January’s been a busy month for skywatchers. Between bright comets, their outbursts and the recent triple shadow transit at Jupiter it’s finally time to catch our collective breath. Maybe hole up in the house and keep warm.
Banish the thought.
Monday night Jan. 26th an obscure asteroid with the moniker 2004 BL86 will make a relatively close pass of Earth, zipping by at 3.1 times the distance of the moon or some 750,000 miles (1.2 million km).
Not a big deal, right? At least once a month a space rock gets this close or closer. Except that this space rock isn’t your typical “tiny house”. 2004 BL86 is 2,230 feet (680 meters) across – more like a space mountain – and big enough and close enough to be easily visible in a small telescope. Even even a Wal-Mart scope will show it. No exaggeration.
At magnitude +9 under a dark sky the asteroid would be faintly visible with a pair of 10×50 binoculars, but the half moon will be out, so you’ll need a 3-inch or larger scope binoculars in the 15×70 range to spot it. The good news is that the object remains close to 9th magnitude from 6 p.m. to midnight (CST) with peak brightness around 10 p.m.
Discovered 11 years ago, hence the “2004” prefix, 2004 BL86 is the largest asteroid to pass closest to Earth until 2027 when 1999 AN10 will beat it by coming within one lunar distance. This will also be the asteroid’s closest approach to our planet for at least the next two hundred years, so if you want to see it before you’re six feet under, now’s the time to put on a coat and toddle out the scope.
All asteroids with well-determined orbits receive a number designation. The very first asteroid discovered, Ceres in 1801, got the #1 spot. Asteroid 13,683 Monty Python (no kidding) was discovered in August 1967. Our featured space mountain numbers 357,439 making its full designation (357439) 2004 BL86. If you’re looking for a new password, this is it.
OK, so let’s talk how to see this speeding “star”. Observers in the Americas, Europe and Africa will have the best seats when the asteroid shines brightest between 7 p.m. and midnight (CST) Monday night from a comfortably high perch in Cancer the Crab not far from the planet Jupiter.
Because 2004 BL86 will be near Earth it will be zipping along at the rate of about 2° or four moon diameters per hour. That means you’ll need to use detailed maps to find and track the asteroid as it moves in real time.
Notice that the 2004 BL86 passes near a couple relatively bright stars and even skirts the edge of the bright Beehive star cluster, also known as M44. These are good places to “lie in wait” for the object to move into the field of view. I usually pick a spot some minutes ahead of where the asteroid will be and familiarize myself with the star field. That way, when it arrives, it really stands out. Remember, you’ll be looking for a star-like object slowly crossing the field of view. In reality, it’s sailing by Earth at around 35,000 mph.
Another thing to remember is that near-Earth asteroids will sometimes be a little bit off a particular track depending on your location. Not much but enough that I recommend you scan not just the single spot where you expect to see it but also nearby in the field of view. Just look for a “star” not plotted on the map and keep an eye on it for movement.
Once you nab your prey, follow it for 10, 15 or 30 minutes. It makes for good sport to watch it brush by stars along its path. The closer it comes to a star the more dramatic its apparent motion appears. You should also watch for changes in its brightness as the asteroid rotates. Depending upon shape and rotation rate (unknown at this point) asteroids can show large enough brightness variations to be seen visually at the telescope.
You won’t be the only one watching. Astronomers plan to use NASA’s Deep Space Network antenna at Goldstone, California and the Arecibo Observatory in Puerto Rico to ping the asteroid with microwaves to generate images of it during the time around its closest approach. We hope to share those pictures as soon as they’re available.
As always, Dr. Gianluca Masi, Italian astrophysicist, will stream live coverage of the event beginning at 1:30 p.m. (19:30 UT) Monday. And don’t worry about Earth getting hit. Not only is this asteroid many thousands of miles away, but our planet’s gravity can’t “pull” it in because the beast is moving rapidly along its own orbit. At worst, Earth’s gravity may alter its orbit some. That’s it – so enjoy!