No. 14 - SPC Report November 2004
After the 1st ring crossing, the main engine fired for 96 minutes to slow down the spacecraft and put it in orbit around the ringed planet. At periape, above the rings, Cassini-Huygens was less than 18000 km above Saturn's cloud top. Once the engine burn was over, the planned science observations started. A series of spectacular images of the rings were obtained before the spacecraft re-oriented itself with its High-Gain Antenna forward for the second ring crossing. Further observations of the rings were then obtained and provided excellent data on the ring dynamics, their interaction with the moons, and composition variation across the various rings.
Science observations highlights
Phoebe: On 11 June, during its approach to Saturn, Cassini-Huygens flew by Phoebe, a distant moon in retrograde orbit around the planet. The closest approach distance was 2000 km. Spectacular data were obtained of what is most likely a captured object.
Titan: Within 36 hours of Saturn orbit insertion, a series of distant observations of Titan were made on 2 and 3 July. During this first so-called "untargeted" encounter, Cassini-Huygens' closest approach to Titan was 339 000 km. Observations of the atmosphere, including detailed observations of a highly variable methane polar cloud, were acquired. Tantalizing images of the surface were obtained by both the Camera (ISS, Imaging Science System) and by the Visual Infrared Mapping Spectrometer (VIMS). Within 2 months, the Titan data set was carefully analysed together with the relevant latest ground-based observations to validate the atmosphere engineering model used for the design of Huygens. The observations confirm that the atmosphere structure is well within the envelope of the engineering model, in fact very close to the nominal profile. The methane and the argon concentration uncertainties have been reduced respectively to the range 1-3 % instead of 1-5% and 0-7% instead of 0-10 %. A model update of Titan's atmosphere is being worked as an input for the final Huygens performance validation before separation.
Saturn: Images of Saturn's atmosphere are regularly obtained while Cassini-Huygens is far from the planet. Lots of atmospheric details are seen in the images, in particular from the southern hemisphere due to current viewing geometry.
Rings and moonlets: Cassini discovered one, possibly two moonlets in the F ring region, and a ring of material associated with Saturn's moon Atlas, between the A and the F rings. The search for new moonlets and the study of their interaction with the rings is a key scientific objective of the Mission. Significant results have already been obtained. Searches will continue for further detections of the newfound body or bodies seen in association with the F ring. If the two objects indeed turn out to be a single moon, it will bring the Saturn moon count to 34. This newly discovered ring adds to the growing number of narrow ringlets around Saturn.
Cassini results are regularly released on a daily basis on the NASA/JPL web site: http://saturn.jpl.nasa.gov/. A selection is also posted on ESA's Huygens science web site http://saturn.esa.int. A series of papers are in preparation for publication in "Science" before the end of 2004.
Huygens operations and engineering activities
On the operations side, two Huygens probe in-flight checkouts have been performed since Saturn orbit insertion, on 14 July and on 14 September. They confirm that the Probe is in good health. A special test was performed on each of the five Lithium sulphur-dioxide batteries on 19 September to remove the passivation chemical layer that formed over the years since their manufacturing. This first of the two so-called "battery passivation" activities demonstrated that all five Huygens batteries are healthy and fully charged.
On the engineering side, the work undertaken over the last few months to validate Huygens's performance during entry is nearing completion. The heat flux and integrated heat load expected to be experienced by Huygens during entry have been re-evaluated and their impact on the Thermal Protection System (TPS) and parachute deployment re-assessed. This work is supported by several expert groups in Europe and in the USA and by Huygens prime contractor Alcatel and its subcontractor EADS. The work is expected to be concluded by end of October. The objective of the on-going work is to re-assess the margins that were included in the Thermal Protection System and to verify that the TPS can cope with the updated entry aerothermodynamic environment. One of the main objectives of the study is also to verify whether the entry corridor is well centred with respect to the Huygens TPS performance with a nominal entry angle of -65°.
On 23 August, the third largest manoeuvre performed by Cassini-Huygens near apoapse during the first 117-day orbit around Saturn placed the spacecraft on its nominal trajectory for the first close encounter with Titan on 26 October, with a planned closest approach at 1200 km. The 2nd close encounter with Titan is planned on 13 December at an altitude of about 2200 km. After release, Huygens trajectory will bring it within 60 000 km of the large moon Iapetus. During the validation process of the Huygens trajectory at JPL, it was found that the mass uncertainty of Iapetus may have an impact on Huygens's targeting accuracy. An alternative reference trajectory is being investigated at JPL that would move Huygens away from Iapetus to about 120 000 km and increase the accuracy with which the Huygens probe will be delivered to Titan. The new reference trajectory incorporates updated orbiter andmajor satellite ephemerides. The major changes from the previous reference trajectory are a decrease in the altitude of the second Titan flyby (Tb, 13 December 2004) from 2200 km to 1200 km, and an increase in the altitude of the fourth Titan flyby (T3, 15 February 2005) from 1000 km to 1577 km. The improved satellite ephemerides resulted in small changes to the timing of events such as Titan flybys and Saturn periapses. The need to change to the alternative trajectory and the Orbiter science impacts are being evaluated. A decision is expected by late October at the latest.
Huygens Mission Readiness review
An independent ESA/NASA Mission Readiness Review of Huygens is being conducted. Preliminary review activities started in early October. The Formal Kick-Off meeting will take place on 21 October. The final board meeting is currently planned at ESTEC on 11 November.
The main upcoming events are:
- First targeted Titan flyby (@ 1200 km altitude) on 26 October
- Probe checkout #16 on 23 November
- Go-NoGo decision for Huygens baseline mission: 6 December 2004
- Second targeted Titan flyby (@ 2200 km altitude, may be reduced to 1200 km) on 13 December
- Go-NoGo for Probe separation preparation activities: 17 December
- Probe separation on the 25 December (separation window extends several days)
- Probe entry on 14 January 2005; Titan flyby at 60000 km
- Third low altitude Titan Flyby (@ 1000 km altitude, may be increased to 1577 km) on 15 February 2005.