Near-Earth Asteroids

Scientists spotted 2016 UR36 days before it passed by Earth. “Killer asteroids” headlines notwithstanding, we knew it would miss our planet by a comfortable margin.

Sooner or later, though, something big will hit Earth: again. We still can’t prevent that, not yet.

  1. Incoming Asteroids
  2. Near-Earth Asteroids: 15,000 and Counting
  3. The Last Really Big One

Why Bother?

(From P. NASA/JPL-Caltech, via Wikimedia Commons, used w/o permission.)
(Orbits of known Potentially Hazardous Asteroids (PHAs) in 2013.)

I’ll be talking about the Chelyabinsk meteor, Tunguska event, and why I think we should get ready for incoming asteroids and comets well before we spot something like the Chicxulub impactor headed our way.

The odds are really good that the next major impact event won’t happen in my lifetime, or near my home; so why should I think it matters?

Like I keep saying I think loving God, loving my neighbor, and seeing everyone as my neighbor, is a good idea. (Matthew 5:4344, 22:3640; Mark 12:2831; Luke 6:31, 10:2527, 2937; Catechism of the Catholic Churchy, 2196)

I believe that God created and is creating a good and ordered universe, that we’re made in the image of God, rational creatures — and that we are stewards of the physical world. (Genesis 1:2728, Psalms 19:2; Wisdom 7:17; Catechism, 1, 341, 373, 1730)

That’s stewards, not owners. We’re in charge of this world, and responsible for its maintenance: but it does not belong to us. Part of our job is managing this world’s resources for future generations. (Catechism, 373, 2415, 2456)

I don’t see the point in believing something unless I act like it matters, so ‘my end of the boat isn’t sinking’ isn’t a viable position for me.

Not that there’s much I can do, personally, about setting up a planetary defense system; apart from writing posts like this.

As an American, I’d like to see my country continue maintaining something like NASA’s Planetary Defense Coordination Office. But I’m relieved to see that asteroid impact avoidance is a very international effort.

Stone Tools and Forest Fires

It wouldn’t take a rock like the Chicxulub impactor to cause real trouble.

Something hit our planet about 800,000 years back, spraying tektites — gravel-size bits of molten glass — over much of Asia and Australia.

We’re not sure where the crater is. None of the ones we’ve found so far are big enough.

Folks were living in that part of the world before the impact, and at least some survived. We’ve found their stone tools, mixed in with charcoal from the tektite-sparked forest fires.1

We’d probably survive another impact like that, but our technology has improved considerably since the days of Oldowan tools. If we use our brains, and have time, I think we can keep the next major impact disaster from happening.

1. Incoming Asteroids

(From P. Carril/ESA, via NPR, used w/o permission.)
(“Asteroids regularly pass by Earth, as depicted here. A new NASA system called Scout aims to identify the ones that will come closest to the planet.”

NASA’s New ‘Intruder Alert’ System Spots An Incoming Asteroid
Joe Palca, NPR (October 30, 2016)

“A large space rock came fairly close to Earth on Sunday night. Astronomers knew it wasn’t going to hit Earth, thanks in part to a new tool NASA is developing for detecting potentially dangerous asteroids.

“The tool is a computer program called Scout, and it’s being tested at NASA Jet Propulsion Laboratory in Pasadena, Calif. Think of Scout as a celestial intruder alert system. It’s constantly scanning data from telescopes to see if there are any reports of so-called Near Earth Objects. If it finds one, it makes a quick calculation of whether Earth is at risk, and instructs other telescopes to make follow-up observations to see if any risk is real.

“NASA pays for several telescopes around the planet to scan the skies on a nightly basis, looking for these objects. ‘The NASA surveys are finding something like at least five asteroids every night,’ says astronomer Paul Chodas of JPL….”

This asteroid’s designation seems to be M.P.E.C. 2016-U84: 2016 UR36. Pan-STARRS on Maui, Hawaii, spotted it on the night of October 25-26.

A few hours later, a Web page by the Smithsonian Astrophysical Observatory’s Minor Planet Center published preliminary details about the asteroid. I put links to some resources and background hear the end of this post.2

NASA’s Scout did a quick analysis of data from Pan-STARRS; LPL/Spacewatch II; Tenagra Observatories; and Steward Observatory, Mt. Lemmon Station; which said that the object was headed for Earth, but would miss us by about 310,000 miles.

Those are the facts. Some of the headlines were quite colorful, and almost accurate, like “EXCLUSIVE: The killer asteroids NASA says are heading towards Earth TODAY.”3

310,000 miles, 498,800 kilometers, is close by cosmic standards: very roughly 1.3 times our moon’s distance from Earth. But that doesn’t make M.P.E.C. 2016-U84: 2016 UR36 a “killer asteroid.” Not this time around, anyway.

The NPR article says this asteroid was between 5 and 25 meters across: about 16.5 to 82 feet. That makes it very roughly the size of the Chelyabinsk meteor.


(From Aleksandr Ivanov, via Wikimedia Commons, used w/o permission.)
(The Chelyabinsk meteor’s fireball, frame from a dashcam video. (February 15, 2003))

A hunk of rock about 20 meters across skimmed into Earth’s upper atmosphere in February of 2013.

Traveling a little over 19 kilometers per second — nearly 56 times the speed of sound, if I did the math right — air couldn’t flow out of its way, building up ram pressure.

The fireball was brighter than the sun before exploding about 23.3 kilometers, 14.5 miles, over Korkino in southern Russia.

Nobody saw it coming, but analyzing CCTV and dashcam video let folks track its path. Before Earth got in its way, this rock had been with the Apollo near-Earth asteroids.4

When it was over, about 7,200 buildings in six cities were damaged, some 1,500 folks needed medical attention, and nobody had been killed.

We were lucky that time.

2. NEOs: 15,000 Asteroids and Counting

(From P. NASA/JPL-Caltech, used w/o permission.)
(“The 15,000th near-Earth asteroid discovered is designated 2016 TB57. It was discovered on Oct. 13, 2016, by observers at the Mount Lemmon Survey, an element of the NASA-funded Catalina Sky Survey in Tucson, Arizona.”

Catalog of Known Near-Earth Asteroids Tops 15,000
Jet Propulsion Laboratory news (October 27, 2016)

“The number of discovered near-Earth asteroids (NEAs) now tops 15,000, with an average of 30 new discoveries added each week. This milestone marks a 50 percent increase in the number of known NEAs since 2013, when discoveries reached 10,000 in August of that year.

“Surveys funded by NASA’s Near Earth Object (NEO) Observations Program (NEOs include both asteroids and comets) account for more than 95 percent of discoveries so far.

“The 15,000th near-Earth asteroid is designated 2016 TB57. It was discovered on Oct. 13 by observers at the Mount Lemmon Survey, an element of the NASA-funded Catalina Sky Survey in Tucson, Arizona. 2016 TB57 is a rather small asteroid — about 50 to 115 feet (16 to 36 meters) in size — that will come closest to Earth on Oct. 31 at just beyond five times the distance of the moon. It will safely pass Earth….”

Finding and cataloging orbits for near-Earth asteroids like 2016 TB57 and 2016 UR36 and comets is important to astronomers: and everyone else living on Earth.5

Sometimes these things get really close.

The Tunguska event is the largest known impact event on Earth since we started keeping records, a few millennia back.

Happily, the East Siberian taiga isn’t prime real estate, and was even less populated in 1908, so property damage was light. We don’t know of any fatalities, although a few folks were close enough to see — and feel — fire in the sky, like a second sun.

The explosion flattened about 2,000 square kilometers, 770 square miles, of forest. If it had happened over a city, I seriously doubt that there would have been survivors near ground zero, and we’d probably still be rebuilding the area.

We’ve learned a great deal about Solar System debris since 1908, and can make reasonable estimates about how often rocks fall out of the sky.

Falling Rocks

(From NASA/Planetary Science, via Wikipedia, used w/o permission.)
(“Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth’s atmosphere.”

The Chelyabinsk, Tunguska, and 1490 Ch’ing-yang event all happened in Asia, which might suggest that someone’s taking potshots at that continent. My guess is that it’s more a matter our having more thorough documentation for that area.

Also, possibly, because a Tunguska-level event might have had a chance of obliterating a culture along with its records until recently. I think it’s easy to forget how small many ancient civilizations were, by today’s standards.

Impact events aren’t the only hazard, of course. Thera exploded somewhere around 35 and 45 centuries back, archeologists and scientists are still working out the exact time. Possibly by coincidence, what we call the Minoan Civilization ended right around the same time.

The Late Bronze Age collapse happened a century or so later, leaving us with tales of king Minos, the Minotaur and labyrinth; pottery; and a lot of questions. Plato’s story of Atlantis may or may not have been inspired in part by the eruption, and that’s another topic.

Nobody saw the Chelyabinsk meteor coming, but we’ve got a pretty good idea of where and when largish rocks fell over the last couple decades: enough to estimate how often such events happen, on average.

Stony asteroid impacts that generate an airburst6
Impactor diameter Kinetic energy at atmospheric entry Airburst energy Airburst altitude Average frequency
4 m (13 ft) 3 kt 0.75 kt 42.5 km (139,000 ft) 1.3 years
7 m (23 ft) 16 kt 5 kt 36.3 km (119,000 ft) 4.6 years
10 m (33 ft) 47 kt 19 kt 31.9 km (105,000 ft) 10.4 years
15 m (49 ft) 159 kt 82 kt 26.4 km (87,000 ft) 27 years
20 m (66 ft) 376 kt 230 kt 22.4 km (73,000 ft) 60 years
30 m (98 ft) 1.3 Mt 930 kt 16.5 km (54,000 ft) 185 years
50 m (160 ft) 5.9 Mt 5.2 Mt 8.7 km (29,000 ft) 764 years
70 m (230 ft) 16 Mt 15.2 Mt 3.6 km (12,000 ft) 1900 years
85 m (279 ft) 29 Mt 28 Mt 0.58 km (1,900 ft) 3300 years

(From Robert Marcus, H. Jay Melosh, Gareth Collins (2010). “Earth Impact Effects Program;” via Wikipedia, used w/o permission.)

It looks like we can expect something as big as the Chelyabinsk meteor every six decades or so, on average.

But let’s remember that “average frequency” is just that: an average. Meteors don’t come on a regular schedule. Tomorrow morning’s news might include a Chelyabinsk-size airburst: or we may not see another for a century.

I hope we’ve got at least a few decades before astronomers notice a city-buster headed for a city, and that someone starts prepping an intercept-and-deflect mission now. If not, the best we may manage is telling folks at the impact site how long they have to get clear.

Rep. Stewart:
“… are we technologically capable of launching something that could intercept [an asteroid]?”

“No. If we had spacecraft plans on the books already, that would take a year—I mean a typical small mission like a Discovery class mission takes four years from approval to start to launch. Okay. Now, a really accelerated military program would be faster than that but that is a couple of years still….”
(United States Congress hearing, via Wikipedia and (April 10, 2013))

Prediction and Preparation

Beecher, Michigan. June 9, 1953, following the June 8 tornado. From NOAA, used w/o permissionHow successful the evacuation is depends on quite a few things: how much time we get before the impact, the impact site’s size and population, and whether folks in charge decide to issue a warning.

I’d like to think that authorities wouldn’t repeat the 1953 Worcester tornado mistake: but I also think that human nature hasn’t changed much.

For events like the 2003 Chelyabinsk meteor, my guess is that a warning few minutes to an hour or so in advance would let folks get inside and away from windows.

That might be enough to avoid most injuries and deaths. Again, nobody died that time, and I think we were lucky.

If we saw something like the Tunguska event coming, and Delhi was at the target footprint’s center — I don’t know how long evacuating the 21,000,000 or so folks who live there would take.

Emergency evacuations are chancy at best, so getting an asteroid impact avoidance system in place seems like a very good idea.

Right now, we’re doing well to notice incoming objects before they hit. I hope we’re ready when the next really big one comes.

3. The Last Really Big One

(From NASA/JPL-Caltech, via BBC News, used w/o permission.)
(“The outer rim (white arc) of the crater lies under the Yucatan Peninsula itself, but the inner peak ring is best accessed offshore”
(BBC News))

Chicxulub ‘dinosaur crater’ investigation begins in earnest
Jonathan Amos, BBC News (October 11, 2016)

Scientists have obtained remarkable new insights into the asteroid impact that wiped out the dinosaurs.

“They have been examining rocks from the crater that the 15km-wide space object dug out of what is now the Gulf of Mexico some 66 million years ago.

“The team says it can see evidence in these materials for how life returned to the scene soon after the calamity.

“Descendants of these small organisms are likely thriving today in amongst the crater’s smashed up materials….”

Their goal is learning more about the Chicxulub impact: how much energy was involved, and what happened to stuff were the crater is now. Drilling happened during April and June of this year. Research has started: with interesting but so far inconclusive results.

The plan is to use about half of the rock samples for analysis now, archiving the rest for study when we’ve developed better lab equipment.

The scientists found DNA which may be from “descendants of these small organisms:” or inadvertent contamination of their samples. They’re apparently still figuring out which.

They’re hoping to find “some vestige of the impactor itself,” and sediment from the enormous tsunamis that sloshed back and forth between North America and what’s now the Yucatán Peninsula.

Extinction Triggers

Learning more about the Chicxulub impact will help us understand the Cretaceous-Paleogene extinction event. Scientists still aren’t quite sure how it happened.

The Chicxulub impact was almost certainly involved, and probably played a major role.

But volcanic eruptions where the Deccan Traps are now were releasing massive quantities of sulfur dioxide and other toxic gasses, which likely enough slowed down recovery after the impact.

There may have been other impact events around the same time: the Boltysh crater impact probably happened a few millennia earlier. The Silverpit crater is roughly the right age, but may be the result of a salt dome’s collapse.

The Shiva crater is even more debatable: partly, I suspect, because that part of the world was passing over the Réunion hotspot which apparently produced the Deccan Traps. There’s been a lot of geologic change there in the last 66,000,000 years.

Earth’s sea level was falling, too, so folks who say there wasn’t just one extinction trigger have a good point.

That was a long time ago, and the odds of big rocks falling are a lot slimmer than for small fry like the Chelyabinsk meteor and Tunguska object. That’s the good news.

The unsettling news is that we apparently don’t know the exact odds, much less when the next impact disaster may happen. Different studies have been coming up with different answers. This is one of the more optimistic ones:

“…Objects with sizes greater than 1 km are considered to produce effects that would be catastrophic, because an impact of such an object would produce global effects. Such meteorites strike the Earth relatively infrequently – a 1 km sized object strikes the Earth about once every million years, and 10 km sized objects about once every 100 million years….”
(“Meteorites, Impacts, and Mass Extinction,” Prof. Stephen A. Nelson, Tulane University (December 1, 2014))

Craters and Statistics

A nickle-iron meteor about 50 meters, 160 feet, across hit North America about 50,000 years back; forming the Barringer Crater. We weren’t living near there until at least a dozen millennia or so after that.

The northwest route to Asia flooded around the start of a warm spell in the current/Quaternary glaciation.

Folks who look a bit like me started arriving a few centuries back, and that’s yet another topic.

Arizona isn’t heavily populated these days, so if another meteor like the object that formed Barringer crater hit exactly the same spot — Western civilization would survive. Folks working at and visiting the Barrihger Crater Company, not so much.

Plugging numbers into “Earth Impact Effects Program;” by Robert Marcus, H. Jay Melosh, Gareth Collins; Imperial College London/Purdue University; I learned that the nearest town should be comparatively safe.

Winslow, 29 kilometers east, would experience a Richter 5.2 earthquake about 5.8 seconds after the impact. Damage would range from minimal to serious, depending on construction quality, with some broken chimneys.

A 30 mile an hour wind gust would arrive a bit over a minute later. It would be a frightening and expensive event, but quite survivable at that distance.

Impacts like that are, happily, few and far between, compared to events like hurricanes and earthquakes. But they do happen.

Stony asteroids that impact sedimentary rock and create a crater6
Impactor diameter Kinetic energy at atmospheric entry Impact energy Crater diameter Average frequency
100 m (330 ft) 47 Mt 38 Mt 1.2 km (0.75 mi) 5200 years
130 m (430 ft) 103 Mt 64.8 Mt 2 km (1.2 mi) 11000 years
150 m (490 ft) 159 Mt 71.5 Mt 2.4 km (1.5 mi) 16000 years
200 m (660 ft) 376 Mt 261 Mt 3 km (1.9 mi) 36000 years
250 m (820 ft) 734 Mt 598 Mt 3.8 km (2.4 mi) 59000 years
300 m (980 ft) 1270 Mt 1110 Mt 4.6 km (2.9 mi) 73000 years
400 m (1,300 ft) 3010 Mt 2800 Mt 6 km (3.7 mi) 100000 years
700 m (2,300 ft) 16100 Mt 15700 Mt 10 km (6.2 mi) 190000 years
1,000 m (3,300 ft) 47000 Mt 46300 Mt 13.6 km (8.5 mi) 440000 years

(From Robert Marcus, H. Jay Melosh, Gareth Collins (2010). “Earth Impact Effects Program;” via Wikipedia, used w/o permission.)

Numbers in that table are estimates, based on what we’ve been learning over the last few decades. I’m pretty sure we’ll fine-tune them as we learn more. But I’m also pretty sure that they’re good enough for a ballpark estimate.

So — if rocks 100 meters across hit every 5,200 years, on average, how come we don’t have records of an impact like that?

Maybe there hasn’t been one since some of us started keeping records: about 5,200 years ago. Again, these are averages; so the next 100-meter impactor isn’t ‘due any time now.’

52 centuries back, the First Dynasty of Egypt was laying the groundwork of Pharaonic Egypt, and the Liangzhu culture was creating jade masks.

If something blasted a kilometer-wide hole back then, the odds are pretty good that it happened far from these comparatively high-tech centers. Besides, about three quarters of Earth is covered in water.

But with falling rocks in the 10-meter range hitting every decade or so, and 100-meter ones every few thousand years, I we should start getting ready to react before experiencing too many more surprise airbursts: or worse.

More of how I see life, the universe, and being human:

1 Stone tools and an impact event:

2 Keeping watch, getting ready:

3 Radar images from last year’s close approach by 2015 TB145 inspired nicknames like “Great Pumpkin” and “Skull Asteroid:” ‘Skull Asteroid’ passing by Earth today.” (WHIO)

4 No relation to the Apollo moon program. Apollo asteroids get their name from 1862 Apollo, the first one spotted. Why it got called “Apollo” I’m not sure. Maybe because it got closer to our star than many asteroids.

5 More about near-Earth objects:

6 From Robert Marcus; H. Jay Melosh; Gareth Collins (2010). “Earth Impact Effects Program,” Robert Marcus, H. Jay Melosh, Gareth Collins; Imperial College London/Purdue University; via Wikipedia.

About Brian H. Gill

I'm a sixty-something married guy with six kids, four surviving, in a small central Minnesota town. I mostly write and make digital art. I'm only interested in three things: that which exists within the universe; that which exists beyond; and that which might exist.
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