On Thursday night, March 18th, a tiny, newly discovered asteroid will make the closest flyby of Earth ever predicted. The object, dubbed 2004 FH, is probably only about 30 meters (100 feet) in diameter, the size of a small office building. An electronic message issued late on March 17th by the Minor Planet Center in Cambridge, Massachusetts, indicates that it will pass by safely about 42,500 kilometers (26,500 miles) from Earth's surface. That's one-ninth the distance of the Moon — and just a few thousand kilometers beyond the orbits of most communications satellites.
When it's closest, at 5 p.m. Eastern Standard Time (22h Universal Time), 2004 FH will have brightened to 10th magnitude and be moving quickly through the constellation Antlia. By 7 p.m. (0h UT), about when darkness falls on the East Coast of North America, it will have faded back to 12th magnitude as it shoots by Sirius near the open star cluster Messier 41.
According to the orbit calculated by Gareth Williams, the MPC's associate director, 2004 FH belongs to the Aten class of asteroids. It circles the Sun in just under 9 months in very nearly the same plane as Earth's orbit. At perihelion it swings well inside the orbit of Venus; when at aphelion (as it is now) it ranges just outside that of Earth.
MIT's Lincoln Near-Earth Asteroid Research (LINEAR) survey discovered this object on March 16th using a robotic telescope in Socorro, New Mexico; observations the following day from the Czech Republic, Germany, and Slovenia helped the Minor Planet Center compute its exact trajectory.
Because 2004 FH will be passing so close, telescope users who want to locate it cannot use a standard ephemeris. Its path across the sky depends greatly on the observer's vantage point on Earth, owing to the parallax effect. Therefore, would-be observers should use the Minor Planet Center's Ephemeris Service, which calculates detailed predictions for any latitude and longitude on Earth.
Subscribers to Sky & Telescope's Minor-Planet AstroAlert service received details about 2004 FH within hours of the Minor Planet Center's announcement. Go here to subscribe to one or more of the 11 AstroAlert lists.