ADR-009 — Free-return flyby as mission scenario

Status · Accepted Date · 2026-04-15 TA anchor · §components/fly-screen

Context

The fly screen needed a mission scenario. The original prototype used a Mars orbit insertion (MOI) scenario — outbound only, 259 days, spacecraft enters Mars orbit. During development this was reconsidered.

Decision

The default mission scenario (ORRERY-1) is a free-return Mars flyby: 259 days outbound, Mars closest approach at ~300 km altitude, 250 days return to Earth. Total 509 days. No landing. No MOI burn.

Rationale

A free-return trajectory is analogous to Artemis II at interplanetary scale — a real mission architecture studied for crewed Mars missions. It is more physically interesting than a one-way cruise: the spacecraft returns to Earth, the return arc is distinct from the outbound arc, and the gravity assist at Mars is meaningful. It also requires less delta-v (no MOI burn), making the mission viable on a Falcon Heavy with 2,500 kg payload — the chosen vehicle.

Alternatives considered

• Mars orbit insertion — simpler single-arc trajectory; less interesting; requires 2.21 km/s MOI burn that consumes most remaining propellant.
• Mars landing scenario — requires atmospheric entry physics not implemented in the physics library; out of scope for v1.
• Multiple selectable scenarios — good long-term feature; adds complexity to the fly screen data model. v2 candidate.

Consequences

Positive: physically richer scenario; two distinct arc segments visible on screen; CAPCOM events span the full 509-day mission including return. Negative: 509-day simulation is longer to scrub through; the "mission arrives" completion state is Earth return, not Mars arrival — some users may expect Mars landing.