The Hiawatha asteroid would have been nearly the size of the comet fragment that slammed into the Atlantic ocean in the 1998 film "Deep Impact." The comet in the film (Wolf-Biederman) was originally 7 miles wide but was broken apart by the nuclear space mission, a 1.5 mile wide fragment of the comet made it's way to Earth and hit the Atlantic ocean, resulting in the famous mega-tsunami scene.
Scientists perform research field work at the edge of the Greenland Ice Sheet while studying the age of the 19-mile-wide (31- km-wide) Hiawatha impact crater that is buried under ice six-tenths of a mile (1 km) thick in 2019.
In the 1998 film Deep Impact, the comet fragment impact generates a wave that is approximately 500 meters (about 1,500 to 1,800 feet) high when it reaches the East Coast and New York City. The initial wave height is roughly 600 feet at the point of impact in the Atlantic Ocean, growing to between 1,500 and 1,800 feet (or around 500 meters) as it slowed and piled up near the coast, towering over skyscrapers and completely submerging landmarks like the Statue of Liberty.
The largest confirmed asteroid to hit Earth is the Vredefort impactor, which struck South Africa about 2 billion years ago. The asteroid is estimated to have been between 20 to 25 kilometers (12 to 15.5 miles) in diameter based on recent studies (previous estimates placed it around 10 to 15 km). It was roughly twice the size of the asteroid that wiped out the dinosaurs (Chicxulub) and occurred long before complex multi-cellular life had evolved. It created the Vredefort Crater, the largest verified impact structure on Earth, with an original estimated diameter of 250 to 300 kilometers (155 to 190 miles), located near what is now Vredefort in the Free State, South Africa. This is the crater from space, you can see the black circular rings that represent the edge of the crater. The Vredefort asteroid impact released an energy equivalent to roughly 6.88 billion Hiroshima atomic bombs detonating simultaneously.
The Manicouagan crater in Québec is a lot older than Chicxulub: 214 million years. Known also as “the eye of Québec”, it became instantly recognizable as an impact crater after flooding of the Manicouagan river by Hydro Québec in the 1960’s. Now anyone can see it on Google Earth and many astronauts, including Canada’s own Chris Hadfield, have photographed it. Manicouagan is the fourth largest impact structure on earth. No tectonic forces have influenced or altered it since its inception. The asteroid hit nearly 1 billion year old Canadian shield rocks. After this event, not much else happened geologically, other than the normal amount of erosion, but it never was an area with high mountains so that wasn’t a whole lot. Then the ice caps of the Quaternary glaciations swept across it, nicely cleaning off debris and making it even more easily visible, so it’s a a rather pure and unchanged. The asteroid responsible for the Crater was approximately 5 kilometers (3 miles) wide.
1.85 billion years ago, a massive asteroid struck Sudbury, Ontario, melting part of Earth’s crust and creating the second-largest confirmed impact crater on Earth, behind only Vredefort in South Africa. 37 million years ago, another asteroid struck nearly the same place. The first impact was enormous. Scientists estimate the asteroid may have been 10–15 kilometers (6-9 miles wide), as big as the asteroid that wiped out the Dinosaurs (Chicxulub). The energy released was so great that vast volumes of rock were instantly melted, forming a huge pool of melted rock within the crater. This was not magma rising from deep inside the Earth. It was the surface rock itself, melted by the force of the collision into what geologists call an impact melt sheet. As that melt slowly cooled, heavy elements separated and concentrated within it, including nickel, copper, palladium, gold, and other platinum-group metals. Instead of remaining scattered through the crust, these materials became gathered and enriched by the cooling melt itself. Dense metal-rich sulfides settled downward, collecting along the base and edges of the structure. In effect, the impact briefly turned this part of Earth into a giant natural smelter, creating the mineral wealth that Sudbury is known for today. Much later, after nearly two billion years of erosion, deformation, and tectonic activity that reshaped the original crater, a second asteroid struck nearly the same place. That impact formed what is now Wanapitei Lake, a circular basin about 8 kilometers across. The younger crater is completely unrelated to the first event. It simply happened to land in terrain already shaped by one of the largest collisions in Earth’s history. Sudbury records two separate cosmic events that reshaped the same landscape, separated by a span of time almost impossible to comprehend.
Theia was a rocky, Mars-sized planetary body (about 5% to 10% of Earth's mass) that shared a similar orbit around the Sun. Theia collided with the early Earth approximately 4.4 billion years ago. The violent impact made the biggest asteroid strikes look like child's play. It blasted a mixture of molten rock and debris from both worlds into orbit, which quickly coalesced to form Earth's Moon. The energy released was approximately 143 trillion times greater than the Tsar Bomba (the most powerful nuclear bomb ever detonated. It released roughly 100 million times more energy than the asteroid impact that wiped out the non-avian dinosaurs. The immense heat and kinetic energy completely melted both bodies (or formed a massive vaporized/molten structure known as a synestia) and liquefied Earth's mantle and surface.
This book about the Chicxulub asteroid impact always sends chills up my spine every time I look at the cover, the blinding flash of light from the moment the asteroid hit. I mean, the impact produced a blinding, hyper-intense flash equivalent to 100 teratons of TNT (about 4.5 billion times the energy of the Hiroshima bomb or millions of nuclear weapons going off simultaneously). Across the daylight side of the planet, the impact generated an instantaneous, blinding mega-flash that outshone the Sun from regional distances and was visible globally. Even thousands of kilometers away, the sky lit up with extreme thermal radiation capable of causing flash burns, ignition of dry vegetation, and temporary or permanent blindness to any creature looking toward the horizon before the shockwave and ejecta rain arrived.
The Earth is about 7,926 miles (12,756 kilometers) wide at the equator. It is estimated by scientists that it would take an asteroid at least 6 to 10-miles-wide to collapse modern human civilization and an asteroid about 60 to 100-miles-wide to completely sterilize the Earth, boiling away the oceans, melting the crust, and destroying all life on our planet, even deep-subsurface extremophile life. It's just mind-bending to think about it. It's just mind-bending to think that the Earth is so big, and all it would take is a 60 to 100 mile-wide rock to end it all.
And just for clarification, no, a single spacecraft like NASA's DART could not deflect an asteroid as large as the Chicxulub impactor if such an asteroid were headed our way. The DART spacecraft weighed about 550 kilograms (1,210 lbs) and targeted Dimorphos, a small moonlet roughly 160 meters across. In contrast, the Chicxulub asteroid was roughly 10 kilometers (6 miles) in diameter and weighed about a trillion metric tons—roughly 200,000 to 250,000 times more massive than Dimorphos. A kinetic impactor like DART transfers momentum through physical crash force. Crashing a probe into a 10-kilometer-wide mountain of rock would barely register a change in its velocity unless executed millions of years in advance. It would be like a mosquito running into a bus. A massive object like the Chicxulub impactor requires early detection spanning many decades or centuries so a tiny change in speed can accumulate into a wide-enough miss distance by the time it reaches Earth's orbital path. Scientific studies indicate that kinetic impactors alone are inadequate for a 10 km scale threat. Deflecting or fragmenting an object of that magnitude would likely require multiple megaton-class nuclear explosive devices delivered via heavy-lift interplanetary spacecraft. Humanity currently lacks the immediate launch capability, pre-built interceptors, or global coordination needed to deploy a high-megaton nuclear deflection mission on short notice.
We are nowhere close to defending Earth from a cataclysmic asteroid
You can't throw a DART at everything in space.
By: Tom Hartsfield
February 17, 2023
An enhanced photograph of ejected plumes caused by DART’s impact on Dimorphos. (Credit: ASI / NASA / APL)
Last September, NASA smashed a spacecraft into a small asteroid. The results were impressive, as shown in the image above.
The Double Asteroid Redirection Test, or DART, was a success as a prototype technological demonstration. But what can we say about the impact of the mission? What does it mean for our chances of defending the planet from cosmic calamity? Unfortunately, not much — not yet, anyway.
NASA’s DART was a successful mission
The spacecraft performed well. A camera aboard DART captured the scene as the craft passed by Didymos, a near-Earth asteroid. DART’s target was Didymos’ satellite, Dimorphos. The target asteroid expanded to fill the frame, and the test ended abruptly with a head-on strike.
Had the craft hit at a wide angle to the geometric center of the rock, the results could have been different. The asteroid might have rotated or wobbled without slowing its translational motion. (Think of how striking the edge of a billiard ball imparts what competitors call English, while striking the center directs the ball purely forward.) Telescopes detected giant plumes of ejected material thrown out into space, another good sign. Mass sprayed off in one direction imparts force in the other direction on the sprayer.
A few days later, NASA announced that the small asteroid’s orbit around the larger one was altered by an amount large enough to clearly measure. The orbital period dropped by about 32 minutes, from 11 hours and 55 minutes to 11 hours and roughly 23 minutes. This was at the high end of estimates going in, so it is good news.
However, that measurement alone does not tell us how well the tactic would work on an asteroid that poses a real threat — which is what we really want to know. Eventually, NASA will have more complete data, allowing researchers to examine details more carefully. For now, our answers are partial. But we can make further estimates and touch on the science behind asteroid deflection.
Interstellar gnats
A planetary defense mission works by altering the motion of an asteroid by a tiny, tiny amount. Let’s say you rolled two bowling balls down one regulation bowling lane. A gnat flies up the lane and smacks into one of the balls. The impact slows that ball by a miniscule amount, too small to reasonably measure. At the end of the lane, the balls will strike at the exact same time, to the human eye. But if the lane is 1,000 miles long, the gnat-slowed ball will arrive slightly tardy at the far end.
The lanes of space are billions of miles long, so you don’t have to hit the ball very hard if you hit it very far in advance.
Let’s say that by smashing a spacecraft into it, we reduce an asteroid’s velocity by 1/10,000 of 1 percent. The distance it travels in a day might now be reduced from, say, 864,000,000 meters to 863,999,136 meters. So, the asteroid’s expected position slows by 864 meters (0.864 km) each day after the impact. Given that Earth is over 12,000 km in diameter (nearly 8,000 miles), it takes several thousand days for the slow-down to accumulate to its intended effect: namely, allowing Earth to slip past the asteroid.
NASA hoped to slow Dimorphos by about one mile per day. In theory, this would be enough to shift an Earth impact to a near-miss, if the asteroid could be struck ten years plus a few weeks in advance of an impending collision.
What we do not know yet is whether DART’s impact slowed the asteroid more than NASA hoped. We can make some estimates by using the minimal eccentricity of Dimorphos’ orbit about Didymos — which is nearly zero, or circular — the decrease in its orbital period after impact, and NASA’s predictions. In the best-case likely scenario, the impact slowed Dimorphos by about 4 miles per day. In the worst-case likely scenario, it might have slowed it by only one mile per day. The middle-of-the-road estimate is somewhere around 2.5 miles per day. Values outside this are of course possible, and we don’t know enough yet to be sure. These numbers are in line with what NASA hoped for going in, although they are on the high end of expectations.
Re-running our simple calculations, in our best-case outcome, striking Dimorphos 2.5 years in advance would give just enough time to shift an impact trajectory to one that scraped the atmosphere but missed. In the worst case, we would need to impact it ten years ahead of time. How much protection does that afford us?
A nice start, but a long way to go
The Tunguska event was likely caused by a rock about 200 feet across. We can roughly estimate that it had less than 1/20 the mass of Dimorphos. The famous Barringer Crater in the Arizona desert, nearly 600 feet deep, was caused by a similarly sized rock of different composition. Either of these could have destroyed a city unlucky enough to sit at the impact point. Rocks of this size could be effectively deflected by DART less than a year before a predicted strike. The catch is that detecting them is very difficult. Ongoing efforts continue to discover new near-earth asteroids on a nearly daily basis. Objects like the Chelyabinsk meteor in 2013 are often detected only when the fireball lights up the sky.
Dimorphos was discovered in 2003. If it had been on a collision course for, say, 2030, we could probably prevent an impact. Didymos, about which it orbits, was discovered in 1996.
This is where the bad news starts.
Let’s say that Didymos was due to hit Earth. Its diameter is about 4.5 times that of Dimorphos. That means Didymos has something like 95 times more mass. In this case, DART would have to impact it somewhere between 190 and 950 years in advance to alter its course sufficiently. We only discovered it 26 years ago. If we needed to deflect it within a few years, we would have to hit it 100 times harder than DART hit Dimorphos. That means mass and velocity, multiplied together, must be on a scale 100 times greater than that of DART. This is possible, but it would stretch our current capabilities. We could add mass through a series of large launches, perhaps using Falcon Heavy launch vehicles. Additional engines could be launched to boost velocity. As for rocks larger than Didymos, they would pose a much greater problem.
The famous Chicxulub impactor that caused a mass extinction 66 million years ago was roughly 10,000 meters in diameter. The mass of this object — about 200,000-fold greater than Didymos — was significant enough that a spacecraft like DART would have to impact it millions of years in advance to avoid an extinction-level impact. No conceivable improved version of DART is capable of preventing such an event. For that, we would need far more powerful countermeasures. An investigation of this problem was carried out by scientists T.J. Ahrens and A.W. Harris, who analyzed the necessary countermeasures against asteroids of varying sizes. For small rocks, they conclude that a mission like DART, perhaps scaled up a bit, could be enough — as DART indeed just showed.
For rocks bigger than Didymos, but much smaller than Chicxulub, the impactor would need thousands of times more momentum. We cannot easily make a craft thousands of times faster or thousands of times more massive than DART with current rocketry technology. Nuclear weapons could impart enough push to do the job, with certain careful considerations about the composition of the asteroid, the location of the blast, how well the blast could be directed, the amount of ejecta blasted away, and so forth.
For giant objects like Chicxulub, the only presently feasible option is a truly enormous nuclear blast. A device of many megatons might give us a chance. Fortunately, we are likely to spot asteroids of this size far in advance.
DART was a useful first step toward asteroid deflection capability. However, Dimorphos is very small. We have a long way to go before we can defend against truly catastrophic collisions.
And also just to clarify, no, a standard Chicxulub-sized asteroid cannot "sneak up" on Earth. Because an object of that scale (10 to 15 kilometers wide) is incredibly massive and reflects a significant amount of light, automated planetary defense telescope systems have already mapped and tracked virtually 100% of all Near-Earth Asteroids (NEAs) of this size. Astronomers can calculate their orbits centuries into the future, and none are currently on a collision course with Earth. However, there are two extreme, highly unlikely astronomical scenarios where a planet-killer could catch us with limited warning:
The Solar Blind Spot
The most significant limitation of Earth's current ground-based optical telescopes is the glare of the Sun. If a previously unknown, highly eccentric asteroid approached Earth directly out of the Sun's direction, it could remain hidden in the daylight sky until it was very close. Even so, because a 10 km object is so large, we would still likely spot it weeks or months in advance as it crossed the inner solar system, rather than discovering it only hours before impact.
Long-Period Comets or Interstellar Objects
The real "wild card" is not a local asteroid, but an object originating from the outermost edges of our solar system (the Oort Cloud) or from another star system entirely.
• Long-Period Comets: These giant chunks of ice and rock spend thousands of years in deep space where they are completely dark and invisible to us. If one shifted into an Earth-crossing orbit, it would travel at extreme speeds.
• Warning Time: We would only detect it once it grew close enough to the Sun to start heating up and forming a visible tail. For an object that massive, we might only get a few months to a couple of years of warning before impact—which, as established, is not enough time for humanity to mount a successful deflection mission.
In summary, the odds of an asteroid the size of the Chixculub impactor sneaking up on us are effectively zero. The odds of a giant, undetected comet appearing out of the deep outer solar system on a direct collision course are mathematically real, but extraordinarily rare, happening only once every tens or hundreds of millions of years.
The History Channel did a 10-part series called "Doomsday: 10 Ways the World Will End", most of it is available on YouTube for viewing. It interviews some of the world's best physicists, paleontologists, and experts and discusses the many threats to our existence, asteroids, Black holes, Gamma Ray bursts, Supervolcanoes, Nuclear War, rogue planets, and even our own sun. Being interested in space, science, physics, and almost everything about life in general, you can't help but wonder how it will all come to an end.
Gamma-ray bursts are quite insane to think about. A gamma-ray burst (GRB) cannot physically blow up or destroy the planet Earth, but a close enough hit could wipe out most life on the surface. A GRB is a hyper-energetic cosmic explosion from a collapsing massive star or merging neutron stars. It releases more energy in a few seconds than our Sun will produce in its entire lifetime. The energy shoots out in narrow, intense beams from the poles of the exploding or merging object. If a beam directly targeted Earth from within a few thousand light-years, the high-energy photons would tear apart nitrogen and oxygen molecules in the upper atmosphere. This destroys the ozone layer. With the ozone layer gone, intense ultraviolet (UV) radiation from the Sun would flood the surface. The ozone layer acts as a global shield by absorbing dangerous solar ultraviolet (UV-B and UV-C) rays. If a nearby gamma-ray burst (GRB) stripped away or severely thinned this layer, that barrier would vanish and our skin would be left completely exposed to the hellish wrath of the sun's radiation. This could also trigger massive ecological collapse, plant and marine die-offs, and a mass extinction event. Some scientists suggest a GRB may have contributed to a past extinction, such as the Ordovician mass extinction 450 million years ago. To cause a mass extinction, the GRB would need to occur relatively close within our own galaxy—typically estimated within a few hundred to a few thousand light-years. According to the European Space Agency, there are no known candidate stars capable of producing a GRB close enough to threaten Earth (with safe zones generally falling within 200 to several thousand light-years). Distant bursts are harmless. Billions of light-years away, GRBs like the record-setting "BOAT" burst observed by NASA/ESA only slightly ionize Earth's upper ionosphere without posing any danger to life on the ground.
Comments
The Hiawatha asteroid would have been nearly the size of the comet fragment that slammed into the Atlantic ocean in the 1998 film "Deep Impact." The comet in the film (Wolf-Biederman) was originally 7 miles wide but was broken apart by the nuclear space mission, a 1.5 mile wide fragment of the comet made it's way to Earth and hit the Atlantic ocean, resulting in the famous mega-tsunami scene.
Scientists perform research field work at the edge of the Greenland Ice Sheet while studying the age of the 19-mile-wide (31- km-wide) Hiawatha impact crater that is buried under ice six-tenths of a mile (1 km) thick in 2019.
In the 1998 film Deep Impact, the comet fragment impact generates a wave that is approximately 500 meters (about 1,500 to 1,800 feet) high when it reaches the East Coast and New York City. The initial wave height is roughly 600 feet at the point of impact in the Atlantic Ocean, growing to between 1,500 and 1,800 feet (or around 500 meters) as it slowed and piled up near the coast, towering over skyscrapers and completely submerging landmarks like the Statue of Liberty.
The largest confirmed asteroid to hit Earth is the Vredefort impactor, which struck South Africa about 2 billion years ago. The asteroid is estimated to have been between 20 to 25 kilometers (12 to 15.5 miles) in diameter based on recent studies (previous estimates placed it around 10 to 15 km). It was roughly twice the size of the asteroid that wiped out the dinosaurs (Chicxulub) and occurred long before complex multi-cellular life had evolved. It created the Vredefort Crater, the largest verified impact structure on Earth, with an original estimated diameter of 250 to 300 kilometers (155 to 190 miles), located near what is now Vredefort in the Free State, South Africa. This is the crater from space, you can see the black circular rings that represent the edge of the crater. The Vredefort asteroid impact released an energy equivalent to roughly 6.88 billion Hiroshima atomic bombs detonating simultaneously.
The Manicouagan crater in Québec is a lot older than Chicxulub: 214 million years. Known also as “the eye of Québec”, it became instantly recognizable as an impact crater after flooding of the Manicouagan river by Hydro Québec in the 1960’s. Now anyone can see it on Google Earth and many astronauts, including Canada’s own Chris Hadfield, have photographed it. Manicouagan is the fourth largest impact structure on earth. No tectonic forces have influenced or altered it since its inception. The asteroid hit nearly 1 billion year old Canadian shield rocks. After this event, not much else happened geologically, other than the normal amount of erosion, but it never was an area with high mountains so that wasn’t a whole lot. Then the ice caps of the Quaternary glaciations swept across it, nicely cleaning off debris and making it even more easily visible, so it’s a a rather pure and unchanged. The asteroid responsible for the Crater was approximately 5 kilometers (3 miles) wide.
Manicouagan crater in the winter as photographed from space in 1983.
1.85 billion years ago, a massive asteroid struck Sudbury, Ontario, melting part of Earth’s crust and creating the second-largest confirmed impact crater on Earth, behind only Vredefort in South Africa. 37 million years ago, another asteroid struck nearly the same place. The first impact was enormous. Scientists estimate the asteroid may have been 10–15 kilometers (6-9 miles wide), as big as the asteroid that wiped out the Dinosaurs (Chicxulub). The energy released was so great that vast volumes of rock were instantly melted, forming a huge pool of melted rock within the crater. This was not magma rising from deep inside the Earth. It was the surface rock itself, melted by the force of the collision into what geologists call an impact melt sheet. As that melt slowly cooled, heavy elements separated and concentrated within it, including nickel, copper, palladium, gold, and other platinum-group metals. Instead of remaining scattered through the crust, these materials became gathered and enriched by the cooling melt itself. Dense metal-rich sulfides settled downward, collecting along the base and edges of the structure. In effect, the impact briefly turned this part of Earth into a giant natural smelter, creating the mineral wealth that Sudbury is known for today. Much later, after nearly two billion years of erosion, deformation, and tectonic activity that reshaped the original crater, a second asteroid struck nearly the same place. That impact formed what is now Wanapitei Lake, a circular basin about 8 kilometers across. The younger crater is completely unrelated to the first event. It simply happened to land in terrain already shaped by one of the largest collisions in Earth’s history. Sudbury records two separate cosmic events that reshaped the same landscape, separated by a span of time almost impossible to comprehend.
Theia was a rocky, Mars-sized planetary body (about 5% to 10% of Earth's mass) that shared a similar orbit around the Sun. Theia collided with the early Earth approximately 4.4 billion years ago. The violent impact made the biggest asteroid strikes look like child's play. It blasted a mixture of molten rock and debris from both worlds into orbit, which quickly coalesced to form Earth's Moon. The energy released was approximately 143 trillion times greater than the Tsar Bomba (the most powerful nuclear bomb ever detonated. It released roughly 100 million times more energy than the asteroid impact that wiped out the non-avian dinosaurs. The immense heat and kinetic energy completely melted both bodies (or formed a massive vaporized/molten structure known as a synestia) and liquefied Earth's mantle and surface.
A simulation showing Theia colliding with Earth 4.4 billion years ago.
This book about the Chicxulub asteroid impact always sends chills up my spine every time I look at the cover, the blinding flash of light from the moment the asteroid hit. I mean, the impact produced a blinding, hyper-intense flash equivalent to 100 teratons of TNT (about 4.5 billion times the energy of the Hiroshima bomb or millions of nuclear weapons going off simultaneously). Across the daylight side of the planet, the impact generated an instantaneous, blinding mega-flash that outshone the Sun from regional distances and was visible globally. Even thousands of kilometers away, the sky lit up with extreme thermal radiation capable of causing flash burns, ignition of dry vegetation, and temporary or permanent blindness to any creature looking toward the horizon before the shockwave and ejecta rain arrived.
The Earth is about 7,926 miles (12,756 kilometers) wide at the equator. It is estimated by scientists that it would take an asteroid at least 6 to 10-miles-wide to collapse modern human civilization and an asteroid about 60 to 100-miles-wide to completely sterilize the Earth, boiling away the oceans, melting the crust, and destroying all life on our planet, even deep-subsurface extremophile life. It's just mind-bending to think about it. It's just mind-bending to think that the Earth is so big, and all it would take is a 60 to 100 mile-wide rock to end it all.
And just for clarification, no, a single spacecraft like NASA's DART could not deflect an asteroid as large as the Chicxulub impactor if such an asteroid were headed our way. The DART spacecraft weighed about 550 kilograms (1,210 lbs) and targeted Dimorphos, a small moonlet roughly 160 meters across. In contrast, the Chicxulub asteroid was roughly 10 kilometers (6 miles) in diameter and weighed about a trillion metric tons—roughly 200,000 to 250,000 times more massive than Dimorphos. A kinetic impactor like DART transfers momentum through physical crash force. Crashing a probe into a 10-kilometer-wide mountain of rock would barely register a change in its velocity unless executed millions of years in advance. It would be like a mosquito running into a bus. A massive object like the Chicxulub impactor requires early detection spanning many decades or centuries so a tiny change in speed can accumulate into a wide-enough miss distance by the time it reaches Earth's orbital path. Scientific studies indicate that kinetic impactors alone are inadequate for a 10 km scale threat. Deflecting or fragmenting an object of that magnitude would likely require multiple megaton-class nuclear explosive devices delivered via heavy-lift interplanetary spacecraft. Humanity currently lacks the immediate launch capability, pre-built interceptors, or global coordination needed to deploy a high-megaton nuclear deflection mission on short notice.
We are nowhere close to defending Earth from a cataclysmic asteroid
You can't throw a DART at everything in space.
By: Tom Hartsfield
February 17, 2023
An enhanced photograph of ejected plumes caused by DART’s impact on Dimorphos. (Credit: ASI / NASA / APL)
Last September, NASA smashed a spacecraft into a small asteroid. The results were impressive, as shown in the image above.
The Double Asteroid Redirection Test, or DART, was a success as a prototype technological demonstration. But what can we say about the impact of the mission? What does it mean for our chances of defending the planet from cosmic calamity? Unfortunately, not much — not yet, anyway.
NASA’s DART was a successful mission
The spacecraft performed well. A camera aboard DART captured the scene as the craft passed by Didymos, a near-Earth asteroid. DART’s target was Didymos’ satellite, Dimorphos. The target asteroid expanded to fill the frame, and the test ended abruptly with a head-on strike.
Had the craft hit at a wide angle to the geometric center of the rock, the results could have been different. The asteroid might have rotated or wobbled without slowing its translational motion. (Think of how striking the edge of a billiard ball imparts what competitors call English, while striking the center directs the ball purely forward.) Telescopes detected giant plumes of ejected material thrown out into space, another good sign. Mass sprayed off in one direction imparts force in the other direction on the sprayer.
A few days later, NASA announced that the small asteroid’s orbit around the larger one was altered by an amount large enough to clearly measure. The orbital period dropped by about 32 minutes, from 11 hours and 55 minutes to 11 hours and roughly 23 minutes. This was at the high end of estimates going in, so it is good news.
However, that measurement alone does not tell us how well the tactic would work on an asteroid that poses a real threat — which is what we really want to know. Eventually, NASA will have more complete data, allowing researchers to examine details more carefully. For now, our answers are partial. But we can make further estimates and touch on the science behind asteroid deflection.
Interstellar gnats
A planetary defense mission works by altering the motion of an asteroid by a tiny, tiny amount. Let’s say you rolled two bowling balls down one regulation bowling lane. A gnat flies up the lane and smacks into one of the balls. The impact slows that ball by a miniscule amount, too small to reasonably measure. At the end of the lane, the balls will strike at the exact same time, to the human eye. But if the lane is 1,000 miles long, the gnat-slowed ball will arrive slightly tardy at the far end.
The lanes of space are billions of miles long, so you don’t have to hit the ball very hard if you hit it very far in advance.
Let’s say that by smashing a spacecraft into it, we reduce an asteroid’s velocity by 1/10,000 of 1 percent. The distance it travels in a day might now be reduced from, say, 864,000,000 meters to 863,999,136 meters. So, the asteroid’s expected position slows by 864 meters (0.864 km) each day after the impact. Given that Earth is over 12,000 km in diameter (nearly 8,000 miles), it takes several thousand days for the slow-down to accumulate to its intended effect: namely, allowing Earth to slip past the asteroid.
NASA hoped to slow Dimorphos by about one mile per day. In theory, this would be enough to shift an Earth impact to a near-miss, if the asteroid could be struck ten years plus a few weeks in advance of an impending collision.
What we do not know yet is whether DART’s impact slowed the asteroid more than NASA hoped. We can make some estimates by using the minimal eccentricity of Dimorphos’ orbit about Didymos — which is nearly zero, or circular — the decrease in its orbital period after impact, and NASA’s predictions. In the best-case likely scenario, the impact slowed Dimorphos by about 4 miles per day. In the worst-case likely scenario, it might have slowed it by only one mile per day. The middle-of-the-road estimate is somewhere around 2.5 miles per day. Values outside this are of course possible, and we don’t know enough yet to be sure. These numbers are in line with what NASA hoped for going in, although they are on the high end of expectations.
Re-running our simple calculations, in our best-case outcome, striking Dimorphos 2.5 years in advance would give just enough time to shift an impact trajectory to one that scraped the atmosphere but missed. In the worst case, we would need to impact it ten years ahead of time. How much protection does that afford us?
A nice start, but a long way to go
The Tunguska event was likely caused by a rock about 200 feet across. We can roughly estimate that it had less than 1/20 the mass of Dimorphos. The famous Barringer Crater in the Arizona desert, nearly 600 feet deep, was caused by a similarly sized rock of different composition. Either of these could have destroyed a city unlucky enough to sit at the impact point. Rocks of this size could be effectively deflected by DART less than a year before a predicted strike. The catch is that detecting them is very difficult. Ongoing efforts continue to discover new near-earth asteroids on a nearly daily basis. Objects like the Chelyabinsk meteor in 2013 are often detected only when the fireball lights up the sky.
Dimorphos was discovered in 2003. If it had been on a collision course for, say, 2030, we could probably prevent an impact. Didymos, about which it orbits, was discovered in 1996.
This is where the bad news starts.
Let’s say that Didymos was due to hit Earth. Its diameter is about 4.5 times that of Dimorphos. That means Didymos has something like 95 times more mass. In this case, DART would have to impact it somewhere between 190 and 950 years in advance to alter its course sufficiently. We only discovered it 26 years ago. If we needed to deflect it within a few years, we would have to hit it 100 times harder than DART hit Dimorphos. That means mass and velocity, multiplied together, must be on a scale 100 times greater than that of DART. This is possible, but it would stretch our current capabilities. We could add mass through a series of large launches, perhaps using Falcon Heavy launch vehicles. Additional engines could be launched to boost velocity. As for rocks larger than Didymos, they would pose a much greater problem.
The famous Chicxulub impactor that caused a mass extinction 66 million years ago was roughly 10,000 meters in diameter. The mass of this object — about 200,000-fold greater than Didymos — was significant enough that a spacecraft like DART would have to impact it millions of years in advance to avoid an extinction-level impact. No conceivable improved version of DART is capable of preventing such an event. For that, we would need far more powerful countermeasures. An investigation of this problem was carried out by scientists T.J. Ahrens and A.W. Harris, who analyzed the necessary countermeasures against asteroids of varying sizes. For small rocks, they conclude that a mission like DART, perhaps scaled up a bit, could be enough — as DART indeed just showed.
For rocks bigger than Didymos, but much smaller than Chicxulub, the impactor would need thousands of times more momentum. We cannot easily make a craft thousands of times faster or thousands of times more massive than DART with current rocketry technology. Nuclear weapons could impart enough push to do the job, with certain careful considerations about the composition of the asteroid, the location of the blast, how well the blast could be directed, the amount of ejecta blasted away, and so forth.
For giant objects like Chicxulub, the only presently feasible option is a truly enormous nuclear blast. A device of many megatons might give us a chance. Fortunately, we are likely to spot asteroids of this size far in advance.
DART was a useful first step toward asteroid deflection capability. However, Dimorphos is very small. We have a long way to go before we can defend against truly catastrophic collisions.
And also just to clarify, no, a standard Chicxulub-sized asteroid cannot "sneak up" on Earth. Because an object of that scale (10 to 15 kilometers wide) is incredibly massive and reflects a significant amount of light, automated planetary defense telescope systems have already mapped and tracked virtually 100% of all Near-Earth Asteroids (NEAs) of this size. Astronomers can calculate their orbits centuries into the future, and none are currently on a collision course with Earth. However, there are two extreme, highly unlikely astronomical scenarios where a planet-killer could catch us with limited warning:
The most significant limitation of Earth's current ground-based optical telescopes is the glare of the Sun. If a previously unknown, highly eccentric asteroid approached Earth directly out of the Sun's direction, it could remain hidden in the daylight sky until it was very close. Even so, because a 10 km object is so large, we would still likely spot it weeks or months in advance as it crossed the inner solar system, rather than discovering it only hours before impact.
The real "wild card" is not a local asteroid, but an object originating from the outermost edges of our solar system (the Oort Cloud) or from another star system entirely.
• Long-Period Comets: These giant chunks of ice and rock spend thousands of years in deep space where they are completely dark and invisible to us. If one shifted into an Earth-crossing orbit, it would travel at extreme speeds.
• Warning Time: We would only detect it once it grew close enough to the Sun to start heating up and forming a visible tail. For an object that massive, we might only get a few months to a couple of years of warning before impact—which, as established, is not enough time for humanity to mount a successful deflection mission.
In summary, the odds of an asteroid the size of the Chixculub impactor sneaking up on us are effectively zero. The odds of a giant, undetected comet appearing out of the deep outer solar system on a direct collision course are mathematically real, but extraordinarily rare, happening only once every tens or hundreds of millions of years.
The History Channel did a 10-part series called "Doomsday: 10 Ways the World Will End", most of it is available on YouTube for viewing. It interviews some of the world's best physicists, paleontologists, and experts and discusses the many threats to our existence, asteroids, Black holes, Gamma Ray bursts, Supervolcanoes, Nuclear War, rogue planets, and even our own sun. Being interested in space, science, physics, and almost everything about life in general, you can't help but wonder how it will all come to an end.
Gamma-ray bursts are quite insane to think about. A gamma-ray burst (GRB) cannot physically blow up or destroy the planet Earth, but a close enough hit could wipe out most life on the surface. A GRB is a hyper-energetic cosmic explosion from a collapsing massive star or merging neutron stars. It releases more energy in a few seconds than our Sun will produce in its entire lifetime. The energy shoots out in narrow, intense beams from the poles of the exploding or merging object. If a beam directly targeted Earth from within a few thousand light-years, the high-energy photons would tear apart nitrogen and oxygen molecules in the upper atmosphere. This destroys the ozone layer. With the ozone layer gone, intense ultraviolet (UV) radiation from the Sun would flood the surface. The ozone layer acts as a global shield by absorbing dangerous solar ultraviolet (UV-B and UV-C) rays. If a nearby gamma-ray burst (GRB) stripped away or severely thinned this layer, that barrier would vanish and our skin would be left completely exposed to the hellish wrath of the sun's radiation. This could also trigger massive ecological collapse, plant and marine die-offs, and a mass extinction event. Some scientists suggest a GRB may have contributed to a past extinction, such as the Ordovician mass extinction 450 million years ago. To cause a mass extinction, the GRB would need to occur relatively close within our own galaxy—typically estimated within a few hundred to a few thousand light-years. According to the European Space Agency, there are no known candidate stars capable of producing a GRB close enough to threaten Earth (with safe zones generally falling within 200 to several thousand light-years). Distant bursts are harmless. Billions of light-years away, GRBs like the record-setting "BOAT" burst observed by NASA/ESA only slightly ionize Earth's upper ionosphere without posing any danger to life on the ground.
Effect of a gamma ray burst on an Earth-like planet.