-Lawrence Livermore National Laboratory (LLNL) scientists are part of a national planetary defense team that designed a conceptual spacecraft to deflect Earth-bound asteroids and evaluated whether it would be able to nudge a massive asteroid – which has a chance of hitting Earth in 2135 – off course. The design and case study are outlined in a paper published recently in Acta Astronautica.
The 9 meter-tall, 8.8-ton spacecraft – dubbed HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle) – features a modular design that would enable it to serve as either a kinetic impactor, essentially a battering ram, or as a transport vehicle for a nuclear device. Its possible mission: Deflect 101955 Bennu, a massive asteroid around 500 meters (more than 5 football fields) in diameter, weighing around 79 billion kilograms (1,664 times as heavy as the Titanic), circling the sun at around 63,000 mph. Based on observation data available, Bennu has a 1 in 270-chance of striking Earth on Sept. 25, 2135, and it is estimated that the kinetic energy of this impact would be equivalent to 1,200 megatons (80,000 times the energy of the Hiroshima bomb).
The researchers evaluated a number of deflection scenarios in this study, ranging from launching 10 years before impact to 25 years before. In the 10-year scenarios, it was determined that it could take between 34 and 53 launches of the Delta IV Heavy rocket, each carrying a single HAMMER impactor, to make a Bennu-class asteroid miss the Earth. If there were 25 years lead time, that number could be reduced to 7 to 11 launches. The exact number would depend on the desired Earth-miss-distance and the impact conditions at the asteroid.
"When many launches are required for a successful deflection, the mission success becomes more difficult, due to the failure rate associated with each individual launch," said Megan Bruck Syal, LLNL physicist and coauthor on the paper. "If we only had ten years from launch, we would need to hit Bennu with hundreds of tons just to barely deflect it off of an Earth-impacting path, requiring dozens of successful launches and impact at the asteroid."
Just how large an asteroid could a single impactor deflect? Researchers determined that a single HAMMER impactor could deflect an object 90 meters in diameter by around 1.4 Earth radii with 10 years of lead time – from the time of launch to anticipated Earth impact. If they needed less of a deflection, around a quarter of an Earth radii, a single impactor could be effective on an object as large as 152 meters in diameter in this same scenario.
The paper ultimately concluded that using a single HAMMER spacecraft as a battering ram would prove inadequate for deflecting an object like Bennu. While recent simulations of nuclear deflection scenarios are not included in this paper – they will be included in a companion paper to be submitted for publication in the near future – the findings suggest that the nuclear option may be required with larger objects like Bennu. The nuclear approach carries the potential to deposit much more energy into an object like Bennu, causing increased change in speed and trajectory.
Unlike popular portrayals of a nuclear deflection mission – like the movie Armageddon – the nuclear deflection approach would consist of detonating a nuclear explosive some distance from the asteroid. This would flood one side of the asteroid with X-rays, vaporizing the surface, which would create propulsion as vaporized material is ejected from the object. Unlike a kinetic impactor, the amount of energy deposited into an asteroid with a nuclear device could be tuned by adjusting how far it is from the asteroid when detonated.
Because Bennu regularly passes close enough to Earth for radar observations, researchers are able to estimate its orbit with enough accuracy to give a few decades warning, if it's bound to impact the Earth. This near-Earth Bennu fly-by happens every six years
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