Moon-1: A Minimum Viable Product for Space Tethers

Author: Vince Cate
Project site: spacetethers.com

This page describes a practical, revenue-capable first space-tether mission: a small, reusable, rotating momentum-exchange tether in lunar orbit. The goal is to keep initial cost and complexity as low as possible while still delivering real value—payloads to the lunar surface—without requiring chemical descent stages.

1. Mission Philosophy

Start small: 10 kg class payloads.
Exploit physics: momentum exchange + high-ISP electric propulsion.
Reusable infrastructure: tether, solar arrays, and thrusters remain in orbit.
Incremental testing: success is measured in steps.

2. Baseline Tether Architecture

Total tether length: ~50 km
Tip radius: ~50,000 m from center of mass
Orbit: Elliptical lunar orbit, inertially fixed
Payload mass: 10 kg
Initial payload count: 50–100

The tether rotates so that at perigee the tip velocity cancels orbital velocity. Payloads are released with near-zero horizontal speed and fall almost vertically.

3. Momentum Accounting

Each payload removes momentum from the system:

Payload mass: 10 kg
Orbital velocity: ~1600 m/s
Momentum loss: 16,000 kg·m/s per payload

4. Electric Propulsion

Turion TIE-20 GEN2

Thrust: 55 mN
Isp: 4500 s
Power: 2000 W
Mass: 23 kg
Cost: $150,000

Time to restore momentum for one payload:

16,000 / 0.055 ≈ 290,000 s ≈ 3.4 days (per thruster)

5. LEO to Lunar Transfer

Total mass: ~1,700 kg
Δv: ~4 km/s

Using 8 TIE-20 thrusters (16 kW total), transfer time is ~7 months.

6. Mass Budget

Subsystem	Mass (kg)	Cost
Tether (50 km)	600	$1.2M
Tip assembly	15	$0.3M
Mobile ballast module	250	$1.0M
8 × TIE-20 thrusters	184	$1.2M
Solar arrays (20 kW)	100	$0.5M
Avionics & robotics	80	$0.4M
Payloads (50 × 10 kg)	500	customer
Total	~1,730	~$4.6M

7. Gravity-Gradient Pumping

The mobile module moves along the tether to trade orbital energy for rotation, avoiding thrusters at the tip and keeping tip mass minimal.

8. Surface Operations & Catch Concept

Early payloads assemble robotic backhoes, solar stations, and a regolith catapult. Payloads are launched upward to be intercepted by a 10 m capture net at ~30 m/s.

9. Economics

Delivered mass: 500 kg
Direct mission cost: ~$6M
Break-even: ~$12,000/kg to lunar surface

10. Why Moon-1 Works

Graceful partial success
No precision landing required
Reusable infrastructure
Immediate economic utility

If Moon-1 only delivers payloads down, it succeeds. If it later catches payloads, it becomes permanent infrastructure.