> I would be curious to know how the math works out...how long will it take those solar panels to gather enough energy to make enough methane to return the rock

> I would be curious to know how the math works out...how long will it take those solar panels to gather enough energy to make enough methane to return the rocket to Earth? Somewhere around a year. A Starship, refuelled in Earth orbit, intends to carry 100 tons to Mars [0] using 1200 tons of propellant [1] with an oxidizer/fuel ratio of 78:22 [1], so 265 tons of methane. Spacex's numbers are likely measured from low Earth orbit to low Mars orbit and assuming aerobraking on the way down. According to [2] that takes 5.8 km/s delta-v, but it's an additional 3.8 km/s to travel from the Mars surface. That's 65% extra, meaning we will need e^0.65 as much fuel because of [3], which is 1.9x, so 500 tons of methane. So the first answer is, it can't be done by refueling a Starship final stage alone - you'd need a booster, or (simpler) reduce payload by half. I'll assume instead we divided the load between two rockets coming back. Methane has a specific energy of 55MJ/kg, [4] so we need 28 TJ of energy to fill the tank. That's 8 GWh. Mars receives about 50% as much sunlight as Earth [5] Spacefaring solar panels weigh 2kg/sqm and produce 350W/sqm in Earth orbit. [6]. Terrestrial solar panels are cheaper to produce, but heavier. That's baseplate capacity, so make it 1/3 of that to average over day and night, and halve it again for running at Mars light levels, giving 60W/sqm or 30W/kg. 100 tons of that is 3 MW. Typical conversion efficiency for power-to-methane is 50%. [7]. This is done via hydrogen and requires carbon dioxide, which is abundant in the Mars atmosphere, and water, which is not, but is hopefully available in the quantities we require. So our solar plant produces 1.5MW of methane. I'm going to assume we get our oxygen for free as a by-product without any loss of efficiency, which seems plausible to me but not well documented. 8 GWh/1.5 MW is 5,300 hours or 225 days (either type) or about 8 earth months. I haven't allowed for the mass cost of the infrastructure for methane refining, or degradation of solar panels in the Mars dust. So all this should be a considered a lower bound for your answer. [0] https://www.spacex.com/human-spaceflight/mars/ https://www.spacex.com/human-spaceflight/mars/ [1] https://www.wevolver.com/specs/spacexs-starship-sn24-bn7 https://www.wevolver.com/specs/spacexs-starship-sn24-bn7 [2] https://i.imgur.com/SqdzxzF.png https://i.imgur.com/SqdzxzF.png [3] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation [4] https://en.wikipedia.org/wiki/Energy_density https://en.wikipedia.org/wiki/Energy_density [5] https://www.quora.com/How-bright-is-sunlight-on-Mars https://www.quora.com/How-bright-is-sunlight-on-Mars [6] https://www.spectrolab.com/DataSheets/Panel/panels.pdf https://www.spectrolab.com/DataSheets/Panel/panels.pdf [7] https://www.sciencedirect.com/science/article/pii/S0306261918311826 https://www.sciencedirect.com/science/article/pii/S030626191...