SCIENCE

How Worried Should We Be About A Huge Asteroid Hitting Earth?

An asteroid explorer answers 13 questions about scary space rocks.

03/08/2016 7:54 AM AEST | Updated August 4, 2016 14:03
John de Dios/UANews
Asteroid explorer Dante Lauretta.

Some media outlets say the asteroid Bennu might be on a collision course with Earth. Others say not to worry.

To learn the truth, The Huffington Post reached out to Dr. Dante Lauretta, professor of planetary science at the University of Arizona and the principal investigator for OSIRIS-REx. That’s the NASA mission scheduled to launch this September with the goal of briefly landing on Bennu in 2018 and returning to Earth in 2023 with samples from its surface.

Here are our questions and Lauretta’s answers, lightly edited:

Is Bennu really on a collision course with Earth?

Bennu is classified as a potentially hazardous asteroid, or PHA. There is a small (1 in 2,700) chance that Bennu will hit Earth late in the 22nd century. If Bennu did hit Earth, it would release the energy equivalent of 1,450 megatons of TNT, creating an impact crater about 5 kilometers (about 3 miles) in diameter. That’s over 70,000 times more energy than was released by the atomic bomb that fell on Hiroshima.

How worried should we be? 

I wouldn’t be worried at all. The impact probabilities are way in the future. It sounds scary, but you’ve crossed the street with those odds. It’s good to be proactive and to keep an eye on Bennu, but it’s just not something to panic about.

What are you hoping to learn from OSIRIS-REx? 

Bennu is a time capsule from the earliest epoch of solar system history. Its surface color suggests a carbon-rich composition, indicating that it may hold clues to the means by which organic molecules were delivered to the surface of the early Earth. By studying samples returned from this asteroid we hope to learn about the origin of life on Earth and the likelihood that it arose elsewhere in the solar system.

What fears do you have about the mission’s success?

Our greatest risk is associated with the sample acquisition event. We have to program the spacecraft to fly autonomously to a precise location on the asteroid surface. Once we reach the surface we have to keep the spacecraft safe while we “vacuum” up many grams of gravel and soil from the surface. This is a tricky sequence and we will spend many months rehearsing before we commit to the actual event.

Once we give the command to go, we will have to sit back and watch events unfold. The one-way light travel time from the spacecraft to Earth at this point will be about 18 minutes. Everything we hear from the spacecraft will have already happened.

NASA/Goddard/University of Arizona
Artist's impression of OSIRIS-REx spacecraft.

Why Bennu and not another asteroid?

Bennu was chosen first based on accessibility. We needed an asteroid with an orbit that required a minimal amount of energy to rendezvous, study, then return to Earth. We also wanted a somewhat large asteroid ― more than 200 meters (about 656 feet) in diameter ― to ensure that loose material was available on the surface for sampling.

Based on these criteria, only 26 out of over 500,000 known asteroids remained as candidates. We made the final selection based on the surface chemistry ― we wanted the best chance at getting that organic material.

How many asteroids are out there? 

There are currently over 700,000 known asteroids in the solar system. There are currently 1,714 known potentially hazardous asteroids, including these that are considered the riskiest.

What is being done to protect Earth from asteroids?

NASA and other agencies have implemented thorough sky surveys to catalog all known potentially hazardous asteroids greater than 140 meters (about 460 feet) in diameter. (Scientists believe an impact by an asteroid this big or bigger could pose a major risk to civilization.) In addition to OSIRIS-REx, other agencies are developing missions to study near-Earth asteroids, including the Hayabusa 2 mission and the Asteroid Impact and Deflection Assessment mission.

The Asteroid Day group (of which I am a member) is seeking to raise awareness of asteroid impact risks and urge more research in this area. This year the United Nations Outer Space Committee endorsed June 30 as International Asteroid Day. This will go to the UN General Assembly for a vote later this year.

What are the biggest misconceptions nonscientists have about asteroids?

Many people envision the asteroid belt to look like the scene in “Empire Strikes Back” where Han Solo is dodging and weaving around hundreds of tumbling space rocks. In reality, asteroids are separated from each other by an average of a million kilometers (about 620,000 miles). Of course, they do occasionally collide with each other, creating a family of smaller asteroids that slowly drift away from each other.

If we were to spot an asteroid on a collision course with Earth, could we do anything to stop it? 

There are many ideas out there for deflecting an incoming asteroid. Most require decades of advance notice.

How do we detect asteroids?

There are several telescope surveys that scan the night skies constantly to detect asteroids and measure their orbits. The most productive of these surveys are the Catalina Sky Survey in Tucson, Arizona, led by the University of Arizona, and the Pan-STARRS survey in Hawaii. It is not difficult but requires constant monitoring. New asteroids are discovered every night.

Where do asteroids come from? 

Asteroids formed in the first 10 million years or so of solar system history. They are the left-over remnants from planet formation. The survivors were prevented from accreting into their own planet because they were constantly perturbed by Jupiter’s massive gravity field. As a result, these remnant objects reside largely in a belt between Mars and Jupiter.

Small objects are escaping from this region and tumbling into the inner solar system, where they become near-Earth objects. Once in the inner solar system, they only survive for a few million years. Most fall into the sun, but others end up impacting the planets. The cratered surfaces of Mars, the moon and Mercury record this cosmic bombardment.

What are asteroids made of, and how fast are they moving?

Asteroids have a wide range of compositions. Objects like Bennu are thought to be composed of hydrated minerals like clays and organic material. Others are mostly stony materials with minerals like olivine and pyroxene (which make up the mantle of the Earth). Many contain iron-nickel metal as a minor component. Still others are made almost entirely of metal. This range is reflected in the diverse compositions of meteorites.

Asteroids orbit the sun with velocities typical of planetary bodies. For example, asteroid Bennu is traveling through the solar system at roughly 25 kilometers per second (roughly 56,000 miles per hour). Earth travels around the sun at roughly 30 kilometers per second (about 67,000 miles per hour).

How did you get interested in asteroids?

I was always interested in the formation of planets and the origin of life. Once I started studying this topic I learned that meteorites are actual fossil rocks from the time period when these events occurred. Once I realized that I could hold a rock in my hand that was 4.5 billion years old, I was hooked.

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