When the asteroid 2004 BL86 sweeps within 1.2m km of the Earth next Monday, 26 January, it will be the largest known body to pass so near to us until 2027. As it approaches to three times the distance of the moon, it should be visible to observers with small telescopes, or possibly large binoculars.
Thought to be about 500 metres wide, the asteroid takes 672 days to orbit the sun, ranging in distance between 134m km, inside the Earth’s orbit, to 315m km, well beyond that of Mars. The one due on 7 August 2027, dubbed 1999 AN10, may be a little larger and will pass 390,000 km from us, but neither object poses any risk of a catastrophic collision with the Earth.
At its closest, at 16:20 GMT on Monday, 2004 BL86 will lie some 20° below Britain’s E horizon minutes before sunset. We might spot it during Monday night, though, as it climbs northwards into the constellation Cancer and passes some 12° to the right (W) of the conspicuous planet Jupiter.
The challenges we face are its faintness and knowing just where to look. On the astronomers’ brightness scale it may shine at only the ninth magnitude and its position against the stars depends on where we are on the Earth. It is also zipping along at more than 2° or four moon-breadths per hour.
There are plans to study the object both optically and by radar. Optical spectroscopy may disclose its surface mineralogy while radar should reveal its true shape, size and rotation, possibly map its surface and indicate whether its bulk is stony or metallic.
