The Lunar Shield: Building a Planetary-Defence Checkpoint On The Moon
It’s only a matter of time before Earth is the random target of another major asteroid impact. However, this can be prevented by investing in and maintaining a planetary-defence system.
Image credit: Emily Garretson on ArtStation
From detection to deflection—how a lunar “guardrail” could backstop Earth against city-killers
Planetary defence already has proof that deflection is possible. NASA’s DART spacecraft struck Dimorphos in September 2022 and measurably shortened its orbital period; peer-reviewed analysis shows the kinetic impact delivered momentum efficiently rather than merely chipping the surface. ESA’s Hera mission is now en route to characterise the crater, mass change and momentum transfer in detail—data that turns a one-off demo into a repeatable tool.
Three recent near misses underline why a faster, more reliable response architecture matters. In July 2019, asteroid 2019 OK—roughly football-field scale—passed about 73,000 km from Earth after only days of warning; it was large enough for city-level damage had it struck. In March 2023, 2023 DZ2 (tens of metres across) passed within ~175,000 km; objects of that size can cause serious regional harm. In June 2024, 2024 MK—estimated around 150 metres—swept by at ~290,000 km; JPL’s radar campaign highlighted the rarity and risk profile of such close approaches for that size class. None hit—but each would have been a severe local-to-regional event had the trajectory been different.
A lunar “checkpoint” cannot replace early discovery, but it can compress timelines between detection and action. The concept is simple: place interceptors, propellant, and spares at Earth–Moon Lagrange points (EML-1 or EML-2), supported by a small launch/assembly node on or near the Moon. From those high ground positions, quick-reaction vehicles can depart in many sky directions without climbing out of Earth’s full gravity well. Academic work argues the Moon is an ideal location to stage intercept missions to catastrophic threats; broader cislunar studies show how depots and L-point hubs reduce response time and mission risk.
The operating stack begins with detection. Today’s blind spot is sunward, where dark asteroids are hard to see from Earth. NASA’s NEO Surveyor—a dedicated infrared telescope planned for the Sun–Earth L1 region—addresses exactly this gap by detecting thermal emission rather than reflected light. Better, earlier catalogues widen the set of encounters that can be addressed by existing physics rather than last-minute gambles.
Next is characterisation, because material properties drive tool choice. Some targets are rubble piles that shed momentum differently; others are monolithic. Hera’s close-range surveying of the DART site is designed to reduce these uncertainties. With a better prior on momentum-enhancement factors, programme managers can plan the number and size of impactors needed to achieve a safe miss distance.
Deflection tools then fall into two families. For years-to-decades warning, gravity tractors—spacecraft that hover and apply a continuous, gentle tug—offer controllable, observable changes; multiple-craft concepts mitigate single-point failures. For shorter warnings, kinetic impactors provide an impulsive push, now validated by DART and to be fully modelled by Hera’s measurements. Nuclear options remain controversial and would require international consent and command chains that do not exist today; the near-term portfolio is impactors and tractors.
Where, precisely, does the Moon add value? First, geometry and cadence. Vehicles staged at EML-1/2 can depart quickly with a smaller required change in velocity than from low Earth orbit, and can be pre-aimed along families of trajectories that cover a wide range of sky positions. Second, logistics. With propellant depots and standardised interceptor buses pre-positioned, campaigns become “startable” in days or weeks, not months, even if Earth launch pads are weathered out, congested, or politically delayed. Third, forensics. Rapid follow-up from cislunar space can confirm a deflection’s effect and—if needed—stage a second shot while Earth-based integration ramps.
The checkpoint would not be a single megastructure. A pragmatic configuration looks like a small platform at EML-1, stocked with a handful of standard kinetic impactors, a reconnaissance chaser for post-impact assessment, and cryogenic propellant tanks sized for multi-year dormancy. A lunar surface node—co-located with other cislunar infrastructure—would provide assembly, checkout, and spares, leveraging lessons from depot work (storage, boil-off management, propellant transfer) that are already being developed for exploration programmes.
What it cannot do is compensate for a lack of warning. The first dollars still belong with NEO Surveyor and ground networks; without early detection, even a ready-to-fire cislunar magazine may be too slow for certain geometries or inbound long-period comets. The checkpoint also does not solve governance by itself. A launch-on-warning authority for deflection attempts, international liability and transparency protocols, and pre-declared debris/fragmentation risk management are prerequisites for public legitimacy. Hera’s data will help on the physics; the policy community must finish the rules.
Cost realism matters. A lunar checkpoint adds capital expenditure—platforms, depots, on-orbit spares—but reduces per-mission scramble cost by avoiding bespoke builds and by reusing buses whose integration and guidance are already certified. It also reduces programme risk by turning low-frequency, high-stress “fire drills” into repeatable operations. The economics resemble insurance: pay steady premiums for readiness to reduce tail risk.
The roadmap can write itself if detection and characterisation are funded first (NEO Surveyor; Hera science return). Cislunar logistics capabilities—long-duration cryogenic storage, autonomous rendezvous and docking at L-points, and quick-reaction small spacecraft launched on short notice—have value in their own right. A pilot checkpoint becomes credible once these elements demonstrate reliability, starting with a modest interceptor magazine and clearly defined rules of engagement. Each increment contributes to broader space operations even if no hazardous asteroid materialises during development.
The strategic case is straightforward. Planetary defence is a probability game: widen the detection funnel; improve the physics; make the response faster and more repeatable. DART and Hera show the first lever works. A lunar checkpoint sharpens the second and third. With city-scale near misses like 2019 OK, 2023 DZ2, and 2024 MK fresh in the record, building a cislunar guardrail looks less like science fiction and more like prudent infrastructure for a low-probability, high-consequence risk.
