wavelength distance knowledge (nanometers at 600nm). The paper uses active laser ranging between surface installations to achieve this. Free-flying spacecraft would need the same laser ranging system PLUS propellant PLUS attitude control. Surface mounting eliminates two of those three requirements. Opposite sides of the sun doesn't work:
Solar orbit puts them in Earth's orbital plane, so Earth is rarely aligned for both simultaneously The Sun itself blocks line of sight for large portions of the orbit Solar radiation pressure and gravitational perturbations constantly change the baseline unpredictably You lose continuous observation capability
The "half orbit wait" critique misunderstands the design: The Rhea-Iapetus baseline changes by <1% over the 79-day period (Section 2.3). Laser ranging tracks this continuously in real-time with nanometer precision. You're observing continuously with a precisely known geometry, not waiting for anything. The slow, predictable variation is actually advantageous - it provides multiple baseline orientations for the same target. Power budget: 20 MW transmission power requires megawatt-class nuclear reactors. Much easier to mount on a surface than on free-flying spacecraft. The surface installation trades mission complexity (landing, drilling, deployment) for operational simplicity (no propellant, stable mounting, unlimited lifetime). For a 30+ year mission, that's the right trade.