NASA has unveiled plans to divert or destroy an asteroid calculated to come disturbingly close to Earth in 2135 using nuclear weapons.
The proposal, published in the journal Acta Astronautica, says that a series of eight-tonne rockets carrying nuclear warheads launched by NASA should be capable of saving humanity from being destroyed by the incoming asteroid named Bennu.
Astronomers have calculated Bennu has a 1-in-2700 chance of striking Earth on September 25, 2135. While those are long odds, in astronomical term they are disturbingly small.
And if Bennu does strike Earth, it will unleash kinetic energy equivalent to that of 1450 megatons — that’s more than 80,000 times more powerful than the Hiroshima bomb.
It would blast a crater some 3km wide into the crust of our planet. Fires would rage out of control over an enormous area. Debris would circle the Earth. Dust and soot would fill the skies.
NASA says humanity may survive — but the fate of civilisation is another matter.
In 2010, NASA astronomers calculating the orbit of Bennu came up with some worrying numbers.
It was big.
It was fast.
It was on a track that would ultimately bring it close to Earth.
While 101955 Bennu is one of the biggest asteroids to ever come our way, it’s odds aren’t among the most disturbing.
NASA’s Centre for Near-Earth Object [NEO] Studies has a hit-list of 73 asteroids which have a 1 in 1600 chance of hitting the Earth.
That includes asteroid 410777. Discovered in 2009, it’s somewhat smaller than Bennu.
“Bennu was selected for our case study in part because it is the best-studied of the known NEOs,” the researchers write. “It is also the destination of NASA’s OSIRIS-Rex sample-return mission, which is, at the time of this writing, en route …”
The Osiris-Rex space probe is expected to arrive in August.
Its objective is to spend two years analysing what the asteroid is made of, even going so far as to bring a sample back to Earth.
This is important as its composition will determine while kind of countermeasures would be most effective.
If it’s a loosely bundled collection of big rocks or even a giant bundle of powder, blasting it apart may actually increase its threat. Bennu is moving at 101,000km/h. Its fragments would continue to hurtle on — possibly increasing the chance of catching Earth with a ‘shotgun’ effect.
If it’s a seemingly solid mass, it will need to be punched — hard.
But there’s also real science to be learnt from the enormous mass of leftovers from the birth of our Solar System. Does it contain the very building blocks of life itself?
NASA and the National Nuclear Security Administration says its HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response) project could save life on Earth as we know it.
Lawrence Livermore National Laboratory (LLNL) researchers have been busily calculating what it would take to budge the 79 billion kilogram lump of rock and ice.
The answer: it depends.
“The push you need to give it is very small if you deflect the asteroid 50 years out,” planetary defence team physicist Kirsten Howley says. “Delay is the greatest enemy of any asteroid deflection mission.”
Their proposal is to build nine-metre long projectiles weighing some 8.8 tonnes.
A single HAMMER impactor could deflect an object 90 metres in diameter by around 1.4 Earth radii with 10 years of lead time — from the time of launch to anticipated Earth impact.
The cost for each of these warhead/Delta Heavy IV missiles has not been calculated. But sending the single OSIRIS-Rex probe to Bennu was in the $A1 billion ballpark.
It is calculated it would take some 7.4 years to build, launch and fly such a missile before it struck its target.
And the longer we wait, the more warheads we will need.
The impactor design is a modular one. The missile can be tailored to carry a different warhead depending on the composition of the asteroid it was targeted at.
The idea is to put a series of warheads in an orbit just ahead of Bennu. The asteroid would then crash into them at speeds greater than 35,000km/h. The kinetic energy (battering ram effect) of such an impact alone is enormous.
But, if necessary, it could also be used to trigger nuclear bombs.
These would not strike the asteroid. They would be set off above its surface, vaporising the material there. This ejecta would thrust the asteroid in the opposite direction.
But even the largest warheads such missiles could carry would need to strike asteroid Bennu decades before it was due to pass Earth. That way, even a small nudge could accumulate to a huge safety gap at the critical time.
“Whenever practical, the kinetic impactor is the preferred approach, but various factors, such as large uncertainties or short available response time, reduce the kinetic impactor’s suitability and, ultimately, eliminate its sufficiency,” the researchers write.
At this point, experts are simply unsure about Bennu’s ultimate course.
“The probability of a Bennu impact may be 1 in 2700 today, but that will almost certainly change — for better or worse — as we gather more data about its orbit,” Howley says.
So putting the issue off could seal Earth’s fate.
Researchers say that any attempt to divert Bennu in 2125 — that’s just 10 years out from impact — would have to involve up to 53 HAMMER warheads.
“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,” says co-author physicist Megan Bruck Syal.
”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.”
An attempt 25 years out would cut the number of warheads needed to between 7 and 11.
NASA is not the only agency working on the problem.
Moscow’s Institute of Physics and Technology (MIPT) earlier this week revealed it had been blowing up artificial asteroids in the lab to determine what it would take to divert different types of asteroids.
Using lasers to simulate nuclear blasts, they believe a 200m asteroid would need a three megaton blast, roughly the equivalent of 20 Hiroshima bombs.
But they’re not worried about Bennu.
“At the moment, there are no asteroid threats, so our team has the time to perfect this technique for use later in preventing a planetary disaster,” study co-author Vladimir Yufa wrote.
“We’re also looking into the possibility of deflecting an asteroid without destroying it and hope for international engagement.”
Not all scientists like the idea of smashing a large asteroid into smaller pieces.
Researchers associated with the NASA study have raised concerns.
They highlight the chances of a single HAMMER warhead being sufficient to divert Bennu are very low. And even radioactive dust showers could have “dire consequences” for Earth below.
The preferred option is to simply ‘bump’ Bennu out of the way, without enough force to break it apart. But such gentle ‘bumps’ have a cut-off point. If we leave it too late, we will need to shatter the asteroid.
“The push you need to give it is very small if you deflect the asteroid 50 years out,” Howley says. But she also adds its much more difficult to hit it when still so far.
Other proposals — such as coating one side of an asteroid with a reflective material to act like a ‘sail’ in the solar winds — need decades to both prepare and produce any noticeable effect.
So, conceding the likelihood that only nuclear weapons have the power to shift Bennu, the researchers urge a need for speed.
“The study aims to help us shorten the response timeline when we do see a clear and present danger so we can have more options to deflect it,” Howley says.
“The ultimate goal is to be ready to protect life on Earth.”
A series of options for asteroid-diverting systems will be presented at the 9th Workshop on Catastrophic Disruption in the Solar System (CD9) being held in Japan in May.
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