At 03:52:50.670 UTC July 26, 2011, CASTOR reached another milestone in its unique satellite catalogue project. This time, it reached its 4,000th satellite, something that even the founder did not know was possible with CASTOR's equipment.

When the catalogue began on January 1, 2007, the founder thought that he could detect up to 2,000 satellites before the detector could detect no more. At the end of 2007, he knew better and continued on his journey to discover that magic number.

At the end of 2009, the catalogue had risen to over 3,100 satellite entries and the new detections still did not stop. CASTOR carried on to see if it could reach 4,000 satellites from all critical orbit types.

Finally, on July 26, 2011, CASTOR reached its new goal of 4,000 individual satellites. This number represents nearly one quarter of all known catalogued satellites now orbiting the Earth.

WHAT WAS THE 4,000TH SATELLITE?

A small piece of debris from the NOAA 11 weather satellite became the "satellite of the hour". CASTOR had planned to make the Hubble Space Telescope the satellite of distinction, but it became CASTOR #3,971 instead.

Another surprise was that CASTOR successfully detected Explorer 7: one of the oldest satellites still in orbit. Explorer 7 was launched on October 13, 1959: just two years after Sputnik. Only 5 other satellites still orbiting are older than Explorer 7. They are the Vanguard payloads and rockets. Explorer 7 became CASTOR #3,981. CASTOR detected this object when it was nearly 800 kilometres in range. This is amazing, considering that the satellite is only 75cm (2.5 feet) in diameter!

Heading for 4,000, CASTOR also detected a second piece of debris from the infamous Iridium 33 - Cosmos 2251 collision. This piece was also from the Iridium 33 satellite.

The CASTOR Satellite Catalogue now stands at 4,016 individual satellites from the LEO, MEO, GEO and HEO orbit realms. Having detected most of the MEO, GEO and HEO satellites, the last frontier (LEO) is still vastly unexplored. This is due to the mammoth amounts of debris floating around between 200 and 1,000km above the Earth's surface.