How to prevent a mid-size asteroid from wiping out a city.

The simplest way to do this is to give the asteroid a bump…to knock it off course.

The physics is simple. In billiards, when a moving ball hits a stationary ball it forces the stationary ball to move, and the first ball is usually knocked off its course.

If ball 1 is moving and hits ball 2 that is also moving both balls with usually be knocked off course. So if a projectile is fired at a moving asteroid, the asteroid will be knocked off course. This is the physics principle.

Monster asteroids can wipe out a planet. The most famous is the Chicxulub asteroid that smashed into the ocean just offshore of Mexico’s Yucatan peninsula. The rock was 7 miles across and the explosion sent tsunami tidal waves that were 300 feet high crashing into Florida and hurling huge boulders up onto land. One study said the tsunami waves might have been 1 mile high.

The explosion lifted so much dust high into the atmosphere that it encircled the earth and blocked the sunshine causing a blackout and freezing temperatures that lasted 10 years.

The dust gradually settled into a thin layer, which over millions of years became a layer of sediment that has a high concentration of iridium, which comes from asteroids. The iridium layer is just an inch or so thick and has been found in many places around the world. I’ve seen it in Colorado.

All of the land-based dinosaurs and three quarters of all plant and animal life went extinct. The year was 66 million years ago and marked the end of the cretaceous geological period.

The crater that was left after the explosion was found by geologists drilling for oil and confirmed only in 1991. The crater was 100 miles across and was found at a depth of 12 miles.

Collisions with earth.
A countless number of asteroids have impacted earth since its beginning. A splendid example can be seen just off the highway between Albuquerque and Flagstaff in Arizona.

95% of monster asteroids have been discovered and ruled out of any collision with earth. But there exist many smaller asteroids undiscovered that could cause havoc on a regional scale, such as the Southwest of USA.

A smaller asteroid, perhaps 100-200 meters across, is big enough to wipe out a city. That would cause extensive damage to a city like Albuquerque. It would leave behind a crater 5/8 miles across and 1/8 mile deep.

The asteroid called Dimorphos in figure 1 is an example of such an asteroid.

Figure 1. Planned collision between Dart and Dimorphos. Source: BBC, NASA, Johns Hopkins Applied Physics Laboratory.

Because of the potential damage to earth, NASA sent up a satellite called Dart that was dedicated to make a head-on collision with a mid-size asteroid called Dimorphos (1/10 a mile across). See Figure 1. Dimorphos is a moon orbiting around a large asteroid called Didymos (1/2 a mile across).

The timing had to be just right because Dimorphos is circling around its parent, and NASA wanted a head-on collision. But NASA is really clever at working the jet thrusters in Dart and they pulled this off.

Dart was destroyed by the collision, but the Italians had a small satellite called LICIACube (see Figure 1) that was trailing Dart and would take photos of the collision and the plume of dust kicked up and the crater left behind.

The data captured by LICIACube will be analyzed by the NASA physicists. They need to know exactly how much of an impact the collision made on Dimorphos, so they can knock a future asteroid heading for earth off its path if they have to.

Dimorphos (and Didymus) weren’t heading for earth so that was never a danger.

The Dart crash wasn’t big enough to knock Dimorphos out of its orbit around Didymus. But it would modify the orbit into a smaller orbit (Figure 1). A smaller orbit means it used to make one revolution in about 12 hours, but the time for a revolution would now shorten by about 10 minutes.

Telescope measurements, such as the James Webb telescope, will be able to confirm the change in the orbit of Dimorphos.

So the Dart crash wouldn’t have affected Dimorphos very much. But it didn’t have to. Even a small change in the trajectory of a future mid-size asteroid heading for earth could make it miss earth entirely, so long as the “nudge” is done while the asteroid is a long way from earth.

If the physicists get the analysis right for the Dart-Dimorphos crash, they will be able to use this analysis to design a new test to knock off its path a deadly asteroid that in the future may be heading straight toward earth.

Figure 2. Chance of earth getting hit by asteroids of different sizes. 140 m = less than 1/10 mile. 1,000 m = 6/10 mile. 10,000 m = 6 miles (monster asteroid). Source: BBC and NASA.

Chance of an asteroid direct hit.
So how anxious should we be about earth getting smashed by an asteroid? The probabilities, calculated by NASA, are given in Figure 2.

Monster asteroids, like Chicxulub 66 million years ago, are very rare. And only 4 have been found in all of earth’s history – over 3 billion years.

But smaller, mid-size strikes like Dimorphos are far more common – every 20,000 years. These can wipe out a city. Only 39% of these have been identified by telescopes.

Since these asteroids can cause massive casualties, NASA physicists want to do tests like Dart and Dimorphos to learn exactly how to defend earthlings against future mid-size asteroids that may yet be discovered hurtling toward a direct hit on earth.
The Gray Nomad ….. It is encouraging to see how brilliant scientists solve some of the calamities of the natural world — a great contrast to political leaders who create calamities.
How terrible for you, teachers of the Law! You have kept the key that opens the door to the house of knowledge; you yourselves will not go in, and you stop those who are trying to go in!
[Gospel of Luke, chapter 11.]

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