PRD-003 — Mission Arc

Status · Draft v0.1 Sources · 02_Project_Concept.md §six-screens · ADR-009 · ADR-010 Audiences · curious learner, STEM student (PA §audiences) Promises · real physics, educational at every level, fail honestly (PA §promises) Principles · physics first, prototype is ground truth (PA §principles) Why this is a PRD · The fly screen is where the physics becomes visceral — the spacecraft actually moves, the distance grows, the signal delay becomes emotional. CAPCOM mode adds the flight controller perspective. This screen is the culmination of the Orrery narrative.

509 days. A person is watching a small white chevron move across a curved arc. The distance from Earth is 187 million kilometres. The signal delay is 10.4 minutes. They press CAPCOM. An event ticker appears. "T+259d · MARS FLYBY · Closest approach: 300 km altitude. Gravity assist initiates return." The chevron is past Mars now. It is on its way home.

The problem

Every Mars mission launches, flies for months, arrives, and does science. The transit — 259 days of deep space cruise — is invisible. There are no images. There is nothing to show. The public understands launches and landings. The 180 million kilometres in between is a blank.

The free-return flyby makes the transit the story. The spacecraft leaves Earth, swings around Mars, and comes home. 509 days. The arc is visible. The signal delay is measurable. The mission has a shape.

The experience

The screen opens on a 3D view. The Sun is at the centre. Earth and Mars orbit in real time. A transfer arc connects them — the outbound leg in blue, the return leg in violet. A white rocket icon moves along the arc, oriented along its velocity vector.

The HUD shows heliocentric velocity (updating from vis-viva), distance from Earth in millions of km and light-minutes, distance from Mars, fuel percentage, estimated arrival, and mission phase.

The user switches to 2D. The orbital view is top-down. The full free-return trajectory is visible — outbound arc solid blue, return arc solid violet, future path dashed. FLYBY and RETURN markers label the endpoints. The rocket chevron points along its direction of travel.

The user presses CAPCOM. Three panels appear alongside the left HUD. The mission event ticker shows all 13 events from LAUNCH to EARTH RETURN, ticking off as time advances. The communications panel shows "LAST TELEMETRY: 10.4 min ago" in real-time — the one-way signal delay computed from the actual Earth distance. The anomaly monitor watches fuel and delta-v margin.

The user speeds up to 90 days/second. The Mars flyby arrives — a pulsing ring appears briefly at closest approach. The return leg begins. Earth grows. The ticker counts down.

Why now

The fly screen is Act 3 — the culmination. The explorer showed the geometry, the configurator showed the cost, and the fly screen shows what it actually looks like to cross that geometry. Without it, the mission planning exercise has no payoff.

Success looks like

A user watches the signal delay grow from 3 minutes to 11 minutes over the outbound arc and says "I never understood why communicating with Mars is so hard until now."

A student uses the CAPCOM ticker to identify the TMI burn, the flyby, and the return trajectory initiation, and can explain what each event means.

A user is emotionally present during the Mars flyby — the pulsing marker and the CAPCOM event create a genuine sense of occasion.

Out of scope

• Mars orbit insertion (free-return trajectory only in v1 — ADR-009)
• Real historical mission telemetry (ORRERY-1 is a fictional free-return scenario)
• Audio
• Multiplayer / shared viewing

The sharpest threat

The CAPCOM mode adds complexity without adding clarity if the event notes are generic. "Trajectory correction manoeuvre" means nothing to a curious learner. Each event note must be written in plain language that makes the event feel real — what it means, why it happens, what it changes.

Open threads

• UXS-003 for full visual contract
• RFC-002 for events array schema in mission JSON
• RFC-004 for URL sharing (/fly?mission=id)
• ADR-009 locks free-return flyby as the scenario
• ADR-010 locks Keplerian arcs

Links

• Prototype: docs/prototypes/P03_mission-arc.html
• Concept: docs/concept/02_Project_Concept.md §six-screens