Apophis is about the same size as the asteroid that blasted
the mile-wide Barringer Crater in Arizona.
Credit: David Roddy, USGS

Fortunately, scientists have already predicted, 20 years in advance, that this is our lucky day: Apophis won't hit the Earth at that time. Rest assured (pretty much).

Discovered in 2004, Apophis is an asteroid about 270 meters across that orbits the Sun at distances ranging from about one astronomical unit (1 AU; the distance between Earth and the Sun) and about three quarters of an AU. Apophis orbits the Sun once every 323 days.

After its initial discovery, before our knowledge of its orbital trajectory had been refined, astronomers had predicted that there was a small chance it could hit the Earth on April 13, 2029, but as we got a clearer picture of its orbit the probability dwindled to practically nothing. Instead, Apophis will pass by Earth no closer than about 18,000 miles. Whew! Disaster averted, and we didn't even have to send Bruce Willis to deal with it.

But wait–that's not all. Though Apophis almost certainly won't hit us in 2029, there's a chance that this close encounter will set the asteroid up for an impact with Earth in 2036–something like 1 in 45,000.

So, if we know there won't be an impact in 2029, why don't we know whether or not there will be one in 2036? Why all the suspense?

Here's where I pull out my pinball analogy. Think of a pinball machine. The play zone around your flippers represents near-Earth space, the various bumpers up in the field represent all the planets, the Sun, and other large asteroids of the Solar System, and the pinball represents a Near Earth Asteroid, like Apophis.

When the pinball inevitably comes into the play zone, there are two possibilities: either it will hit (or be hit by) one of your flippers and thus be deflected back into the field where it will bounce around some more between bumpers, or it will sail right through that dreaded "window" between the tips of the flippers and fall into the end pocket–which represents Terra Firma and a catastrophe if a NEA falls there. As any pinball player knows, it's nearly impossible to predict exactly what path the pinball will follow into the play zone until it gets close.

It's a lot like that with a NEA in the Solar System: as it orbits around the Sun, its course is influenced by the gravitational pull of planets, large asteroids, and potentially smaller asteroids that it might pass close to. A very small deviation in a NEA's direction or speed can, over time, "amplify" into a very large difference in position much farther down the road.

Given the 2029 close encounter with Earth, though we're reasonably confident Apophis won't hit us on that pass, we don't know precisely how that encounter will alter Apophis' orbit. The gravitational interaction between Earth and a NEA passing close by is a complex one, with many variables, not the least of which is Earth's non-uniform gravitational field.

If Apophis passes Earth through precisely the right "window" in 2029–say, right between the flipper tips–then it could be set up for an impact at its 2036 encounter. That window, called a gravitational keyhole, is only about 600 meters across for the 2029 encounter.

As we gather more data on Apophis, we'll get a better prediction for what may happen in 2036–but right now the odds are that it will ultimately miss us at that time. That's a good thing, too, because at that time Bruce Willis will be 81 years old… and even John Glenn was only 80 when he returned to space…

A few weeks ago, this asteroid came really close to hitting Earth.

Called “2009 DD45”, the asteroid was estimated to be around the same size as the one that exploded in the atmosphere near the Podkamennaya Tunguska River in remote Siberia on June 30th, 1908, flattening 80 million trees across eight hundred square miles of remote forest. Of course, if an asteroid of this size were to hit a city or in an ocean offshore from a populated area, tens of thousands of people would likely die.

Then, just as the last of the night sky observers were completing their collective sighs of relief, on March 17th, 2009 another Tunguska-class asteroid, 2009 FH, passed by about 53,000 miles from Earth. Thankfully, neither of these asteroids actually hit us. But astronomers didn’t even observe 2009 DD45 until 4 days before its closest approach. It's orbit was calculated and it was determined that it would miss the Earth. But it's likely that asteroids of this size are fairly frequently buzzing by the Earth. And until recently, most of them have been undetected.

In 1998, NASA started the Spaceguard Survey which set out to discover 90% of those Near Earth Asteroids (NEAs) 1 km in diameter and larger. An impact by an asteroid this size would likely cause global destruction and an end to much of life as we know it so it’s definitely reassuring that 10 years after its inception, the Spaceguard Survey had found about 80% (CK) of them. But unfortunately, once we’ve found them, there’s still no international concensus or infrastructure in place in how to deflect or destroy them. But the Survey is limited by its mandate to find those mass extinction-sized asteroids as well as by the size and sophistication of the telescopes that are dedicated to searching the skies.

As former Apollo 9 astronaut, Rusty Schweickart said in a recent phone conversation, "in the process of finding the big ones, you also find a bunch of small ones, and the smaller ones are obviously far more numerous than the large ones." But it will take many more resources and new telescopes to continue searching for and tracking the smaller ones. And unfortunately, once we’ve found them, there's still no international consensus or infrastructure in place in how to deflect or destroy them. Raising awareness and building alliances amongst governments and space agencies is Schweikart's current "mission". He founded the B612 Foundation and Association of Space Explorers to tackle these goals on different fronts.

The message that I hope is conveyed with the Asteroid Hunters TV segment is that we are not immune from asteroid impacts here on Earth. Rusty Schweikart puts it best in a portion of his interview that didn’t make it into the final program:

"Well, asteroids and comets are good news and bad news, you know? But for them we wouldn’t be here, and on the other hand, if we don't actually take some action now, at some point we won’t be here anymore, because there's no question that we will be hit by asteroids, and we’ll probably be hit by, we would be hit by comets as well. Unless, we use the technology that we have and the brains that we have in order to protect the Earth from asteroid impacts, and we can do that. We can basically now, with current technology, assure that no asteroid ever hits the Earth again. That can do any serious damage."
-Rusty Schweikart

Here's a little exercise from Rusty that you can do to get a sense of what we know today about exactly what's out there:

• Go to: neo.jpl.nasa.gov/risk
• See two tables, the first table says "Recently Observed Objects" and the table below says "Objects not recently observed." You’ll notice in the bottom table that Apophis is the 4th one listed.
• Click on "Apophis". At the top you see a bunch of boxes, like the diameter at .27 km, or 270 meters.
• Down below that you see 3 lines, those are the 3 potential impacts. The first one is April 13, 2036. Go over to the right on that line you'll see the column "Impact Probability" is 2.3 x 10-5 – click on that. So there is the probability, 1 in 43,000 of that particular impact.
• Now if you go back to the main table you can do the same thing with every single one of those lines.
• Now go to the very top of the page and hit "Discovery Statistics." Scroll down to a blue and red graph "Known Near-Earth Asteroids". This shows the discovery rate beginning back in 1980 going up to almost current time. Notice the knee in that curve in 1998 – that’s when the Spaceguard Survey began.
• Scroll down to table just below the graph and look across that table to the far right side, to see that a a total of 6166 NEOs (of ALL sizes) have been discovered.

Rusty concludes that, "…what we really care about is not only the things that large, we care about things that can hurt us. Things that can hurt us go down to 40 to 45 meters or so. Instead of there being 940 of them, there are more like 600,000 of them. So the new charge for NASA, which they have so far ignored, is to find 90% of the objects 140 meters and larger by 2020. You can't reasonably set a goal to find everything down to 40 meters because it's just beyond the capability of telescopes and the money available. So NASA, working with Congress, set the goal at 140 meters. Now nevertheless, when you are looking for 140 meter objects, it’s going to take bigger telescopes than the ones to find a kilometer. Therefore we are going to find many many smaller objects as well. So 10 to 15 years from now, instead of that number on the far right hand column being 6000, it will be 1 million."

Watch the Asteroid Hunters television story online.