The battle between life on Earth, and asteroids so far has been largely one-sided. But that is not to be. Soon we will be able to stop asteroids from causing trouble with Earth. Although conventional wisdom has taught us that intercepting an asteroids is easier if they are farther away, it doesn't mean we have sufficient advance notice.
New research suggests that we may be able safely to destroy potentially dangerous rocky interlopers even if they are closer to Earth than we would like.
The dilemma of asteroids is a problem facing humanity. Although we have identified many potentially dangerous asteroids, not all, particularly smaller ones, has been done so. There are many small asteroids that go undiscovered and can cause serious damage. Asteroids can cause damage not only due to their size but also because of their velocity and angle of impact. Purdue University's asteroid impact simulator is available. A typical asteroid of the size of a football can wipe out New York City.
NASA and other agencies are concerned about Near-Earth Objects and Potentially Hazardous Objects. NASA was mandated by the US Congress to catalog 90% of NEOs larger than 140m (460ft) in size. NASA will launch the NEO Surveyor mission in 2026 to search for more asteroids within our area. It is unlikely that we will ever be able to see all the potential asteroids that could cause us harm. There are many surprises in the Universe.
Artist's conception of an NEO asteroid orbiting around the Sun. Credit: NASA/JPL
A non-explosive, kinetic impactor is the best way to deal with an asteroid heading for Earth. To do this, we must have advance warning of asteroids approaching. We can launch a low mass impactor if we know for decades that an asteroid will be hitting Earth. What if an asteroid is headed straight for Earth, and we don't have enough time to prepare? Is it possible to wait less than one year before impact?
If we spot a dangerous object that is likely to strike the Earth, it would be too late to divert it safely. Our best option would be to smash it up so much the fragments would miss the Earth.
We will have to blow it up as best we can, and pray that no fragments hit Earth.
It is risky to blow up an asteroid as it approaches Earth. It is possible for the asteroid to split into dangerous fragments. There are also technical risks. There are risks involved in attaching an explosive nuclear device on a rocket and then launching it into orbit.
The issue has been addressed by a team of researchers. The study is entitled Late-time small bodies disruptions for planetary defense and was published in Acta Astronautica. Patrick King, Johns Hopkins University Applied Physics Laboratory is the lead author. The study uses the term "late-time" to mean less than one year after impact.
In some ways, it might not be difficult to blow up an asteroid. The authors wanted to examine what happens when an asteroid is destroyed. What happens to all the pieces? We are focusing on tracking the orbits of all the fragments after a hazardous body is disrupted on an Earth-impact trajectory. If they do, we will estimate the magnitude of the consequences. This is especially important because many asteroids are rubble-pile and not tightly held together.
The study showed a 100-meter-long asteroid approaching Earth, and then being disintegrated with a one-megaton explosive device. Although the explosive device would not actually hit the asteroid it would detonate a few meters higher than the surface. This type of detonation doesn't cause the asteroid to disappear, it just fragments it into smaller pieces. If all goes well, this should make it less dangerous.
Because Bennu is so well-studied, the asteroid was used in these simulations. This image shows Bennu from a distance 24 km (15 miles) captured by the PolyCam OSIRIS-REx. This mosaic is made up of 12 images. Image Credit: NASA/Goddard/University of Arizona.
Five different impact scenarios were also created using five real-world asteroids. Because both Apophis (and Bennu) are well-studied, they chose Apophis. Because it is in a retrograde orbit, has a high relative velocity and passes within 0.5 AU from Earth, 343158 Marsyas was chosen by the team. This makes it an unusual and extreme scenario. Because of its extremely inclined orbit and the fact that it was within 0.08 AU in 1973, 5496 1973 NA (a minor world) was chosen. They also used PDC 2019 (a hypothetical asteroid) in the 2019 Planetary Defence Conference.
The study's table below outlines the conditions that each scenario requires. Image Credit: King et al 2021.
The simulation results are promising.
Lead author King stated in a press release that one of the difficulties in assessing disruption is the need to model all fragment orbits. This is usually much more difficult than modeling a simple deflection. These challenges must be overcome if disruption is to be considered as a strategy.
Five scenarios were used to simulate the 100-meter-wide asteroid. The disrupting device was activated when the asteroid was approximately two months from Earth in all five scenarios. In all five scenarios, 99.9% missed Earth.
Spheral's hydro simulation provided the basis for this analysis: 1 Megaton at just a few meters from an asteroid measuring 100 meters in diameter (with Bennu form). Colours denote velocities. The legend is cm/us which is equal to 10 km/s. Image Credit: King et al 2021
However, this study does not change the fact that kinetic impact is preferred for asteroid deflection. The farther away an asteroid is detected, the better. It is sufficient to have a smaller kinetic impactor. This study is for late-time small bodies. If we don't have enough lead time, there is not enough time to launch a large-mass kinetic impactor.
It does however show that nuclear weapons can be used in the fight against asteroids.
He said that we focused on late disruptions. This means that the impacting body has been disintegrated shortly before it impacts. It is preferred to use kinetic impactors to deflect the impacting bodies when you have a lot of time, which can often be a decade long.
This study focuses on late disruptions. These are situations in which we don't have enough time to send an impactedor. Understanding the consequences of shattering an approaching asteroid into pieces is crucial. Michael Owen, a co-author of the study, wrote Spheral software to help them understand this. Spheral was capable of modeling not only the nuclear disintegration of the asteroid, but also the resulting fragments.
This video shows the hydro simulation that was performed in Spheral, which provided the basis for this work. Credit: King et al 2021.
The fragments became subject to gravity, as well as the gravity of the Earth and Moon. The team discovered that the fragments had formed a coherent stream of spacetime.
Owen stated that if we spot a dangerous object that is likely to strike Earth too late to divert it safely, the best option would be to smash it into small pieces so it would miss Earth. Although this is a complex orbital question, it's possible to break down an asteroid into smaller pieces. Each fragment will follow its own path around the sun and interact with the other planets. The cloud will form a curving stream of fragments that follows the original path taken by the asteroid. The speed at which these pieces spread (along with the time it takes for the cloud to cross Earth's path) will tell us how many will hit the Earth.
NASA is testing the kinetic impactor method for asteroid protection using its Double Asteroid Redirection Test mission (DART). It is expected to launch on November 24, 2021.
The double in the name refers the Didymos double asteroid. Didymos measures approximately 780 meters in size, but it also has a companion called Didymoon. Didymoon, which is only 160 meters in diameter, is DART's target. Didymoon is the most dangerous asteroids. The kinetic impactor is actually the DART spacecraft. It will collide with Didymoon while sensitive telescopes monitor and record what happens.
Artists impression showing NASA's Double Asteroid Redirection Test spacecraft speeding towards the smaller of two bodies in the Didymos Asteroid System. Credit: NASA/Johns Hopkins University Applied Physics Laboratory
However, there is no way to test the nuclear detonator on an asteroidal object. One day, perhaps we will be able to test it on an asteroid near another planet. This is what sci-fi enthusiasts can see. However, it is not a good idea to test an asteroid in real life. There are many possible problems. It would be unacceptable.
We will continue to use simulations such as the one in this study for now. Future studies will build on it and refine it.
Bruck Syal, co-author, stated that our group is continually improving its modelling methods for nuclear disruption and deflection. This includes ongoing improvements in X-ray energy-deposition modelling which sets the shock and initial blowoff conditions for a nuclear disruption issue. This paper shows how modern multiphysics tools can be used for simulating this problem across multiple relevant physics regimes.
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