Lunar Momentum Exchange Tether
First Mission Concept (MVP)

Mission Goal: Deploy a 30 km rotating tether system in lunar orbit capable of delivering 50-100 small payloads (10 kg each) to the Moon's surface, with eventual capability to catch payloads launched from the surface.

Mission Overview

This design represents a Minimum Viable Product (MVP) for space tether technology—a first mission that can generate revenue while proving the fundamental concepts needed for future, larger-scale tether operations. Unlike pure demonstration missions, this system will perform useful work from day one by delivering commercial and scientific payloads to the lunar surface at a fraction of traditional costs.

Core Concept

The system consists of a 30 km tether rotating in an elliptical lunar orbit. At perigee (closest approach to the Moon), the tether's tip velocity exactly cancels the orbital velocity, bringing payloads to near-zero velocity relative to the lunar surface at altitudes of 100-1000 meters. Payloads are then released to fall gently to the surface, experiencing impact velocities of only 18-58 m/s—survivable with simple crush protection.

Key Innovation: Rather than placing heavy thrusters and solar panels at both ends of the tether, we use a movable module that can traverse the tether length. This keeps the tip end light (minimizing tether mass) while enabling gravity gradient pumping to transfer orbital energy into rotational energy.

Propulsion System

SpaceX Argon Hall-Effect Thruster Specifications

Thruster Configuration

For LEO to lunar orbit transfer in under 8 months plus momentum recovery between payload releases, we specify 6 thrusters operating simultaneously:

Transfer Orbit Performance

With initial system mass of approximately 1,450 kg (see mass budget below) and continuous thrust of 1.02 N:

Momentum Recovery Between Releases

After each 10 kg payload release at 1,600 m/s:

This 5-day interval allows plenty of time for gravity gradient pumping to restore rotational energy and for orbital phasing to position the next payload delivery site.

System Architecture

Tether Design

The tether is sized to handle the stress from a 10 kg payload at the tip rotating at sufficient velocity (approximately 1,600 m/s tip speed) to cancel orbital velocity at perigee:

Design principle: Tether mass is approximately 10× the tip payload mass due to the centrifugal loading. Keeping the tip end light is critical to system efficiency.

Movable Module (Main Spacecraft Bus)

This module houses all major systems and can traverse the entire tether length:

Tip Assembly (Lightweight Design)

Minimizing mass here is critical:

Payloads

Complete Mass Budget

Component Mass (kg)
Tether (30 km) 100
Movable Module 538
Tip Assembly 35
Payloads (100 × 10 kg) 1,000
Contingency (15%) 251
Total Launch Mass 1,924 kg

Launch Cost Analysis

Launch Vehicle Cost per kg Total Launch Cost Cost per Payload Delivered
Falcon 9 (future pricing) $1,000/kg $1,924,000 $19,240
Starship (target pricing) $200/kg $384,800 $3,848

Cost Advantage: Traditional lunar landers cost $500,000 to several million per kg delivered. Even with development costs, this tether system offers 10-100× cost reduction per payload delivered, with costs dropping further on subsequent missions that reuse the tether infrastructure.

Operational Phases

Phase 1: Deployment and Transit (Month 0-6)

  1. Launch to LEO as secondary payload or dedicated small launcher
  2. Deploy solar panels and activate thrusters
  3. 6-month spiral trajectory to lunar orbit using continuous low thrust
  4. Deploy tether to full 30 km length in lunar orbit
  5. Begin rotation using gravity gradient and active pumping

Phase 2: Initial Payload Deliveries (Month 7-12)

  1. Release first payloads from 1,000 m altitude (58 m/s impact velocity)
  2. Deliver infrastructure payloads: robot backhoe components, solar panels, catapult parts
  3. Gradually decrease release altitude as confidence grows (target: 100 m / 18 m/s)
  4. Deliver 50-70 payloads during this phase

Phase 3: Surface Infrastructure Assembly (Month 13-18)

  1. Robot backhoes assemble themselves from delivered components
  2. Solar panels deployed to power surface operations
  3. Catapult assembled from delivered parts
  4. Backhoes begin filling regolith bags to standard 10 kg mass
  5. Practice catapult launches without attempting catches

Phase 4: Catch Development (Month 19-24)

  1. Refine tether tracking using corner cube reflectors and surface LIDAR
  2. Practice net positioning at progressively lower altitudes
  3. Attempt first catches of catapulted regolith bags
  4. Once reliable, begin two-way payload exchange operations

Phase 5: L5 Transfer Demonstrations (Month 24+)

  1. Adjust tip velocity to 2.5 km/s (lunar escape velocity to L5 transfer)
  2. Release small satellites with course correction thrusters toward L5
  3. Demonstrates capability for future Moon-L5 cargo operations

Gravity Gradient Pumping

This technique, developed by Forward, Hoyt, and Tethers Unlimited, allows the tether to gain rotational energy from orbital energy without expending propellant:

  1. As the tether rotates with tip pointing away from Moon, the movable module pulls itself toward the tip, shortening the effective length
  2. As the tip rotates toward the Moon, the module moves back, lengthening the tether
  3. This asymmetric lengthening exploits the gravity gradient to pump energy into rotation
  4. In 1-2 days, sufficient tip velocity can be achieved for payload release

The movable module also controls the rotational phase so the tip points downward (toward Moon) precisely at perigee for payload release.

Payload Delivery Mechanics

Drop Trajectory

Payloads released at near-zero velocity relative to the surface from various altitudes:

Release Altitude Impact Velocity Risk Level
1,000 m 58 m/s Initial tests
500 m 41 m/s Medium confidence
200 m 26 m/s Standard operations
100 m 18 m/s Optimized operations

Impact Protection

Each payload incorporates:

Surface Operations

Robot Backhoe Specifications

Lunar Surface Mobility Requirements

The Moon rotates once every ~28 days. The tether's orbital plane is fixed relative to the stars, so perigee sweeps around the Moon. To keep the backhoe/catapult at the perigee point and remain in sunlight:

Additionally, the tether orbit must precess ~1°/day to maintain perigee alignment with the solar terminator, easily accomplished with periodic small thruster burns.

Catapult System

Catch Operations

Net Design

Tracking and Precision

Mission Economics

Revenue Model (Falcon 9 Launch Scenario)

Item Amount
Launch Cost (F9 @ $1,000/kg) $1,924,000
Hardware Development & Integration $5,000,000
Mission Operations (2 years) $2,000,000
Total Mission Cost $8,924,000

Crowdfunding Revenue Potential

Package Payload Mass Price Number Sold Revenue
Basic 1 kg $50,000 30 $1,500,000
Standard 10 kg $400,000 20 $8,000,000
Reserved infrastructure — — 40 payloads $4,000,000
Total Potential Revenue $9,500,000

Business Case: With 50 customer payloads at these prices, mission costs are fully covered. The remaining 50 payload slots support infrastructure (backhoes, solar panels, catapult) with potential for additional revenue. Subsequent missions reusing the tether infrastructure would require only propellant and new payloads, dramatically reducing costs.

Potential Customers

Key Advantages of This Approach

  1. Lower mass than chemical alternatives: High-ISP thrusters reduce propellant needs by 5-10×
  2. Reusable infrastructure: Future missions bring only propellant and payloads
  3. Incremental testing: Each payload delivery validates systems for the next
  4. Revenue from day one: Not just a technology demonstration
  5. Scalable design: Successful MVP leads naturally to larger payload versions
  6. Multiple fallback modes: Mission succeeds even if catch capability never works
  7. Technology pathfinder: Proves concepts needed for Earth-LEO and Moon-L5 tethers

Risk Mitigation

Technical Risks

Mission Success Criteria

Future Evolution

Mission 2: Extended Operations

Mission 3+: Water Propellant

Scaled Operations

Technology Pathfinder Value

This mission proves critical technologies needed for all future tether operations:

Success here validates the tether approach for much larger systems that could revolutionize space transportation economics.

Conclusion

This lunar momentum exchange tether represents an achievable first step toward operational space tether systems. With launch costs under $2M (Falcon 9) or $400K (Starship), development costs of ~$5M, and strong crowdfunding potential, the project is financially viable. More importantly, it delivers real value from the first mission while proving technologies essential for future applications.

The incremental approach—beginning with simple payload drops, progressing to surface infrastructure, then attempting catches—provides multiple success criteria and fallback positions. Even partial success validates the concept and paves the way for scaled operations.

Most significantly, this isn't just a technology demonstration. It's a working transportation system that can deliver payloads at 10-100× lower cost than alternatives, with costs decreasing further as infrastructure is reused across multiple missions.

Next Steps: Detailed engineering design, trajectory optimization, tether material selection, payload standardization, partner/customer development, and crowdfunding campaign planning.

For more information on space tether concepts, visit spacetethers.com