Comet 67P/Churyumov–Gerasimenko from 14 miles up as seen by the ESA Rosetta spacecraft on September 29, 2016—the day before the spacecraft was deliberately crashed into the comet. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Anywhere from 60 to 100 tons of material falls to Earth every day. Most of it is in the form of dust and grain-sized particles and is harmless, but it's a reminder that a lot of stuff is out there. The weathering on the International Space Station provides startling evidence of that.
So what do we do if a not-so-harmless object is hurtling towards us?
Although a doomsday asteroid is a frightening prospect, don’t worry—NASA has a plan. The agency actively monitors space for dangerous objects and has conducted research into the best way to repel or destroy a space invader. Today, it is actively developing missions to do just that, and even has a department to deal with the problem: the Planetary Defense Coordination Office. But just how fast could the agency deal with an actual catastrophe? Here’s an inside look into NASA’s emergency planning system.
FIRST WE FIND IT.
NASA has several ongoing projects to survey the solar system for new celestial objects. In 2009, the agency launched an infrared telescope called the Wide-field Infrared Survey Explorer (WISE). Its mission, run by NASA's astrophysics division, was to create an infrared map of the entire sky. After the completion of its primary mission, NASA's planetary science directorate asked to extend the life of the spacecraft, re-purposing it as an asteroid hunter in 2013. NEOWISE was born. Over the course of its life, what the spacecraft has found is terrifying―hundreds of new near-Earth objects, and scores of potentially hazardous ones. In other words, the solar system is a lot scarier than we thought. Here on Earth, there are several observatories that work together with a goal of discovering, tracking, and characterizing this population of renegade asteroids and comets.
A small body called TB145―the "Great Pumpkin asteroid"―exemplifies how the discovery of a potentially hazardous object works in practice. On October 10, 2015, the Panoramic Survey Telescope and Rapid Response System (PAN-STARRS) in Hawaii spotted an object approximately 600 meters across that was speeding perilously toward Earth. The Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia imaged it, and the Goldstone Deep Space Network telescope also took radar images. The Infrared Telescope Facility in Hawaii provided spectrometry. In a very short amount of time, scientists knew a lot about this scary new cosmic neighbor. The object was soon identified as the dead nucleus of a comet, its volatiles having been burned away. Moreover, scientists identified boulders several meters in size sitting on the object's surface. Those boulders matter because they can help steer the object away from Earth. We weren't in danger from it; its trajectory was well understood, and even at its closest pass, it was 300,000 miles away from the Earth.
THEN WE TRY TO MOVE IT.
Two of the rapidly maturing projects of the still very nascent asteroid deflection program are the Asteroid Impact Deflection Assessment and the Asteroid Redirect Mission. These programs use two different techniques to attempt to change the orbit of space objects, kinetic deflection, and enhanced gravity tractoring.
The Asteroid Impact & Deflection Assessment is a collaboration between NASA and the European Space Agency. It recently completed its concept study phase and has moved into design. The goal is to build a rendezvous spacecraft called the Asteroid Impact Monitor (AIM) that would fly to an asteroid called Didymos, which is easily reached from Earth but does not cross our orbital path. (In other words, if something goes terribly wrong with this experiment, we don't risk creating the potentially hazardous object we want to deflect.) Didymos is about a half-mile in diameter, and even has its own small moon, informally called Didymoon. Then NASA will launch a spacecraft called the Double Asteroid Redirection Test (DART). DART is a "kinetic impactor": It will plow into Didymoon and demonstrate how much energy can be imparted, and how much it changes the moon's orbital period. The hope is to test the effectiveness of a technique called "kinetic deflection," which would enable scientists to redirect an asteroid were it on an impact trajectory with Earth (provided they discovered the asteroid quickly enough).
Another such project in development is the Asteroid Redirect Mission, run by NASA's Human Exploration and Operations directorate. That mission is an element of NASA's "journey to Mars," and will further the development of solar electric propulsion, a technology designed to push large masses around the inner solar system—things like Mars habitat modules and cargo and, as a bonus, asteroids.
The asteroid redirect vehicle demonstrates the “gravity tractor” planetary defense technique on a hazardous-size asteroid. The gravity tractor method leverages the mass of the spacecraft to impart a gravitational force on the asteroid, slowly altering the asteroid’s trajectory. The demonstration is conducted after capturing the boulder and is referred to as the “enhanced gravity tractor” because the additional mass of the boulder enhances the force that can be transmitted to the asteroid. Image Credit: NASA
In fact, the near-Earth object observation program of the Planetary Defense Coordination Office helped identify places to test out the Asteroid Redirect Mission’s capabilities. When it launches, a robotic spacecraft will fly to asteroid 2008 EV5, a potentially hazardous object close to Earth that has been tentatively selected as the mission's target. The spacecraft will approach the asteroid's surface and survey it for boulders. Once scientists identify a suitable boulder, the robot will touch down on the surface using long landing legs, and then deploy grappling arms to grab hold of the boulder. With the boulder firmly in hand, the spacecraft will lift off from the asteroid surface.
Before flying back to Earth's orbit with the asteroid (for astronauts to study safely once it’s in a new, safe, lunar orbit), the spacecraft will first perform an "enhanced gravity tractor" maneuver—another kind of asteroid redirection. By flying near one side of the asteroid, the mass of the spacecraft and the tens-of-tons boulder will use gravity to gently and gradually alter the trajectory of the asteroid.
AND IF THAT DOESN'T WORK, WE BLOW IT UP.
In a pinch, there's the nuclear option [PDF]. If scientists discover an asteroid on an impact course with Earth and find that there's no time to build a spacecraft, study the object, and adjust its course with "slow push deflection/migration" techniques such as the gravity tractor, they can crack their knuckles and resort to "impulsive migration" techniques. The beauty of using a nuclear device on an asteroid is that you don't need to know much about the asteroid in advance. In a time-sensitive situation, this is your go-to option, and there are four ways of deploying it.
A standoff nuclear detonation involves a flyby of a hazardous object and using a proximity sensor to detonate a nuclear device. The explosion would push the asteroid off course. This technique is orders of magnitude less effective than plowing the nuke into the asteroid and pressing the red button, but it has the advantage of not fragmenting the asteroid. Fragments are bad. Remember the meteorite explosion over Chelyabinsk, Russia?
That rock was a dinky 20 meters in diameter. If we created a sustained bombardment of such asteroid fragments, we would be in for a pretty bad time.
The standoff technique also allows for a progressive adjustment of an asteroid's course. We wouldn't be limited to launching a single nuke; we would launch several. (It's not like we're running low on nuclear weapons.) Rather than correct the asteroid's course in a single dramatic blast, we could more precisely adjust its course with a series of detonations.
Other nuclear use tactics are surface, subsurface, and delayed. A nuclear surface is like dropping a nuke on the asteroid. When it touches the asteroid's surface, it detonates. Subsurface is like the DART half of the Asteroid Impact Deflection Assessment mission―the impactor drives a nuclear explosive deep into the asteroid, and it detonates. A delayed nuclear technique is just that: The nuke is landed on the asteroid and waits for scientists to detonate it when the time is right.
All of this can be done with conventional explosives as well, though it's unlikely that conventional explosives would pack enough punch to make much of a difference.