No space-tracking system whether in orbit or on Earth saw the Chelyabinsk rock before it entered the atmosphere. Why was that? Two reasons have been given. The first is that we don’t pick up on space rocks coming at us from all orbital angles particularly if coming in from the direction of the Sun in daylight. The second is that the Chelyabinsk rock was too small for Earth-based telescopes to detect and that it was a stony chondrite composed of silicates and dark materials which made it non-reflective. Our best estimate on size was that Chelyabinsk was between 12 and 13,000 metric tons and 20 metres or 66 feet in diameter.
It approached the atmosphere during daylight, flying from east to west, which made it virtually invisible. Even if it had hit the atmosphere on the nightside of the planet, our ground-based telescopes would not have detected it until it was within 135,000 kilometres (less than 84,000 miles) of Earth. Travelling at 69,000 kilometres (42,700 miles) per hour, that would have given us less than 2 hours warning before it struck.
Chelyabinsk broke up before fragments hit the ground. It was its explosion that caused damage to buildings but fortunately didn’t result in any human deaths for the city of 1.1 million.
Asteroid 2023 NT1 is another story. It was first detected two days after it made its closest approach, within 100,000 kilometres or 61,000 miles from Earth (a quarter of the distance from the Moon). Described to be as large as a commercial passenger jet, it measured over 60 metres (200 feet) in diameter. Like the Chelyabinsk meteorite, it came at us from the Sun making it undetectable. If it had entered our atmosphere, the impact would have been significantly larger than what occurred over Russia in 2013.
NASA’s DART (Double Asteroid Redirection Test) mission launched in November 2021, was the first test of what could become standard practice to defend the planet from errant space rocks. In September 2022, the spacecraft deliberately plowed into Dimorphos. a companion space rock to the near-Earth asteroid named Didymos. The collision was a planetary defence test to see if the kinetic energy created by the impact of a projectile could alter the trajectory of its target. Based on observations since the collision, Dimorphos has seen its orbit of Didymos change. Four years from now a European Space Agency mission will once more visit Didymos to measure the changes DART caused.
Today, we can account for approximately 40% of the space rocks that are near Earth. That means objects are flying by our planet, often well within the orbit of the Moon, for which we have no record. We remain blind to space rocks as big as a bus or a commercial passenger jet when approaching from the daylight side of our planet.
A proposed method to defend the planet from a space rock first detected only hours from Earth has been given the acronym, PI. It stands for “Pulverize It.” This is different than DART which was designed to nudge a space rock off course. PI (see image above) would involve launching a kinetic mass at hypervelocity to smash an incoming meteorite, comet, or small asteroid. Based on the angle of intercept, the fragments created would either burn up in the atmosphere before striking the Earth or miss the surface of the planet entirely. The resulting airbursts would produce atmospheric shockwaves with far less effect than Chelyabinsk. The longer the warning time, the less likely the impacted and shattered object will intersect with our atmosphere. But, in the case where the window of warning is short like Chelyabinsk or Asteroid 2023 NT1, the damage to anything on the surface would be dramatically mitigated.
What PI proposes is heightened detection combined with short-term mitigation systems. Objects the size of Chelyabinsk impact Earth about once a century. Objects like 2023 NT1 could strike Earth about once a millennia. The authors of the PI strategy write:
“While 2023 NT1 currently poses no threat to Earth, it is conceivable that similar objects could go undetected and result in short-warning (or no-warning) impacts. In situations like these, deflection-based mitigation strategies fall short of preventing damage if warning times are not sufficiently long; the time taken to plan, assemble, launch, and deflect is likely to be on the scale of years if not decades.”
Hence the DART strategy that NASA has demonstrated is insufficient. What is needed, therefore, is “a robust planetary defence system” prepared to deal with a range of threat sizes and warning times, that is layered, reliable, and tested.
]]>DART was launched in November of 2021. Its target was the binary asteroid system explicitly chosen to demonstrate a method of deflecting a space rock should one be on a collision course with Earth at some future date.
The choice of the binary, Didymos-Dimorphos, was deliberate because its orbit does not intersect with Earth and, therefore, poses no near-term threat. As a binary system, it could effectively demonstrate the redirection strategy being tested as a planetary defence.
The DART spacecraft (see illustration above) was accompanied on the mission by a small companion satellite, LICIACube, the latter to observe and measure the test. The impact was timed to occur when Didymos-Dimorphos was 11 million (7 million miles) from Earth.
Dimorphos’ mass was estimated to be 5 billion kilograms (11 billion pounds). Its known orbital period around Didymos was 11 hours and 55 minutes. DART at 570 kilograms (1,260 pounds) struck the moonlet travelling at a speed of 6.6 kilometres per second or 22,530 kilometres per hour (4 miles per second or 14,000 mph). In theory, the impact’s kinetic energy would alter the orbit moving Dimorphos closer to Didymos. It was believed that the kinetic force applied during the impact would not be enough to disrupt or destroy the asteroid.
Since the September 26th impact date, NASA has been analyzing the data. Dimorphos orbit is now 32 minutes (plus or minus 2) shorter. In a NASA press release on October 11, 2022, Lori Glaze, Director of NASA’s Planetary Science Division in Washington, pointed out that NASA “will be able to better assess whether, and how, a mission like DART could be used in the future to help protect Earth from a collision with an asteroid if we ever discover one headed our way.”
The LICIACube that accompanied DART will continue to monitor the impact results assessing changes to Dimorphos’ mass and shape. So far the impactor has created a plume of debris that stretches well into surrounding space as can be seen in the image appearing at the top of this posting. It looks like a comet’s tail.
A follow-up mission by the European Space Agency, called Hera is planned for four years from now to observe the changes to the binary asteroid system, study the impact crater, and measure the moonlet’s change in mass.
While we currently do not know of asteroids larger than 140 metres (approximately 460 feet) in diameter that will collide with us in the next hundred years, we can only account for about 40% of the rocks in our near-Earth space.
A look at recent history, however, suggests that asteroid strike occurrences are not rare. For example, in the 21st century, we had a 2008 4-metre (13-foot) rock weighing 80 tons explode over Sudan raining meteorite fragments across the Nubian desert. In 2013 a 20-metre (66-feet) diameter rock did the same over the city of Chelyabinsk in Russia causing injuries to thousands and damaging buildings and infrastructure. The Chelyabinsk rock was greater in mass than the Eiffel Tower and the airburst equalled 500 kilotons of TNT or 20 to 30 times the energy of the Hiroshima atomic bomb. Less than 24 hours later another space rock, 30 metres (approximately 100 feet) in diameter passed Earth at a distance of 27,700 kilometres (17, 211 miles).
Going back over a century, an asteroid fragment in 1908 exploded over the Tunguska region of Siberia, flattening forests for tens of kilometres. Further back in time an asteroid fragment 50,000 years ago impacted near where Winslow, Arizona is today. Its estimated size was between 30 and 50 metres with an impact force equivalent to a 15-megaton atomic bomb (1,200 times more powerful than the bomb that fell on Hiroshima).
And the most famous of all is the 66-million-year-ago dinosaur-killing impactor that was probably 10 kilometres (6 miles) in diameter packing 100 million megatons of force and leading to a worldwide extinction event.
Are dinosaur-killing asteroids rare? Based on recent research, the occurrence rate of space rocks bigger than 5 kilometres (3 miles) across is between 16 and 32 impacts every billion years which is ten times the rate previously thought.
That’s why the DART experiment is significant as is the mapping of near-Earth space which to date has catalogued over 28,000 rocks passing through the inner Solar System (December 20, 2021 count).
The Jet Propulsion Laboratory in Pasadena, California, in cooperation with the U.S. Space Force, hosts a website that tracks and studies near-Earth objects (NEOs). What remains hard to track are the space rocks that enter near-Earth space from the outer Solar System or from interstellar space as one-time visitors. Detecting them with enough advanced warning to launch an impactor would be a challenge. And it should be noted that sometimes a space rock comes out of the Sun from a completely unexpected direction as was the case with the Chelyabinsk rock.
]]>Could what happened 66 million years ago occur again? A study published in 2021 used a computer model to see how asteroids from the Belt lying between Mars and Jupiter could find their way to Earth. It simulated the thermal and gravitational forces within the Solar System and determined that about once every 250 million years a 10-kilometre (6-mile) sized asteroid could be flung from the Belt into a collision path with Earth. The frequency of this type of event happening was five times greater than previously thought.
A similar event 466 million years ago likely saw an asteroid from the Belt dislodge and break up into a rain of meteorites that caused Earth’s atmosphere to fill with dust leading to an Ice Age and enormous sea level drops. What followed is known as the Great Ordovician Biodiversification Event, a period in Earth’s history that saw an explosion of new marine and land animal life almost as impactful as the Cambrian Explosion that saw almost every form of modern life emerge less than 100 million years earlier.
Knowing what happened in the past has motivated NASA and the European Space Agency (ESA) to endeavour to develop a planetary defence strategy to intercept asteroids using kinetic impactors to deflect the largest of these rocks from striking Earth. This week saw a first demonstration when the Double Asteroid Redirection Test (DART) impacted a small moonlet named Dimorphos circling Didymos, a near-Earth asteroid. Check out the last five minutes of its flight at this Nature link.
The two bodies have never been in an orbit that would threaten Earth which made them ideal candidates for the demonstration test. So impacting Dimorphos would demonstrate the technology concept without consequences to our planet. DART’s onboard camera gave NASA a bird’s-eye-view of the spacecraft’s final 90,000-kilometre (56,000-mile) journey before it intentionally crashed at a speed of 22,530 kilometres per hour (14,000 mph). ESA’s companion CubeSat, called LICIACube) captured images of DART’s impact showing a cloud of ejected matter. In the next few weeks, NASA researchers will measure the orbit of Dimorphos around Didymos to see if the impact has altered it.
A follow-up ESA spacecraft, Hera, is planned four years from now to visit the asteroid and its moonlet to measure Dimorphos’ mass, study the impact crater left by DART and validate the effectiveness of using kinetic impactors as a planetary defence strategy.
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NASA has created the Planetary Defense Coordination Office to begin providing for a planetary-wide defence strategy that includes:
NASA’s Jet Propulsion Laboratory in Pasadena, California, is helping in this strategy by managing the agency’s near-Earth object study which tracks comets and space rocks in our orbital neighbourhood. The agency is also using the NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer) satellite to survey Earth’s close neighbours and has identified more than 26,000 objects so far. A future NASA satellite, the NEO Surveyor, will be deployed to find even more potential space rock threats.
NASA is joined by organizations like the International Astronomical Union (IAU), a body of academics and researchers from 90 countries that are employing several Earth-based observatories and space telescopes to track potential impactors.
Then there is B612, a non-profit organization created by two former NASA astronauts, Dr. Edward Lu, an astrophysicist, who flew on three space missions during the construction of the International Space Station, and his partner, Rusty Schweickart, who served on the crew of Apollo 9. The work they are doing is less about detection and more about developing defences. Why? Because B612 sees the planet’s vulnerability to space rock impactors.
Fortunately, most space rocks that enter our atmosphere are small and burn up harmlessly from friction and heat. About every five years, however, Earth intercepts one of at least 6 metres (20 feet) in diameter, that either detonates in the atmosphere or makes it to the planet’s surface. The vast majority of these fall into the ocean arriving unnoticed.
About once a century a larger space rock collides with Earth measuring up to hundreds of metres in diameter. If one of these were to strike an urban area the energy and shockwave produced would be more powerful than the largest hydrogen bomb ever tested.
Finally, about every 15 million years we see a space rock equivalent to a dinosaur killer. These are rocks with the potential to wipe out much of life on Earth.
A recent study describes a period in Earth’s history in which city-sized asteroids struck our planet at frequency rates far greater than anything recent in the geological record. Between 2.5 and 3.5 billion years ago, these space rocks, the size of cities and larger continuously pummeled Earth. These were rocks equivalent or greater in size than the asteroid of the Cretaceous-Paleogene event that formed the Chicxulub Crater in the Yucatan. We still see evidence of these impactors on the lunar surface which was equally bombarded during that period and also received post-impact debris expelled into space from Earth. The remaining evidence here are spherules, molten glassy globs embedded in ancient rocks. The rest has worn away over time.
So how can we defend the planet from a future space rock strike as menacing as the one that took out the dinosaurs? NASA and JPL have a plan that will begin to unfold this fall with the launch of the Double Asteroid Redirection Test (DART).
DART will target a companion moonlet of the near-Earth asteroid, Didymos. The asteroid spans 780 metres (2,500 feet). Its moonlet is 160 metres (over 500 feet), more typical, states NASA, of the types of rocks that have struck Earth in the past. DART will crash into the moonlet. It is a kinetic energy weapon that will intercept its target in September 2022 with the asteroid about 11 million kilometres (6.8 million miles) from Earth. The impact speed will be 6.6 kilometres (4.1 miles) per second and the results should be captured by DART’s onboard camera with the signal beamed to Earth. In addition, Earth-based telescopes will monitor post-impact results which NASA hopes will show a small change to the moonlet’s orbit increasing it by several minutes.
As critical as DART will be in the evolving plan to defend Earth, it only works on potential impactors we can detect while still far away. We know of at least one recent space rock, however, that arrived here on Earth virtually undetected. That was the estimated 17-metre (56-foot) space rock that weighed 10,000 metric tons and travelled at a speed of 17.88 kilometres (11.11 miles) per second before exploding and breaking up over the Russian city of Chelyabinsk in February 2013. It came out of nowhere and had never been tracked because its orbit was atypical.
Another concern we will need to address is probably still decades away when commercial space mining operations begin to exploit the potential riches found within space rocks. Asteroid mining is seen as being worth tens of trillions of dollars.
An unintended consequence could be the altering of the orbit of one of these asteroids during a mining operation. What miners will need to ensure is they don’t cause a change that could send pieces or the entire asteroid on a collision course with Earth. Because the most likely mining candidates will be near-Earth asteroids and not those in the Asteroid Belt between Mars and Jupiter, the risks of such an accident are that much greater.
That risk is the subject of a conference paper presented at the 7th IAA Planetary Defense Conference, held in April of this year, in Vienna, Austria, where authors Aaron Boley and Michael Byers, from the University of British Columbia, describe a need to actively manage asteroid orbits as a precautionary principle.
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