Miami - The largest asteroid in more than a century will whizz safely past Earth on September 1 at a safe but unusually close distance of about 7 million kilometres, Nasa says. The asteroid was discovered in 1981, and is named Florence after the famed 19th century founder of modern nursing, Florence Nightingale. "Florence is the largest asteroid to pass this close to our planet since the first near-Earth asteroid was discovered over a century ago," said a US space agency statement.It is one of the biggest asteroids in the Earth's vicinity, and measures about 4.4km wide - or about the size of 30 Egyptian pyramids stuck together."While many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller," said Paul Chodas, manager of Nasa's Centre fpr Near-Earth Object Studies. Scientists plan to study the asteroid up close when it passes, using ground-based radar imaging in California and Puerto Rico."The resulting radar images will show the real size of Florence and also could reveal surface details as small as about 10m," Nasa said. This pass will be Florence's closest "since 1890 and the closest it will ever be until after 2500", added the US space agency.Asteroids are small, natural rocky bodies that orbit the Sun.Large asteroid collisions with Earth are rare.A car-sized asteroid hits Earth's atmosphere about once a year and burns up before reaching the surface."About every 2 000 years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the area," said Nasa."Finally, only once every few million years, an object large enough to threaten Earth's civilisation comes along."Scientists are confident that Florence will not be one of them.