Astronomers following the so-called "doomsday" asteroid Apophis which could collide with Earth have discovered it is 20% bigger than previously thought.
Previous estimates put the asteroid's average diameter at 270 metres (877 feet) representing a mass that would equal the energy release of a 506-megatonne bomb, according to Nasa figures.
The European Space Agency (ESA) said its Herschel telescope had scanned the space rock as it headed towards its closest fly-by with the planet in years on Wednesday.
In a two-hour observation, Herschel returned a diameter of 325 metres (1,056 feet), with a range of 15 metres (48.75 feet) either way, the ESA said.
"The 20% increase in diameter, from 270m to 325m translates into a 75% increase in our estimates of the asteroid's volume or mass," said Thomas Mueller, of the Max Planck Institute for Extra-terrestrial Physics in Garching, Germany, who led the data analysis.
Named after the god of evil, darkness and destruction in Egyptian mythology, Apophis sparked a scare when it was first detected in 2004.
Early calculations suggested a 2.7% chance of the space rock hitting Earth in 2029 - the highest ever for an asteroid, but the risk was swiftly downgraded after further observations.
It is expected to pass even closer to the planet on April 13, 2036, according to Nasa 's Jet Propulsion Laboratory (JPL) in Pasadena, California.
The object is being tracked by astronomers to help them fine tune the 2029 and 2036 risks of a collision.
Herschel, using thermal sensors, also found that Apophis is darker than previously thought, the ESA added.
Only 23% of light that falls on it is reflected, and the rest is absorbed by the asteroid. Previous estimates of this reflectivity, known as albedo, were put at around 33%.
This discovery is important because asteroids experience something called the Yarkovsky effect, or an increase in thrust that comes from alternate heating and cooling as the rock slowly turns in space.
Over time, this momentum can change the body's trajectory as it moves through the Solar System.
On February 15, a 57-metre (185-feet) asteroid, 2012 DA14, will skim the planet at just 34,500km (21,600 miles), making the narrowest approach so far of any detected asteroid.