commit 655f1c9af633cabf717d8844f9e7fe9a03c1ed25
Author: Lucy <lucy.wonderful@proton.me>
Date:   Thu May 21 20:31:17 2026 +0200

    Initial commit: Keplerian orbital mechanics with patched-conics SOI transitions
    
    - orbital_elements.py: elliptical + hyperbolic orbit support
    - orbit_drawer.py: orbit point generation with SOI truncation
    - soi_calculator.py: SOI crossing time calculator
    - frame_transition.py: reference frame switching
    - test_orbital.py: 147 assertions, all passing
    - visual_test.py: pygame flyby visualization

diff --git a/frame_transition.py b/frame_transition.py
new file mode 100644
index 0000000..b4d1817
--- /dev/null
+++ b/frame_transition.py
@@ -0,0 +1,90 @@
+"""
+Reference-frame transitions for patched-conics SOI handling.
+
+When a body enters a planet's sphere of influence its orbital
+elements must be re-expressed relative to the planet.  On exit
+the reverse transition occurs.
+"""
+
+from typing import Tuple
+
+from orbital_elements import OrbitalElements, orbital_elements_from_cartesian
+
+
+def sun_state_to_planet_orbit(
+    body_x: float,
+    body_y: float,
+    body_vx: float,
+    body_vy: float,
+    planet_x: float,
+    planet_y: float,
+    planet_vx: float,
+    planet_vy: float,
+    mu_planet: float,
+    epoch: float,
+) -> OrbitalElements:
+    """
+    Convert a body's sun-frame state to a planet-relative orbit.
+
+    Parameters
+    ----------
+    body_*  — body position & velocity in sun frame
+    planet_* — planet position & velocity in sun frame
+    mu_planet — planet's gravitational parameter
+    epoch — time of transition
+
+    Returns
+    -------
+    OrbitalElements describing the body's trajectory around the planet.
+    """
+    x_rel = body_x - planet_x
+    y_rel = body_y - planet_y
+    vx_rel = body_vx - planet_vx
+    vy_rel = body_vy - planet_vy
+    return orbital_elements_from_cartesian(
+        x_rel, y_rel, vx_rel, vy_rel, mu_planet, epoch,
+    )
+
+
+def planet_orbit_to_sun_state(
+    orbit_planet: OrbitalElements,
+    planet_x: float,
+    planet_y: float,
+    planet_vx: float,
+    planet_vy: float,
+    t: float,
+) -> Tuple[float, float, float, float]:
+    """
+    Convert a planet-relative orbit back to sun-frame state at time t.
+
+    Returns (x, y, vx, vy) in sun frame.
+    """
+    x_rel, y_rel, vx_rel, vy_rel = orbit_planet.compute_state_at(t)
+    return (
+        x_rel + planet_x,
+        y_rel + planet_y,
+        vx_rel + planet_vx,
+        vy_rel + planet_vy,
+    )
+
+
+def planet_orbit_to_sun_orbit(
+    orbit_planet: OrbitalElements,
+    planet_x: float,
+    planet_y: float,
+    planet_vx: float,
+    planet_vy: float,
+    mu_sun: float,
+    t: float,
+) -> OrbitalElements:
+    """
+    Full round-trip: planet-relative orbit → sun-frame orbital elements.
+
+    Used after SOI exit to put the body back into a heliocentric orbit.
+    """
+    x_sun, y_sun, vx_sun, vy_sun = planet_orbit_to_sun_state(
+        orbit_planet, planet_x, planet_y, planet_vx, planet_vy, t,
+    )
+    return orbital_elements_from_cartesian(
+        x_sun, y_sun, vx_sun, vy_sun, mu_sun, t,
+    )
diff --git a/orbit_drawer.py b/orbit_drawer.py
new file mode 100644
index 0000000..323c398
--- /dev/null
+++ b/orbit_drawer.py
@@ -0,0 +1,77 @@
+"""
+Orbit trajectory point generation for rendering.
+
+generate_orbit_points() — full orbit (elliptical) or truncated
+  hyperbolic path within a given cut-off radius.
+"""
+
+import math
+from typing import List, Optional, Tuple
+
+from orbital_elements import OrbitalElements, TAU
+
+
+def generate_orbit_points(
+    orbit: OrbitalElements,
+    num_points: int = 200,
+    max_radius: Optional[float] = None,
+) -> List[Tuple[float, float]]:
+    """
+    Generate *num_points* world-space positions along the orbit.
+
+    Elliptical orbits → full closed loop, equally spaced in E.
+    Hyperbolic orbits → samples H ∈ [−H_max, +H_max], truncated
+      at *max_radius* (e.g. the planet's SOI edge).
+
+    Parameters
+    ----------
+    max_radius : float or None
+        Only used for hyperbolic orbits.  Points with r > max_radius
+        are omitted.  If None, samples out to r ≈ 5·|a| from focus.
+    """
+    if orbit.is_hyperbolic:
+        return _hyperbolic_points(orbit, num_points, max_radius)
+    return _elliptical_points(orbit, num_points)
+
+
+def _elliptical_points(
+    orbit: OrbitalElements, num_points: int,
+) -> List[Tuple[float, float]]:
+    points = []
+    for i in range(num_points):
+        E = TAU * i / num_points
+        points.append(orbit.position_from_E(E))
+    return points
+
+
+def _hyperbolic_points(
+    orbit: OrbitalElements,
+    num_points: int,
+    max_radius: Optional[float],
+) -> List[Tuple[float, float]]:
+    """
+    Sample the hyperbolic branch that passes through periapsis.
+
+    Truncates at *max_radius* by solving for the H values where
+    r = max_radius, then sampling uniformly between those limits.
+    """
+    r_limit = max_radius if max_radius is not None else 5.0 * abs(orbit.a)
+
+    # Find H where r = r_limit.
+    # r = |a|(e·cosh(H) − 1)  →  cosh(H) = (r/|a| + 1) / e
+    abs_a = abs(orbit.a)
+    cosh_Hmax = (r_limit / abs_a + 1.0) / orbit.e
+    if cosh_Hmax < 1.0:
+        return []  # entire orbit is outside the radius
+    H_max = math.acosh(cosh_Hmax)
+
+    points = []
+    for i in range(num_points):
+        frac = -1.0 + 2.0 * i / (num_points - 1) if num_points > 1 else 0.0
+        H = frac * H_max
+        x, y = orbit.position_from_H(H)
+        r = math.hypot(x, y)
+        if r <= r_limit + 1e-9:
+            points.append((x, y))
+
+    return points
diff --git a/orbital_elements.py b/orbital_elements.py
new file mode 100644
index 0000000..0334295
--- /dev/null
+++ b/orbital_elements.py
@@ -0,0 +1,303 @@
+"""
+2D Keplerian orbital elements — elliptical and hyperbolic.
+
+Provides OrbitalElements dataclass with cached derived quantities for
+efficient position/velocity computation, plus cartesian ↔ orbital
+conversion.
+
+Supports both elliptical (e < 1, a > 0) and hyperbolic (e > 1, a < 0)
+orbits using the standard Keplerian formulation.
+"""
+
+import math
+from dataclasses import dataclass
+from typing import Tuple
+
+TAU = 2.0 * math.pi
+KEPLER_TOL = 1e-12
+KEPLER_MAX_ITER = 50
+
+
+@dataclass
+class OrbitalElements:
+    """
+    2D Keplerian orbital elements.
+
+    a       — semi-major axis.  a > 0 for ellipses,  a < 0 for hyperbolas.
+    e       — eccentricity.  e ≥ 0.  e < 1 = ellipse,  e > 1 = hyperbola.
+    omega   — argument of periapsis (rad), +x → periapsis.
+    M0      — mean anomaly at epoch (rad).  Wrapped to [0, 2π) for ellipses;
+              unbounded (can be any real) for hyperbolas.
+    epoch   — reference time.
+    mu      — gravitational parameter of central body.
+    """
+    a: float
+    e: float
+    omega: float
+    M0: float
+    epoch: float
+    mu: float
+
+    def __post_init__(self) -> None:
+        if self.e < 0.0:
+            raise ValueError("Eccentricity must be ≥ 0")
+
+        self._hyperbolic: bool = self.e > 1.0
+
+        if self._hyperbolic:
+            if self.a > 0:
+                raise ValueError("Hyperbolic orbits require a < 0")
+            self._abs_a = abs(self.a)
+            self._p = self._abs_a * (self.e * self.e - 1.0)  # semi-latus rectum
+            self._sqrt_mu_over_p = math.sqrt(self.mu / self._p)
+            self._n = math.sqrt(self.mu / self._abs_a ** 3)  # mean motion
+            self._sqrt_em1 = math.sqrt(self.e * self.e - 1.0)
+        else:
+            if self.a <= 0:
+                raise ValueError("Elliptical orbits require a > 0")
+            self._p = self.a * (1.0 - self.e * self.e)  # semi-latus rectum
+            self._sqrt_mu_over_p = math.sqrt(self.mu / self._p)
+            self._n = math.sqrt(self.mu / self.a ** 3)   # mean motion
+            self._sqrt_1me2 = math.sqrt(1.0 - self.e * self.e)
+            self._sqrt_mu_a = math.sqrt(self.mu * self.a)
+
+    # ── Convenience properties ──────────────────────────────────
+
+    @property
+    def period(self) -> float:
+        """Orbital period (s).  +inf for parabolas, meaningless for hyperbolas."""
+        if self._hyperbolic:
+            return float("inf")
+        return TAU / self._n
+
+    @property
+    def periapsis(self) -> float:
+        """Distance at closest approach."""
+        if self._hyperbolic:
+            return self._abs_a * (self.e - 1.0)
+        return self.a * (1.0 - self.e)
+
+    @property
+    def apoapsis(self) -> float:
+        """Distance at farthest point (ellipses only)."""
+        if self._hyperbolic:
+            return float("inf")
+        return self.a * (1.0 + self.e)
+
+    @property
+    def is_hyperbolic(self) -> bool:
+        return self._hyperbolic
+
+    def radius_range(self) -> Tuple[float, float]:
+        """(minimum, maximum) distance.  Max is inf for hyperbolas."""
+        return self.periapsis, self.apoapsis
+
+    # ── Kepler solvers ──────────────────────────────────────────
+
+    def solve_kepler(self, M: float) -> float:
+        """Solve Kepler's equation.  Returns E (ellipse) or H (hyperbola)."""
+        if self._hyperbolic:
+            return self._solve_kepler_hyperbolic(M)
+        return self._solve_kepler_elliptic(M)
+
+    def _solve_kepler_elliptic(self, M: float) -> float:
+        """M = E - e·sin(E)  →  E."""
+        e = self.e
+        if e < 1e-14:
+            return M
+        if e < 0.8:
+            E = M
+        else:
+            E = M + 0.85 * e * math.sin(M)
+        for _ in range(KEPLER_MAX_ITER):
+            dE = (E - e * math.sin(E) - M) / (1.0 - e * math.cos(E))
+            E -= dE
+            if abs(dE) < KEPLER_TOL:
+                break
+        return E
+
+    def _solve_kepler_hyperbolic(self, M: float) -> float:
+        """M = e·sinh(H) − H  →  H."""
+        e = self.e
+        if abs(M) < 1e-14:
+            return 0.0
+        # Robust initial guess
+        H = math.asinh(M / e)
+        for _ in range(KEPLER_MAX_ITER):
+            dH = (e * math.sinh(H) - H - M) / (e * math.cosh(H) - 1.0)
+            H -= dH
+            if abs(dH) < KEPLER_TOL:
+                break
+        return H
+
+    # ── Mean anomaly at time ────────────────────────────────────
+
+    def mean_anomaly_at(self, t: float) -> float:
+        """Mean anomaly M at time t."""
+        M = self.M0 + self._n * (t - self.epoch)
+        if self._hyperbolic:
+            return M           # unbounded — can be any real
+        return M % TAU          # wrap to [0, 2π)
+
+    # ── Position / state computation ────────────────────────────
+
+    def compute_state_at(self, t: float) -> Tuple[float, float, float, float]:
+        """World-space (x, y, vx, vy) at time t."""
+        M = self.mean_anomaly_at(t)
+        anomaly = self.solve_kepler(M)  # E for ellipse, H for hyperbola
+        if self._hyperbolic:
+            return self._state_from_H(anomaly)
+        return self._state_from_E(anomaly)
+
+    def position_at(self, t: float) -> Tuple[float, float]:
+        """World-space (x, y) at time t."""
+        x, y, _, _ = self.compute_state_at(t)
+        return x, y
+
+    # ── Elliptical: state from eccentric anomaly E ─────────────
+
+    def _state_from_E(self, E: float) -> Tuple[float, float, float, float]:
+        cosE = math.cos(E)
+        sinE = math.sin(E)
+
+        # Perifocal position
+        x_pf = self.a * (cosE - self.e)
+        y_pf = self.a * self._sqrt_1me2 * sinE
+
+        # Perifocal velocity
+        inv_denom = 1.0 / (1.0 - self.e * cosE)
+        sqrt_mu_over_a = math.sqrt(self.mu / self.a)
+        vx_pf = -sqrt_mu_over_a * sinE * inv_denom
+        vy_pf = sqrt_mu_over_a * self._sqrt_1me2 * cosE * inv_denom
+
+        return self._rotate_to_world(x_pf, y_pf, vx_pf, vy_pf)
+
+    def position_from_E(self, E: float) -> Tuple[float, float]:
+        cosE = math.cos(E)
+        sinE = math.sin(E)
+        x_pf = self.a * (cosE - self.e)
+        y_pf = self.a * self._sqrt_1me2 * sinE
+        cos_om = math.cos(self.omega)
+        sin_om = math.sin(self.omega)
+        return (
+            x_pf * cos_om - y_pf * sin_om,
+            x_pf * sin_om + y_pf * cos_om,
+        )
+
+    # ── Hyperbolic: state from hyperbolic anomaly H ────────────
+
+    def _state_from_H(self, H: float) -> Tuple[float, float, float, float]:
+        coshH = math.cosh(H)
+        sinhH = math.sinh(H)
+
+        # Perifocal position  x = |a|(e − coshH),  y = |a|√(e²−1)·sinhH
+        x_pf = self._abs_a * (self.e - coshH)
+        y_pf = self._abs_a * self._sqrt_em1 * sinhH
+
+        # Distance  r = |a|(e·coshH − 1)
+        r = self._abs_a * (self.e * coshH - 1.0)
+
+        # True anomaly trig
+        cos_nu = x_pf / r
+        sin_nu = y_pf / r
+
+        # Perifocal velocity (unified ν-formula, works for both orbit types)
+        vx_pf = -self._sqrt_mu_over_p * sin_nu
+        vy_pf = self._sqrt_mu_over_p * (cos_nu + self.e)
+
+        return self._rotate_to_world(x_pf, y_pf, vx_pf, vy_pf)
+
+    def position_from_H(self, H: float) -> Tuple[float, float]:
+        coshH = math.cosh(H)
+        sinhH = math.sinh(H)
+        x_pf = self._abs_a * (self.e - coshH)
+        y_pf = self._abs_a * self._sqrt_em1 * sinhH
+        cos_om = math.cos(self.omega)
+        sin_om = math.sin(self.omega)
+        return (
+            x_pf * cos_om - y_pf * sin_om,
+            x_pf * sin_om + y_pf * cos_om,
+        )
+
+    # ── Rotate perifocal → world frame ─────────────────────────
+
+    def _rotate_to_world(
+        self, x_pf: float, y_pf: float, vx_pf: float, vy_pf: float,
+    ) -> Tuple[float, float, float, float]:
+        cos_om = math.cos(self.omega)
+        sin_om = math.sin(self.omega)
+        return (
+            x_pf * cos_om - y_pf * sin_om,
+            x_pf * sin_om + y_pf * cos_om,
+            vx_pf * cos_om - vy_pf * sin_om,
+            vx_pf * sin_om + vy_pf * cos_om,
+        )
+
+
+# ── Cartesian → Orbital Elements ─────────────────────────────────
+
+
+def orbital_elements_from_cartesian(
+    x: float,
+    y: float,
+    vx: float,
+    vy: float,
+    mu: float,
+    epoch: float = 0.0,
+) -> OrbitalElements:
+    """
+    Convert (x, y, vx, vy) to Keplerian orbital elements.
+
+    Handles both elliptical (energy < 0) and hyperbolic (energy > 0)
+    trajectories.
+    """
+    r = math.hypot(x, y)
+    v_sq = vx * vx + vy * vy
+    energy = 0.5 * v_sq - mu / r
+    r_dot_v = x * vx + y * vy
+
+    # Semi-major axis (negative for hyperbolas)
+    if abs(energy) < 1e-15:
+        raise ValueError("Parabolic trajectory not supported (energy ≈ 0)")
+    a = -mu / (2.0 * energy)   # negative for hyperbolic, positive for elliptical
+
+    # Eccentricity vector
+    factor = v_sq / mu - 1.0 / r
+    ex = factor * x - r_dot_v * vx / mu
+    ey = factor * y - r_dot_v * vy / mu
+    e = math.hypot(ex, ey)
+
+    # Argument of periapsis
+    omega = math.atan2(ey, ex) % TAU
+
+    # True anomaly
+    if e > 1e-14:
+        cos_nu = (ex * x + ey * y) / (e * r)
+        cos_nu = max(-1.0, min(1.0, cos_nu))
+        nu = math.acos(cos_nu)
+        if r_dot_v < 0:
+            nu = -nu
+    else:
+        nu = 0.0
+
+    # Mean anomaly
+    if e < 1.0:  # elliptical
+        cos_E = (e + math.cos(nu)) / (1.0 + e * math.cos(nu))
+        cos_E = max(-1.0, min(1.0, cos_E))
+        sin_E = math.sqrt(max(0.0, 1.0 - cos_E * cos_E))
+        if nu < 0:
+            sin_E = -sin_E
+        E = math.atan2(sin_E, cos_E)
+        M0 = (E - e * math.sin(E)) % TAU
+    else:  # hyperbolic
+        # cosh(H) = (e + cos(ν)) / (1 + e·cos(ν))
+        cosh_H = (e + math.cos(nu)) / (1.0 + e * math.cos(nu))
+        cosh_H = max(1.0, cosh_H)
+        H = math.acosh(cosh_H)
+        if nu < 0:
+            H = -H
+        M0 = e * math.sinh(H) - H  # unbounded, no wrap
+
+    return OrbitalElements(
+        a=a, e=e, omega=omega, M0=M0, epoch=epoch, mu=mu,
+    )
diff --git a/soi_calculator.py b/soi_calculator.py
new file mode 100644
index 0000000..580c9b2
--- /dev/null
+++ b/soi_calculator.py
@@ -0,0 +1,197 @@
+"""
+SOI (Sphere of Influence) crossing-time calculator.
+
+Given two bodies in Keplerian orbits around the same parent,
+find all times when the distance between them equals a given
+SOI radius — i.e., when one body enters or exits the other's
+gravitational sphere of influence.
+
+Algorithm:
+  1. Quick radial-overlap check (rejects impossible cases).
+  2. Coarse time-scan across the search window.
+  3. Newton-Raphson refinement of each sign-change bracket.
+"""
+
+import math
+from typing import List, Optional, Tuple
+
+from orbital_elements import OrbitalElements
+
+
+def find_soi_crossings(
+    body: OrbitalElements,
+    planet: OrbitalElements,
+    soi_radius: float,
+    window_start: float,
+    window_end: float,
+    n_steps: int = 500,
+) -> List[Tuple[float, float]]:
+    """
+    Find all (enter_time, exit_time) SOI crossing pairs.
+
+    Parameters
+    ----------
+    body, planet : OrbitalElements
+        Two bodies orbiting the same central body (same mu).
+    soi_radius : float
+        Radius of the planet's sphere of influence.
+    window_start, window_end : float
+        Search window in simulation time.
+    n_steps : int
+        Number of coarse-scan steps (default 500).
+
+    Returns
+    -------
+    List of (enter, exit) time pairs, sorted chronologically.
+    Empty list if no crossings found.
+    """
+    # ── Quick rejection: orbital radius ranges must overlap ──
+    b_min, b_max = body.radius_range()
+    p_min, p_max = planet.radius_range()
+
+    if b_max + soi_radius < p_min or p_max + soi_radius < b_min:
+        return []  # orbits never come close enough
+
+    # ── Coarse time scan ────────────────────────────────────
+    duration = window_end - window_start
+    dt = duration / n_steps
+
+    crossings: List[Tuple[float, float]] = []
+    prev_dist: Optional[float] = None
+    enter_bracket: Optional[Tuple[float, float]] = None
+
+    for i in range(n_steps + 1):
+        t = window_start + i * dt
+
+        bx, by = body.position_at(t)
+        px, py = planet.position_at(t)
+        dist = math.hypot(bx - px, by - py)
+
+        if prev_dist is not None:
+            prev_t = t - dt
+            was_outside = prev_dist > soi_radius
+            is_inside = dist <= soi_radius
+
+            if was_outside and is_inside:
+                # ── Entering SOI — save narrow bracket ──
+                enter_bracket = (prev_t, t)
+
+            elif not was_outside and not is_inside and enter_bracket is not None:
+                # ── Exiting SOI — refine both crossings ──
+                exit_bracket = (prev_t, t)
+                enter_t = _refine(
+                    body=body, planet=planet, soi_radius=soi_radius,
+                    bracket=enter_bracket, entering=True,
+                )
+                exit_t = _refine(
+                    body=body, planet=planet, soi_radius=soi_radius,
+                    bracket=exit_bracket, entering=False,
+                )
+                crossings.append((enter_t, exit_t))
+                enter_bracket = None
+
+        prev_dist = dist
+
+    # ── Still inside SOI at window end? ─────────────────────
+    if enter_bracket is not None:
+        t_end = window_end
+        exit_bracket = (window_end - dt, window_end)
+        enter_t = _refine(
+            body=body, planet=planet, soi_radius=soi_radius,
+            bracket=enter_bracket, entering=True,
+        )
+        exit_t = _refine(
+            body=body, planet=planet, soi_radius=soi_radius,
+            bracket=exit_bracket, entering=False,
+        )
+        crossings.append((enter_t, exit_t))
+
+    return crossings
+
+
+# ── Newton-Raphson refinement ───────────────────────────────────
+
+
+def _refine(
+    body: OrbitalElements,
+    planet: OrbitalElements,
+    soi_radius: float,
+    bracket: Tuple[float, float],
+    entering: bool,
+) -> float:
+    """
+    Newton-Raphson refinement of an SOI crossing time.
+
+    Solves  |r_body(t) − r_planet(t)| − soi_radius = 0.
+
+    Parameters
+    ----------
+    bracket : (t_low, t_high)
+        Narrow bracket around the crossing (ideally one scan-step wide).
+    entering : bool
+        True  → distance *drops* to SOI (outer → inner).
+        False → distance *rises*  to SOI (inner → outer).
+    """
+    t_low, t_high = bracket
+
+    # Start from the side closest to the expected root.
+    # For entering: outside edge (t_low, where dist > soi).
+    # For exiting:  outside edge (t_high, where dist > soi).
+    t = t_low if entering else t_high
+
+    newton_tol = 1e-8
+    max_iter = 30
+
+    for _ in range(max_iter):
+        bx, by, bvx, bvy = body.compute_state_at(t)
+        px, py, pvx, pvy = planet.compute_state_at(t)
+
+        dx = bx - px
+        dy = by - py
+        dvx = bvx - pvx
+        dvy = bvy - pvy
+
+        dist = math.hypot(dx, dy)
+        f_val = dist - soi_radius
+
+        if dist < 1e-14:
+            break  # co-located — degenerate case
+
+        fprime = (dx * dvx + dy * dvy) / dist
+
+        if abs(fprime) < 1e-14:
+            break  # stationary w.r.t. SOI boundary
+
+        dt_correction = -f_val / fprime
+        t += dt_correction
+
+        # Stay inside bracket
+        t = max(t_low, min(t_high, t))
+
+        if abs(dt_correction) < newton_tol:
+            break
+
+    return t
+
+
+# ── Convenience: next SOI event ──────────────────────────────────
+
+
+def find_next_soi_event(
+    body: OrbitalElements,
+    planet: OrbitalElements,
+    soi_radius: float,
+    from_time: float,
+    max_orbits: int = 3,
+) -> Optional[Tuple[float, float]]:
+    """
+    Find the first (enter, exit) SOI crossing after *from_time*.
+
+    Searches *max_orbits* × the longer of the two orbital periods.
+    Returns None if no crossing found in that window.
+    """
+    window = max_orbits * max(body.period, planet.period)
+    crossings = find_soi_crossings(
+        body, planet, soi_radius, from_time, from_time + window,
+    )
+    return crossings[0] if crossings else None
diff --git a/test_orbital.py b/test_orbital.py
new file mode 100644
index 0000000..71ee241
--- /dev/null
+++ b/test_orbital.py
@@ -0,0 +1,346 @@
+"""
+Tests for orbital mechanics, SOI detection, and frame transitions.
+
+Run with: python3 test_orbital.py
+or:       python3 -m pytest test_orbital.py -v
+"""
+
+import math
+import sys
+from orbital_elements import OrbitalElements, orbital_elements_from_cartesian, TAU
+from soi_calculator import find_soi_crossings
+from orbit_drawer import generate_orbit_points
+from frame_transition import (
+    sun_state_to_planet_orbit,
+    planet_orbit_to_sun_state,
+    planet_orbit_to_sun_orbit,
+)
+
+_assertions = 0
+_failures = 0
+
+
+def check(condition: bool, msg: str = "") -> None:
+    global _assertions, _failures
+    _assertions += 1
+    if not condition:
+        _failures += 1
+        print(f"  FAIL: {msg}", file=sys.stderr)
+    else:
+        print(f"  ok: {msg}")
+
+
+def approx(a: float, b: float, tol: float = 1e-9) -> bool:
+    return abs(a - b) < tol
+
+
+# ═══════════════════════════════════════════════════════════════
+#  Elliptical orbit tests (unchanged)
+# ═══════════════════════════════════════════════════════════════
+
+def test_circular_orbit_position() -> None:
+    print("test_circular_orbit_position")
+    orbit = OrbitalElements(a=100.0, e=0.0, omega=0.0, M0=0.0, epoch=0.0, mu=1_000_000.0)
+    for t in [0.0, 10.0, 50.0, orbit.period / 4, orbit.period / 2]:
+        x, y = orbit.position_at(t)
+        check(approx(math.hypot(x, y), 100.0), f"t={t:.1f} dist={math.hypot(x,y):.6f}")
+
+
+def test_elliptical_periapsis_apoapsis() -> None:
+    print("test_elliptical_periapsis_apoapsis")
+    a, e = 200.0, 0.3
+    mu = 500_000.0
+    orbit = OrbitalElements(a=a, e=e, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    x, y = orbit.position_at(0.0)
+    check(approx(x, a*(1-e)), f"periapsis x={x} == {a*(1-e)}")
+    check(approx(y, 0.0), f"periapsis y={y}")
+    half_T = orbit.period / 2
+    x2, y2 = orbit.position_at(half_T)
+    check(approx(x2, -a*(1+e)), f"apoapsis x={x2} == {-a*(1+e)}")
+    check(approx(y2, 0.0, 1e-8), f"apoapsis y={y2}")
+
+
+def test_omega_rotation() -> None:
+    print("test_omega_rotation")
+    orbit = OrbitalElements(a=100.0, e=0.2, omega=math.pi/2, M0=0.0, epoch=0.0, mu=1_000_000.0)
+    x, y = orbit.position_at(0.0)
+    check(approx(x, 0.0, 1e-8), f"omega=pi/2: x={x} ≈ 0")
+    check(approx(y, 80.0), f"omega=pi/2: y={y} == 80")
+
+
+def test_period() -> None:
+    print("test_period")
+    a, mu = 200.0, 1_000_000.0
+    orbit = OrbitalElements(a=a, e=0.0, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    expected = 2*math.pi*math.sqrt(a**3/mu)
+    check(approx(orbit.period, expected), f"period {orbit.period} == {expected}")
+    x0, y0 = orbit.position_at(0.0)
+    x1, y1 = orbit.position_at(orbit.period)
+    check(approx(x0, x1) and approx(y0, y1), "closed after 1 period")
+
+
+def test_cartesian_roundtrip() -> None:
+    print("test_cartesian_roundtrip")
+    orbit1 = OrbitalElements(a=300.0, e=0.25, omega=1.2, M0=2.5, epoch=1000.0, mu=1_500_000.0)
+    x, y, vx, vy = orbit1.compute_state_at(2000.0)
+    orbit2 = orbital_elements_from_cartesian(x, y, vx, vy, orbit1.mu, epoch=2000.0)
+    check(approx(orbit2.a, orbit1.a, 1e-6), f"a: {orbit2.a} == {orbit1.a}")
+    check(approx(orbit2.e, orbit1.e, 1e-6), f"e: {orbit2.e} == {orbit1.e}")
+    omega_diff = abs(orbit2.omega - orbit1.omega) % TAU
+    check(omega_diff < 1e-6 or abs(omega_diff-TAU) < 1e-6, f"ω: {orbit2.omega} ≈ {orbit1.omega}")
+    x2, y2, vx2, vy2 = orbit2.compute_state_at(2000.0)
+    check(approx(x, x2, 1e-6) and approx(y, y2, 1e-6), "position at epoch")
+    check(approx(vx, vx2, 1e-6) and approx(vy, vy2, 1e-6), "velocity at epoch")
+
+
+def test_orbit_points() -> None:
+    print("test_orbit_points")
+    orbit = OrbitalElements(a=100.0, e=0.0, omega=0.0, M0=0.0, epoch=0.0, mu=100_000.0)
+    points = generate_orbit_points(orbit, 128)
+    check(len(points) == 128, f"{len(points)} == 128")
+    for x, y in points:
+        if not approx(math.hypot(x, y), 100.0, 1e-8):
+            check(False, f"point at {math.hypot(x,y):.9f} ≠ 100.0")
+            break
+    else:
+        check(True, "all 128 points at correct distance")
+
+
+def test_radial_overlap_quick_reject() -> None:
+    print("test_radial_overlap_quick_reject")
+    mu = 1_000_000.0
+    inner = OrbitalElements(a=100.0, e=0.0, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    outer = OrbitalElements(a=500.0, e=0.0, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    crossings = find_soi_crossings(inner, outer, 30.0, 0.0, 200.0)
+    check(len(crossings) == 0, f"non-overlapping: {len(crossings)} crossings")
+
+
+# ═══════════════════════════════════════════════════════════════
+#  SOI crossing tests
+# ═══════════════════════════════════════════════════════════════
+
+def test_soi_crossing_detection() -> None:
+    print("test_soi_crossing_detection")
+    mu = 1_500_000.0
+    planet = OrbitalElements(a=350.0, e=0.02, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    body = OrbitalElements(a=320.0, e=0.25, omega=math.pi*0.7, M0=1.0, epoch=0.0, mu=mu)
+    soi_r = 50.0
+    window = 3*max(body.period, planet.period)
+    crossings = find_soi_crossings(body, planet, soi_r, 0.0, window)
+    check(len(crossings) > 0, f"found {len(crossings)} crossing(s)")
+    for i, (enter_t, exit_t) in enumerate(crossings):
+        dur = exit_t - enter_t
+        check(enter_t <= exit_t, f"crossing {i}: {enter_t:.3f} ≤ {exit_t:.3f}")
+        for label, t in [("enter", enter_t), ("exit", exit_t)]:
+            bx, by = body.position_at(t)
+            px, py = planet.position_at(t)
+            d = math.hypot(bx-px, by-py)
+            check(approx(d, soi_r, 0.5), f"{label} d={d:.3f} ≈ {soi_r}")
+        if dur > 0.2:
+            mid_t = (enter_t+exit_t)/2
+            bx, by = body.position_at(mid_t)
+            px, py = planet.position_at(mid_t)
+            d_mid = math.hypot(bx-px, by-py)
+            check(d_mid < soi_r-1.0, f"deep crossing midpoint d={d_mid:.1f}")
+        else:
+            print(f"  (crossing {i} is a graze, dur={dur:.3f}s)")
+
+
+def test_deep_soi_crossing() -> None:
+    print("test_deep_soi_crossing")
+    mu = 1_500_000.0
+    planet = OrbitalElements(a=350.0, e=0.02, omega=0.5, M0=0.0, epoch=0.0, mu=mu)
+    body = OrbitalElements(a=345.0, e=0.02, omega=0.5, M0=-0.35, epoch=0.0, mu=mu)
+    soi = 80.0
+    window = 5*max(body.period, planet.period)
+    crossings = find_soi_crossings(body, planet, soi, 0.0, window, n_steps=800)
+    check(len(crossings) >= 1, f"found {len(crossings)} crossing(s)")
+    for i, (enter_t, exit_t) in enumerate(crossings):
+        dur = exit_t-enter_t
+        check(enter_t < exit_t, f"crossing {i}: {enter_t:.2f} < {exit_t:.2f} dur={dur:.2f}s")
+        for label, t in [("enter", enter_t), ("exit", exit_t)]:
+            bx, by = body.position_at(t)
+            px, py = planet.position_at(t)
+            d = math.hypot(bx-px, by-py)
+            check(approx(d, soi, 1.0), f"{label} d={d:.2f} ≈ {soi}")
+        if dur > 0.5:
+            mid_t = (enter_t+exit_t)/2
+            bx, by = body.position_at(mid_t)
+            px, py = planet.position_at(mid_t)
+            d_mid = math.hypot(bx-px, by-py)
+            check(d_mid < soi-2.0, f"midpoint d={d_mid:.1f} < {soi}")
+
+
+# ═══════════════════════════════════════════════════════════════
+#  Hyperbolic orbit tests
+# ═══════════════════════════════════════════════════════════════
+
+def test_hyperbolic_orbit_position() -> None:
+    """Body at periapsis at t=0 on a hyperbolic trajectory."""
+    print("test_hyperbolic_orbit_position")
+    mu = 50_000.0
+    # a < 0 for hyperbolas, e > 1
+    orbit = OrbitalElements(a=-40.0, e=2.5, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+
+    # At t=0 (H=0), should be at periapsis
+    x, y = orbit.position_at(0.0)
+    r_peri = abs(orbit.a)*(orbit.e - 1.0)  # = 40*1.5 = 60
+    check(approx(x, r_peri), f"hyperbolic periapsis x={x:.3f} == {r_peri}")
+    check(approx(y, 0.0), f"y={y:.3f} ≈ 0")
+
+
+def test_hyperbolic_velocity() -> None:
+    """Velocity at periapsis should match the vis-viva equation."""
+    print("test_hyperbolic_velocity")
+    mu = 50_000.0
+    orbit = OrbitalElements(a=-40.0, e=2.5, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    _, _, vx, vy = orbit.compute_state_at(0.0)
+
+    r_p = abs(orbit.a)*(orbit.e - 1.0)  # 60
+    # v_p = sqrt(mu * (2/r_p + 1/|a|))
+    v_expected = math.sqrt(mu * (2.0/r_p + 1.0/abs(orbit.a)))
+    v_actual = math.hypot(vx, vy)
+    check(approx(v_actual, v_expected, 1e-6),
+          f"v_peri: {v_actual:.3f} == {v_expected:.3f}")
+
+
+def test_hyperbolic_cartesian_roundtrip() -> None:
+    """Hyperbolic orbit → cartesian → same hyperbolic orbit."""
+    print("test_hyperbolic_cartesian_roundtrip")
+    mu = 50_000.0
+    orbit1 = OrbitalElements(a=-30.0, e=3.0, omega=1.0, M0=0.5, epoch=0.0, mu=mu)
+    x, y, vx, vy = orbit1.compute_state_at(10.0)
+    orbit2 = orbital_elements_from_cartesian(x, y, vx, vy, mu, epoch=10.0)
+    check(approx(orbit2.a, orbit1.a, 1e-6), f"a: {orbit2.a} == {orbit1.a}")
+    check(approx(orbit2.e, orbit1.e, 1e-6), f"e: {orbit2.e} == {orbit1.e}")
+    omega_diff = abs(orbit2.omega - orbit1.omega) % TAU
+    check(omega_diff < 1e-6 or abs(omega_diff-TAU) < 1e-6, f"ω: {orbit2.omega} ≈ {orbit1.omega}")
+    x2, y2 = orbit2.position_at(10.0)
+    check(approx(x, x2, 1e-6) and approx(y, y2, 1e-6), "position at epoch")
+
+
+def test_hyperbolic_orbit_points() -> None:
+    """generate_orbit_points should truncate at max_radius."""
+    print("test_hyperbolic_orbit_points")
+    mu = 50_000.0
+    orbit = OrbitalElements(a=-20.0, e=2.0, omega=0.0, M0=0.0, epoch=0.0, mu=mu)
+    points = generate_orbit_points(orbit, num_points=100, max_radius=80.0)
+    check(len(points) > 0, f"generated {len(points)} hyperbolic points")
+    for x, y in points:
+        d = math.hypot(x, y)
+        check(d <= 80.0 + 1e-6, f"point at r={d:.3f} ≤ 80.0")
+
+
+# ═══════════════════════════════════════════════════════════════
+#  Frame transition tests
+# ═══════════════════════════════════════════════════════════════
+
+def test_frame_transition_roundtrip() -> None:
+    """
+    Body in sun frame → planet frame → back → should match original.
+    """
+    print("test_frame_transition_roundtrip")
+    mu_sun = 1_500_000.0
+    mu_planet = 30_000.0
+
+    planet = OrbitalElements(a=350.0, e=0.02, omega=0.5, M0=0.0, epoch=0.0, mu=mu_sun)
+    body_sun = OrbitalElements(a=380.0, e=0.25, omega=2.0, M0=1.0, epoch=0.0, mu=mu_sun)
+
+    t_transition = 50.0
+
+    # Original body position in sun frame
+    bx, by, bvx, bvy = body_sun.compute_state_at(t_transition)
+    px, py, pvx, pvy = planet.compute_state_at(t_transition)
+
+    # Transition to planet frame
+    body_planet = sun_state_to_planet_orbit(
+        bx, by, bvx, bvy, px, py, pvx, pvy, mu_planet, t_transition,
+    )
+
+    # Body's planet-relative position should be the vector difference
+    bx_rel, by_rel = body_planet.position_at(t_transition)
+    check(approx(bx_rel, bx-px, 1e-6), f"rel x: {bx_rel:.3f} == {bx-px:.3f}")
+    check(approx(by_rel, by-py, 1e-6), f"rel y: {by_rel:.3f} == {by-py:.3f}")
+
+    # Transition back to sun frame
+    x_back, y_back, vx_back, vy_back = planet_orbit_to_sun_state(
+        body_planet, px, py, pvx, pvy, t_transition,
+    )
+    check(approx(x_back, bx, 1e-6) and approx(y_back, by, 1e-6), "position roundtrip")
+    check(approx(vx_back, bvx, 1e-6) and approx(vy_back, bvy, 1e-6), "velocity roundtrip")
+
+
+def test_frame_transition_hyperbolic_flyby() -> None:
+    """
+    Body on planet-crossing orbit → planet frame at SOI entry.
+    Verify the planet-relative orbit is hyperbolic (flyby, not capture).
+    """
+    print("test_frame_transition_hyperbolic_flyby")
+    mu_sun = 1_500_000.0
+    mu_planet = 10_000.0   # v_rel ≈ 20.4, critical ≈ 16.9 → hyperbolic ✓
+
+    # Planet on near-circular orbit
+    planet = OrbitalElements(a=350.0, e=0.02, omega=0.5, M0=0.0, epoch=0.0, mu=mu_sun)
+
+    # Asteroid: crossing orbit (verified to intersect planet's SOI)
+    asteroid = OrbitalElements(a=380.0, e=0.35, omega=2.5, M0=0.8, epoch=0.0, mu=mu_sun)
+
+    # Find first SOI crossing
+    soi = 70.0
+    window = 4 * max(asteroid.period, planet.period)
+    crossings = find_soi_crossings(asteroid, planet, soi, 0.0, window, n_steps=1000)
+
+    check(len(crossings) > 0, f"found {len(crossings)} SOI crossing(s)")
+
+    if crossings:
+        enter_t = crossings[0][0]
+
+        # Get states at SOI entry
+        ax, ay, avx, avy = asteroid.compute_state_at(enter_t)
+        px, py, pvx, pvy = planet.compute_state_at(enter_t)
+
+        # Transition to planet frame
+        body_planet = sun_state_to_planet_orbit(
+            ax, ay, avx, avy, px, py, pvx, pvy, mu_planet, enter_t,
+        )
+
+        # This SHOULD be hyperbolic (flyby, energy > 0)
+        check(body_planet.is_hyperbolic,
+              f"planet-relative orbit is hyperbolic (e={body_planet.e:.3f})")
+
+        # Verify position matches relative to planet
+        bx_rel, by_rel = body_planet.position_at(enter_t)
+        d_rel = math.hypot(bx_rel, by_rel)
+        check(approx(d_rel, soi, 2.0),
+              f"rel distance at SOI entry: {d_rel:.1f} ≈ {soi}")
+
+        # Verify the orbit goes out past the SOI (will exit)
+        check(body_planet.apoapsis > soi,
+              f"hyperbolic: path extends past SOI (max r={body_planet.apoapsis})")
+
+
+# ═══════════════════════════════════════════════════════════════
+#  Runner
+# ═══════════════════════════════════════════════════════════════
+
+if __name__ == "__main__":
+    print("=== Orbital Mechanics Tests ===\n")
+
+    test_circular_orbit_position()
+    test_elliptical_periapsis_apoapsis()
+    test_omega_rotation()
+    test_period()
+    test_cartesian_roundtrip()
+    test_orbit_points()
+    test_radial_overlap_quick_reject()
+    test_soi_crossing_detection()
+    test_deep_soi_crossing()
+    test_hyperbolic_orbit_position()
+    test_hyperbolic_velocity()
+    test_hyperbolic_cartesian_roundtrip()
+    test_hyperbolic_orbit_points()
+    test_frame_transition_roundtrip()
+    test_frame_transition_hyperbolic_flyby()
+
+    print(f"\n---\n{_assertions} assertions, {_failures} failures")
+    sys.exit(0 if _failures == 0 else 1)
diff --git a/visual_test.py b/visual_test.py
new file mode 100644
index 0000000..8f64164
--- /dev/null
+++ b/visual_test.py
@@ -0,0 +1,363 @@
+"""
+Pygame visualisation: Keplerian flyby with SOI frame transition.
+
+Shows:
+  - Star at centre (yellow)
+  - Planet (blue) in near-circular orbit, SOI bubble
+  - Asteroid (red) on crossing orbit
+  - When asteroid enters SOI: switches to planet-relative hyperbolic
+    trajectory, orbit trail shows truncated hyperbola inside SOI
+  - On exit: switches back to sun frame
+  - Countdown in window title
+
+Controls:
+  SPACE      — pause
+  LEFT/RIGHT — jump ±10 s
+  UP/DOWN    — speed ×2 / ÷2
+  ESC        — quit
+"""
+
+import math
+import sys
+from typing import List, Optional, Tuple
+
+import pygame
+
+from orbital_elements import OrbitalElements, orbital_elements_from_cartesian
+from orbit_drawer import generate_orbit_points
+from soi_calculator import find_soi_crossings
+from frame_transition import (
+    sun_state_to_planet_orbit,
+    planet_orbit_to_sun_orbit,
+)
+
+# ═══════════════════════════════════════════════════════════════
+#  Simulation parameters
+# ═══════════════════════════════════════════════════════════════
+
+MU_SUN = 1_500_000.0
+MU_PLANET = 10_000.0
+SOI_RADIUS = 70.0
+
+PLANET = OrbitalElements(
+    a=350.0, e=0.02, omega=0.5, M0=0.0, epoch=0.0, mu=MU_SUN,
+)
+
+ASTEROID_SUN = OrbitalElements(
+    a=380.0, e=0.35, omega=2.5, M0=0.8, epoch=0.0, mu=MU_SUN,
+)
+
+SEARCH_ORBITS = 5
+
+# ═══════════════════════════════════════════════════════════════
+#  Display
+# ═══════════════════════════════════════════════════════════════
+
+WIDTH, HEIGHT = 1000, 1000
+CENTER = (WIDTH // 2, HEIGHT // 2)
+FPS = 60
+
+STAR_COLOUR = (255, 220, 50)
+STAR_RADIUS = 14
+PLANET_COLOUR = (70, 130, 230)
+PLANET_RADIUS = 8
+SOI_COLOUR = (70, 130, 230, 35)
+ASTEROID_COLOUR = (230, 80, 50)
+ASTEROID_INSIDE_COLOUR = (50, 230, 80)
+ASTEROID_RADIUS = 5
+ORBIT_PLANET_COLOUR = (50, 80, 150)
+ORBIT_ASTEROID_COLOUR = (150, 50, 30)
+ORBIT_FLYBY_COLOUR = (80, 200, 100)
+UI_COLOUR = (200, 200, 200)
+BACKGROUND = (12, 12, 24)
+GRID_COLOUR = (28, 28, 44)
+
+
+def to_screen(x: float, y: float) -> Tuple[float, float]:
+    return CENTER[0] + x, CENTER[1] - y
+
+
+# ═══════════════════════════════════════════════════════════════
+#  SOI exit-time finder
+# ═══════════════════════════════════════════════════════════════
+
+def find_soi_exit_time(
+    orbit_planet: OrbitalElements,
+    soi_r: float,
+    from_time: float,
+    max_search: float = 500.0,
+) -> Optional[float]:
+    """
+    Given a planet-relative orbit, find the next time *after* from_time
+    when distance = soi_r (the exit point).
+
+    Uses coarse stepping + Newton refinement, similar to soi_calculator.
+    """
+    dt = 0.1  # fine step
+    t = from_time + dt
+
+    # Walk forward until we cross outside SOI
+    for _ in range(int(max_search / dt)):
+        bx, by = orbit_planet.position_at(t)
+        r = math.hypot(bx, by)
+        if r >= soi_r:
+            # Refine with Newton
+            return _refine_soi(orbit_planet, soi_r, t - dt, t)
+        t += dt
+    return None
+
+
+def _refine_soi(
+    orbit: OrbitalElements,
+    soi_r: float,
+    t_low: float,
+    t_high: float,
+) -> float:
+    """Newton-Raphson for r(t) = soi_r (exiting side)."""
+    t = t_high  # start from outside
+    tol = 1e-8
+    for _ in range(30):
+        bx, by, bvx, bvy = orbit.compute_state_at(t)
+        r = math.hypot(bx, by)
+        f_val = r - soi_r
+        if r < 1e-14:
+            break
+        fprime = (bx * bvx + by * bvy) / r
+        if abs(fprime) < 1e-14:
+            break
+        t -= f_val / fprime
+        t = max(t_low, min(t_high, t))
+        if abs(f_val / fprime) < tol:
+            break
+    return t
+
+
+# ═══════════════════════════════════════════════════════════════
+#  Main
+# ═══════════════════════════════════════════════════════════════
+
+def main() -> None:
+    pygame.init()
+    screen = pygame.display.set_mode((WIDTH, HEIGHT))
+    pygame.display.set_caption("Orbital Flyby — initialising…")
+    clock = pygame.time.Clock()
+    font = pygame.font.Font(None, 18)
+
+    # ── Pre-compute sun-frame orbit trails ─────────────────
+    trail_planet = [to_screen(x, y) for x, y in
+                    generate_orbit_points(PLANET)]
+    trail_asteroid_sun = [to_screen(x, y) for x, y in
+                          generate_orbit_points(ASTEROID_SUN)]
+
+    # ── Pre-compute SOI entry times ────────────────────────
+    window = SEARCH_ORBITS * max(ASTEROID_SUN.period, PLANET.period)
+    print(f"Scanning {window:.0f}s for SOI crossings…")
+    sun_crossings = find_soi_crossings(
+        ASTEROID_SUN, PLANET, SOI_RADIUS, 0.0, window, n_steps=1000,
+    )
+    print(f"Found {len(sun_crossings)} SOI crossing(s):")
+    for i, (enter, exit_) in enumerate(sun_crossings):
+        print(f"  [{i}] enter={enter:.1f}s  (sun-frame)")
+
+    # ── Simulation state ───────────────────────────────────
+    # Start 10 s before the first SOI entry so the user sees
+    # the countdown tick down without waiting long.
+    _first_entry = sun_crossings[0][0] if sun_crossings else 0.0
+    sim_time: float = max(0.0, _first_entry - 10.0)
+    sim_speed: float = 1.0
+    paused: bool = False
+
+    in_soi: bool = False
+    soi_enter_time: Optional[float] = sun_crossings[0][0] if sun_crossings else None
+    soi_exit_time: Optional[float] = None
+
+    # Asteroid orbits (one per frame)
+    asteroid_sun: OrbitalElements = ASTEROID_SUN
+    asteroid_planet: Optional[OrbitalElements] = None
+
+    
+
+    cross_idx: int = 0  # index into sun_crossings
+
+    running = True
+    while running:
+        dt = clock.tick(FPS) / 1000.0
+
+        # ── Input ─────────────────────────────────────────
+        for event in pygame.event.get():
+            if event.type == pygame.QUIT:
+                running = False
+            elif event.type == pygame.KEYDOWN:
+                if event.key == pygame.K_ESCAPE:
+                    running = False
+                elif event.key == pygame.K_SPACE:
+                    paused = not paused
+                elif event.key == pygame.K_RIGHT:
+                    sim_time += 10.0
+                elif event.key == pygame.K_LEFT:
+                    sim_time = max(0.0, sim_time - 10.0)
+                elif event.key == pygame.K_UP:
+                    sim_speed = min(16.0, sim_speed * 2.0)
+                elif event.key == pygame.K_DOWN:
+                    sim_speed = max(1 / 16, sim_speed / 2.0)
+
+        if not paused:
+            sim_time += dt * sim_speed
+
+        # ── SOI state machine ─────────────────────────────
+        # Check entry
+        if (not in_soi and soi_enter_time is not None
+                and sim_time >= soi_enter_time):
+            # ── ENTER SOI ──
+            in_soi = True
+            print(f"\n>>> SOI ENTRY at t={sim_time:.1f}s")
+
+            px, py, pvx, pvy = PLANET.compute_state_at(sim_time)
+            ax, ay, avx, avy = asteroid_sun.compute_state_at(sim_time)
+            asteroid_planet = sun_state_to_planet_orbit(
+                ax, ay, avx, avy, px, py, pvx, pvy, MU_PLANET, sim_time,
+            )
+            print(f"    Planet-relative orbit: a={asteroid_planet.a:.1f}, "
+                  f"e={asteroid_planet.e:.3f}, "
+                  f"hyperbolic={asteroid_planet.is_hyperbolic}")
+
+            # Compute exit time
+            soi_exit_time = find_soi_exit_time(asteroid_planet, SOI_RADIUS, sim_time)
+            if soi_exit_time:
+                print(f"    Computed SOI exit: t={soi_exit_time:.1f}s "
+                      f"(dur={soi_exit_time - sim_time:.1f}s)")
+            else:
+                print("    WARNING: could not find SOI exit time!")
+
+            
+
+        # Check exit
+        if in_soi and soi_exit_time is not None and sim_time >= soi_exit_time:
+            # ── EXIT SOI ──
+            in_soi = False
+            print(f">>> SOI EXIT at t={sim_time:.1f}s")
+
+            px, py, pvx, pvy = PLANET.compute_state_at(sim_time)
+            asteroid_sun = planet_orbit_to_sun_orbit(
+                asteroid_planet, px, py, pvx, pvy, MU_SUN, sim_time,
+            )
+            print(f"    New sun orbit: a={asteroid_sun.a:.1f}, "
+                  f"e={asteroid_sun.e:.3f}")
+
+            asteroid_planet = None
+            soi_exit_time = None
+
+            # Advance to next crossing
+            cross_idx += 1
+            while cross_idx < len(sun_crossings):
+                if sun_crossings[cross_idx][0] > sim_time + 0.1:
+                    soi_enter_time = sun_crossings[cross_idx][0]
+                    break
+                cross_idx += 1
+            else:
+                soi_enter_time = None
+
+        # ── Compute current positions ──────────────────────
+        px, py = PLANET.position_at(sim_time)
+
+        if in_soi and asteroid_planet is not None:
+            # Asteroid in planet frame → convert to sun frame
+            ax_rel, ay_rel = asteroid_planet.position_at(sim_time)
+            ax = ax_rel + px
+            ay = ay_rel + py
+        else:
+            ax, ay = asteroid_sun.position_at(sim_time)
+
+        sp = to_screen(px, py)
+        sa = to_screen(ax, ay)
+
+        # ── Draw ──────────────────────────────────────────
+        screen.fill(BACKGROUND)
+
+        # Grid
+        for g in range(-400, 401, 100):
+            sx1, sy1 = to_screen(g, -400)
+            sx2, sy2 = to_screen(g, 400)
+            pygame.draw.line(screen, GRID_COLOUR, (sx1, sy1), (sx2, sy2), 1)
+            sx1, sy1 = to_screen(-400, g)
+            sx2, sy2 = to_screen(400, g)
+            pygame.draw.line(screen, GRID_COLOUR, (sx1, sy1), (sx2, sy2), 1)
+
+        # Sun-frame asteroid trail (always shown)
+        if len(trail_asteroid_sun) > 1:
+            pygame.draw.lines(screen, ORBIT_ASTEROID_COLOUR, True,
+                              trail_asteroid_sun, 1)
+
+        # Planet orbit trail
+        if len(trail_planet) > 1:
+            pygame.draw.lines(screen, ORBIT_PLANET_COLOUR, True,
+                              trail_planet, 1)
+
+        # Flyby trail (planet-relative hyperbolic arc, inside SOI)
+        if in_soi and asteroid_planet is not None:
+            pts = generate_orbit_points(
+                asteroid_planet, num_points=200, max_radius=SOI_RADIUS,
+            )
+            if len(pts) > 1:
+                # pts are in planet frame — offset by planet world position
+                offset_trail = [to_screen(x + px, y + py) for x, y in pts]
+                pygame.draw.lines(screen, ORBIT_FLYBY_COLOUR, False,
+                                  offset_trail, 2)
+
+        # Planet SOI bubble
+        soi_surf = pygame.Surface((WIDTH, HEIGHT), pygame.SRCALPHA)
+        pygame.draw.circle(
+            soi_surf, SOI_COLOUR,
+            (int(sp[0]), int(sp[1])), int(SOI_RADIUS),
+        )
+        screen.blit(soi_surf, (0, 0))
+
+        # Star
+        pygame.draw.circle(screen, STAR_COLOUR, CENTER, STAR_RADIUS)
+        pygame.draw.circle(screen, (255, 240, 160), CENTER, STAR_RADIUS + 4, 2)
+
+        # Planet
+        pygame.draw.circle(screen, PLANET_COLOUR,
+                           (int(sp[0]), int(sp[1])), PLANET_RADIUS)
+
+        # Asteroid (green inside SOI)
+        acol = ASTEROID_INSIDE_COLOUR if in_soi else ASTEROID_COLOUR
+        arad = ASTEROID_RADIUS + (2 if in_soi else 0)
+        pygame.draw.circle(screen, acol, (int(sa[0]), int(sa[1])), arad)
+
+        # ── Title / countdown ─────────────────────────────
+        if in_soi and soi_exit_time is not None:
+            remaining = soi_exit_time - sim_time
+            title = f"⚫ INSIDE SOI — flyby exit in {remaining:.1f}s"
+        elif soi_enter_time is not None:
+            countdown = soi_enter_time - sim_time
+            title = (f"SOI entry in {countdown:.1f}s  "
+                     f"(crossing {cross_idx + 1}/{len(sun_crossings)})")
+        else:
+            title = "No upcoming SOI crossing"
+        pygame.display.set_caption(title)
+
+        # ── HUD ───────────────────────────────────────────
+        lines = [
+            f"t={sim_time:.1f}s  speed={sim_speed:.1f}×  "
+            f"{'PAUSED' if paused else 'RUNNING'}  "
+            f"{'[SOI]' if in_soi else '[SUN]'}",
+            f"Planet   T={PLANET.period:.1f}s  "
+            f"r=[{PLANET.periapsis:.0f}, {PLANET.apoapsis:.0f}] px",
+            f"SOI={SOI_RADIUS:.0f} px  μ_planet={MU_PLANET}",
+            "SPACE=pause  ←→=skip  ↑↓=speed  ESC=quit",
+        ]
+        y = 12
+        for line in lines:
+            surf = font.render(line, True, UI_COLOUR)
+            screen.blit(surf, (12, y))
+            y += 22
+
+        pygame.display.flip()
+
+    pygame.quit()
+    sys.exit()
+
+
+if __name__ == "__main__":
+    main()