Venus

The first trajectory correction manoeuvre was scheduled for 9 November 1997. The spacecraft terminated its solar-inertial orientation and adopted the proper attitude to make the burn. The 34.6-second main engine firing was monitored by real-time telemetry, and Draper reported that the engine had performed "very, very nicely". Afterwards, the solar-inertial orientation was automatically resumed. The 2.7-metre-per-second burn fine-tuned the trajectory for the first Venus fly-by. The rest of the year was routine, with Cassini periodically reporting its excellent state of health.39 Throughout January and February 1998 the Deep Space Network refined the spacecraft's location to enable JPL's navigators to work out the trajectory correction manoeuvre for 3 March. The change in velocity was so small that the hydrazine thrusters were used instead of the main engine. In late March, the spacecraft survived its scorching perihelion at 0.676 AU, slightly within Venus's orbit. The plan included an option for a final trajectory adjustment in early April, but this proved to be unnecessary.40

Monitored by the Deep Space Network antennas at Goldstone in California and Madrid in Spain, Cassini performed its first Venus fly-by, skimming 284 kilometres above the planet's surface on 26 April. The closest approach was at 06:45 Pacific Daylight Time, but even at light-speed the signal took 7 minutes to reach the Earth. As the spacecraft crossed the limb, as viewed from the Earth, the radio occultation provided information on the planet's atmosphere. For several hours, the Radio and Plasma Wave Spectrometer listened for evidence of lightning in Venus's atmosphere. The excellent sensitivity of the negative result placed a strong constraint on lightning caused by rapid vertical convection in the dense atmosphere.41,42,43 "If lightning exists in the Venusian atmosphere, then it is either extremely rare or is very different

North Ecliptic View Groundtrack

North Ecliptic View Groundtrack

from terrestrial lightning," reflected team leader D.A. Gurnett. "If terrestrial-like lightning were occurring in the atmosphere of Venus within the region viewed by Cassini, it would have been easily detectable." Indeed, in its subsequent fly-by of the Earth, the instrument detected lightning continuously while Cassini was within 14 Earth radii, at rates of up to 70 impulses per second. In the case of Venus, the clouds are at very high altitude (above 40 kilometres) and so there is unlikely to be electrical discharges to the surface. If there is lightning, it will therefore be either from cloud to cloud or up into the ionosphere. Nevertheless, dust rising in volcanic plumes in the stagnant dry lower atmosphere may produce discharges to the surface. If so, then as Cassini flew by there were no active volcanoes.44 When close to Venus, the radar illuminated the surface as an engineering test of the instrument's functionality, but the geometry was not conducive to receiving the 'bounce'. Operational limitations ruled out conducting a comprehensive study of the planet using the spacecraft's full suite of instruments.

The fly-by gave Cassini an 'assist' of 7,055 metres per second, sufficient to ease its aphelion out beyond the Earth's orbit. ''The accuracy achieved by our navigators is roughly equivalent to shooting a basket from Los Angeles to London for a swish shot,'' proclaimed Spehalski proudly. With Cassini firmly on track, Spehalski retired on 5 June. As a veteran of four decades at JPL, during which he had played crucial management roles in both the Galileo and Cassini missions - which JPL director E.C. Stone described as ''the last great flagship planetary flights of discovery of the twentieth century'' - Spehalski received NASA's highest honour, the Distinguished Service Medal. Robert T. Mitchell, who was at that time running Galileo's 'extended mission', was reassigned to manage the Cassini mission.

On 3 December 1998, four days before the 1.58-AU aphelion point of the orbit formed by the first Venus encounter, and out near Mars's orbit, Cassini performed a 88-minute Deep Space Manoeuvre to reduce its speed by 450 metres per second and slip the ensuing perihelion within Venus's orbit. ''The performance of the spacecraft and the team in performing this manoeuvre, was just perfect,'' Mitchell reported. ''We couldn't have asked for anything better.'' The refinement would maximise the effect of the slingshot during the second encounter.45

A depiction of Cassini's Deep Space Manoeuvre, firing its main propulsion system.

On 11 January 1999, while performing instrument tests, the spacecraft sensed a potential error in its orientation and 'safed' itself by adopting a minimum-power configuration, with its big dish aimed at the Sun for a thermally benign environment. An analysis of the telemetry established that the cause was a problematic geometry experienced during a very slow roll. The actual orientation is constantly compared to the Attitude and Articulation Control System's model, and the 'safing' was triggered when a reference star very near the edge of the sensor's field of view was lost. After resuming normal operations on 15 January, Cassini made an 11.6-metre-per-second manoeuvre on 4 February to refine its aim for Venus and on 24 March it activated its Cosmic Dust Analyser, which was to remain operational through most of the interplanetary cruise.46

In early March, 60 members of the science community met at JPL to consider opportunities to study Venus, Earth and Jupiter en route to Saturn, and to consider the optimal plan for the orbital tour of the Saturnian system.

A 0.24-metre-per-second trajectory correction manoeuvre was made on 18 May to fine-tune the fly-by of Venus, which took place at 13:30 Pacific Daylight Time on 24 June. The closest point of approach was at an altitude of 603 kilometres - some two seconds earlier and 4 kilometres higher than planned. This time, most of the scientific instruments were active to make observations for the final few hours of the approach and for two days thereafter, with the data being stored for later replay. On 29 June, Cassini made its 0.7166 AU perihelion passage.

North Ecliptic View

G round track closest approach

' ialaafli

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment