Asteroid 2004 BL86 Flies by Earth

January 29, 2015

Earlier this week, Earth had the opportunity to meet (from afar) Asteroid 2004 BL86. Checking in at an estimated 0.5 kilometres in size and zipping by just 1.2 million kilometres from Earth on 26 January, it was the closest an asteroid of this size is expected to approach Earth until 2027.

At its closest approach, the asteroid was about three times the distance between Earth and the Moon, or about 289 times the distance from New Zealand to Australia.

Near-Earth Objects are detected and tracked by special teams that coordinate with NASA to identify objects that are potentially hazardous (PHOs = Potentially Hazardous Objects). Using the MOID method (Minimum Orbit Intersection Distance) scientists calculate the collision risk using the distance between the closest points of the orbits of the two space bodies concerned, e.g. a planet and an asteroid. An object that is potentially hazardous to Earth has a MOID of less than 0.05AU (astronomical unit), which is about 7.5 million kilometres. There are many other elements to consider when calculating MOID like orbit timing, and size of the asteroid. So, even though this asteroid has a MOID of 0.00817AU, it is not a risk to Earth.

While the asteroid passed relatively close to Earth, it wasn’t possible to view it with the naked eye. David Britten, Stardome Astronomy Educator, explains: “At more than a million kilometres away and less than a kilometre in size, the asteroid will not be visually exciting. It will be incredibly difficult to see, even with binoculars. A small telescope will just show a small, very faint ‘star’ moving so slowly it will barely be noticeable.

“What will be exciting is the planned radio imaging using the giant Arecibo Radio Telescope in Puerto Rico and the Goldstone telescope in California. By bouncing microwave pulses off the asteroid, echo ‘sonograms’ will provide details of the asteroid’s shape and rotation.”

Additional information can be found at

Update: The asteroid has a Moon. Image credit: NASA/JPL-Caltech

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