A modest-sized near-Earth asteroid will pass 1,770,000 miles (2,860,000 km) from our planet on the night of Thursday–Friday, September 13–14. That’s a little more than seven times the Earth-Moon distance.
That's far enough to be completely harmless. However, 2012 QG42, estimated to be about 800 feet across, is classified as a PHA (potentially hazardous asteroid) because there's a small chance that it will hit Earth some time in the distant future. The Catalina Sky Survey in Arizona discovered the asteroid on August 26th, giving unusually long advance notice for a rock that’s so small in astronomical terms.
The asteroid is expected to shine at magnitude 14.5 at closest approach, making it a challenging object for backyard telescopes. But because of the orbital geometry, it will be brighter during the preceding week, peaking at magnitude 13.9 two or three nights before closest approach. That’s a little brighter than Pluto, so if your sky conditions are good, 2012 QG42 should be visible through Wednesday night using backyard telescopes with mirrors 10 inches or more in aperture.
Asteroid 2012 QG42 is ideally placed for observing — well up in the late evening sky — from every inhabited place on Earth. The chart below shows its path through the constellations. The tick marks are for 0 hours Universal Time, which falls on the evening of the previous date for the time zones of the Americas.
Locating this asteroid won’t be easy; it requires excellent chart-reading skills and planetarium software capable of showing stars down to magnitude 14.5. You can download ephemerides (coordinates) for the asteroid from JPL Horizons. Make sure you type in “2012 QG42” in upper case, and select an observing location near you. The asteroid is close enough so that its position in the sky varies quite a lot depending on where you’re standing.
At closest approach the asteroid will be moving almost one arcsecond per clock second, so you should actually be able to watch it move across the field of view at high magnifications.
Don’t wait too long; the asteroid will fade rapidly after closest approach, reaching magnitude 15.2 just 24 hours later, making it an exceedingly challenging target.
If you don’t have a telescope, dark skies, or a clear night, there’s another option. The Virtual Telescope Project, a remote-controlled, 17-inch robotic telescope, will broadcast the asteroid’s closest approach live starting at 22:00 UT (6 p.m. EDT) on Wednesday, September 12th.