Thursday, 5 February 2015

Near Earth Asteroid (357439) 2004 BL84 discovered to have a small moon.


Asteroid (357439) 2004 made a close approach to the Earth on Monday 26 January 2015, enabling scientists at NASA's Deep Space Network antenna at Goldstone, California to collect radar images of the object. These revealed that the 325 m asteroid has a small moon, which is approximately 70 m in diameter. This is not the first asteroid to be found to possess such a moon; it is thought that abut 16% of Near Earth Asteroids larger than 200 m have such a companion body, and the figure may be similar or even higher for asteroids further out in the Solar System (though this is harder to assess).

(357439) 2004 BL84 and its newly discovered moon. NASA/Deep Space Network.

The close encounter also enabled scientists at NASA’s InfraredTelescope Facility on Mauna Kea, Hawaii, to study the spectroscopy of the asteroid, with initial results suggesting it has a surface composition similar to that of Vesta. (357439) 2004 BL84 is of particular interest as it is a Potentially Hazardous Asteroid (an asteroid larger than 150 m in diameter that is known to come within 0.05 AU of the Earth), and this was considered an excellent opportunity for observation as it is the closest predicted flyby of such a large object until 2027, when 1999 AN10, thought to be between 800 m and 1.8 km in diameter, will pass us at about 390 000 km (roughly the same distance as the Moon), and encounter that should be visible through binoculars.

Asteroid (357439) 2004 BL84 passed by the Earth at a distance of 1 200 000 km (3.1 times the average distance between the Earth and the Moon, or 0.8 % of the average distance between the Earth and the Sun), at about 4.20 pm GMT on Monday 26 January 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented significant minor threat. With an estimated equivalent diameter of 325 m (i.e. it is estimated that a spherical object with the same volume would be 325 m in diameter)(357439) 2004 BL84would be expected to be capable of passing through the atmosphere reasonably intact, impacting the ground in an explosion equivalent to about 1800 megatonnes of TNT (roughly 106 000 times the energy of the Hiroshima bomb) and creating a crater about 5 km in diameter. Such an event would cause devastation over a wide area, and could cause climatic problems for decades.

(357439) 2004 BL84was discovered on 30 January 2004 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2004 BL84 implies that it was the 2111th asteroid (asteroid L84) discovered in the second half of January 2004 (period 2004B).The longer designation, 357439 indicates that the asteroid was the 357 439th asteroid ever discovered. Asteroids are not given this longer designation immediately, to avoid duplicate or false sightings

The calculated orbit of (357439) 2004 BL84.  JPL Small Body Database.

(357439) 2004 BL84has a 672 day orbital period and an eccentric orbit tilted at an angle of 23.7° to the plane of the Solar System, which takes it from 0.89 AU from the Sun (i.e. 89% of the average distance at which the Earth orbits the Sun) to 2.10 AU from the Sun (i.e. 2.10% of the average distance at which the Earth orbits the Sun, considerably outside the orbit of Mars). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This means that close encounters between the asteroid and Earth are extremely common, with the last having occurred in September 2014 this year and the next predicted in February 2026. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (357439) 2004 BL84is also classified as a Potentially Hazardous Asteroid.

See also…

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