1. The long way down

Andy Weir’s The Martian is mostly about staying alive, but its engine is the problem of returning from a place that does not want to let you go. Every solution is improvised from available mass, energy, and time. Deorbiting a spacecraft is less dramatic, but the same variables apply: how much impulse do you have, how patient can you be, and what happens if the plan fails.

Entry 274 argued that end-of-life is an architectural requirement. This entry looks at the first branch of that requirement: the actual deorbit.

2. The physics in one paragraph

A spacecraft in LEO stays up because it is moving fast enough that the Earth’s surface curves away beneath it. To come down, it must lose energy. The cheapest way is to let atmospheric drag do the work. The fastest way is to use propulsion to drop perigee into dense air. The safest way is to control where and when the reentry happens.

Those three goals — cheap, fast, safe — are usually in tension.

3. Propulsive deorbit

The most controlled method is a retrograde burn that lowers perigee enough for rapid reentry. For a typical LEO orbit, a few tens of metres per second of delta-v is enough to guarantee reentry within one orbit.

Advantages:

  • Predictable reentry time and ground track.
  • Low casualty risk if aimed at an ocean.
  • Works regardless of solar activity or atmospheric density.

Disadvantages:

  • Requires propulsion system to be functional at end of life.
  • Consumes propellant that might otherwise extend mission life.
  • Adds failure modes: if the burn is incomplete, the orbit becomes elliptical and unpredictable.

For the desktop, propulsive deorbit is the baseline. The platform already has propulsion for station-keeping and collision avoidance. Using the last of the fuel for disposal is the standard move.

4. Drag augmentation

If propulsion is exhausted or unreliable, drag can be increased artificially. Options include:

  • Drag sails: thin membranes deployed from the spacecraft to increase area-to-mass ratio.
  • Inflatable balloons: larger cross-section, shorter orbital lifetime.
  • Electrodynamic tethers: a conducting tether interacts with Earth’s magnetic field and ionosphere to generate drag.

Advantages:

  • Does not consume propellant.
  • Can be passive once deployed.
  • Works well for small, low-mass objects.

Disadvantages:

  • Lifetime depends on atmospheric density, which varies with solar activity.
  • Deployment mechanisms can fail.
  • Less controlled than propulsive deorbit.

A drag sail might be a useful backup for the desktop, especially if the propulsion attachment has been removed or is non-functional at end of life.

5. Natural decay

Do nothing and wait. Atmospheric drag will eventually bring the spacecraft down. The timeline ranges from months to centuries depending on altitude and area-to-mass ratio.

This is the default for debris, but it is not a plan. It creates uncertainty, increases collision risk during the wait, and may violate operator obligations or national regulations.

The desktop should not rely on natural decay as its primary disposal strategy.

6. Controlled reentry constraints

A controlled deorbit must satisfy several constraints:

  • Casualty risk: the probability of human injury from surviving debris must be below a threshold, typically 1 in 10,000.
  • Ground track: the reentry must avoid populated landmasses, shipping lanes, and air routes.
  • Timing: the deorbit must occur when ground stations can monitor it and when emergency services can respond if needed.
  • Survivability: the spacecraft should be designed to break up and burn as completely as possible.
  • Notification: other space operators and aviation authorities must be informed.

These constraints mean that deorbit planning starts years before the event.

7. Desktop-specific considerations

The desktop’s modularity changes the deorbit equation:

  • If attachments have been replaced over the years, the final mass and centre of gravity may differ from the original design.
  • Customer payloads may need to be removed before deorbit, either for return or separate disposal.
  • Hazardous materials, such as batteries and propellant, must be passivated first.
  • The robotic arm may be used to jettison or stow components that would not burn up cleanly.

A modular platform can also be partially salvaged before disposal. Entry 277 will look at that option.

8. The backup stack

A robust end-of-life plan has layers:

  1. Primary: propulsive controlled deorbit with reserved fuel.
  2. Secondary: drag augmentation if propulsion fails.
  3. Tertiary: active debris removal contract if both fail.
  4. Emergency: design for demise to minimise surviving debris if reentry is uncontrolled.

The desktop should have at least the first two layers.

What this changes

  • Propulsive deorbit is the baseline for the desktop, using reserved end-of-life fuel.
  • Drag augmentation is a useful backup, especially if propulsion is unavailable.
  • Natural decay is not an acceptable primary strategy.
  • Controlled reentry must satisfy casualty risk, ground track, timing, and notification constraints.
  • Modularity affects final mass, attachments, and hazardous-material handling before disposal.
  • The next entry will cover passivation and debris mitigation.