Apophis

 "Difficulties strengthen the mind, as labor does the body" - Lucius Annaeus Seneca

It's time to discuss Apophis - pronounced Epp-eff-iss or App-eff-iss, depending on how you like to pronounce the Greek translation of the ancient Egyptian diety of "uncreation" or destruction.

Apophis is a massive earth-crossing asteroid that was only recently discovered - in 2004.  It caused a great deal of consternation at the time, because preliminary orbital calculations showed that it had a 2.7% chance of impacting Earth in just 25 years - 2029.

If this asteroid *ever* impacts the earth, it will cause incredible weather effects and of course major issues near the impact site.  Apophis does not compare in size to the asteroid that wiped out the dinosaurs, which is believed to have been about 10 km in diameter.  Instead Apophis is elongated, and measures 450 m x 170 m.  (1480 ft x 550 ft).  That's still pretty large for a potential impactor.

Bear in mind that the asteroid that broke apart over Chelyabinsk, Russia was a mere 18 meters in diameter, and many nearby Russians believed that they were under nuclear attack.  It was estimated to have released a 0.5 MT air burst of energy.  Below is a compilation of video clips at the moment the shock wave arrived.

 The estimated energy that would be delivered by an Apophis impact with earth is 1,200 Megatons (MT) of TNT.  For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, was measured at 50 MT.

Below: The largest nuclear test ever performed by the US.  Castle Bravo test (15 Megatons)

 The eruption at Mt. Saint Helens below (from 35 miles away) was estimated at 26 MT.  By Rocky Kolberg - Rocky Kolberg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15044411

The volcanic explosion of Krakatoa (or Krakatau) in 1883 was estimated at 200 MT.  By the way, there is a new island that has emerged from the underwater crater left by the 1883 explosion.  The new island is "Anak Krakatau" or "Child of Krakatau".  It is still a belligerent volcano.

Below:  Size comparison of asteroid Apophis with some familiar tall structures, courtesy: https://commons.wikimedia.org/wiki/User:Phoenix_CZE

The closest known approach of this asteroid to earth is expected to occur April 13, 2029, at 21:46 Universal Time.  It will pass 31,000 km or 19,600 miles above the surface of the earth.  This is inside the orbit of geosynchronous satellites, which orbit the earth at an altitude of 35,786 km or 22,236 miles.  This is a mere 8% of the distance between the earth and the moon.  Scientists consider anything inside 500% the distance to the moon to be of concern.

While that 19,600 mile distance might seem large, it's a hair-width in an astronomical sense.  The close encounter will permanently alter the orbit of Apophis.  There was initially a great deal of concern that it would hit a "key hole" that would bring it onto a collision course with earth during one of the next few approaches.  That possibility has been ruled out as the orbit has been mathematically refined better since the initial discovery.

The path of Apophis will permanently change following the 2029 fly-by, and become a much wider orbit.  Phoenix7777 - Own work Data source: HORIZONS System, JPL, NASA

 Below:  Showing how the near-miss with the earth's gravity changes the path of Apophis

Note that even though the orbit of Apophis will be changed significantly, that hasn't altered the fact that it will still make very close approaches to the earth.  Astronomers are only saying that it won't impact the earth over the next 100 years.  Our grandchildren may still very well need to be concerned about an impact.

 As dysfunctional as governments have become, I would not hold my breath expecting them to spend the necessary money to make an attempt at re-directing Apophis into a safer orbit.

There is a neat orbital viewer here, run by JPL.  You can enter any object you want (Halley's Comet, Hale-Bopp, , and it will show you the orbit, for as long as you care to watch it.

Here is a higher quality - from a graphic standpoint - orbital viewer, run by NASA.  It not only shows the planets, but it also shows a few of the more important space probes and asteroids that are wandering about!

***UPDATE***

That was a timely blog post!  We have a new potential contender for impact with earth, currently calculated at a 1 in 71 chance of impact on December 23, 2032.  

NASA estimates the size of this one at just 55 meters across, but it will have a very high velocity, so the kinetic energy of the impact will be similar to a much larger object.  If this object is metallic, it could create a crater similar to the one in Arizona.  If it's a pile of smaller rocks, it could create a "Tunguska Event".  More information on 2024 YR4 is here on Wikipedia.