In the summer of 1999, millions of people across Europe peered upwards at the sky in an
effort to see one of Nature’s wonders – a total eclipse of the Sun. On 25 January, European
engineers and scientists witnessed an eclipse of a different kind – the passage of four Cluster
spacecraft through the outer part of the Moon’s shadow.

This was yet another in the long line of firsts registered by Cluster – the first time that a lunar eclipse had been studied simultaneously by a fleet of four identical scientific spacecraft.

The unique event began at 06:15 GMT (07:15 CET), when the Cluster quartet were about 100,000 km from the Earth. The Salsa spacecraft was the first to sweep into the Moon’s shadow, closely followed by Rumba, Tango and Samba. However, the duration of the eclipse was different for each spacecraft. First to emerge once more into full sunlight was Tango, with Salsa, Samba and Rumba trailing behind.

During the entire 53 minutes of eclipse, instruments on the Cluster quartet sent back a stream of data that gave scientists a unique opportunity to study the changes taking place around each of the spacecraft. Of particular interest were the data returned by the PEACE plasma electron spectrometer.

Before and after the eclipse, the spacecraft were in full sunlight and being bombarded by solar radiation. Under such conditions, ultraviolet light from the Sun knocks electrons from the spacecraft’s skin, creating a cloud of “photoelectrons” close to each Cluster satellite.

This lingering swarm of charged particles acts as a form of pollutant, spoiling the measurements by sensitive instruments such as PEACE. However, most of this electron cloud should disappear when the spacecraft shelter in the lunar shadow and the ultraviolet light from the Sun is temporarily dimmed.

“We expected to see a drop in the number of low energy electrons around each spacecraft as they moved out of strong sunlight, leading to reduced photo-electron levels and reduced spacecraft electric potential on the spacecraft,” said PEACE Principal Investigator Andrew Fazakerley from Mullard Space Science Laboratory, UCL, in the UK.

“This was, in fact, what we saw. It was particularly noticeable in the data from the Rumba and Salsa spacecraft, which showed rather pronounced reductions in measured counts during the penumbra crossing. This was probably because they both passed deeper into the Moon’s shadow.”

The eclipse also proved to be something of a challenge for the team at the Joint Science Operations Centre (JSOC) in Rutherford Appleton Laboratory, UK.

“We had to change the planning of scientific payload operations specially to observe the eclipse,” explained JSOC scientist Mike Hapgood. “It was the first special operation we have carried out and proved to be a very valuable learning experience.”

Although no further lunar eclipses are foreseen for Cluster, the mission operations team is well prepared for much longer, more hazardous, eclipses by the Earth. Last September, the Cluster commissioning programme was delayed by a series of Earth eclipses, and further passes through our planet’s shadow are predicted for the end of February and September this year. Since the lack of sunlight falling on their solar panels shuts off the normal power supply, each spacecraft carries five silver-cadmium batteries that enable them to survive eclipses lasting up to four hours. At such times, most of the scientific instruments are switched off to conserve power.