add parabolic orbit enhancements to future work and clarify altitude parameter docs

docs/future_work.md

@@ -188,6 +188,10 @@ This document outlines planned enhancements and future development areas for the
 - Asteroid flyby simulations
 - Gravity assist maneuvers
 
+### Parabolic Orbit Enhancements
+- Altitude parameter support for parabolic orbits: parse `altitude` and convert to `semi_latus_rectum = parent_radius + altitude`
+- Explicit perihelion parameter: add `perihelion` to config file, derive `semi_latus_rectum = 2 * perihelion` for parabolic orbits
+
 ## Performance Optimizations
 
 ### Adaptive Timestepping

docs/parabolic_union_implementation.md

@@ -1,152 +0,0 @@
-# Parabolic Orbit Union Implementation Plan
-
-## Overview
-Add support for parabolic orbits (e≈1.0) using semi-latus rectum parameter `p` instead of the current hacky `semi_major_axis = 1.0e30` infinity approximation.
-
-## Problem
-Current implementation uses `semi_major_axis = 1.0e30` to approximate infinity for parabolic orbits, causing:
-1. Numerical precision issues with extremely large distances (~6.68e18 AU)
-2. Velocities approaching zero (1.6e-08 km/s instead of ~42 km/s escape velocity)
-3. Test failures due to floating-point equality (final_distance ≈ initial_distance)
-
-## Solution
-Use a union in `OrbitalElements` struct to support both `semi_major_axis` (for elliptical/hyperbolic) and `semi_latus_rectum` (for parabolic).
-
-## Mathematical Background
-For parabolic orbits (e=1.0), the semi-major axis is theoretically infinity. Using semi-latus rectum `p` is mathematically correct:
-
-Position: `r = p / (1 + cos(ν))`
-Velocity: `v = √(2μ / r)`
-
-Where:
-- `p` = semi-latus rectum
-- `ν` = true anomaly
-- `μ` = GM (gravitational parameter)
-
-For parabolic orbits: `p = 2q` where `q` is perihelion distance
-
-## Implementation Steps
-
-### Phase 1: Update OrbitalElements Struct
-**File: `src/orbital_mechanics.h`**
-
-```cpp
-struct OrbitalElements {
-    union {
-        double semi_major_axis;       // for elliptical (e<1) and hyperbolic (e>1)
-        double semi_latus_rectum;     // for parabolic (e≈1)
-    };
-    double eccentricity;
-    double true_anomaly;
-    double inclination;
-    double longitude_of_ascending_node;
-    double argument_of_periapsis;
-};
-```
-
-### Phase 2: Update Config Loader
-**File: `src/config_loader.cpp`**
-
-Add to `parse_toml_body()` and `parse_toml_spacecraft()`:
-
-1. Parse both `semi_major_axis` and `semi_latus_rectum` from orbit table
-2. Initialize union field based on which is specified
-3. Validate exactly one is present per eccentricity range
-
-**Validation Logic:**
-```cpp
-bool has_semi_major = (semi_major.type == TOML_FP64);
-bool has_semi_latus = (semi_latus.type == TOML_FP64);
-
-if (fabs(elements.eccentricity - 1.0) < 0.005) {
-    // Parabolic orbit - requires semi_latus_rectum
-    if (!has_semi_latus) {
-        printf("Error: Parabolic orbit requires 'semi_latus_rectum'\n");
-        return false;
-    }
-    if (has_semi_major) {
-        printf("Error: Parabolic orbit cannot have 'semi_major_axis'\n");
-        return false;
-    }
-    elements.semi_latus_rectum = semi_latus.u.fp64;
-} else {
-    // Elliptical or hyperbolic - requires semi_major_axis
-    if (!has_semi_major) {
-        printf("Error: Elliptical/hyperbolic orbit requires 'semi_major_axis'\n");
-        return false;
-    }
-    if (has_semi_latus) {
-        printf("Error: Elliptical/hyperbolic orbit cannot have 'semi_latus_rectum'\n");
-        return false;
-    }
-    elements.semi_major_axis = semi_major.u.fp64;
-}
-```
-
-### Phase 3: Update orbital_mechanics.cpp
-**File: `src/orbital_mechanics.cpp`**
-
-Update parabolic case (line 21-23):
-
-```cpp
-} else if (fabs(e - 1.0) < 0.005) {
-    double p = elements.semi_latus_rectum;
-    r = p / (1.0 + cos(nu));
-    v_mag = sqrt(2.0 * mu / r);
-}
-```
-
-Remove the `2.0 * a` approximation that requires `a=1.0e30`.
-
-### Phase 4: Update Test Configs
-**File: `tests/configs/parabolic_comet.toml`**
-
-Replace `semi_major_axis = 1.0e30` with `semi_latus_rectum = 1.496e11` (p = 1 AU):
-
-```toml
-[[bodies]]
-name = "ParabolicComet"
-mass = 1.0e14
-radius = 5.0e3
-parent_index = 0
-color = { r = 0.7, g = 0.8, b = 0.9 }
-orbit = {
-    semi_latus_rectum = 1.496e11,
-    eccentricity = 1.0,
-    true_anomaly = 0.0
-}
-```
-
-### Phase 5: Update Documentation
-**File: `docs/technical_reference.md`**
-
-1. Update `OrbitalElements` struct documentation to show union
-2. Add note about `semi_latus_rectum` being required for parabolic orbits (e≈1.0)
-3. Document `semi_latus_rectum` in config format section
-
-**File: `docs/unified_orbital_elements_plan.md`**
-
-Mark union implementation as complete in Phase 7 status.
-
-## Validation Steps
-
-1. Build: `make clean && make`
-2. Run parabolic test: `./orbit_test '[parabolic]'`
-3. Verify velocity is correct: should be ~42,127 m/s escape velocity at 1 AU
-4. Verify energy is ~0 (parabolic orbits have total energy = 0)
-
-## Decisions Made
-
-### Default Behavior
-No backward compatibility for `semi_major_axis` on parabolic orbits - require explicit `semi_latus_rectum` for all parabolic configs. This is cleaner than trying to auto-convert `p = 2*a`.
-
-### Spacecraft Altitude Parameter
-Spacecraft `altitude` parameter is not supported for parabolic orbits in this implementation. If user specifies `altitude` with `eccentricity ≈ 1.0`, the config loader will require `semi_latus_rectum` instead and reject `altitude` or `semi_major_axis`. Added to future todos for Phase 8+.
-
-### Parabolic Detection Tolerance
-Using `|e - 1.0| < 0.005` as threshold for detecting parabolic orbits. This matches tolerance used elsewhere in the codebase.
-
-## Future Enhancements (TODO)
-
-- Spacecraft `altitude` parameter for parabolic orbits: parse `altitude` and convert to `semi_latus_rectum = parent_radius + altitude` when eccentricity is parabolic
-- Consider adding explicit `perihelion` parameter to config file, then derive `semi_latus_rectum = 2 * perihelion` for parabolic orbits

docs/technical_reference.md

@@ -298,7 +298,7 @@ orbit = {
 }
 ```
 - Uses `orbit` table for orbital elements (same as bodies)
-- Altitude can be used instead of `semi_major_axis` (convenience feature, added to parent radius)
+- Altitude can be used instead of `semi_major_axis` for non-parabolic orbits (convenience feature, added to parent radius)
 
 **Config format (TOML) - Maneuvers:**
 ```toml