1. The hard rain

Neal Stephenson’s Seveneves begins with the Moon exploding. The fragments first cause wonder, then inconvenience, then a storm of impacts that makes low Earth orbit unusable for thousands of years. The book’s title refers to what comes after the rain: the long, slow project of rebuilding. The immediate lesson is that a single breakup in orbit can outlast the civilisation that caused it.

The desktop will not break the Moon. But it could break itself, and a self-inflicted fragmentation is one of the most embarrassing ways to fail. This entry is about preventing that.

2. What passivation means

Passivation is the process of making a dead spacecraft safe. A satellite that has run out of purpose but still contains stored energy is a hazard. Passivation removes or isolates that energy so the object becomes inert.

For the desktop, passivation includes:

  • Discharging batteries.
  • Venting propellant and pressurant.
  • Depressurising tanks.
  • Isolating the power bus.
  • Releasing stored mechanical energy, such as reaction wheel momentum or tape-spring tension.
  • Securing deployable structures so they cannot release unexpectedly.

Passivation is not the same as deorbit. A passivated object can still be orbital debris. But it is debris that will not explode.

3. Battery discharge

Batteries are the most common source of in-orbit explosions. Overcharged cells, thermal runaway, or internal short circuits can turn a battery into shrapnel.

The desktop’s passivation plan should:

  • Discharge all batteries to a safe state of charge at end of life.
  • Open contactors to isolate battery strings from the bus.
  • Provide thermal management during discharge so the process itself does not cause runaway.
  • Use cell-level protection where possible.

If a battery attachment is replaced during service, the old battery should be discharged and isolated before it is stored or jettisoned.

4. Propellant and pressurant venting

A propulsion system left pressurised after shutdown is a pressure vessel waiting for a crack. The desktop should:

  • Vent residual propellant through thrusters or dedicated vent lines.
  • Depressurise tanks and manifolds.
  • Leave valves in a safe, open or vented configuration.
  • Record the passivation state so future operators know what was done.

For storable propellants, venting must be controlled to avoid creating a small debris cloud from ice or unburned fuel. For cold gas systems, the risk is lower but the procedure is similar.

5. Power bus isolation

A live power bus can arc if connectors separate due to thermal cycling or impact. At end of life:

  • All solar array strings should be open-circuited or shunted.
  • The main bus should be de-energised.
  • Capacitors should be discharged.
  • Pyrotechnic devices, if any, should be left in a safe state.

The goal is to remove electrical energy from every part of the spacecraft that does not need it for the disposal manoeuvre itself.

6. Design for demise

Design for demise means choosing materials and constructions that burn up completely during reentry. The desktop is large enough that some parts might survive unless they are designed not to.

Principles:

  • Use materials with low melting points for non-structural parts.
  • Avoid dense, heat-resistant components such as titanium tanks or steel reaction wheels where possible.
  • Ensure that joints fail predictably, allowing parts to separate and heat more evenly.
  • Place critical dense items inside structures that ablate before they do.

Design for demise conflicts with design for long life. A material that survives radiation and thermal cycling for fifteen years may also survive reentry. Trade-offs are inevitable.

7. Debris mitigation during operations

Passivation is the endgame, but debris mitigation starts at launch:

  • Collision avoidance: the desktop should have enough propulsion and autonomy to manoeuvre around conjunctions.
  • Tracking and registration: the platform must be catalogued and its state vectors published.
  • Operational lifetime: shorter-lived objects in crowded orbits should be deorbited sooner.
  • Release constraints: avoid releasing anything intentionally, including fasteners, covers, or test objects.
  • Attachment design: every replaceable part should have a retention and disposal plan.

The desktop’s modularity can help. Attachments that fail or are retired can be removed and deorbited separately, reducing the total time hazardous components spend in orbit.

8. The cascade threshold

Space debris is a threshold problem. Each new fragment increases the collision risk for everyone. The individual contribution of one desktop is small, but the collective contribution of many desktops could be significant.

The Resident therefore treats debris mitigation as a public good. It is not enough to meet the minimum regulatory requirement. The desktop should be designed so that its operator can defend the claim: “This platform made orbit safer, or at least no more dangerous, than it would have been without us.”

What this changes

  • Passivation is defined as discharge, venting, depressurisation, and isolation of stored energy.
  • Batteries, propellant systems, pressurant tanks, and power buses all have end-of-life safe states.
  • Design for demise is acknowledged as a trade-off against long-life materials.
  • Operational debris mitigation includes collision avoidance, tracking, controlled releases, and attachment disposal plans.
  • The desktop should aim to leave orbit no more hazardous than it found it.
  • The next entry will consider reuse and salvage before final disposal.