A dry run to save the planet: NASA DART probe closing in on asteroid impact

As NASA nears the climax of its Double Asteroid Redirection Test (DART), the head of the Planetary Defense Coordination Office wants you to know that their strategy for dealing with an Earth-bound asteroid is nothing like “the ways depicted in all the Hollywood movies.”

Lindley Johnson, who oversees the Planetary Defense Coordination Office (PDCO), says NASA has no plans to violently detonate any near-Earth asteroids, as depicted in Bruce Willis’s mission in Armageddon. Instead, NASA’s latest step to prepare for such a catastrophe is more akin to a carefully-engineered pool shot in the form of the Double Asteroid Redirection Test (DART).

If the DART spacecraft is the cue ball, the target billiard ball in question is a double asteroid system called Didymos.

Called the “kinetic impaction” technique, this method of deflection involves smashing a spacecraft into an asteroid, and the DART mission will be the first real-life test of its viability. The spacecraft, launched on November 24, 2021, will finally collide with Didymos on September 26 at 7:14 pm Eastern Daylight Time.

Didymos is merely a test candidate, not a genuine hazard; at its closest, the system will still be some 11 million km from Earth. The planned deflection will not affect the path of an asteroid towards Earth, but rather disrupt the orbit of Didymos’ moon about its central body. If all goes well, the moonlet, Dimorphos, will be nudged such that its orbit around Didymos is shortened by several minutes. If this prediction is correct, NASA will prove not only that they can change an asteroid’s orbit, but also that they can control its outcome.

The Planetary Defense Coordination Office

NASA has been tracking asteroids since 1998, when scientists at Cambridge’s Minor Planet Center discovered an asteroid they believed to be on an Earth-bound path. While the asteroid proved not to be a real threat, it served as a wake-up call to astronomers, who soon established a near-Earth object surveillance program.

The PDCO, established in 2016, takes the threat of an asteroid-driven apocalypse even more seriously than its predecessor. The PDCO’s mission is four-fold: find, warn, coordinate, and mitigate.

The “find” facet of the strategy may sound less glamorous than “mitigate,” but sufficiently early awareness of hazardous asteroids is key. In an interview with OnlySky, Lindley Johnson emphasized the important role that NASA’s NEO Surveyor will play in planetary defense strategy:

With a space-based capability, we could find the population of asteroids down to 140 meters or so in size within just 10 years, and have a good understanding of what the threat could really be out there decades in advance. That provides us sufficient time then to develop what we would need to do to deflect an object off of the hazardous trajectory.

Lindley Johnson, Planetary Defense Officer, NASA

Given that only about 40% of potentially hazardous asteroids over 140 m have been discovered as of yet, the PDCO’s discovery efforts serve as the foundation for their entire planetary defense strategy. 

Johnson also spoke highly of the coordination between the PDCO and other countries’ space programs, particularly through the International Asteroid Warning Network. The hunt for hazardous objects, he said, “is actually a very nice area for nations to participate, collaborate together… Over 30 countries are participating in trying to find these objects and will be able to help in characterizing their sizes and what the threat really is.”

DART’s origins

While DART was not officially a NASA project until 2017, the idea of kinetic impaction via spacecraft is decades old. 

Scientists have used computer simulations to test this concept extensively. For example, MIT researchers simulate kinetic impaction using possible impact trajectories of actual asteroids. Though these asteroids are not a real threat, astronomers can learn much about the timing necessary for accurate asteroid deflection through experimenting with “scenarios in which the asteroids may be headed toward a gravitational keyhole”—i.e approaching certain points along Earth’s gravitation field such that gravity would send them hurtling directly at us.

With a space-based capability, we could find the population of asteroids down to 140 meters or so in size within just 10 years, and have a good understanding of the threat decades in advance.

PLANETARY DEFENSE OFFICER Lindley Johnson

Despite the importance of these kinds of simulations—which allow scientists to vary factors such as asteroid mass, size, and speed—theoretical experimentation can only be so useful when the real-life technology still lags behind.

Bringing this mission to life was first conceived by the European Space Agency in the mid-2000s through the Don Quijote mission—named for the 17th-century fictional hero who charges at high speed into a windmill. Don Quijote’s mission was to send a double spacecraft system—the kinetic impactor and an observer spacecraft—to collide with an asteroid, then observe its changed orbital path. 

But by 2011, the project had been deemed too costly. Instead, the idea of DART was developed under the new Asteroid Impact and Deflection Assessment (AIDA) collaboration between the ESA and NASA, which promised a more efficient approach.

By instead crashing into a moonlet asteroid orbiting a larger asteroid, scientists hoped to observe the orbital effects of kinetic impaction much more quickly than Don Quijote would have allowed.

Given Didymos’ proximity and its relatively well-characterized qualities, it was the perfect fit for such a project. Its moonlet, Dimorphos, has a diameter of about 160 m—just above the minimum size for an asteroid to be considered “potentially hazardous”—while Didymos is around 780 m. Since Dimorphos loops around Didymos every 12 hours, any change to this orbital period can be observed far more rapidly than if a singular asteroid’s lengthy orbit around the Sun were affected.

Global collaboration will be vital to measuring Dimorphos’ trajectory after its impact this month. While the Europeans’ facet of the AIDA collaboration, called the Asteroid Impact Mission, was later canceled, the ESA will now contribute via the Hera probe. Hera will approach Didymos four years after the impact, providing a more accurate glimpse of Dimorphos’ surface, mass, and resulting orbit. DART itself will be accompanied by the Italian LICIACube, a small imaging satellite that will capture the impact. 

If all goes well, the moonlet, Dimorphos, will be nudged such that its orbit around Didymos is shortened by several minutes. If this prediction is correct, NASA will prove not only that they can change an asteroid’s orbit, but also that they can control its outcome.

Judging DART’s success

According to a 2021 paper, in order for DART to be considered a success, it must achieve the following five criteria: 

1. Impact Dimorphos
2. Cause at least a 73-second change in its orbital period
3. Measure this orbital change with sufficient accuracy (within one standard deviation)
4. Measure Dimorphos’s “momentum transfer,” a useful measurement that takes into account post-impact momentum (and therefore velocity)
5. Use observational data to determine dynamical changes to Dimorphos

While the first item on the list is what we’ve been counting down the days to, the science doesn’t stop there. After DART’s impact, a number of Earth-based telescopes—including Arizona’s Lowell Discovery Telescope and Chile’s Magellan Telescope—will track Dimorphos’ new orbit. 

To do so, astronomers will use a method similar to the planetary transit method used to find exoplanets. As Dimorphos zips around Didymos, the brightness of the system dims as one asteroid momentarily eclipses the other. Thus, scientists can track varying brightness levels to determine each time that Dimorphos loops back around. As telescopes observe the system for a number of orbits, researchers will be able to calculate Dimorphos’ new, presumably shorter, orbital period.

Calculating Dimorphos’ momentum transfer value will also be helpful in understanding its post-impact behavior. The momentum transfer is dependent on factors such as the material of the asteroid, the amount of material ejected as a result of the impact, and the spacecraft’s incoming speed. This measurement, in conjunction with criterion #5, will provide even more information about the collision’s effects on the moonlet.

Deflect, deflect, deflect

DART’s impact on the 26th, as the first real-life test of asteroid deflection, will lead the way in future applications of deflection techniques.

While the search for potentially hazardous asteroids is first and foremost – Johnson estimates with the help of the NEO Surveyor we’ll have a complete catalog of near-Earth objects by the late 2030s—studies of other possible deflection techniques are also already underway.

In comparison with DART’s kinetic impaction technique, the “gravity tractor” method will function more slowly and more carefully. This technology is dependent on “the mutual gravitational interaction between [a] spacecraft and the asteroid,” said Johnson, “maintaining a precise position from the asteroid and using nature’s tug rope… to just slowly tug that asteroid off the offending trajectory.”

Another spin on this technique is the “enhanced gravity tractor,” which would steal a multi-ton boulder from the asteroid itself in order to increase the mass of the spacecraft, therefore imparting an even greater tug.

And while Johnson asserted that the Armageddon method was not so accurate—particularly the idea that an asteroid would be discovered mere weeks before impact—using a nuclear device may still be a possibility. However, simply detonating an asteroid would not necessarily mitigate the threat of collision, as its ejected debris could still pose a problem. 

Instead, a nuclear device would be detonated some distance from the surface of the asteroid, so that it “superheats the asteroid, and causes things to vaporize and blow off of the surface… that imparts a force on the asteroid and again changes its velocity in its orbit and then changes its orbit.”

Looking forward

Regardless of DART’s outcome, astronomers worldwide will continue on the case to find, model, and engineer away any Earth-bound threat.

Really, compared to the slew of ground-based threats we face every day, asteroids are relatively easy. Johnson took the optimistic angle of this outlook: “Folks should be aware that of all the natural disasters—earthquakes, volcanoes, hurricanes—we don’t really know how to prevent those from happening yet. But we know how to prevent this kind of natural disaster from occurring… This can be undertaken at a relatively modest cost and humankind does not have to worry about getting impacted by an asteroid in the future.”

Seems we don’t have to worry about this particular brand of Armageddon.