Cape Canaveral - An asteroid measuring about half a kilometre in diameter will make a relatively close, but harmless pass by Earth on Monday night, Nasa said.The asteroid will pass about 1.2m kilometres from Earth, roughly three times farther away than the moon.Amateur and professional astronomers are preparing to watch the flyby, which will be most visible between 20:00 on Monday and 01:00 on Tuesday from the Americas, Europe and Africa.A small telescope or binoculars will be needed to see the asteroid, which is known as 2004 BL86."While it poses no threat to Earth for the foreseeable future, it's a relatively close approach by a relatively large asteroid, so it provides us a unique opportunity to observe and learn more," astronomer Don Yeomans, with Nasa's Jet Propulsion Laboratory in Pasadena, said in a statement.The asteroid, which orbits the sun every 1.84 years, was discovered 11 years ago by the Lincoln Near-Earth Asteroid Research, or Linear, telescope in New Mexico.Scientists plan to map the asteroid's surface with radar during the flyby in hopes of learning more about its size, shape, rate of rotation and other features."At present, we know almost nothing about this asteroid, so there are bound to be surprises," astronomer Lance Benner with Nasa's Jet Propulsion Laboratory in Pasadena, California, told SpaceWeather.com.Largest asteroidAsteroid 2004 BL86 will be the largest asteroid to pass this close to Earth until asteroid 1999 AN10 flies by in 2027, Nasa said.Nasa currently tracks more than 11 000 asteroids in orbits that pass relatively close to Earth. The US space agency says it has found more than 95% of the largest asteroids, those with diameters 0.65 miles or larger, with orbits that take them relatively close to Earth.An object of that size hit the planet about 65 million years ago in what is now Mexico's Yucatan peninsula, triggering a global climate change that is believed to be responsible for the demise of the dinosaurs and many other forms of life on Earth.Two years ago, a relatively a small asteroid exploded in the atmosphere over Chelyabinsk, Russia, leaving more than 1 500 people injured by flying glass and debris. That same day, an unrelated asteroid passed just 17 200 miles from Earth, closer than the networks of communication satellites that ring the planet.Websites planning live coverage of Monday night's flyby include Nasa's Solar System Exploration Research Virtual Institute, Slooh.com and The Virtual Telescope Project 2.0.