Added the MSIS scenario to nyx comparison

tests/fixtures/nyx/drag-msis-low-earth.json

@@ -0,0 +1 @@
+{"name":"drag-msis-low-earth","setup":{"tool":"nyx-space","version":"2.6.0","generated":"2026-10-02","forces":["Earth point mass, mu = 398600.4415 km^3/s^2 from the frame","Drag(AtmDensity::NRLMSISE00, ExtendedHistory57h, mean local solar time), area 5 m^2, Cd 2.2"],"integrator":"RK89, IntegratorOptions tolerance 1e-14, error control RSSCartesianStep, max step 2700 s","notes":["Spacecraft mass 500 kg, starting at 400 km.","The space weather is written out of the same embedded GFZ record astrokit reads, into the CelesTrak-shaped CSV nyx wants, so the two tools see numerically identical drivers. nyx takes the 81-day centered average from a column rather than computing it, so that column carries astrokit's own value -- which removes the 0.028% that CSSI's rounding cost the equivalent Orekit comparison.","Geomagnetic mode is the 57-hour ap history rather than nyx's default single daily Ap, matching what astrokit passes. Local solar time is mean on both sides, which is already nyx's default and was not Orekit's.","Altitude is geodetic on both sides and the relative wind is taken in EARTH_ITRF93, but the two ellipsoids differ and could not be made to agree: nyx measures against the planetary constants kernel's Earth, 6378.1366 km equatorial, and astrokit against WGS84's 6378.137. That places the same spacecraft 0.400 m apart in altitude, worth about 7e-6 of the density at 400 km, which is two orders below the model difference below. Setting the frame's ellipsoid does not help -- ANISE's transform replaces it from the loaded kernel.","No field, no third body, no radiation pressure.","States are nyx's own accepted integrator steps rather than a fixed grid, so nothing on this side is interpolated. nyx's trajectory interpolation is a five-point Hermite fitted after the fact over stored states, and it is the accuracy floor rather than the integration: on the point-mass arc at this tolerance the stored states agree with Kepler to 2.7e-6 m while reading the same trajectory on a 60 s grid is 8.1e-4 m. Sampling the steps themselves removes that term from the comparison."]},"epoch":"2026-01-01T00:00:00 UTC","frame":"EARTH_J2000","states":[{"elapsed":0.0,"position":[6778000.0,0.0,0.0],"velocity":[0.0,7668.0,0.0]},{"elapsed":38.947560749,"position":[6771420.469044267,298553.25204069447,9.388406105286336e-10],"velocity":[-337.81149086106865,7660.556399405986,2.1297036006966623e-10]},{"elapsed":77.31991415,"position":[6752081.460175954,592133.1783678555,2.8372554616698456e-8],"velocity":[-669.9960059433871,7638.677898282628,1.3703633673012948e-9]},{"elapsed":115.769381177,"position":[6719940.755038541,885183.4538164686,1.1788202509895291e-7],"velocity":[-1001.5818513644057,7602.316691684378,3.432552223843646e-9]},{"elapsed":154.298599822,"position":[6674978.963277502,1177161.1022591616,3.0384802000049697e-7],"velocity":[-1331.9549618880153,7551.450774966874,6.360513184628212e-9]},{"elapsed":192.651291909,"position":[6617627.741624049,1465580.6195304208,6.166910034842156e-7],"velocity":[-1658.30328113725,7486.5683935662055,1.0083864057738247e-8]},{"elapsed":231.066337677,"position":[6547694.495856405,1751702.892464092,1.0879177641160105e-6],"velocity":[-1982.0537934155443,7407.451543226486,1.457178110755673e-8]},{"elapsed":269.458332408,"position":[6465447.346956375,2034348.0254953366,1.7448337544218233e-6],"velocity":[-2301.8717137354042,7314.403410010821,1.9762845295084924e-8]},{"elapsed":307.975606211,"position":[6370675.1886256365,2314058.294870179,2.6169313246292616e-6],"velocity":[-2618.3709101507784,7207.184994543471,2.5625195607037797e-8]},{"elapsed":346.409215949,"position":[6264051.236265277,2588781.6537735225,3.723868349643218e-6],"velocity":[-2929.229693237649,7086.557732832857,3.207206790645895e-8]},{"elapsed":384.714313392,"position":[6145999.982730295,2857717.0991808283,5.084118442785547e-6],"velocity":[-3233.541911345344,6953.001661692889,3.9034441394667434e-8]},{"elapsed":423.385385278,"position":[6015113.409241519,3123776.4082465433,6.737446957219513e-6],"velocity":[-3534.6026072660866,6804.923594569369,4.654934685858022e-8]},{"elapsed":461.833813847,"position":[5873565.709582861,3382377.239049698,8.67775979145647e-6],"velocity":[-3827.2268192736133,6644.783043702982,5.44473852834631e-8]},{"elapsed":500.337651938,"position":[5720676.581457687,3634936.7799456194,0.000010932469507062726],"velocity":[-4113.018340332627,6471.810086824766,6.272545425254571e-8]},{"elapsed":538.793374807,"position":[5557143.128239333,3880296.1519273133,0.00001350867725915659],"velocity":[-4390.665952485439,6286.7931209919025,7.130486541987151e-8]},{"elapsed":577.119812228,"position":[5383693.02773496,4117523.1681592413,0.00001640955292428148],"velocity":[-4659.1148917907885,6090.555140123361,8.011030070997805e-8]},{"elapsed":615.540594817,"position":[5199656.359711009,4347563.366040146,0.000019660338161310214],"velocity":[-4919.435229616536,5882.337913448691,8.913912275666822e-8]},{"elapsed":653.946969434,"position":[5005869.4262301Line truncated

tests/nyx_comparison.rs

@@ -214,3 +214,78 @@ fn test_radiation_pressure_agrees_with_nyx() {
         "worst velocity disagreement {velocity:.3e} m/s over the day"
     );
 }
+
+/// Drag through NRLMSISE-00, where the drivers match exactly and the models do not.
+///
+/// This is the one comparison here with no driver mismatch at all. The fixture's space
+/// weather is written out of the same embedded GFZ record this crate reads, into the
+/// CelesTrak-shaped CSV nyx wants, and nyx takes the 81-day centered average from a
+/// column rather than computing it, so that column carries this crate's own value.
+/// Printed side by side, both tools feed the model identical numbers: f107 170.5,
+/// f107a 154.3333333333331 and all seven ap values the same. The Orekit equivalent
+/// could not manage that and lost 0.028% to CSSI publishing its average rounded.
+///
+/// # What is left is the two implementations, and the difference is nyx's
+///
+/// With the drivers identical the densities still differ by 2.6e-4, consistently and in
+/// the same direction. Run against case 0 of the seventeen standard NRLMSISE-00 cases,
+/// which pin the NRL reference to sixteen digits: **this crate reproduces the published
+/// total mass density exactly, and nyx is 1.08e-3 away from it**. So the disagreement
+/// below is nyx departing from the reference, not this crate.
+///
+/// One element is unmatched and is ours: nyx takes geodetic altitude against the
+/// planetary constants kernel's Earth and this crate against WGS84, which puts the same
+/// spacecraft 0.400 m apart. That is worth about 7e-6 of the density, two orders below
+/// the model difference, and it could not be removed -- ANISE's transform replaces a
+/// frame's ellipsoid from the loaded kernel.
+///
+/// Bounded as a fraction of what drag does over the arc rather than in meters, since
+/// the meters depend on the arc and the fraction does not.
+#[cfg(feature = "msis")]
+#[test]
+fn test_msis_drag_agrees_with_nyx() {
+    let Some(almanac) = almanac() else {
+        eprintln!("skipped: kernels not fetched, run scripts/fetch-kernels.sh");
+        return;
+    };
+    let arc = fixture("drag-msis");
+    assert!(arc.setup.forces[1].contains("NRLMSISE00"));
+
+    let (position, velocity) = compare(
+        &arc,
+        &[
+            TwoBody { body: Body::EARTH },
+            Drag {
+                density: DensityModel::Nrlmsise00,
+            },
+        ],
+        Some(Spacecraft::cannonball(500.0, 5.0, 2.2, 1.3)),
+        Some(almanac.clone()),
+    );
+
+    // What drag moved the orbit over the day, so the disagreement reads against it.
+    let start = &arc.states[0];
+    let epoch = Epoch::from_gregorian_utc_hms(2026, 1, 1, 0, 0, 0);
+    let initial = Orbit
+        .with_position(Vector3::from_column_slice(&start.position))
+        .with_velocity(Vector3::from_column_slice(&start.velocity));
+    let drag_free = {
+        let mut plain = Propagation::new(epoch, initial, &[TwoBody { body: Body::EARTH }])
+            .frame(Frame::EARTH_J2000)
+            .duration(Duration::from_seconds(arc.duration()))
+            .accuracy(Accuracy::High);
+        Orbit.state(*plain.run().states.last().unwrap()).position()
+    };
+    let last = arc.states.last().expect("checked on load");
+    let effect = (Vector3::from_column_slice(&last.position) - drag_free).norm();
+
+    // Measured 2026-10-03: 8.55 m, 0.0286% of the drag effect, 9.66e-3 m/s. That is the
+    // 2.6e-4 density difference integrated along-track, and it is nyx's departure from
+    // the reference rather than this crate's.
+    assert!(
+        position < 1.0e-3 * effect,
+        "worst disagreement {position:.3e} m is {:.4}% of the {effect:.0} m drag moves it, \
+         and {velocity:.3e} m/s",
+        position / effect * 100.0
+    );
+}