A nuclear-powered space probe is preparing to zip by Earth today to gain momentum for the last leg of its seven-year voyage to Saturn.

Cassini was launched by NASA two years ago. Cassini is about two-storeys tall. It is the largest and most expensive unmanned probe ever put in space. On August 17, 1999, the Cassini will come within 1,200 kilometres of Earth. Earth's gravity will give Cassini an ``assist''. A "gravity assist" is when a spacecraft uses a planet's gravity to swing itself faster on its way to a farther planet.

Cassini will get two gravity assists from Venus, one from Earth, and one from Jupiter. Only then will it be fast enough to make it to Saturn.

Cassini is expected to reach Saturn in 2004, where it will study the planet's moons and rings.

Cassini will approach Earth at roughly 56,000 kilometres an hour. By swinging around earth speed will increase by more than 17,000 kilometres an hour.

At its closest point over the Earth, NASA says the probe might be visible from the Easter Islands in the South Pacific.

Some anti-nuclear activists are concerned because Cassini is carrying plutonium.

Plutonium is a radioactive element, often used for fuel or as a power source in nuclear reactors. It is extremely dangerous. Exposure to plutonium can damage the body's ability to produce blood cells .

Cassini is not powered by plutonium. It uses plutonium power in its dozen scientific instruments.

Activists fear navigation or communications problems may cause the craft to burn up in the Earth's atmosphere, showering the planet with poisonous plutonium.

But NASA says Cassini poses almost no risk. The space agency estimates that the chance of accidental re-entry is about 1 in 1.2 million.

NASA has used Earth's gravity to project its probes through space since 1973. Galileo, also powered by plutonium, passed by Earth on the way to Jupiter in the early 1990s .

1. What is preparing to zip by Earth today to gain momentum for the last leg of its seven-year voyage to Saturn?