Asteroid impact avoidance facts for kids
Protecting Earth from Asteroids is about finding ways to stop space rocks, called near-Earth objects (NEOs), from crashing into our planet. A big collision could cause huge tsunamis or massive firestorms. It could also create an "impact winter," where dust and debris block sunlight, making Earth very cold. Scientists believe a crash 66 million years ago, involving an object about 10 kilometers (6 miles) wide, created the Chicxulub crater. This event led to the Cretaceous–Paleogene extinction event, which caused the disappearance of all non-bird dinosaurs.
While a major collision isn't likely to happen very soon, it's almost certain one will happen eventually. That is, unless we take steps to protect ourselves. Events like the Shoemaker-Levy 9 comet hitting Jupiter and the 2013 Chelyabinsk meteor have shown us these threats are real. More and more near-Earth objects are being found and tracked on the Sentry Risk Table. The movie Don't Look Up also helped people think more about avoiding NEOs. We've learned a lot in recent years, but there's still much to do to keep Earth safe.
In 2016, a NASA scientist warned that Earth wasn't ready for such an event. In 2018, the B612 Foundation, a group of scientists and astronauts, said it's "100 percent certain we'll be hit by a devastating asteroid." Also in 2018, famous scientist Stephen Hawking wrote in his last book that an asteroid collision was the biggest threat to our planet.
Scientists have described two main ways to avoid an asteroid impact. The first way is to change the asteroid's path so it misses Earth. The second way is to break the asteroid into smaller pieces. These smaller pieces would then either miss Earth or be small enough to burn up safely in our atmosphere.
However, in 2019, scientists reported that asteroids might be harder to break apart than once thought. An asteroid could even reassemble itself due to gravity after being broken up. In 2021, NASA astronomers said it could take 5 to 10 years to prepare for a mission to stop a potential impact. This was based on a practice exercise at the 2021 Planetary Defense Conference.
In 2022, NASA's DART spacecraft successfully hit Dimorphos. This impact shortened the asteroid moon's orbit by 32 minutes. This was the first successful mission to change an asteroid's path. China also plans to launch a mission in 2027 to deflect the near-Earth object 2015 XF261. Its impact is expected in April 2029.
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Ways to Change Paths
Experts told the United States Congress in 2013 that NASA would need at least five years to get ready for a mission to stop an asteroid. In 2018, the US National Science and Technology Council warned that the United States was not ready for an asteroid impact. They created a plan called the "National Near-Earth Object Preparedness Strategy Action Plan" to help prepare.
Most efforts to deflect a large space object need a warning time of a year to several decades. This allows enough time to plan and carry out a project to avoid a collision. Currently, no special planetary defense equipment has been built yet. Scientists estimate that even a very small change in an asteroid's speed can make it miss Earth, if done many years before a possible impact. For example, a tiny speed change of about 10-6 meters per second could have deflected 99942 Apophis years before its close approach.
NASA's DART mission is the world's first full-scale test to protect Earth from asteroids or comets. It launched on a SpaceX Falcon 9 rocket from California.
A 10-kilometer (6-mile) asteroid hitting Earth has historically caused events that led to the disappearance of many life forms. Comets entering our inner Solar System also pose a threat. A long-period comet would hit much faster than a near-Earth asteroid, causing more damage. Also, we might only have a few months' warning for a comet. Even objects as small as 50 meters (164 feet) can cause huge regional damage, like the Barringer crater.
Knowing what an object is made of helps decide the best strategy. Missions like the 2005 Deep Impact probe and the Rosetta spacecraft have given us important information. In 2022, a new method was suggested to map the inside of an asteroid. This would help find the best spot to hit it.
Finding Asteroids: A History
Early efforts to avoid asteroid impacts focused on searching the sky. In 1992, a NASA workshop looked at how to stop space objects that could hit Earth. A report in 1992 suggested a program called Spaceguard to find and track Earth-crossing asteroids. This program aimed to find 90% of objects larger than one kilometer (0.6 miles) within 25 years.
In 1998, NASA officially set a goal to find and list 90% of all near-Earth objects (NEOs) that are one kilometer or larger by 2008. These are objects that could pose a collision risk. The one-kilometer size was chosen because impacts from smaller objects might cause local damage but likely not a worldwide disaster. However, an object much larger than one kilometer could cause global damage and even threaten all life on Earth. NASA's commitment led to funding for many NEO search efforts. By 2008, great progress was made, but new discoveries of large NEOs after 2008 showed there were still many to find.
Congressman George Brown Jr. supported planetary defense projects. He said that if we could change an asteroid's path to stop it from hitting Earth, it would be "one of the most important accomplishments in all of human history." A law was even named after him: The George E. Brown Jr. Near-Earth Object Survey Act. This law, passed in 2005, asked NASA to find, track, and study near-Earth objects 140 meters (460 feet) or larger. The goal was to find 90% of these objects within 15 years.
Current Projects
The Minor Planet Center in Massachusetts has been listing the paths of asteroids and comets since 1947. Other surveys now specialize in finding near-Earth objects (NEOs). Many of these are funded by NASA's Spaceguard program. One well-known program is LINEAR, which started in 1996. By 2004, LINEAR was finding tens of thousands of objects each year. It used two one-meter telescopes and one half-meter telescope in New Mexico.
The Catalina Sky Survey (CSS) is another important program. It uses telescopes near Tucson, Arizona. Since 2005, CSS has found more NEOs and dangerous asteroids each year than LINEAR. In 2008, CSS found 564 NEOs.
Spacewatch uses a 90-centimeter (35-inch) telescope in Arizona. It was started in 1980 by Tom Gehrels and Robert S. McMillan. The project now uses a larger 1.8-meter (71-inch) telescope to search for NEOs.
Other programs that track near-Earth objects include Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), and Pan-STARRS.
The Asteroid Terrestrial-impact Last Alert System (ATLAS) constantly scans the sky. It aims to detect asteroids late in their orbit, when it's too late for deflection. However, it can still give enough warning for people to evacuate and prepare.
The European Union supports a project called NEOShield. This project studies ways to prevent NEOs from hitting Earth. It focuses on developing technology for guiding spacecraft near asteroids and understanding what NEOs are made of.
"Spaceguard" is a general name for these programs. A 2003 NASA study suggested spending money to find 90% of all near-Earth asteroids 140 meters (460 feet) and larger by 2028.
In 2013, the United Nations Committee on the Peaceful Uses of Outer Space approved steps to deal with asteroid impacts. This included creating an International Asteroid Warning Network (IAWN). This network shares information about dangerous asteroids. The Space Missions Planning Advisory Group (SMPAG) coordinates studies on deflection technologies. As of January 2024, 34,274 NEOs are known. About 2,395 of these are larger than 140 meters (460 feet) and come within 8 million kilometers (5 million miles) of Earth's orbit. However, only about 44% of these larger NEOs have been found so far.
NEODyS is an online database of known NEOs.
Sentinel Mission
The B612 Foundation is a private group in the United States. It works to protect Earth from asteroid impacts. Scientists, former astronauts, and engineers from places like NASA lead it.
This group has researched how to find NEOs that could hit Earth and how to change their paths. The foundation had planned to build a privately funded space telescope called Sentinel. It was meant to launch in 2017–2018 but was canceled in 2015. If launched, Sentinel would have helped find dangerous NEOs by listing 90% of those larger than 140 meters (460 feet).
Sentinel's data would have been shared with groups like NASA and the Minor Planet Center. The foundation also suggested using gravity tractors to deflect dangerous NEOs. This idea was partly created by the group's CEO, physicist and former NASA astronaut Ed Lu.
Future Projects
The Vera C. Rubin Observatory is expected to start a detailed sky survey in the fall of 2025.
Detecting from Space
In 2007, NASA officials suggested using the Wide-field Infrared Survey Explorer (WISE) to find NEOs. WISE surveyed the sky using infrared light, which helps detect asteroids that absorb sunlight. It was expected to find about 400 NEOs within its one-year mission.
NEOSSat, a small satellite launched by the Canadian Space Agency (CSA) in 2013, also searches for NEOs in space. In September 2013, NEOWISE, an extension of the WISE mission, continued to hunt for asteroids and comets close to Earth's orbit.
Deep Impact Mission
In 2009, scientists published research about how they identified an asteroid in space before it entered Earth's atmosphere. This allowed computers to predict where it came from and when and where its pieces would land on Earth. The four-meter (13-foot) asteroid, called 2008 TC3, was first seen by the Catalina Sky Survey telescope in October 2008. Predictions correctly showed it would hit 19 hours later in the Nubian Desert of northern Sudan.
Some potential threats have been identified, like 99942 Apophis. In 2004, it briefly had a 3% chance of hitting Earth in 2029. However, more observations later showed it would not hit.
Double Asteroid Redirection Test (DART)
On September 26, 2022, the DART spacecraft hit Dimorphos. This impact reduced the asteroid moon's orbital period by 32 minutes. This mission was the first successful attempt to change an asteroid's path.
China's 2019 VL5 Asteroid Deflection Mission
In 2025, China's CNSA plans to launch a mission to deflect the near-Earth object 2019 VL5, a 30-meter (98-foot) wide asteroid. The mission will use a Long March 3B rocket and carry both a spacecraft to hit the asteroid and one to observe it.
How Impact Probability Changes
The diagram shows how the predicted path of an asteroid changes as we get more information. At first, with only a few observations, the possible error in the asteroid's path is very large and includes Earth. As more observations are made, this error area shrinks. If Earth is still within this smaller error area, the predicted chance of impact goes up. This is because Earth now covers a larger part of the possible impact zone. Finally, even more observations (often using radar) make the error area shrink even more. This often shows that Earth is actually outside the path, and the chance of impact becomes almost zero.
For asteroids that are truly on a path to hit Earth, the predicted chance of impact keeps increasing as more observations are made. This similar pattern makes it hard to tell the difference between asteroids that will just pass close by and those that will actually hit. This also makes it hard to decide when to warn people. Getting more certainty takes time, which leaves less time to react. But warning people too soon could cause a false alarm and make people less likely to believe warnings later, like in the story of the Boy Who Cried Wolf.
Ways to Avoid a Collision
When choosing how to avoid a collision, scientists consider cost, risk of failure, how complex the method is, and how well it works. Methods can be grouped by how they work: by breaking the object apart (fragmentation) or by changing its path (delay). They also differ by the energy source used (like kinetic, gravity, or nuclear) and how they approach the object.
Strategies fall into two main types: breaking up the object (fragmentation) or changing its arrival time (delay). Fragmentation aims to make the asteroid harmless by breaking it into pieces that either miss Earth or are small enough to burn up in the atmosphere. Delay uses the fact that both Earth and the asteroid are moving in space. An impact happens when both reach the same point at the same time. Since Earth is about 12,750 kilometers (7,920 miles) wide and moves at about 30 kilometers per second (18.6 miles per second) in its orbit, it travels its own diameter in about seven minutes. So, delaying or speeding up the asteroid's arrival by just a few minutes can make it miss Earth.
Collision avoidance methods can also be direct or indirect. Direct methods, like using powerful blasts or hitting the asteroid, quickly change its path. These are often preferred because they are usually faster and less costly. They work for threats with short or long warning times and are best for solid objects. However, for asteroids that are loose piles of rocks, hitting them might just break them apart without changing their overall path enough. Indirect methods, like gravity tractors or attaching rockets, are much slower. They involve traveling to the object, matching its speed, and then slowly changing its path over a longer time.
Many NEOs are thought to be "flying rubble piles," which are only loosely held together by gravity. A typical spacecraft hitting such an asteroid might just break it up without changing its course enough. If an asteroid breaks into pieces, any piece larger than 35 meters (115 feet) across would not burn up in the atmosphere and could still hit Earth. Tracking thousands of small pieces would be very difficult. However, breaking up an asteroid is still better than doing nothing and letting a large one hit Earth.
Computer simulations in 2011–2012 showed that if enough energy is delivered quickly to a rubble pile asteroid, like from a special nuclear blast, the pieces would not come back together. Instead, they would fly apart and move away from an Earth-impact path.
Using Powerful Blasts
Using a nuclear explosive device near, on, or slightly under the surface of a threatening space object is a possible way to deflect it. The best height for the blast depends on what the object is made of and its size. The goal isn't to vaporize the whole object. If the object is a "rubble pile" (loose rocks), a blast from a distance can prevent it from breaking apart. The energy from the blast heats up the surface of the object, turning some of it into gas. This gas shoots away like a rocket exhaust, pushing the object in the opposite direction. This "nudge" can change the object's orbit enough to make it miss Earth.
A project called Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER) has been suggested. NASA's 2023 Planetary Defense Strategy and Action Plan says it's important to keep studying nuclear energy for deflecting or destroying asteroids. This is because it might be the only option if we don't have much warning time (only a few months or years). The report also notes that we need to research the rules and laws about using nuclear energy in space.
Blasts from a Distance
If a very large object is a loose pile of rocks, one idea is to detonate one or more nuclear devices a short distance (20 meters or more) from its surface. This way, the object won't break apart. If this is done far enough in advance, the force from enough nuclear blasts would change the object's path enough to avoid an impact. This has been shown in computer simulations and experiments.
In 1967, students at Massachusetts Institute of Technology (MIT) designed a mission to stop the asteroid 1566 Icarus from hitting Earth. This asteroid sometimes comes close to Earth. Their study, called Project Icarus, suggested using powerful nuclear devices carried by rockets. This study later inspired the 1979 film Meteor.
A NASA study in 2007 said that nuclear blasts from a distance are "10–100 times more effective" than non-nuclear methods. Other ways of using nuclear explosives on or under the surface might be more efficient, but they risk breaking the asteroid apart.
Research in 2021 showed that for a nuclear deflection mission to work well, we need a lot of warning time, ideally several years. More warning time means less energy is needed to change the asteroid's path. The study also said that deflecting an asteroid is safer than destroying it, as there's less chance of debris falling to Earth. Researchers suggested that adjusting the energy from the nuclear blast could be the best way to deflect an asteroid.
Blasts on or Under the Surface
In 2011, Dr. Bong Wie from Iowa State University began studying ways to deal with asteroids 50 to 500 meters (164-1,640 feet) wide when there's less than a year until impact. He concluded that only a nuclear explosion or a similar powerful event could provide the energy needed in such a short time.
This led to the idea of a conceptual Hypervelocity Asteroid Intercept Vehicle (HAIV). This vehicle would first use a kinetic impactor (like a spacecraft hitting it) to create a hole. Then, a nuclear device would detonated inside that hole. This would make the nuclear energy very efficient at pushing the asteroid. Another idea is to use a surface blast to create the first hole, then use that hole like a rocket nozzle for later nuclear blasts.
Dr. Wie claimed his computer models showed that a 300-meter (984-foot) wide asteroid could be destroyed with one HAIV if there was 30 days' warning. The models also showed that less than 0.1% of the asteroid's debris would reach Earth.
A 2020 study suggested that a nuclear blast near an asteroid's surface could be effective. The blast would cover one side of the asteroid with X-rays, pushing it in a chosen direction. The lead researcher, Dave Dearborn, said nuclear impacts offer more flexibility because the energy can be adjusted for the asteroid's size and location.
Stopping Comets Too
— Lord Byron
After the Shoemaker-Levy 9 comet hit Jupiter in 1994, Edward Teller suggested a very powerful nuclear device could be designed. This device, weighing about 25–30 tons, could be lifted by the Energia rocket. It could instantly vaporize a one-kilometer (0.6-mile) asteroid. It could also change the paths of very large asteroids (over 10 kilometers wide) with only a few months' warning. With a year's notice, it could even deal with rare comets from the Kuiper belt that pass Earth within two years.
In 2013, scientists from the US and Russia signed a deal to work together on asteroid defense.
What We Can Do Now
As of late 2022, the most likely and effective way to deflect an asteroid does not involve nuclear technology. Instead, it uses a kinetic impactor, which showed great promise in the NASA DART mission. For nuclear technology, simulations have been run to study using energy from a nuclear device to redirect an asteroid. These simulations look promising, but there hasn't been a real-world test yet.
Hitting it Hard: Kinetic Impact
Hitting an asteroid with a massive object, like a spacecraft, is another possible solution. If the asteroid is far from Earth, a spacecraft can be sent to collide with it, knocking it off course.
A NASA study in 2007 said that non-nuclear kinetic impactors are the "most mature approach." They could be used to deflect smaller, solid asteroids.
This method, used by DART and NASA's Deep Impact probe, involves launching a spacecraft into the near-Earth object. The asteroid's speed changes because of the law of conservation of momentum. This law states that when two objects collide, their total momentum stays the same.
Here, V1 is the spacecraft's speed, V2 is the asteroid's speed before impact, and V3 is the speed after impact. M1 and M2 are the masses of the spacecraft and the asteroid.
The European Union's NEOShield-2 Mission also studies the Kinetic Impactor method. The idea is that the asteroid is deflected after being hit by an impactor spacecraft. The impactor crashes into the NEO at a very high speed (10 kilometers per second or more). The impactor's momentum is transferred to the NEO, changing its speed and slightly altering its path.
The modified AIDA mission has been approved. NASA's Double Asteroid Redirection Test (DART) spacecraft launched in November 2021. Its goal was to hit Dimorphos, the 180-meter (590-foot) asteroid moon of 65803 Didymos. The impact happened in September 2022. Earth-based telescopes observed the event. The impact changed Dimorphos's orbital speed and period, showing that we can change the orbit of a small asteroid. This is important for future protection. The second part of the AIDA mission, the ESA HERA spacecraft, was approved in 2019. It will reach the Didymos system in 2026 to measure the mass of Dimorphos and the exact effect of the DART impact.
The Gravity Tractor
Another way to deflect an asteroid is to move it slowly over time. A small but constant push can add up to change an object's path enough. Edward T. Lu and Stanley G. Love suggested using a large uncrewed spacecraft hovering near an asteroid. The spacecraft would use its own gravity to gently pull the asteroid into a safer orbit. Even though both objects pull on each other, the spacecraft can use its engines (like an ion thruster) to stay in place. This way, the asteroid is slowly pulled towards the spacecraft, slightly changing its orbit. This method is slow, but it works no matter what the asteroid is made of or how fast it spins. It would likely need to stay next to the asteroid for several years to be effective.
A NASA study in 2007 said that "slow push" methods are the most expensive and less ready for use. They would also be limited unless missions could last many years or decades.
Using Ion Beams
Another way to deflect an asteroid without touching it is to use a low-power ion thruster. This thruster would be on a spacecraft hovering near the asteroid. The ions hitting the asteroid's surface would create a small but steady force. This force could deflect the asteroid in a similar way to the gravity tractor, but with a lighter spacecraft.
Using Sunlight
Scientists H. J. Melosh and I. V. Nemchinov suggested deflecting an asteroid or comet by focusing sunlight onto its surface. This would heat up the surface, causing material to vaporize and create a push. This method would need a space station with large, curved mirrors, like those used in solar furnaces.
Using highly focused sunlight could deflect a global-threatening object within a year, even with short warning. This could be useful if a dangerous asteroid is found late. V.P. Vasylyev proposed a "ring-array concentrator" design for the mirrors. This design would avoid the mirrors being shadowed by the asteroid. If the sunlight is concentrated about 5,000 times, it could create a strong enough push to deflect a 0.5-kilometer (0.3-mile) asteroid in several months. For larger NEOs (1.3 to 2.2 kilometers wide), the mirrors would need to be about the same size as the asteroid. If there's more warning time, smaller mirrors could be used.
The Mass Driver
A mass driver is a system that could be placed on an asteroid to throw material into space. This would give the asteroid a slow, steady push and reduce its mass. A mass driver uses the asteroid's own material as fuel to change its path.
The Modular Asteroid Deflection Mission Ejector Node (MADMEN) is an idea for small, unmanned vehicles (like space rovers) to land on an asteroid. They would drill and break up small rocks, which would then be ejected very fast. Because there are no forces acting on the asteroid these rocks will push the asteroid off course at a very slow rate. This process takes time but could be very effective if implemented correctly.
Using Rocket Engines
Attaching any spacecraft propulsion device, like a rocket engine, to an asteroid would also give it a push. This could force the asteroid onto a path that takes it away from Earth. These rocket engines would likely be very efficient electric engines, such as ion thrusters.
Laser Blasts
Similar to nuclear devices, it might be possible to focus enough laser energy on an asteroid's surface. This would cause the surface material to vaporize, creating a push or removing mass from the asteroid. This idea, called asteroid laser ablation, was discussed in the 1990s. Such systems would need a lot of power, possibly from a Space-Based Solar Power Satellite.
One proposal is the DE-STAR project. This idea from the University of California, Santa Barbara, suggests a modular laser array powered by solar energy. The plan is for the array to eventually be about one kilometer (0.6 miles) square. It could be launched in pieces and assembled in space. Even a small version could deal with smaller targets, help solar sail probes, and clean up space debris.
Other Ideas
- Wrapping the asteroid in a sheet of special reflective plastic, like a solar sail. This would use sunlight to push the asteroid.
- "Painting" the asteroid with white titanium dioxide to increase how much sunlight it reflects, or with black soot to change its path using the Yarkovsky effect. This effect describes how sunlight can create a tiny push on a rotating asteroid.
- Planetary scientist Eugene Shoemaker suggested releasing a cloud of steam in the asteroid's path to gently slow it down.
Important Things to Consider
Carl Sagan, in his book Pale Blue Dot, worried about deflection technology. He noted that any method strong enough to push asteroids away from Earth could also be used to push harmless objects towards our planet. He felt that Earth might be at greater risk from a human-caused impact than a natural one if this technology was misused. Sagan suggested that deflection technology should only be developed when there's a real emergency.
All methods that deliver low energy to an asteroid have very precise control. This means it's possible to add just the right amount of energy to steer an asteroid that was originally going to pass close by, towards a specific target on Earth.
According to former NASA astronaut Rusty Schweickart, the gravity tractor method is a bit controversial. When it changes an asteroid's path, the point on Earth where it might hit would slowly shift across different countries. So, the threat to the whole planet would be reduced, but at the cost of some specific countries' safety. Schweickart believes choosing how to "drag" the asteroid would be a difficult diplomatic decision.
There are also legal concerns about launching nuclear technology into space. In 1992, the United Nations passed a resolution with strict rules about sending nuclear technology to space. These rules aim to prevent space contamination and protect people on Earth from potential fallout. As of 2022, the UN is still looking at the safety and legal issues of using nuclear power in space, especially as more private groups get involved in space travel. The UN Committee on Peaceful Uses of Outer Space stressed that it's up to member countries to ensure everyone's safety regarding nuclear power in space.
Timeline of Planetary Defense
- In their 1964 book, Islands in Space, Dandridge M. Cole and Donald W. Cox talked about the dangers of planetoid impacts. They argued for listing these objects and developing ways to land on, deflect, or even capture them.
- In 1967, students at MIT did a study called "Project Icarus." It looked at a mission to stop the asteroid 1566 Icarus from hitting Earth. This study brought asteroid impact into the public eye for the first time.
- In the 1980s, NASA studied past impacts on Earth and the risks of future ones. This led to a program that maps objects in our Solar System that cross Earth's orbit and are large enough to cause serious damage.
- In the 1990s, the US Congress held hearings about these risks. This led to an annual budget for programs like Spaceguard and the near-Earth object program, managed by NASA and the USAF.
- In 2005, a group of astronauts called the Association of Space Explorers asked for a united effort to protect Earth from cosmic collisions.
- In 2007, it was estimated that about 20,000 objects 140 meters (460 feet) or larger could cross Earth's orbit. On average, one of these might hit Earth every 5,000 years. By 2008, 90% of objects one kilometer (0.6 miles) or larger were expected to be found and watched. The goal to find all objects 140 meters or larger was expected around 2020. By April 2018, over 8,000 near-Earth asteroids at least 460 feet (140 meters) wide had been spotted, but about 17,000 more were still hidden. By 2019, over 19,000 near-Earth asteroids of all sizes had been found, with about 30 new ones discovered each week.
- The Catalina Sky Survey (CSS) is one of NASA's funded surveys. In 2007, CSS found an asteroid called 2007 WD5. It was first thought to have a chance of hitting Mars, but later observations showed it would miss.
- In January 2012, after object 2012 BX34 passed close by, a paper discussed the "NEOShield" project. This project involves researchers from many countries working together.
- In November 2021, NASA launched the Double Asteroid Redirection Test (DART) mission. This mission aimed to crash a small spacecraft into an asteroid to deflect it, rather than destroy it.
- In January 2022, the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) became the first system to search the entire dark sky every 24 hours for near-Earth objects. ATLAS now has four telescopes, including two new ones in South Africa and Chile.
- As of March 1, 2023, NASA confirmed that DART worked. It successfully hit and redirected an asteroid moving at high speeds. This showed that we can move an asteroid up to half a mile (0.8 kilometers) wide without using nuclear means.
Images for kids
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This image shows a secondary fireball created during a nuclear test in 1954. Scientists used special tubes to study the energy released by these early nuclear devices.
See also
