We’ve been warned by an asteroid. The next one might hit.

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by Fabius Maximus

Summary: We have had several near-misses – asteroids passing close by with little warning from our sensors. This reminds us that asteroid and comet impacts have changed the course of life on Earth, and will again unless we stop them. Which we will, eventually, either when we go deeper into space – or after we are hit. This post discusses this risk and what steps we can take now to better prepare. Perhaps it is humanity’s role to defend the planet.

“The dinosaurs became extinct because they didn’t have a space program. And if we become extinct because we don’t have a space program, it’ll serve us right!”
— Science fiction author Larry Niven, as quoted by Arthur C. Clarke.

Asteroid approaching Earth

We can’t say that we weren’t warned

WaPo: “‘It snuck up on us.’ Scientists stunned by ‘city-killer’ asteroid that just missed Earth.

How much would an asteroid impact hurt?

Even a small asteroid could devastate a city. What would impacts of different sizes do to your community? See the stunning results at Purdue U’s Impact Earth website. A presentation by NASA’s David Kring gives examples and consequences of impacts.

How likely is an impact? One could hit tomorrow.

The U.S. Government’s sensors recorded at least 556 meteors entering the atmosphere (fireballs, technically bolides) from 1994-2013. The largest in this record was a 20 meter asteroid near Chelyabins in central Russia on 15 February 2013 (details here), an explosion equivalent to 440- 500 kilotons of TNT.

The size of the dots on this NASA map represents the meteor’s optical radiant energy. The smallest dot on the map is 1 billion Joules (1 GJ), the equivalent of roughly 5 tons of TNT. The dots for 100, 10,000 and 1,000,000 GJ convert to 300 tons, 18,000 tons and one million tons of TNT. The Hiroshima blast was equivalent to 15,000 tons.

NASA map of bolide events

Scientists have accumulated enough data to estimate the odds of impacts from space.

“Every day Earth is bombarded with more than 100 tons of dust and sand-sized particles from space. About once a year, an automobile-sized asteroid hits Earth’s atmosphere, creating a spectacular fireball (bolide) event as the friction of the Earth’s atmosphere causes them to disintegrate – sometimes explosively.

“Studies of Earth’s history indicate that about once every 5,000 years or so on average an object the size of a football field hits Earth and causes significant damage. Once every few million years on average an object large enough to cause regional or global disaster impacts Earth. Impact craters on Earth, the Moon and other planetary bodies are evidence of these occurrences.

“Meteor Crater near Winslow, Arizona, is evidence of the impact with Earth’s surface of a 50-meter asteroid about 50,000 years ago. Impact of the metal-rich object released energy equivalent to a 10 megaton explosion and formed a 1.2 kilometer-diameter crater.” {Source: NASA.}

The National Research Council published a typically magisterial analysis of this threat: “Defending Planet Earth: Near-Earth Object Surveys & Hazard Mitigation Strategies“ (2010). Here are the numbers, comforting or terrifying, depending on your perspective. Thirty-five million years ago, a 5-8 km impactor blasted out the Popigai crater – at the time of the Eocene–Oligocene extinction event. The dinosaurs were killed by an object 11-81 km in diameter.

Frequency of asteroid impact on Earth, by size of object.

Books and films about how this happens and how we respond

Stories about collisions with space objects go back to the 19thC. Perhaps the best story about doom from the sky is When Worlds Collide by Philip Wylie and Edwin Balmer (1933). Earth is hit by a rogue planet. But there is good news!

An example of an optimistic science-fiction story in this genre is Rendezvous with Rama by Arthur C. Clarke (1973). A city is destroyed. Humanity says “never again” and creates Project Spaceguard – sending us into space. An ounce of prevention is worth …etc.

Lucifer’s Hammer by Larry Niven and Jerry Pournelle (1977) is gripping disaster porn about a comet hitting Earth. “Cities were turned into oceans; oceans turned into steam. It was the beginning of a new Ice Age and the end of civilization. But for the terrified men and women chance had saved, it was also the dawn of a new struggle for survival.”

Hollywood has given the inspiring stories about humanity defending the world against doom from space.

What can we do to prepare?

“Find all asteroid threats to human populations and know what to do about them.”
— NASA’s Grand Challenge, 18 June 2013.

The Apollo program burned billions of dollars) but did little for America. Since then, the manned space program has done even less. The reason is simple: we lacked a good reason to put people in space. An asteroid or comet will eventually provide the motivation – either to prevent another impact or mitigate its effects. We have the technology and money to begin preparations.

Here are the four kinds of space threats, with warning times ranging from decades to days. Buying warning time is the key to preventing impacts or minimizing their damage, but it will take time to build the necessary sensor systems. As a first step, in 2016 NASA created the Planetary Defense Coordination Office. Its staff supervises NASA’s programs to detect and track potentially hazardous objects, issues notices of close passes and warnings of any detected potential impacts, and coordinates the US government’s efforts to prepare for impact threats. See their website, which has a wealth of information.

Other nations have similar programs. NASA is a member of the International Asteroid Warning Network.

What happens after we detect an object on a collision course with Earth? A presentation by NASA’s Dan Mazanek describes deflection strategies. This NASA video shows what a mission to intercept a threatening space object might look like.

The longer the warning time and the better the preparations, the higher the odds of success. Here are some ways to defend Earth: a Gravity Tractor, a Kinectic Impactor, and a Blast Deflection. This graphic shows which works best for various combinations of warning time and asteroid size. For short warning times, we can use only what we have ready to launch.

NRC - asteroid mitigation measures

From the NRC report (2010). Graphic by Tim Warchocki. Copyright © NAS.

A last note about these threats

“Estimates of the frequency of space-rock strikes are just estimates, and may not tell us anything about when the next impact will occur – it could be an eon, it could be tomorrow. Floods, earthquakes, volcanoes, and tsunamis are sure to happen more frequently, but humanity will survive these events; we might not survive an impact from space. Meanwhile, nothing can be done to prevent earthquakes, volcanoes, and tsunamis. But space strikes appear to be entirely avoidable, and not necessarily with “massive repositioning of government funding.” A fraction of the money NASA wants to waste on a moon base would likely be sufficient.”

— By Gregg Easterbrook in The Atlantic, September 2008.

Impact of comet or asteroid




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