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Asteroid hurtling toward near-miss with Earth
BY MARK ARMSTRONG
ASTRONOMY NOW

Posted: 14 February 2013


Let's hope for clear skies on Friday night as the Near-Earth Asteroid 2012 DA14 makes a really close approach to the Earth, passing at a distance of around 28,000 kilometres, which is just below the ring of Earth's geostationary satellites.


The path of the rapidly moving NEO 2012 DA14 as it streaks through the sky. AN graphic by Greg Smye-Rumsby. See a larger image.
 
The time of closest approach is 19.25 UT. 2012 DA14 will become the record holder as the largest known object to come this close to Earth. Despite the fact that this is close enough for the 40-50 metre sized body to be perturbed slightly by Earth's gravity, professional astronomers have confirmed there is absolutely no chance of an impact. As you might imagine this object has been subject to exhaustive study and observation and astronomers around the globe have been eagerly anticipating this event since DA14 was discovered in February 2012 and its orbit determined.

Observers in the UK will have a great chance to see this NEA using just binoculars or small telescopes but don't expect an easy ride, despite its predicted peak magnitude of +7.2. It will be moving so fast across the sky, whizzing along at over 40 arcminutes per minute (0.8 degrees) around closest approach so it won't be that bright for long. In fact 2012 DA14 hurtles from south-west Leo to the Draco/Camelopardalis border in the first four hours of visibility as seen from UK shores from 20h UT, and that's taking into account a marked deceleration after 21h. A couple of hours either side of close approach, it traverses 120 degrees of declination from -60 to +60 degrees in just four hours!

I have tried to input 2012 DA14's orbital elements into my various planetarium programmes, as have many others, but this time there's no way they can handle plotting accurate positions as the asteroid is too close to the Earth and the software does not account for perturbations by Earth's gravity. The JPL website will plot accurate positions (ephemerides) for your specific location and chosen time. You can specify to have positions down to the nearest minute and I have used their information for this article.

2012 DA14 is visible low in the east from around 8pm when it will be 15 degrees up and shining around magnitude +7.2. As it rockets north it keeps close to the Right Ascension 12h line, fading as it goes. By 21h it lies south-east of the 'Bowl' of the Plough some 42 degrees up and still just in range of tripod mounted 10 x 50 binoculars at magnitude +8.5, although 15 x 70 binoculars or a small telescope working at low magnification would be better. GOTO telescopes can slew to a specified position where 2012 DA14 is predicted to cross the field-of-view using the handset or planetarium software, but don't rely on planetarium programmes, in which you have previously charted the NEA, to slew your telescope to the position it predicts. Instead use the position provided by the JPL ephemeris. At close approach the asteroid is covering the diameter of the Moon every 45 seconds or so and by 9pm that has sped up to every 30 seconds or so, making for 'real time' movement against the background stars akin to a faint satellite.


The path of asteroid 2012 DA14 through the ring of geostationary satellites during its closes approach. Image: NASA/JPL–Caltech. See a larger image.
 
Experienced NEA chasers recommend catching the asteroid as it crosses a set declination and then sweeping in right ascension close to the predicted position until the NEA is detected. As luck will have it 2012 DA14 crosses the +50 degree mark just after 9pm and this would seem a good chance to nab it. The 40 degree mark is reached 25 minutes earlier at 20.35 UT with DA14 some 35 degrees up. By 10pm it will have faded to magnitude +9.5 and to +10.9 by midnight, when it crosses into Camelopardalis and lies 15 degrees from the Pole Star.

If you are successful in making an observation of 2012 DA14 then muse that many telescopes around the world will be trained on this chunk of rock to try to further refine its orbital parameters so astronomers can accurately predicted future close approaches. When DA14 was discovered it had just passed seven times the Moon's distance from the Earth with an orbital period of about 368 days. This close approach will shorten that period to around 317 days and it will not come as close again for at least 30 years. So make the most of any clear skies to observe this ancient piece of debris left over from the formation of the Solar System.

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