Astronomy & Celestial Mechanics
the sky, computed
Concept

Phasing orbit — where it appears

A temporary orbit of slightly different size from a target's, used to change a spacecraft's position along the orbit relative to the target. The difference in period makes the spacecraft gain or lose ground each revolution until it reaches the right phase.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Catching a target 40° ahead. A phasing manoeuvre. Dropping into an orbit 6% lower shortens the period to 0.9553 of the target's, so the chaser gains 16.1° each lap and closes 40° in 3 revolutions. Speeding up would have lost ground instead.

Catching up by slowing down, which cost Gemini 4 its fuel

To reach something ahead in the same orbit, a spacecraft must fire backwards. Pointing at the target and thrusting makes the gap grow, and a crew found that out in orbit before anyone had flown the correct manoeuvre.

spaceflight · Rendezvous Two satellites moved apart by the attitude they hold. The along-track separation of two identical 5 kg satellites at 475 km, starting together, when one flies broadside to the airflow and the other edge-on for 15 days and then they swap, so that their relative drift ends at zero. The separation grows quadratically and then levels off. At solar minimum the manoeuvre moves the pair 2,797 km apart; mid-cycle 8,299; at solar maximum 24,620 — 206° of the orbit's 43,060 km circumference. No propellant is spent; the same command produces a tenfold different result depending on the state of the Sun.

A rendezvous paid for in air

Two satellites in the same low orbit can be moved along it relative to each other with no propellant at all, by turning one broadside to the thin air and the other edge-on. The draggier one sinks and speeds up, the gap grows as the square of the time, and swapping attitudes halfway brings the drift back to zero. The cost moves from fuel to time, to altitude, and to knowing an atmosphere that changes tenfold with the Sun.

spaceflight · Rendezvous

Named alongside it

The objects these essays reach for when they reach for this one.

Atmospheric dragBallistic coefficientDifferential dragFeedbackLaunch windowOrbital rendezvousR-bar approachRelative motionRendezvousSatellite constellationSolar cycleSynodic period

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