1. The flip-and-burn habit

James S.A. Corey’s Leviathan Wakes spends a lot of pages on orientation. The Canterbury and later the Rocinante do not drift through space like boats; they burn, flip, and burn again. The crew is always aware of which way is “down,” because down is wherever the engines are pointing. Pointing is not a cosmetic choice. It is how you get somewhere, how you avoid killing yourself, and how you live inside the machine.

The desktop has no crew to keep alive, but the same principle applies. Its attitude determination and control system — ADCS — is the subsystem that decides which way the body faces and enforces that decision.

2. What pointing means in LEO

A desktop in low Earth orbit is moving at roughly 7.6 km/s. It is also rotating once per orbit if it wants to keep one face toward Earth, or it may want to stay fixed relative to the stars, or it may want to keep its solar arrays aimed at the Sun. Each of these is a different pointing mode, and ADCS is what makes any of them happen.

ADCS has two jobs:

  • Attitude determination: figure out which way the desktop is currently pointing, using sensors.
  • Attitude control: apply torque to change or maintain that pointing, using actuators.

Without ADCS, the desktop is a tumbling box. With it, the desktop becomes a platform that can look at things on purpose.

3. Why ADCS is its own attachment

At first glance, ADCS is just software and a few small moving parts. It could be absorbed into propulsion, or power, or the computer. But it deserves its own attachment because it sits at the centre of almost every other subsystem.

  • The imager needs nadir pointing with low jitter to take useful pictures.
  • The solar arrays need sun pointing to generate power.
  • The radiators need cold-sky pointing to reject heat.
  • The star trackers need a clear view away from Earth and Sun.
  • The antennas need to point at ground stations or other spacecraft.
  • The thrusters need the body oriented so their plumes miss deployables.

ADCS is the negotiator between these competing demands. It cannot satisfy all of them at once, so it must sequence them and trade them off.

4. Pointing modes the desktop needs

From the arcs already written, the desktop needs at least these modes:

  • Nadir pointing: the body z-axis points at Earth’s centre. Used for imaging, some communications, and Earth-relative operations.
  • Sun pointing: the solar array normal tracks the Sun. Used for power generation and battery charging.
  • Inertial pointing: the body stays fixed relative to the stars. Used for astronomy, some observation campaigns, and momentum dumping.
  • Target pointing: the body or an instrument tracks a specific ground target, another spacecraft, or a celestial object.
  • Sun-safe / survival: a safe orientation that keeps solar arrays roughly sunward and radiators shaded while the desktop recovers from a fault.

Each mode has different accuracy and stability requirements. Nadir pointing for a medium-resolution imager might tolerate a few degrees of error. A star tracker or laser communicator might need arc-second stability.

5. Accuracy and stability

Pointing accuracy is how close the actual aim is to the intended aim. Pointing stability is how still it stays once it gets there. The desktop’s current payload does not need arc-second performance, but it does need to know what performance it needs.

A rough hierarchy:

  • Coarse: ±1 to ±5 degrees. Enough for solar array capture and safe mode.
  • Moderate: ±0.1 to ±1 degree. Enough for nadir imaging and routine communications.
  • Fine: ±1 to ±100 arcseconds. Needed for high-resolution imagers, lasercom, or precision science.

The desktop probably lives in the moderate band for most operations, with coarse backup for safe mode.

6. The ADCS questions to answer

The next entries should work through:

  • What sensors are needed to determine attitude?
  • What actuators are needed to control it?
  • How is the control law partitioned between coarse and fine modes?
  • How does ADCS recover from sensor faults, actuator saturation, and safe mode?

These are ordinary questions, but the answers decide whether the desktop can do any of the things the other attachments were built for.

What this changes

  • ADCS is now the next attachment to define, following the propulsion arc.
  • The desktop needs multiple pointing modes: nadir, sun, inertial, target, and survival.
  • Pointing requirements come from the imager, solar arrays, radiators, star trackers, antennas, and thrusters.
  • The desktop likely needs moderate pointing accuracy for routine operations, with coarse backup for safe mode.
  • The next entry will define the sensor suite.