What is Planetary Defense?

What is Planetary Defense?

The term “planetary defense” might sound like the stuff of science fiction. It conjures images of nervous scientists watching an encroaching asteroid on a grainy radar screen, a clock ticking down the seconds until a doomsday-bringing impact, or perhaps even tense negotiations between world leaders and hostile alien invaders. While these tropes make for good cinema, the reality of this emerging field is far more practical.  In short, planetary defense refers to the collective effort of detecting, tracking, and mitigating risks from near-Earth objects (NEOs) like asteroids or comets that could impact Earth.  I met with Lowell astronomer and asteroid expert Dr. Nick Moskovitz to gain some insight on what exactly planetary defense entails, as well as the role that the observatory currently plays in this research area.

The Five “Slices” of Planetary Defense

According to NASA’s Planetary Defense Coordination Office, Planetary Defense can be divided up into five “slices”: 

• Search, Detect, and Track: Searching for, locating, and monitoring near-Earth asteroids (an asteroid that comes within 0.3 AU of Earth, where 1 AU is the distance between Earth and the Sun).
• Assess: Calculating the potential risk associated with an object. Could it hit Earth? If so, where and with what effects? 
• Characterize: Measuring properties like size, what the object is made of, how fast it spins, and how its orbit or trajectory in the solar system will evolve in the future. This is primarily where Lowell comes in. 
• Plan and Coordinate: The diplomatic side, coordinating dissemination of scientific data and findings to the UN, government agencies, and world leaders.

“It’s a bit of astronomy, a bit of planetary science, a bit of practical engineering, and a bit of risk assessment crossing into the realm of FEMA and emergency managers,” says Moskovitz. “You mash that all together into one discipline.”

Search, Detect, and Track

This is the “census” phase: searching for, detecting, and building a catalog of known objects. The NASA-funded project Lowell Observatory Near-Earth Object Search (LONEOS), which ran from 1993 to 2008, was one of the earliest dedicated asteroid-search programs. The project was run by Lowell astronomer Ted Bowell from one of Lowell’s research sites at Anderson Mesa and is credited with the discovery of more than 22,000 objects including 291 near-Earth objects, thousands of main belt asteroids (small bodies that orbit the Sun within the asteroid belt between Mars and Jupiter), and Mars-crossers (asteroids with orbits that cross that of Mars).  The asteroid monitoring game is set to receive a serious boost when two new projects come online in the next year or two: Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST) and NASA’s NEO Surveyor mission.  LSST will be an unparalleled wide-field astronomical survey using a 3.2 gigapixel camera, a powerful supercomputing cluster, a sophisticated data processing and distribution network, and a massive 8-meter telescope. The camera will image the entire visible sky every few nights over a ten-year period, capturing changes over time to create a time-lapse movie of the universe.  NEO Surveyor is an infrared space telescope scheduled to launch in September 2027. The telescope is designed to discover and characterize at least two-thirds of potentially hazardous asteroids and comets larger than 140 meters (460 ft.) that come within 0.3 au (29.8 million miles) of Earth’s orbit.

Assess

Once an object is found, scientists determine its threat level.   “You have to assess every object independently,” says Moskovitz. “We ask: Is it possible that it could hit the Earth? When? What would the consequences be?”  Risk assessment is primarily handled by two NASA facilities: the Center for Near-Earth Object Studies (CNEOS), JPL’s facility for computing asteroid and comet orbits and their probability of Earth impact.  CNEOS hosts the Sentry System, which automatically scans asteroid catalogs and estimates which asteroids have a chance of hitting Earth in the next 100 years. It is also home to the Scout System, which monitors newly detected objects for possible Earth-impacts before their discoveries are even confirmed. Once a threat is identified, the Asteroid Threat Assessment Project (ATAP) at Ames Research Center takes the lead on modeling the possible consequences. According to the NASA Advanced Supercomputing Division, their work includes:

• Impact Modeling: Developing computer models of asteroid atmospheric entry, airbursts, and surface impact to understand potential destruction.
• Asteroid Characterization: Studying the physical properties (size, shape, density, and composition) to determine how an asteroid might break up or be deflected.
• Support for Mitigation: Providing key analysis to the Planetary Defense Coordination Office (PDCO) to help determine the best actions to mitigate a threat.

Characterize

This is Lowell’s primary role in the planetary defense process. Once an object has been found, it needs to be classified based on its physical properties like size and composition. While survey telescopes are well-suited for finding objects, they are not necessarily designed to identify what they are made of and function more as the modern equivalent of LONEOS — automated facilities that continuously monitor the sky for near-Earth objects.  “If we can measure the composition of an object, like if it’s a retired spacecraft with a hollow metal body like a tin can, that has a very different impact hazard than a solid block of iron metal,” says Moskovitz. The 4.3-meter Lowell Discovery Telescope (LDT), Lowell Observatory’s flagship research instrument, is remarkably well-equipped to measure NEO compositions. Its instruments are co-mounted, meaning scientists can switch from taking a picture of an object to analyzing its spectrum in a matter of minutes. It can accommodate five instruments that can be switched almost instantly, while most telescopes of a similar size require long periods of downtime to change between instruments.  Unlike many large research telescopes, which typically cannot observe targets less than 15–20 degrees above the horizon, the LDT was designed with cometary studies in mind and can point as low as five degrees above the horizon. This capability allowed Lowell astronomer Dr. Qicheng Zhang to capture the first ground-based image of Comet 31-Atlas earlier this year.  The LDT’s greatest boon as a planetary defense tool is its rapid response capability. It can be operated remotely from anywhere in the world, allowing researchers to take over the telescope quickly during an emergency.  “This rapid response could be vital when we have short notice before a potential impact,” says Moskovitz. 

Plan and Coordinate

This is the diplomatic layer of planetary defense, where hard science meets international relations and emergency management. NASA’s Planetary Defense Coordination Office (PDCO) serves as the official conduit for all asteroid-related information in the United States, ensuring that data from telescopes like the LDT are translated into actionable intelligence for the government. Crucially, this office remains a civilian entity.  Keeping the discipline under NASA’s jurisdiction rather than the U.S. Space Force or the Department of War (formerly the Department of Defense) ensures that the work of scientists is viewed through a lens of global transparency and scientific cooperation rather than a tactical one. As Moskovitz notes, “It demilitarizes our activity… we can start with the science before it ever goes to the military.” Because an asteroid strike is a global threat, the response must be international. This is handled by the Space Mission Planning Advisory Group (SMPAG), a UN-convened body where global space agencies coordinate their asteroid response playbooks. Here, representatives operate on strict, agreed-upon thresholds before the alarm is raised.  “We’re not going to bother the heads of state if a little tiny rock is going to come in,” explains Moskovitz. “But if it gets large enough to be a concern, then absolutely, you want to notify heads of state. Particularly if they’re in the landing zone.” As an object approaches the 140-meter threshold, the size of a football stadium and the point at which an impact becomes catastrophic, the UN sends out the call for leaders to put their emergency plans into action.  In addition to public safety, the flow of information is vital for preventing global conflict. During the 2013 Chelyabinsk Event, a 20-meter asteroid exploded in the air above the southern Ural region in Russia with the force of 30 nuclear bombs.  At a time of strained international relations, the fact that this was immediately recognized as a natural event and not a military strike was a triumph of planetary defense communication. By framing these events as natural disasters akin to earthquakes or hurricanes rather than “attacks,” scientists and diplomats can help to ensure that an asteroid doesn’t inadvertently trigger a conflict between countries.  Once a threat is confirmed to be unavoidable, astronomers pass the baton to emergency managers. Just as they would for a hurricane, FEMA handles the ground-level response. This includes everything from mobilizing emergency personnel to disseminating life-saving information, such as the “Don’t look out the window” directive used to prevent shattered glass injuries from an asteroid’s shockwave, which left over 1500 people hospitalized in the aftermath of the Chelyabinsk event.  The final, and perhaps most difficult, part of planetary defense coordination is managing public perception. In an era of clickbait headlines, Moskovitz and his peers often find themselves performing “science therapy.” “People can get really worked up,” says Moskovitz. “They need a therapist, almost. They say, ‘I’m losing sleep over this. What do I do? How bad is it?’ Much of that is just the way articles are written to sensationalize things beyond the reasonable hazard that actually exists.” The ultimate goal of planetary defense communication is to move the public from a state of fear to a state of preparation. By replacing sensationalist headlines with clear, grounded facts, the scientific community ensures that if the day of an impact ever comes, people will feel prepared rather than panicked. 

Mitigate

This section is where the “defense” in planetary defense becomes literal. According to Moskovitz, mitigation is a game of precise orbital mechanics and long-term planning, not the exciting nuclear explosion a Hollywood portrayal might prefer.  Based on the results of the DART mission, we now know that a DART-sized kinetic impactor (a spacecraft designed to intentionally crash into an object to change its orbit) can deflect an asteroid 160-meters in size by about 100 kilometers per year. “That means you would have to impact an object of that size 60 years before impact to deflect it by a distance equal to Earth’s radius,” says Moskovitz. DART was the first-ever mission dedicated to investigating and demonstrating one method of asteroid deflection by changing an asteroid’s motion in space through kinetic impact. While the “kinetic nudge” used by DART is currently our only proven tool, it is far from the only option in the planetary defense toolkit. For missions with decades of lead time, scientists have proposed more elegant, “slow-push” methods that could offer much higher precision. These include gravity tractors, which involves a heavy spacecraft simply hovering near an asteroid and using its own tiny gravitational pull to tug it off course, and laser ablation, where the surface of an asteroid is zapped with a laser to create a jet of vaporized rock that acts like a rocket thruster. There is also the ion-beam shepherd, a concept that uses the exhaust from a spacecraft’s ion engines to provide a gentle “head wind” to slowly push an asteroid away over time. The choice between these methods depends on what the asteroid is made of, which relates back to the critical work being done with the LDT. Without the telescope’s ability to characterize an object’s composition and structural integrity, choosing a mitigation strategy would be pure guesswork. After all, you don’t want to use the hammer-like force of a kinetic impactor on an object that requires the more delicate method of a gravity tractor. While it may be some time before we need to launch a kinetic impactor or evacuate an imperiled city, the work being done at facilities like the LDT ensures that we stay one step ahead of cosmic threats. By replacing asteroid anxiety with actionable plans and international cooperation, experts like Moskovitz ensure that when we look to the stars, we can do so with the confidence that we have the tools, plans, and resources to protect our home.