Salsa and Samba in perfect step
21 July 2000After one of the most complex series of manoeuvres ever carried out by Earth-orbiting spacecraft, the first Cluster pair have successfully reached their final elliptical orbit. Salsa (FM 6) and Samba (FM 7) are now dancing in step between 16 869 km (perigee) and 121 098 km (apogee) above the Earth.
The last apogee raising manoeuvre was performed on 20 July at 00:59 CEST (FM 6 - Salsa) and 01:25 (FM 7 - Samba), out of contact with any ground stations. As a result, the spacecraft were placed in an elongated orbit with 246 km perigee and 119 523 km apogee.
By firing their small onboard thrusters, the spacecraft then completed a slew manoeuvre at 06:00 (FM 6 - Salsa) and 09:17 CEST (FM 7 - Samba). This changed their attitude with respect to the Sun from 86 to 109 degrees and placed them in the correct alignment for the final major engine burn.
In order to bring the spacecraft to their operational polar orbit, the main engines on both Cluster spacecraft were ignited last night, (21 July) at 01:02 (FM 6 - Salsa) and 02:04 CEST (FM 7 - Samba) over the Perth and Villafranca ground stations.
This fifth main engine burn in four days, which lasted for approximately 18 minutes, dramatically altered the spacecraft orbits. Not only was the perigee (lowest point) raised to 16 869 km, but the spacecraft performed a 'dog-leg' manoeuvre which shifted their orbital inclination from 64.9 degrees to 90 degrees.
Thanks to a new computer visualisation system known as Oratos, the Cluster operations team were able to watch events unfold in real time on a giant screen in the main control room at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
"Using telemetry from the spacecraft, this computer model is able to couple the orbital dynamics - the positions of the Earth, Sun, stars and Moon - together with the orbits, so that we can see how the spacecraft changes when we do a manoeuvre," said Manfred Warhaut, Cluster Ground Segment Manager at ESOC.
"The perigee raising and inclination change manoeuvre went perfectly, so all of the major orbital manoeuvres have now been successfully achieved," said Alberto Gianolio, Cluster Deputy Project Manager. "Apart from a few minor adjustments, we will now be babysitting the spacecraft until they are joined by their two companions."
These minor adjustments include a small reduction in spin rate to 15 rpm that was completed this morning, and another minor slew manoeuvre to modify the attitude of the spacecraft that is planned for this afternoon. Meanwhile, the commissioning of onboard solid state data recorders has been completed. Removal of the protective covers on two of the experiments (ASPOC and CIS) is planned for 25 July 2000.
The quick fire sequence of steps to achieve operational configuration will be completed on Monday, 24 July, when the first Cluster pair will begin their orbital drift phase.
With Salsa and Samba already dancing in unison, the Cluster team is waiting eagerly for the second pair to join them and complete the formation. The launch of the second duo is currently planned for 9 August at 13:11 CEST.