This photo depicts the safe flyby of asteroid 2012 TC4 as it passes under Earth on Oct. 12, 2017. While scientists cannot yet predict exactly how close it will approach, they are certain it will come no closer than 4,200 miles (6,800 kilometers) from Earth's surface.

This photo depicts the safe flyby of asteroid 2012 TC4 as it passes under Earth on Oct. 12, 2017. While scientists cannot yet predict exactly how close it will approach, they are certain it will come no closer

No photos of asteroid 2012 TC4 exist, but this image of Itokawa, another near-Earth asteroid, helps visualize its approximate size: next to Itokawa, which is a third of a mile long, TC4 would appear about the same size as the 'bunny tail' feature visible on the left.

No photos of asteroid 2012 TC4 exist, but this image of Itokawa, another near-Earth asteroid, helps visualize its approximate size: next to Itokawa, which is a third of a mile long, TC4 would appear about the

On Oct. 12, 2017, asteroid 2012 TC4 will safely fly past Earth. Even though scientists cannot yet predict exactly how close it will approach, they are certain it will come no closer to Earth than 4,200 miles (6,800 kilometers).

On Oct. 12, 2017, asteroid 2012 TC4 will safely fly past Earth. Even though scientists cannot yet predict exactly how close it will approach, they are certain it will come no closer to Earth than 4,200 miles

GALLERY: NASA's costliest space missions

GALLERY: NASA's costliest space missions

Date:2012-`

Mission:Explore Mars.

Cost in 2017 dollars:$2.6 billion

Date:2012-`

Mission:Explore Mars.

Cost in 2017 dollars:$2.6 billion

Date: 1958-1963

Mission: Put an American into Earth's orbit before the Soviet Union.

Cost in 2017 dollars: $2.13 billion

Date: 1958-1963

Mission: Put an American into Earth's orbit before the Soviet Union.

Cost in 2017 dollars: $2.13 billion

Project Gemini

Date:1962-1967

Mission:Develop technology and practice manuevers that would be used to go to the Moon.

Cost in 2017 dollars:$9.4 billion

Project Gemini

Date:1962-1967

Mission:Develop technology and practice manuevers that would be used to go to the Moon.

Cost in 2017 dollars:$9.4 billion

Date:1961-1972

Mission: Land the first humans on the Moon.

Cost in 2017 dollars: $162.4billion

Date:1961-1972

Mission: Land the first humans on the Moon.

Cost in 2017 dollars: $162.4billion

Date:2011-2017

Mission:Explore Jupiter

Cost in 2017 dollars:$1.21 billion

Date:2011-2017

Mission:Explore Jupiter

Cost in 2017 dollars:$1.21 billion

Date:1998-

Mission: Establish a oribiting science laboratory

Cost in 2017 dollars:$148 billion

Date:1998-

Mission: Establish a oribiting science laboratory

Cost in 2017 dollars:$148 billion

Date:1981-2011

Mission:Carry cargo and astronauts into low Earth orbit

Cost in 2017 dollars:$232.7 billion

Date:1981-2011

Mission:Carry cargo and astronauts into low Earth orbit

Cost in 2017 dollars:$232.7 billion

Date:Set to launch in 2018

Mission:Replace the Hubble telescope

Cost in 2017 dollars:$8.8 billion

Date:Set to launch in 2018

Mission:Replace the Hubble telescope

Cost in 2017 dollars:$8.8 billion

Date:1997-2017

Mission:Explore Saturn and its moons.

Cost in 2017 dollars:$4.59 billion

Date:1997-2017

Mission:Explore Saturn and its moons.

Cost in 2017 dollars:$4.59 billion

Date:1977-

Mission:Explore outer planets and beyond.

Cost in 2017 dollars:$3.9 billion

Date:1977-

Mission:Explore outer planets and beyond.

Cost in 2017 dollars:$3.9 billion

Date:1975-1982

Mission:Explore Mars.

Cost in 2017 dollars:$4.5 billion

Date:1975-1982

Mission:Explore Mars.

Cost in 2017 dollars:$4.5 billion

How prepared are we for an asteroid impact? NASA is conducting a test to find out

In the event of an impending asteroid impact, just how prepared are the inhabitants of Earth? NASA hopes to find out in an upcoming exercise using an actual space rock.

The asteroid 2012 TC4 will pass close to Earth as it hurls through the great beyond. As this is astronomy, which deals in light years and universes, close means about 4,200 miles from the planet's surface, at best.

Dozens of observatories, universities and labs around the world will participate in the preparatory exercise on Oct. 12, which is intended to uncover the "strengths and limitations of our planetary defense capabilities," said Vishnu Reddy, a University of Arizona professor and coordinator of the upcoming campaign, in a press release.

NASA has rehearsed preparedness for an asteroid impact in the past, but using only hypothetical impactors. Reddy proposed to enhance the reality of the exercise by centering it around the approach of a real near-Earth asteroid (NEA).

The effort begins by pinning down the NEA's exact path (uncertainty in an asteroid's orbit increases as time passes). Then, various drills, such as deflection, evacuation and disaster relief, will be rehearsed by the coordinating agencies in the hopes of avoiding a future "Armageddon"-esque disaster.

ReadMichelle Robertsons latest storiesandsend her news tips atmrobertson@sfchronicle.com.

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How prepared are we for an asteroid impact? NASA is conducting a test to find out - SFGate