Saturn orbit insertion

spaceweb@oulu.fi - last update: 6th March 2006 (JV)

Cassini orbit insertion was executed in 1st of July 2004. The most critical event after launch was succeed, and the interplanetary probe changed its status to Saturnian satellite. The orbit insertion burn was automated operation, since the possible operation commands from Earth, if something would have gone wrong would be too late in Saturn. The approach to Saturn and orbit insertion burn time was so critical, that most of the instruments were in the safest mode available. For CAPS instrument it ment, that the actuator was not allowed to use. However the CAPS instrument made measurements during approach. During the actual orbit insertion burn the instrument was switch off, and turned on after critical phase, and continued to measure as soon as it was possible.

The orbit insertion was the closest approach of Saturn ever. The CAPS instrument data was used to study plasmas just above the ring planes after closest approach. Molecular and atomic oxygen ions were detected above the rings, and this detection was used to prove that neutral oxygen atmosphere exist above the rings. In this content, you have to note the difference of the atmosphere definition in ordinar language and plasma physicist. The detected atmosphere is very very thin, and it is only notable as a oxygen ion source. The ionisation occurs when sun light ionize neutral molecylar or atomic oxygen. The collisions of neutral thin gas atoms may ionize atoms. Also the neutral gas interaction with rings when icy ring particles collide with neutral atoms and molecules may ionize them. When the neutrals are ionized they start to interact with Saturnian magnetosphere. As a ionized plasma component the neutral gas cloud then can be detected indirectly using plasma instrument like CAPS, which was the firs one in the place. In the CAPS/IMS data the precence of oxygen neutrals appears as a dramatical increase of the amount of the molecular oxygen, and slight increase of the amount of the oxygen ion just above the B and A ring after the orbit insertion.

The following figure shows the Cassini orbit during orbit insertion, and shows using colors where the CAPS instrument was in measurement mode, and when it was direcctly above the specific ring.

When considering the CAPS measurements further from the Saturn the magnetosphere can roughly be divided to outter and inner magnetosphere (see figure below). The outer magnetosphere contain co-rotative warm ions, which are mainly hydrogen. The approach flight of the Cassini passsed through the Titan orbit too far higher latitudes to observe any nitrogen originating from Titan. The inner magnetosphere, plasmasphere, was roughly divided to three regions. Outer region (A and Ap (p for outbound) in the figure) contains mainly corotative hydrogen ions. However the water group ions (OH+ H₂O+ H₃O+) amount is less than ten percent, and oxygen ions close to one percent. In the middle region in plasmasphere (B and Bp in the figure) the water group and oxygen ions became more popular than hydrogen ions. In the inner region (C and Cp in the figure) in plasmasphere the hydrogen amount still decrease compared to water group and oxygen ions. When the ion origins in the plasma sphere is considered the icy satellites and rings forms the main ion source, the water group and oxygen are mainly from there. The hydrogen in outer magnetosphere is from solar wind as well as from Saturn ionosphere. The Titan, or its atmosphere and ionosphere, produce nitrogen and ammonia ions around its orbit. This is called Titan plasma wake. During the inbound and outbound, near orbit insertion, the nitrogen was not detected at Titan distance. However the inner plasmasphere shows some nitrogen ions. If those are from Titan, they must travel inward as neutrals, since ions would be captured to the corotative magnetospheric plasma. The other nitrogen source in the inner Saturnian system could also explain the nitrogen founding in inner plasmapshere.

This figure shows the Cassini orbit during orbit insertion directly from north axis of the Saturn. The colors in Cassini orbit shows four different regions detected using CAPS instrument.

The artistic view (J. Tubbin, Los Alamos Natinal Laboratory) of the Saturn magnetosphere. Solar wind shows the sun direction.

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