Asteroid 2012 TC₄ To Zip Past Earth This Week

The close pass of near-Earth asteroid 2012 TC₄ this week will give NASA a chance to test planetary defense coordination.

A small space rock designated 2012 TC₄ will pass 50,150 kilometers (31,160 miles) from Earth over the South Pacific Ocean on Thursday, October 12, 2017, at 5:41 Universal Time (1:41 a.m. EDT). Moving at 7.6 km per second relative to Earth at closest approach, this house-sized rock is roughly 15 meters (50 feet) across, though it's likely quite elongated in shape.

This asteroid has a unique history. Discovered by the PanSTARRS 1 telescope on Maui's Haleakala, 2012 TC₄ passed 94,960 km 59,000 miles) from Earth on October 12, 2012, eight days after its discovery. At that time dynamicists showed increased interest in the Earth-crossing asteroid when they realized it could pass just under three Earth radii from us this week.

More observations often make for a better refinement for an asteroid's orbit, and 2012 TC₄ was recovered by the European Southern Observatory's Very Large Telescope earlier this year on July 27th, at the extraordinarily faint magnitude 27. The asteroid was 0.4 astronomical unit (37 million miles or 60 million km) away at the time of recovery, and this week's close pass marks the first documented case of an asteroid passing us closer than the Earth-Moon distance twice.

For perspective, the 31,160-mile pass on Thursday is roughly 13% of the Earth-Moon distance and not quite 50% farther than the ring of geosynchronous satellites around the Earth.

The pass is close enough that Earth's gravity will modify the orbit of 2012 TC₄, which goes around the Sun once every 609 days, and perhaps its spin rate. It's also a fairly fast tumbler: Observers know from analyzing its light curve that it spins once every 12.2 minutes. This is typical for small space rocks, but the rapidity record goes to 2014 RC, which spins at a dizzying once every 16 seconds.

Usually, we only see smaller asteroids such as 2012 TC₄ shortly before closest approach. Some, like the Chelyabinsk meteor that slammed into the atmosphere over Russia on February 15, 2013, approach Earth from our sunward blind spot (recall that we were all watching the Valentine's Day passage of asteroid 2012 DA₁₄ just the night before) and strike with no warning at all.

2012 TC₄ will also pass 278,000 km (173,000 miles) from the Moon just 13½ hours after its closest approach to Earth. A NASA bulletin from the ongoing 2012 TC₄ Observing Campaign out last month states that a “new orbit solution precludes a possible impact in 2050.” The next future close pass for 2012 TC₄ is on December 1, 2079, at about 1 million km distant. Of course, expect refinements in the orbit of 2012 TC₄ made from observations during this week's pass to narrow down these future parameters a bit more.

“Scientists have always appreciated knowing when an asteroid will make a close approach to and safely pass the Earth because they can make preparations to collect data to characterize and learn as much as possible about it,” says Michael Kelley (NASA Headquarters) in a recent press release. “This time we are adding another layer of effort, using this asteroid flyby to test the worldwide asteroid detection and tracking network.”

NASA researchers will ping 2012 TC₄ with radar from the Goldstone tracking station this week. Arecibo Observatory, recently battered by Hurricane Maria, will also ping asteroid 2012 TC₄ on October 12th.

Observing Prospects

Even at its closest approach, 2012 TC₄ will shine at only magnitude 12 to 13, meaning you'll need a telescope with at least an 8-inch aperture and dark skies to see it. The Moon also reaches last quarter phase on October 12th, and the best viewing prospects for North America come the evening prior on October 11th, as 2012 TC₄ glides southward through the constellations Capricornus, Microscopium and Sagittarius. But at 10:00 p.m. EDT (2:00 UT on the 12th) 2012 TC₄ will still be 103,000 km distant, shining at around 15th magnitude. South American observers get the very best view, as 2012 TC₄ heads sunward in the predawn hours of October 12th.

At its closest approach, 2012 TC₄ will move across the sky at a whopping 1° every 4 minutes, fast enough to see it move in real time at the eyepiece. Such a close pass will also show a large amount of parallax shift, about 15° from pole-to-pole. Although planetarium programs take this into account, they usually don't incorporate the deflection of such a close pass due to Earth's gravity. Your best bet for nabbing 2012 TC₄ is to use the JPL Horizons web-interface to generate an ephemeris for your specific location.

And though it's a frequent visitor to Earth's neighborhood, 2012 TC₄ isn't a great candidate for some future asteroid-retrieval mission (ARM), due to its relatively high velocity compared to Earth. For example, Osiris-REX, which made a gravity assist past Earth on September 22nd headed for asteroid 101955 Bennu, requires a "delta V" of only 1.4 km/s for rendezvous and Earth return.

Clouded out? The good folks over at the Virtual Telescope Project have got you covered, with a webcast from Italy tracking 2012 TC₄ starting October 11th at 19:00 UT (3:00 p.m EDT) and then the Tenagra observatory in Arizona starts at 2:00 UT on October 12th (10:00 p.m EDT on the 11th).

We can breathe easy as 2012 TC₄ gives us a miss . . . for now.