June 3, 2009

SPOT 2 deorbiting

After being exploited during nineteen and a half years, the second satellite -  Spot 2 (Earth Observation System) - of the Earth Observation Programme decided by the Belgian, Swedish and French governments reached its end-of-life.

The manoeuvres started on July 16th 2009 by CNES' technical teams and led to the definitive deorbiting on July 30th. Placed into orbit the perigee of which was below the 600 km, the satellite had been passivated and then considered like a space debris. On this new low orbit, Spot 2 was submitted to stronger frictions and progressively lost altitude to finally disintegrate into the atmosphere The re-entry should take place in 25 years at the latest, i.e. approximately 6 years after its predecessor Spot 1 which had been deorbited by CNES in 2003.

A 6.5-million image collection

L'une des dernières images prises par Spot 2 : Marseille en couleurs naturelles en juin 2009 - Crédit : CNES
One of the last images taken by Spot 2: Marseilles in natural colours in June 2009 - credits: CNES

In approximately 20 years, the Spot 2 satellite collected a total of 6.5 million "scenes" covering almost 23.4 billion km2, i.e. approximately 46 times the complete Earth's surface.

The day after being launched on January 23rd 1990, the second satellite of the Spot series took the first pictures above Marseilles and the Lake Garda in Italy.

By comparison, similar pictures were taken shortly before Spot Image stopped the satellite's commercial exploitation on June 30th 2009.

Keep the space clean

Even if Spot 2 was still able to send quality pictures, the mission was initially scheduled to last 3 years and reached its end-of-life. Like for Spot 1 in 2003, CNES decided to use the last hydrazine supply of Spot 2 to move it on a lower orbit. In fact, without any interference and given its operational orbit altitude, Spot 2 would take more than 200 years to enter the atmosphere.

However, since 2002 the IADC (Inter Agency Space Debris Coordination Committee) recommended that, henceforth, any low orbiting satellite would have to re-entry the atmosphere in less than 25 years. Even if this recommendation does not formally apply for the previous Spot 1 & 2, CNES decided to apply it in a bid to give the example and keep the space clean for the future generations.

La désorbitation de Spot 2 a démarré le 16 juillet 2009 - Crédit : CNES
The Spot 2 deorbiting started on July 16th 2009 - credits: CNES

Daily braking manoeuvres

Managed by the Toulouse Space Center's teams, this deorbiting phase was made up of several steps. “On Wednseday, 16th July, we performed a first manoeuvre to remove the satellite from its operational orbit”, Frédéric Tavera, Head of the exploitation mission of the Spot satellites at CNES, explained. “It consisted in making the altitude decrease by 15 km using two successive braking thrusts at peak and perigee to maintain a quasi circular orbit.” The second step consisted in performing from July 20th to 29th daily braking manoeuvres using 1000-second thrusts at peak. Every day, the perigee of Spot 2 was losing approximately 25 km to reach the objective placed at an altitude of less than 600 km.

Frédéric Tavera added that Spot 2 was entirely passivated since July 30th. He explained that the satellite's power supply had been cut off and the reservoirs emptied. In early July, about sixty out of the 160 kilos of hydrazine were still available for the deorbiting operation. The stock available resulted from the satellite's low consumption during all the exploitation because of its good flight specifications. Indeed, Spot 2 did not experience any anomaly which required to be put in survival mode during the mission. This mode would have been the most hydrazine demanding one.

Spot 2 turned into a space debris on July 30th, 2009. It started then to follow an elliptic orbit with a perigee of less than 600 km and a peak of approximately 800 km. It is planned to disintegrate into the atmosphere in approximately 25 years without putting the population at risk.

Spot 2 at a glance

Spot 2 - Crédit : CNES
Spot 2 - Credit: CNES
  • 6.5 million: number of scenes collected by Spot 2, i.e. 23.4 billion km2 and 46 times the planet's surface.
  • 19.5: number of years of exploitation.
  • 123 billion: number of rotations performed without maintenance by each of the three nominal spinning gyroscopes of Spot 2, a world record for a man-made instrument.
  • 100,000: number of orbits travelled by Spot 2 at 7.4 km/s, i.e. more than 30 times the Sun-Earth distance.
  • 1: number of important breakdown experienced by Spot 2. In 1993, when the electron magnetic recorders (EMR ) were lost, the programmes had to be executed within visibility of the reception stations.
La station de poursuite d'Issus Aussagel - Crédit : CNES
The tracking station of Issus Aussagel - credits: CNES

The station network drafted

The tracking station network of CNES (antenna set up in Issus Aussaguel in South Toulouse, France, in Hartebeestoek in South Africa and on the Kerguelen islands) completed by the Swedish station of Kiruna and assisted by the Canada's in the North Pole, observed and controlled Spot 2 during these operations.

If this deorbiting mission translated the end of a adventure for the teams working on the Spot 2 mission, two other satellites of the same series (Spot 4 & 5) kept successfully collecting Earth's images which were then commercialised by Spot Image. After Spot, CNES designed and was the prime contractor of the Pleiades programme based on two new-generation satellites. The Pleiades' dual system was aimed to simultaneously provide civil and military data thanks to its 70-cm resolution. The first flight model of the satellite was delivered in Autumn 2009 to be launched in 2010.

See also

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