1. The Hermes problem
Andy Weir’s The Martian is a long exercise in orbital mechanics as plot device. The Hermes spacecraft does not go where the characters want; it goes where the planets and propellant allow. Every rescue plan is a negotiation between delta-v, time, and mass. The lesson is that orbit is not a location; it is a budget. Once you spend it, you cannot unspend it.
This entry puts the desktop’s orbit choices into that budget frame.
2. The requirements
The first-generation desktop must:
- Get to orbit cheaply enough that the programme survives.
- Survive long enough to prove the attachment architecture.
- Talk to the ground often enough to debug and operate.
- Keep its thermal and power budgets closed without heroic radiation hardening.
These are LEO-shaped requirements. They favor access, heritage, and forgiveness over persistence or special geometry.
3. The comparison
| Orbit | Launch cost | Radiation | Drag | Ground access | Latency | Thermal | Persistence |
|---|---|---|---|---|---|---|---|
| LEO | Low | Moderate | Significant | Frequent passes | Low | Moderate cycling | Minutes per pass |
| HEO | Moderate | High (belt crossings) | Low at apogee | Long dwell over one hemisphere | Moderate to high | Large swings | Hours per day over region |
| MEO | Moderate to high | High (belts) | Negligible | Longer gaps | Moderate | Stable | Hours per pass |
| GEO | High | High (outer belt) | Negligible | Continuous from fixed ground sites | High (~240 ms round trip) | Hot eclipses | Permanent over one point |
For the desktop’s first phase, the dominant column is launch cost. LEO wins because rideshare is abundant and the energy barrier is low. The other orbits only win when persistence or dwell becomes the dominant requirement.
4. When the higher orbits win
HEO wins when the customer needs hours of continuous coverage over high latitudes and can tolerate the radiation dose. GEO wins when the customer needs continuous equatorial coverage and has a ground station to match. MEO wins when the mission is navigation, timing, or a constellation that trades individual pass length against global coverage.
None of these customers exist for the first desktop. The first desktop’s customer is the programme itself, and the programme needs flight heritage more than it needs persistence.
5. What this changes
- LEO is reconfirmed as the first-generation orbit.
- HEO, MEO, and GEO are shelved with explicit activation conditions.
- Orbit is added to the list of things that are not immutable but are deferred.
- The next entry closes the arc.