This chart models the fissile fuel mass required to capture a 1,000-tonne near-Earth asteroid into lunar orbit, given a 3 km/s velocity change. The x-axis is the acceptable capture time horizon; the y-axis is the required U-235 or Pu-239 fuel mass in kilograms, on a logarithmic scale.

The model

The rocket equation sets the propellant mass for each variant. For a 1,000-tonne rock and 3 km/s Δv:

  • Single cell + nozzle (Isp ≈ 400 s) consumes ~535 tonnes of silicate propellant.
  • Three candles, no nozzle (Isp ≈ 250 s) consumes ~706 tonnes.
  • Self-burrowing candle (Isp ≈ 300 s) consumes ~639 tonnes.
  • Single cell, H₂ from rock water (Isp ≈ 800 s) consumes ~317 tonnes.

Vaporizing silicate rock costs roughly 10 MJ/kg. Fission releases ~80 TJ/kg of fissile material, of which ~5% is usable in a practical reactor, giving ~4,000 MJ/g of consumed fuel.

The longer the capture timeline, the more heat leaks into the surrounding rock rather than escaping with the exhaust. Each variant has a different heat-loss fraction per year: 15% for the high-Isp hydrogen case, 20% for the single nozzle, 30% for the self-burrowing candle, and 40% for the three distributed candles.

What the curves show

The curves rise with time. A fast one-year capture needs the least fissile fuel; a ten-year slow burn needs more because the candle is bleeding heat into the rock the whole time.

The high-Isp hydrogen variant is consistently the most fuel-efficient because it consumes the least propellant. The three-candle no-nozzle variant is the least efficient because it consumes the most propellant and loses the most heat.

At one year, the required fuel ranges from under 1 kg to about 2.5 kg. At ten years, the spread widens from ~2 kg to nearly 9 kg.

Caveats

This is a simplified model, not an engineering design. It assumes constant Isp, constant heat-loss rates, and perfect reactor control. Real asteroid capture would involve variable geometry, tumbling dynamics, and material constraints that this chart ignores.

The point is comparative: the variant and the timeline matter more than the total energy budget alone.