PERIAPSIS / FIELD NOTES
A little orbital
understanding.
The paths are calculated. The stakes are imaginary.
Fly the first minute
Choose Fly guided mission to watch a complete flight. To take the controls, launch the probe, tune a burn, inspect the gold prediction, and execute. The launch button places you at the mission’s stated insertion conditions; ground ascent is outside this model.
Positive prograde adds speed along the local orbit tangent. Positive radial points away from Earth. Changes pause flight so your maneuver stays reviewable. Load flight solution supplies a useful burn, including the arrival node after a transfer.
Read the orbit
Periapsis is closest approach; apoapsis is farthest. Both readouts are altitudes above the 6,371 km reference sphere. Cyan shows the current orbit. Dashed gold shows the planned result. The green corridor is your target altitude.
Use Plan view for a view perpendicular to the orbit. In the oblique view, the orbital plane is foreshortened. The corridor is drawn wider for visibility; the numeric mission tolerance is unchanged.
The model
A massless probe moves around a fixed spherical Earth in one plane: a = −μ r / |r|³. Earth’s gravitational parameter is 398600.435507 km³/s², from JPL’s DE440 parameter table ↗.
Velocity-Verlet advances in steps of at most 2 simulated seconds. Prediction and flight share the same integrator. Burns are instantaneous velocity changes. The delta-v budget is the sum of burn-vector magnitudes.
This excludes atmosphere, drag, oblateness, other bodies, finite thrust, attitude, real navigation uncertainty, and launch engineering. Earth uses locally bundled NASA Blue Marble imagery ↗. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio; Blue Marble data courtesy of Reto Stockli (NASA/GSFC) and NASA Earth Observatory. Lighting is illustrative. An open trajectory ends when it crosses the 120,000 km geocentric boundary outward.
Why the second burn?
A Hohmann transfer follows an ellipse tangent to two circular orbits. At the far end, another tangential burn matches the new circular speed. See NASA: Trajectories ↗ and NASA: Planetary Orbits ↗.
SPACE Pause / resumeR Reset flightG Guided flightENTER Execute focused button← → Adjust focused sliderESC Close manual
Read the complete flight manual & verification method ↗