# Gravity Assist Swingby > Use when you must analyze a gravity-assist swing-by maneuver of a spacecraft past a planet or moon: compute the periapsis speed from the hyperbolic excess velocity with the vis-viva energy integral, the flyby turn angle, the delta-v gain for the heliocentric velocity change, and the outgoing direction for outside or inside passes, and check close approach feasibility against the body radius and minimum altitude. Produces the single-flyby summary with periapsis speed, turn angle, delta-v gain and the pass verdict that gates interplanetary trajectory design. Trigger: gravity assist, swing-by, hyperbolic excess velocity, turn angle, periapsis speed, delta-v gain, patched conic flyby, close approach altitude. - Skill: `ashfordeou/gravity-assist-swingby` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/gravity-assist-swingby` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/gravity-assist-swingby/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/gravity-assist-swingby --- # Gravity Assist Swing-by (space-systems/orbit-mechanics/gravity-assist-swingby) Use when the task is analyzing a single gravity-assist (swing-by) flyby of a spacecraft past a planet or moon: the spacecraft arrives on a hyperbolic approach with excess speed v_inf, swings through periapsis at radius rp, and leaves with the same excess speed magnitude but a rotated direction. This leaf computes the periapsis speed from the vis-viva energy integral, the flyby turn angle, the heliocentric delta-v gain, the outgoing excess velocity direction, and the close-approach feasibility verdict against the body radius plus a minimum altitude. It implements the standard single-flyby patched-conic model in pure Python, stdlib only. It pairs with space-systems/orbit-mechanics/lambert-transfer and space-systems/orbit-mechanics/hohmann-transfer as the transfer method alternatives around the flyby, and with space-systems/orbit-mechanics/orbital-perturbations for the flyby perturbation context. ## Domain quick reference - Periapsis speed from the energy integral: vp = sqrt(v_inf^2 + 2*mu/rp), with mu the body gravitational parameter. At periapsis the excess speed is purely radial, so the vis-viva equation collapses to this form. - Hyperbola eccentricity: e = 1 + rp*v_inf^2/mu. The excess speed fixes the specific energy, the periapsis radius fixes the angular momentum, and together they set the eccentricity. - Turn angle: delta = 2*asin(1/e), the rotation of the excess velocity vector during the flyby. Fast, distant passes turn little; slow, grazing passes can turn beyond 90 degrees. - Delta-v gain: dv = 2*v_inf*sin(delta/2). For a single unpowered flyby the excess speed magnitude is unchanged, so the change of the heliocentric velocity vector has this magnitude. - Outgoing direction: outgoing = incoming + turn_sign * delta, with turn_sign +1 for the outside pass and -1 for the inside pass geometry relative to the central body. - Feasibility: altitude = rp - body_radius; the pass is feasible when altitude >= min_alt and rp >= body_radius (the flyby clears the surface by the minimum altitude). - Units are SI throughout: m/s, m, m^3/s^2, radians and degrees. - ECSS frames the mission analysis context (ECSS-E-ST-10 series); the relations above are standard engineering methodology, summary-only. ## Workflow 1. Gather the flyby inputs: the arrival hyperbolic excess speed v_inf_ms, the periapsis radius rp_m, and the body gravitational parameter mu_body (default MU_EARTH = 3.986004418e14 m^3/s^2). 2. Get the periapsis speed with periapsis_speed: vp = sqrt(v_inf^2 + 2*mu/rp). This is the peak speed the spacecraft sees at closest approach, the driver for the thermal and delta-v bookkeeping. 3. Compute the turn angle with turn_angle_rad and convert to degrees for reporting; this is how much the excess velocity vector swings. 4. Get the delta-v gain with dv_gain(v_inf_ms, delta_rad), the heliocentric velocity change the planet imparts for free. 5. Resolve the outgoing excess velocity direction with outgoing_direction_deg, picking turn_sign +1 (outside pass) or -1 (inside pass) for the flyby geometry at hand. 6. Check the geometry with feasibility(rp_m, body_radius_m, min_alt_m): the altitude above the body surface and the pass verdict against the minimum altitude. 7. Run the full summary with analyze(v_inf_ms, rp_m, incoming_deg, mu_body, body_radius_m, min_alt_m) and report vp, delta, dv, outgoing direction, altitude and the pass verdict together. 8. Confirm the deterministic checks with the contract test scripts/test_gravity_assist_swingby.py. ## Worked example Earth swing-by with v_inf = 3000 m/s at rp = 7000 km, body radius 6371 km, mu = 3.986004418e14 m^3/s^2. - Periapsis speed: vp = sqrt(9e6 + 2*3.986004418e14/7e6) = sqrt(1.22886e8) = 11085.4 m/s. - Turn angle: e = 1 + 7e6*9e6/3.986004418e14 = 1.15806, so delta = 2*asin(1/1.15806) = 2*asin(0.86351) = 119.43 deg. - Delta-v gain: dv = 2*3000*sin(59.714 deg) = 6000*0.86351 = 5181.1 m/s. - Altitude: 7000 - 6371 = 629 km, above the 200 km minimum, so the pass is feasible. - Outgoing direction: incoming 0 deg with turn_sign +1 gives 119.43 deg. - Second case: v_inf = 5000 m/s at the same rp gives vp = 11785.0 m/s, delta = 88.04 deg, dv = 6949 m/s; the faster flyby turns less and gains more heliocentric speed. ## Pitfalls - Confusing periapsis speed with the excess speed: vp = sqrt(v_inf^2 + 2*mu/rp) is the peak speed at closest approach (11085.4 m/s in the worked example), far above the 3000 m/s excess; the excess speed magnitude is unchanged across the flyby, only its direction rotates. - Forgetting the turn_sign geometry: an outside pass uses +1 and an inside pass -1 relative to the central body; swapping the sign sends the outgoing direction the wrong way around the planet. - Planning a flyby that grazes the planet: feasibility checks altitude = rp - body_radius against min_alt and rejects periapsis inside the body radius when supplied; a 629 km pass clears a 200 km minimum, an embedded one does not. - Ignoring the speed-trade of the flyby: a faster excess turns less and gains more heliocentric speed (5000 m/s gives delta 88.04 deg and dv 6949 m/s versus 119.43 deg and 5181.1 m/s at 3000 m/s), so the turn angle and the delta-v gain trade against each other. - Using the wrong body mu: the functions take mu_body explicitly and default to Earth (MU_EARTH); a Mars or lunar swing-by with the Earth default silently changes vp, e, and the turn angle. - Treating a single patched-conic flyby as a trajectory: this leaf sizes one unpowered pass; the legs before and after belong to the lambert-transfer and hohmann-transfer leaves. ## Verification - Confirm periapsis_speed(3000.0, 7000e3, MU_EARTH) returns 11085.4 m/s within 0.5 and analyze on the same case returns delta 119.43 deg, dv 5181.1 m/s, altitude 629 km and pass True. - Confirm the 5000 m/s case: vp 11785.0 m/s, delta 88.04 deg, dv 6949 m/s. - Confirm the energy-integral round trip: vp^2 - v_inf^2 equals 2*mu/rp for any valid input. - Confirm ValueError rejection of v_inf below zero, rp at or below zero, mu at or below zero, a flyby periapsis inside the body when the body radius is supplied, and a turn_sign other than +1 or -1. - Run the contract test offline: python3 scripts/test_gravity_assist_swingby.py (33 tests, deterministic). ## Related leaves - space-systems/orbit-mechanics/lambert-transfer: the two-position boundary-value transfer that replaces or follows the flyby. - space-systems/orbit-mechanics/hohmann-transfer: the coplanar two-impulse alternative for the transfer budget. - space-systems/orbit-mechanics/orbital-perturbations: flyby effects and third-body context around the patched-conic assumption. - space-systems/mission-design/launch-window-analysis: the interplanetary geometry that sets the incoming excess speed. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_gravity_assist_swingby.py The test covers both worked examples (periapsis speed, turn angle and delta-v gain at 3 km/s and 5 km/s excess speed), the Mars flyby feasibility case, periapsis-speed bounds and the energy-integral round-trip identity, turn angle bounds, the dv-gain formula identity, inside and outside pass outgoing directions, feasibility verdicts at and below the minimum altitude and inside the body, the full analyze summary dict, and ValueError rejection of negative excess speed, non-positive periapsis radius or gravitational parameter, flybys inside the body, and invalid turn signs. ## Compliance - Standards referenced, not reproduced: ECSS (ECSS-E-ST-10 series) is a free ESA download (ecss.nl/standards); the swing-by relations above are standard engineering methodology, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.