1. The authority of the electron

Robert Heinlein’s The Moon is a Harsh Mistress is about a colony where water, air, and power are the currencies of survival. Whoever controls the mass drivers and the solar farms controls the society. On the desktop, power is not political in that sense, but it is still the constraint everything else queues behind.

Entry 236 closed the sensor arc and announced that power is next. This entry defines what the power attachment actually does.

2. The three jobs

The power attachment has three jobs:

  • Generate electricity from sunlight.
  • Store enough electricity to survive eclipse and peak loads.
  • Distribute electricity to every other attachment in the right voltage, current, and reliability class.

These sound like one subsystem, but they are often designed by different teams with different heritage. Solar arrays are deployable structures. Batteries are electrochemical devices with thermal and cycling limits. Power distribution units are boxes of switches, converters, and protection circuits. The attachment model treats them as one logical unit because the desktop’s economics depend on the sum.

3. Generation

The desktop needs enough solar array area to produce more power than it consumes on average, so the batteries can recharge during sunlight and the system can survive degradation.

Generation must also cope with:

  • Eclipse. In LEO, a spacecraft spends roughly 35 minutes of each 90-minute orbit in Earth’s shadow. During that time the arrays produce nothing.
  • Degradation. Solar cells lose output from radiation damage and atomic oxygen erosion over the mission life.
  • Pointing. Body-mounted panels produce less than sun-tracking panels, but sun-tracking panels add mechanism and control complexity.
  • Season. The angle of sunlight varies through the year, changing the effective array area.

The generation subsystem is sized for the worst-case combination of these effects, not for nominal conditions.

4. Storage

Batteries carry the desktop through eclipse and provide surge capacity for peak loads. The key parameters are:

  • Energy capacity: how many watt-hours can be stored.
  • Depth of discharge: how much of that capacity can be used without shortening battery life.
  • Cycle life: how many charge-discharge cycles the battery can survive.
  • Charge and discharge rate: how fast energy can move in and out.
  • Temperature range: batteries perform poorly when too cold and age faster when too hot.

The storage subsystem is sized so the desktop can survive the longest eclipse plus a margin, while keeping depth of discharge within the range that gives the desired mission life.

5. Distribution

Power distribution is the least glamorous job and the one that causes the most integration arguments. It must:

  • Convert the battery voltage to the voltages each attachment needs.
  • Switch attachments on and off independently.
  • Protect against overcurrent, undervoltage, and reverse polarity.
  • Provide telemetry so operators can see who is drawing what.
  • Manage inrush current when attachments wake up.
  • Support hot-swap or isolation of failed attachments without killing the bus.

The distribution architecture is where the desktop’s promise of modular attachments meets reality. If every attachment needs a different voltage and a custom connector, the grid is not modular. If the distribution unit is too rigid, it cannot host unexpected payloads.

6. Peak load management

The desktop’s attachments do not all run at full power simultaneously. Imaging, high-rate downlink, compute-intensive processing, and robotics each create peak loads. The power attachment must either supply the sum of the peaks or coordinate the loads so the peaks do not overlap.

Coordination is usually cheaper than oversizing. The operations scheduler can delay a non-urgent processing job until after a high-rate downlink, or pause imaging during a battery-critical eclipse exit. The power attachment provides the telemetry and switch control that makes this coordination possible.

7. Why this is an attachment

Calling power an attachment, rather than part of the platform bus, keeps the desktop’s architecture honest. The power subsystem has a defined interface, a defined mass and volume budget, and a service contract with the rest of the grid. It can be upgraded or replaced. It can be supplied by one vendor and integrated by another.

It also makes the economics visible. Power is not a hidden overhead; it is a billed resource. Customer payloads pay per watt. The desktop’s own subsystems pay per watt in the internal accounting. This discipline prevents the platform from silently subsidising inefficient attachments.

What this changes

  • The power attachment covers generation, storage, and distribution as one logical unit.
  • Generation is sized for eclipse, degradation, pointing, and seasonal effects.
  • Storage is sized for eclipse duration plus margin, with cycle life and depth of discharge as constraints.
  • Distribution provides voltage conversion, switching, protection, telemetry, and load coordination.
  • Treating power as an attachment makes its cost and upgrade path visible.
  • The next entry can estimate the desktop’s total power demand and the solar array area required to meet it.