There was no way of knowing where any wreckage might have landed, or even if any had survived the flaming fall.
Mr Howard said at a press conference that by the time Mr Clinton phoned he had already chaired a meeting of the National Security Council which had called on the Defence Forces to “make preparations to deal with any emergency”.
A Defence Department spokesman, Colonel Andrew Reynolds, said space debris emergency search teams had been put on standby.
Armed with radiation detectors and special containers, it would have been their job to find and collect any debris contaminated with plutonium 238.
Mars 96 was launched from Kazakhstan on Sunday morning, Sydney time, aboard a Proton rocket. Its fourth stage failed, stranding the craft in an 87-minute Earth orbit.
With friction slowing the craft, it completed about 19 orbits before falling to Earth.
Death of a complex craft just one of many failures
Russia’s Mars 96 would have been one of the most complex spacecraft yet fired at the red planet.
Launched on Sunday morning, Sydney time, it was supposed to circle the world once before the fourth stage of its Proton rocket was to reignite, sending the 6.7-tonne craft on its 10-month journey to Mars.
It was really five spacecraft bolted together. Five days before reaching Mars it would have released two landing craft, each weighing 75 kilograms.
The landing craft were carrying cameras as well as instruments designed by Finland, France and the United States.
Each landing craft was fitted with microchips carrying the names of 100,000 members of The Planetary Society, an international group of space enthusiasts, as well as CD-ROMs loaded with novels, art work and even a recording of the radio broadcast War of the Worlds.
Mars 96 also carried two arrow-like penetrators, designed to bury themselves six metres below the surface, where some scientists believe water, or even life, might be found. Each landing craft and penetrator was powered by a tiny pencil-sized container of plutonium.
The main body of Mars 96 was to map the planet from orbit.
Russia has attempted at least 18 Mars missions. The first two, in 1960, failed even to reach Earth‘s orbit. Another three, launched in 1962 and 1971, fell back after being stranded in Earth’s orbit – failures repeated at the weekend by Mars 96.
In 1971 Mars 2 crashed while trying to land on the planet. Mars 3 landed safely, but all contact was lost seconds later.
Russia launched four Mars probes in 1973 but only one, an orbiter, was a complete success. Two Russian missions to land on Phobos, a moon of Mars, failed in 1989. Russia is planning one more Mars probe, in 2001.
An American probe was launched earlier this month to map Mars from orbit. Another NASA mission is to be launched on December 2.
Not much room left as junk goes round in circles
Russia’s failed explorer, Mars 96, which crashed back to Earth yesterday, was just one of more than 8,000 man-made objects circling the globe yesterday.
Every object larger than 10 centimetres is tracked by the US Space Command’s global radar network.
“The US Space Command is the world’s most sophisticated, if not only, capability to survey space and predict such re-entries,” a spokesman, Lt Col Don Planalp, said.
On Saturday, the Space Command was keeping tabs on 8,674 man-made objects in space, including 2,296 satellites, 6,301 bits of debris from burnt-out rocket stages to spacecraft window covers, and 77 probes travelling out beyond Earth orbit.
Since the dawn of the space age, on October 4, 1957, when the then Soviet Union launched Sputnik 1, 15,948 objects have already fallen back to Earth, including 2,430 satellites.
“This is a frequent occurence,” Lt Col Planalp said of falling satellites.
“It happens a couple of times a week. When there are 8,000 objects in orbit around the Earth something is coming down all the time.”
Most burn up in the atmosphere. Others, like Mars 96, which carry electric generators powered by plutonium, attract more attention.
In 1964, an American plutonium-powered satellite crashed into the Atlantic after failing to reach orbit. Four years later, a Nimbus weather satellite crashed after a launch failure but its plutonium generators were found and used again.
The Apollo 12 to 17 missions all carried plutonium generators to power instruments left on the Moon. Apollo 13’s nuclear generator crashed into the Pacific Ocean, along with the remains of its lunar module. All efforts to trace the plutonium failed.
Several dozen Soviet spy satellites, designed to track the US Navy, carried plutonium generators. After completing their missions, rockets blasted the generators into high Earth orbit, where they should remain for hundreds of years.
However, several malfunctioned and fell back to Earth. In January 1978, one nuclear spy satellite, Cosmos 954, crashed into Canada, spewing plutonium over a wide area.
Mars 96 carried its 200 grams of plutonium in four pencil-sized containers to power four probes to be dropped on to the Martian surface.
The Voyager missions to the outer planets and Galileo, now circling Jupiter, all have nuclear generators.
Cassini, a NASA mission to be launched next October to orbit Saturn, will also carry a plutonium power system.