An 325m-wide asteroid passed as close as 1.2 million kilometres - or three lunar distances - from Earth in a rare flyby on Tuesday.
2004 BL86, which made its closest approach at 12.19am (Singapore time), poses no threat but is the largest-known asteroid to come this close until 2027 - when 999 AN10 is expected to fly past within one lunar distance.
Most near-Earth objects range from 15m to 30m in diameter, which makes this incident particularly special.
Here are some things you might not have known about asteroids:
1. What is an asteroid?
Too small to be called planets, they are also known as planetoids or minor planets and are small, airless rocky worlds covered in dust that orbit the sun.
Most are irregularly shaped, although a few have enough gravity to be spherical. Ceres, the first asteroid to be discovered in 1801, was first thought to be a planet because of its size (950km in diameter).
Large enough asteroids might have moons orbiting them. New images over the past week have revealed that 2004 BL86 has its own small moon (about 70m across).
2. What's in an asteroid's name?
In the case of 2004 BL86, the numbers and characters are a provisional designation as it has not been named. 2004 denotes the year it was discovered by the Lincoln Near-Earth Asteroid Research (Linear) set up to track near-Earth objects. It has been assigned a number though - 357439.
Asteroids are named through a detailed process overseen by the Smithsonian Astrophysics Observatory's Minor Planets Center. The person who discovers one usually proposes a name; in the past, they have been named after characters, pets (this was eventually banned) and famous musicians.
In 1971, 2309 Mr Spock was named after the discoverer's cat, while 4150 Starr, discovered in 1984, was named in honour of ex-Beatle Ringo Starr.
3. Where can they be found?
Most of them lie in a vast ring between the orbits of Mars and Jupiter, which is known as the main asteroid belt.
It is estimated to hold more than 200 asteroids larger than 100km in diameter, with more than 750,000 smaller ones larger than 1km in diameter. There also also millions of smaller ones.
More than 300,000 of them have been identified and catalogued.
4. What are they made of?
Asteroids are typically classified by their composition, which are varied. There are three main categories:
- C-type or carbonaceous: Most common type which makes up nearly 80 per cent of all known asteroids and consist of clay and stony silicate rocks. These are usually greyish in colour and inhabit the main belt's out regions.
- S-type of silicaceous: They account for about 17 per cent of known ones and range from greenish to reddish in colour. They appear to be made of silicate materials and nickel iron.
- M-type or metallic: Found in the middle region of the belt, they are believed to be made up of nickel-iron and are reddish in colour.
5. Are they dangerous to life on Earth?
Theories abound that an asteroid impact killed off the dinosaurs, but dangerous asteroids are extremely rare. The National Aeronautics and Space Administration (Nasa) estimates that an asteroid capable of global disaster would have to be at least 402m wide. Asteroids of that size strike Earth once every 1,000 centuries on average.
Nasa's near-Earth Object Program is dedicated to tracking, monitoring and studying asteroids and comets whose obits periodically bring them close to Earth.
As of Jan 25, 2015, 12,165 Near-Earth objects have been discovered, with 868 of these with a diameter of about 1km or larger. Nasa has classified 1,544 of them as Potentially Hazardous Asteroids.
The most recent incident occurred on Feb 15, 2013, when an asteroid thought to have measured 2m wide slammed into the atmosphere over the Russian city of Chelyabinsk. The resulting shockwave injured about 1,200 people and damaged hundreds of buildings.
Sources: Space.com, Nasa, Sky & Telescope, Universe Today, EarthSky
