On April 13, 2029, an icy chunk of space rock larger than the Eiffel Tower, will barrel past Earth at 67,108 mph (108,000 kmh) grazing the planet’s sphere of geostationary satellites. This will be the closest approach by one of the largest asteroids crossing Earth’s orbit in the next decade.
Observations of the asteroid, known as 99942 Apophis, for the Egyptian god of chaos, once suggested that its 2029 flyby would take it through a gravitational keyhole — a location in Earth’s gravity field that would tug the asteroid’s trajectory such that on its next flyby, in the year 2036, it would likely make a devastating impact.
More recent observations have fortunately confirmed the asteroid will safely sling by Earth without incident in both 2029 and 2036.
Nevertheless, most scientists believe it is never too early to consider strategies for deflecting an asteroid if one were ever on a crash course with Earth.
Now MIT researchers have created a doomsday computer program for deciding which type of mission would be most successful in deflecting an incoming asteroid.
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The scientists’ decision method takes into account an asteroid’s mass and momentum, its proximity to a gravitational keyhole, and the amount of warning time that scientists have of an impending collision.
All three factors have degrees of uncertainty, which the researchers also factored in to identify the most successful mission for a given asteroid.
The researchers applied their method to Apophis, and Bennu, another near-Earth asteroid which is the target of OSIRIS-REx, an operational NASA mission that plans to return a sample of Bennu’s surface material to Earth in 2023.
REXIS, an instrument designed and built by students at MIT, is also part of this mission and its task is to characterise the abundance of chemical elements at the surface.
The researchers have use their decision map to lay out the type of mission that would likely have the most success in deflecting Apophis and Bennu, in various scenarios in which the asteroids may be headed toward a gravitational keyhole.
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They say the method could be used to design the optimal mission configuration and campaign to deflect a potentially hazardous near-Earth asteroid.
Dr Sung Wook Paek, lead author of the study, said: “People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth.
“I’m interested in preventing keyhole passage well before Earth impact. It’s like a preemptive strike, with less mess.”
NASA concluded in a 2007 report submitted to the US Congress that in the event that an asteroid were headed toward Earth, the most effective way to deflect it would be to launch a nuclear bomb into space.
The force of its detonation would blast the asteroid away, though the planet would then have to contend with any nuclear fallout.
The use of nuclear weapons to mitigate asteroid impacts remains a controversial issue in the planetary defence community.
The second best option was to send up a “kinetic impactor” — a spacecraft, rocket, or other projectile that, if aimed at just the right direction, with adequate speed, should collide with the asteroid, transfer some fraction of its momentum, and veer it off course.
Dr Paek added: “The basic physics principle is sort of like playing billiards.”
For any kinetic impactor to be successful, however, de Weck, a professor of aeronautics and astronautics and engineering systems, says the properties of the asteroid, such as its mass, momentum, trajectory, and surface composition must be known “as precisely as possible.”
In designing a deflection mission, scientists must consequently take uncertainty into account.