Cruising On

On 3 April 2002, in the first burn since the trajectory refinement after the Jovian fly-by, Cassini fired its main engine for 9.85 seconds. This was routine maintenance, as the engineers did not wish to allow more than 400 days to pass without firing the engine to clear its propellant lines. On this occasion it had been decided to test new

Manoeuvre Automation Software designed to simplify the process of drawing up the command sequence for a manoeuvre. As another test, the sequence began and ended with the high-gain antenna pointed at Earth, as would be the case when orbiting Saturn, rather than at the Sun, as it did during the interplanetary cruise to provide thermal protection. On 21 October Cassini's narrow-angle camera took filtered imagery for a natural-colour view of Saturn from a range of 285 million kilometres. ''This is an emotional event for the mission,'' noted project scientist D.L. Matson. ''We now have Saturn in our sights.'' However, the resolution was still inferior to that provided by the Hubble Space Telescope.

A fly-by of Phoebe on the way into the Saturnian system had always been on the flight plan in order to use that moon's gravity to slow the spacecraft, save propellant in the Saturn Orbit Insertion manoeuvre, and ultimately extend the mission at Saturn beyond 4 years. The baseline fly-by range was 52,000 kilometres, but this had been considerably reduced as a consequence of the 9-day delay in launching the mission, and a trajectory tweak inbound to Jupiter had refined that slingshot to further reduce it. ''A small burn is required shortly before Phoebe, and another one afterwards, but if Phoebe hadn't been there we'd have had to make a fair-sized manoeuvre two or three weeks prior to Saturn Orbit Insertion as a system test, so the net cost of going closer to Phoebe is practically zero,'' pointed out Robert Mitchell. After meetings of the project science group in October-November, it was decided to make the fly-by at an altitude of 2,000 kilometres. Some had argued for a closer encounter in order to better chart the moon's gravity field and thereby gain insight into its internal structure, but this was dismissed owing to the possibility that dust in the vicinity of Phoebe could damage the spacecraft sufficiently to prevent it proceeding with the Saturn Orbit Insertion manoeuvre.

A new moon was discovered on 5 February 2003.143 It was discovered by Scott S. Sheppard, David C. Jewitt and Jan Kleyna using a wide-field digital camera on the Subaru telescope in Hawaii. It was the 14th satellite to be discovered in an eccentric distant retrograde orbit, and the first such discovery for nearly 3 years. It increased to 31 the total of listed satellites. The International Astronomical Union designated it S/2003S1. On the assumption that its surface was dark, its diameter was estimated at about 8 kilometres.

On 17 June 2003, with Earth now within 5 degrees of the Sun as viewed from the spacecraft, Cassini began its radio science solar conjunction experiment. When the angle had reduced to 3 degrees, the Deep Space Network began to send sequences of 'no op' commands 10 to 20 times per day, as it had during each conjunction period after the Earth fly-by, so as to gather statistics on the reliability of reception of commands as functions of conjunction angle and heliocentric range. This data

Cassini's first view of Saturn (with Titan), taken on 21 October 2002.

An image of Saturn taken by Cassini on 9 November 2003 (bottom), and an auroral display at the south pole in the ultraviolet viewed by the Hubble Space Telescope in January 2004. Note the different vantage points. The ultraviolet emission is due to solar wind electrons flowing in the magnetic field in the polar region and exciting atomic and molecular hydrogen. There was an increase in the electron flux on 25 January 2004.

would form the basis for deciding the duration of the hiatus in uplinking for the conjunction that would occur shortly after Saturn Orbit Insertion. On 1 May, Cassini fired its engine for 17.53 seconds to flush the propellant pipes for its 400-day routine maintenance. In this case, it also verified software intended for use during the orbital tour, and the ability of the Deep Space Network to upload and execute a burn during a single communications session. In September, after tests had indicated that there would be an adequate power margin, it was decided that the data-relay system of the Huygens probe should be powered up 4 hours prior to the calculated time of entry into Titan's atmosphere in order to warm up and stabilise the circuitry. On 10 September a minor manoeuvre tested the Manoeuvre Automation Software's ability to specify burns to use the thrusters of the reaction control system - as would be done often once the spacecraft was in orbit around Saturn. A 21.8-second burn of the bipropellant engine on 1 October demonstrated the yaw-steering that was to be utilised during Saturn Orbit Insertion, and the energy-based algorithm that would terminate that burn.

On 9 November 2003, one year after its 'first look', and now at a range of 111 million kilometres, Cassini took an image of Saturn showing the banded structure of its atmosphere. ''For all of us who have worked for more than a decade preparing for this mission, seeing Saturn grow larger in Cassini cameras is a bit like the feelings children have as they come downstairs on Christmas morning to see what gifts are waiting for them under the tree,'' noted Anthony DelGenio of the Goddard Institute for Space Studies in New York City.

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