No. 10 - Cassini-Huygens Periapsis Raising Manoeuvre
Prior to the PRM the Cassini-Huygens spacecraft had been in a highly elliptical orbit around Saturn following the SOI manoeuvre of 1 July. The orbital elements with respect to Saturn's equatorial plane were:
|Semi-major axis||4 585 959 km|
|Periapsis radius||80 731 km|
|Apoapsis radius||9 091 186 km|
If left in this orbit the spacecraft would not encounter Titan with the correct velocity to make further orbital changes for the successful deployment of the Huygens probe, and would return to a periapsis point 20 000 km above Saturn's cloud tops and also a second ring plane crossing.
The PRM was performed when Cassini-Huygens was near the outer point in its orbit at about 9 million kilometers from Saturn. Here the orbital velocity had reduced to a mere 325 ms-1 compared to the 30 000 ms-1 after the SOI engine burn on 30 June.
The PRM engine burn imparted an increase in the orbital velocity of between 350 and 390 ms-1. Later confirmation will give the exact new orbital parameters which were targeted to be:
|Semi-major axis||4 790 340 km|
|Periapsis radius||498 970 km|
|Apoapsis radius||9 081 700 km|
The new periapsis is located in the safe, tenuous outer regions of Saturn's E-ring instead of only 20 000 km above Saturn's cloud tops. This periapsis is of little significance because the Titan encounter on 26 October, at an altitude of only 1200 km, will change the orbital parameters again. Following the Titan A encounter the apoapsis will be reduced to 4.7 million km and the periapsis to 370 000 km.
During the first three orbits around Saturn (Orbit A, B and C) Cassini-Huygens will make three encounters with Titan. The Huygens probe will be released on 25 December 2004 to descend in Titan's atmosphere on 14 Januari 2005.
Each of the three encounters with Titan will further alter the orbit of the Cassini-Huygens spacecraft.
prior to encounter
|Titan A||26.10.2004||around 4 months||48 days (3 Titan orbital periods)|
|Titan B||13.12.2004||48 days (3 Titan orbital periods)||32 days (2 Titan orbital periods)|
|Titan C||14.01.2005||32 days (2 Titan orbital periods)|