United We Survive: Planetary Defense and NEOs/NEAs
Four Types of Near-Earth Asteroids (Courtesy of NASA/JPL-Caltech)
Every now and then, an asteroid will remind us of the perilousness of our existence. Earlier this summer, an asteroid, 2019 OK, just missed impacting Earth. While it is very common for asteroids to travel closely past Earth, 2019 OK’s flyby of our homeworld came as a complete surprise to astronomers; this asteroid’s trajectory had been disguised by the Sun. If it had been on a collision path with Earth, we wouldn’t have known until it was too late.
Known as a Near-Earth Asteroid (NEA)—a subtype of a Near-Earth Object (NEO), Asteroid 2019 OK is the type of space rocks that planetary scientists are weary of. If they are on a collision path with Earth, these asteroids can completely wipe out a city and devastate an entire region. Due to the significant harm that could result from an asteroid strike, the discovery and identification of NEOs has always been a top priority for space agencies like the National Aeronautics and Space Administration (NASA). Through a 1998 Congressional mandate, NASA was able to formalize its NEO tracking program and, as of September 20, 2019, has identified over 20,900 NEOs.
While I have discussed space rocks before, with so many NEOs in the sky above us, I want to explore this topic in-depth as well. First, I will provide a brief background on NEO and the classifications within this family of space rocks. After providing this primer, I will chart the latest developments in planetary defense against the threat of NEOs. Finally, I will provide my policy take on protecting humanity against disastrous encounters with these space rocks.
Near-Earth Object Categories
A Near-Earth Object is defined as any orbital space rock whose trajectory or orbit will take it within 1.3 astronomical units of our Sun when measured by its perihelion distance—the closest point of its orbit to the Sun. One astronomical unit (AU) is the average distance between the Earth and the Sun, which is defined as exactly 149,597,870,700 meters or approximately 150 million km/93 million miles. NEOs are further divided into Near-Earth Asteroids (NEAs) and Near-Earth Comets (NECs).
Near-Earth Asteroids (NEA)
Most NEOs are Near-Earth Asteroids (NEAs); of the 20,900 NEOs that NASA has discovered so far, 20,792 are NEAs. Of these NEAs, 900 have a diameter of about one kilometer or bigger and 8,782 have a diameter of 140 meters or bigger. Depending on their characteristics, NEAs are classified as one of four types: Amors, Apollos, Atens, and Atiras.
Amors are NEAs whose orbits fully encapsulate Earth’s orbit around the Sun. Many Amors have a perihelion distance that is between Earth’s and Mars’ perihelion distances; in other words, its AU is usually between 1.0 and 1.38. The Amor class of NEAs is named after 1221 Amor (1932 EA1), which was first discovered by the Belgian astronomer Eugene Delporte on March 12, 1932. Currently, NASA has identified 7,784 Amors.
A NEA with an orbit around the Sun that crisscrosses that of Earth’s is either an Apollo or an Aten. An Apollo has the additional characteristic that its semi-major axis (the longest radius length of its elliptical orbit) is greater than that of Earth. Because of this feature, an Apollo has an orbit that is wider than that of Earth and tends to spend more time outside of Earth’s orbit around the Sun. Hence, Apollos are more likely to be found at least 1 AU away from the Sun. Apollos take their namesake from 1862 Apollo (1932 HA), first documented by the German astronomer Karl Reinmuth on April 24, 1932. 11,426 Apollos have been discovered so far.
Like Apollos, Atens’ orbits also crisscross that of Earth’s. However, An Aten has a semi-major axis less than that of Earth, and, therefore, would most likely to be less than 1 AU away from the Sun at any given time. Atens are named after 2062 Aten (1976 AA), first located by the American astronomer Eleanor Helin on January 7, 1976. To date, there are 1,562 known Atens.
An Atira is a NEA whose orbit is completely within Earth’s orbit around the Sun. As such, An Atira’s distance to the Sun will always be less than 1 AU. NASA has only identified 20 Atiras. The Lincoln Laboratory Near-Earth Asteroid Research Team at Socorro, New Mexico, first discovered the namesake Atira, 163693 Atira (2003 CP20), on February 11, 2003.
Near-Earth Comets (NEC)
Other than NEAs, Near-Earth Comets (NECs) make up the remaining NEOs. A Near-Earth Comet (NEC) has an orbital period less than 200 years and typically travels at a much higher speed than a NEA. As I described previously, comets are similar to asteroids but have certain unique features—such as a coma and a tail—and tend to have a more elliptical orbit. So far, NASA has discovered 108 NECs with 23 of them having traveled within 0.102 AU of Earth.
Potentially Hazardous Objects
A Potentially Hazardous Object (PHO)* is a subtype of NEO whose trajectory can take it threateningly close to Earth and, in the event of a collision, can cause catastrophic damages. Any NEO that has (1) a Minimum Orbit Intersection Distance (MOID) with Earth of less than 0.05 AU and (2) an absolute magnitude of less than 22 is considered a PHO. *You might hear the term “PHA” (Potentially Hazardous Asteroids) used interchangeably with PHO, as most PHOs are PHAs*
The MOID between two orbital objects is measured by the shortest distance between their orbits. 0.05 AU is about 7,500,000 km or 4,600,000 miles and is equivalent to about 19.5 lunar distances (the average distance between the Earth and the Moon).
Meanwhile, the size of a NEO is formulated from its Absolute Magnitude, the luminosity (the absolute radiant power) of an object. Absolute Magnitude is measured by the apparent magnitude of an object when viewed from a distance of 10 parsecs (32.6 light-years). Because this magnitude is measured on an inverse logarithmic scale, the brighter an object is, the lower its Absolute Magnitude. Since all objects reflect a certain amount of solar radiation, a factor, albedo, is used to discount for the increase in luminosity due to this effect. NASA applies an albedo factor of 0.14 for the purposes of identifying PHOs. After these calculations are taken into consideration, an Absolute Magnitude of 22 or brighter would translate into a PHO having a diameter of at least 140 meters (460 feet).
Therefore, a NEO will be classified as a PHO if it is (1) within 19.5 lunar distances to Earth and (2) has a diameter that is equal to or greater than 140 meters (460 feet). As of September 20, 2019, NASA has identified 2020 PHOs. Of these, 156 have a diameter of 1 kilometer (0.62 miles) or greater.
Passive Monitoring: Detecting PHOs
While PHOs are persistent threats in the sky, the danger might not be immediate. If a PHO’s orbit is well-known, astronomers can calculate when the risk of collision becomes probable. Through Sentry, NASA/Jet Propulsion Laboratory (JPL)’s automated collision monitoring system, scientists are continuously monitoring asteroids that have a possibility of impacting Earth. As of September 2019, there are only 24 PHOs that have a Palermo Rating equal to or greater than -4 with higher than a 0.00001% chance (1 in 10,000,000 odds) of crashing into Earth within the next century. Palermo Rating is a logarithmic scale used to measure the harm resulting from a PHO’s collision with Earth; NASA/JPL considers any PHO with a rating of -2 or greater to warrant careful monitoring.
Of the 24 PHOs, only two have a cumulative Palermo Scale greater than -2. (29075) 1950 DA, an Apollo PHO, has the highest cumulative Palermo Rating at -1.42. In the next 100 years, it has a 0.012% (1 in 8,300) chance of colliding with Earth. Meanwhile, 101955 Bennu (1999 RQ36), another Apollo PHO, while having the second highest cumulative Palmero Rating at -1.71, has a higher chance of impact at 0.037% (1 in 2,700).
NASA/JPL also keeps a more “frontline” monitoring system, Scout, which conducts threat analysis on newly discovered objects on Minor Planet Center’s Near-Earth Object Confirmation Page (NEOCP). Because objects on NEOCP have not been confirmed (some might just be artifacts), Scout does a preliminary analysis and assign a rating of 0 (negligible) to 4 (elevated). If it turns out to be a false positive, the “object” will be removed from both NEOCP and Scout; otherwise, the NEO will eventually be classified and analyzed in Sentry.
Active Measures: Redirecting PHOs
Apart from monitoring PHOs, planetary scientists are also working on defensive solutions that can redirect a PHO’s trajectory through direct impact. While sounding like something out of science fiction, this idea could very much become a reality through the Double Asteroid Redirection Test (DART) Mission.
Jointly-sponsored by NASA and the John Hopkins Applied Physics Laboratory, DART has moved to final design and assembly phase and is expected to launch onboard a Falcon 9 in July 2021. Costing approximately 69 million dollars, DART will be powered by NASA’s NEXT-C (Evolutionary Xenon Thruster—Commercial) propulsion system. Using a solar-powered electric thruster, the 1,100 lb (500 kg) DART spacecraft will be intentionally crashed, at about 3.7 miles (6 km) per second into the moon, Didymos B, of an Apollo NEA known as Didymos A. Expecting to shift Didymos B’s orbit by 10 minutes, scientists are hoping that this test will demonstrate the feasibility of kinetic impact in redirecting a PHO’s trajectory.
Because DART’s impact on Didymos B is likely to kick up a large cloud of space dust, visual information might be limited shortly after the collision. Hence, DART will be carrying a CubeSat named LICIACube: Light Italian CubeSat for Imaging of Asteroids for imminent monitoring. Contributed by the Italian Space Agency, LICIACube will be deployed five days prior to DART’s controlled crash into Didymos B and is expected to capture the initial impact as well as helping researchers to get a close look at the impact crater immediately post-collision.
Planetary Defense Policies: A Shared Responsibility for Our Homeworld
Detecting and protecting Earth from PHOs should be a shared responsibility across the globe. Hence, we should enact policies that will foster collaborations among different nation-states, among governments and the private sector, and among budding start-ups and well-capitalized companies. Only by using the full spectrum of humanity’s innovations can we guarantee our survival and the protection of our only homeworld.
Of all causes, planetary defense is one of the more persuasive rationales for nation-states to put aside their differences. Although a PHO impact might be limited to a specific country, the aftermath can be felt everywhere. The resulting devastation could lead to both geopolitical instability that promulgates itself across neighboring states as well as detrimental economic effects that can ripple across continents. Therefore, an impact would concern all nations and all states should be united in addressing this common existential threat.
While the DART mission is an example of how countries are working together to face this challenge, more can be done. In DART’s case, while the Italian Space Agency provided the LICIACube for post-mission analysis, the European Space Agency (ESA) was originally supposed to participate on a much grander scale. Via the Asteroid Impact Mission (AIM), ESA had plans to send an independent spacecraft to assist with DART’s after-action report. But AIM was later cancelled when its funding was not secured; unfortunately, planetary defense is still a low-hanging fruit for most nations because of its low probability of occurrence. Therefore, a diffusion of responsibility exists in this area where many countries expect others to take the lead. While nation-states with more resources should contribute more, I believe planetary defense can only succeed if it were a joint-project around the world; only by doing this together, can we hope to capitalize on the sum-total of human ingenuity needed to solve a worldwide borderless issue.
Additionally, we should encourage more partnerships between the public sector and private enterprises in the defense of our homeworld. Governments should incentivize the private sector in developing new technologies related to planetary defense. Just as NASA ensured SpaceX’s survival, which led to the rise of reusable rockets, space agencies should assist companies in taking on the challenge of coming up with new methods that can detect and/or deflect PHOs from hitting the Earth. Assistance from the government can come in the form of (i) financial incentives such as funding in foundational basic research that are necessary but not commercially viable, (ii) competitions and contests that encourage well-established companies to enter into collaborations with start-ups, and (iii) access to resources like satellite datasets and expertise from government scientists. Public policies should be crafted so that we can fold the full panoply of innovators and industrial leaders into this field.
Earth is our only known homeworld; it’s the birthplace of our humanity and the ultimate denominator of our common heritage. Hence, we, as a species, share a joint responsibility in protecting our only known home for both our survival and for our legacy. For without it, we will be forever lost in the dark vastness of the cosmic universe.
Double Asteroid Redirection Test (DART) Mission: https://www.nasa.gov/planetarydefense/dart
John Hopkins Applied Physics Laboratory: https://www.jhuapl.edu/
JPL Small-Body Database Browser: https://ssd.jpl.nasa.gov/sbdb.cgi#top
Minor Planet Center: https://minorplanetcenter.net/
Much ado about Space Rocks: https://alexsli.com/thespacebar/2018/10/21/much-ado-about-space-rocks
NASA’s Center for Near Earth Object Studies: https://cneos.jpl.nasa.gov/
Near-Earth Object Confirmation Page: https://minorplanetcenter.net//iau/NEO/toconfirm_tabular.html
NEOs Discovery Statistics: https://cneos.jpl.nasa.gov/stats/totals.html
Scout: NEOCP Hazard Assessment: https://cneos.jpl.nasa.gov/scout/#/
Sentry: Earth Impact Monitoring: https://cneos.jpl.nasa.gov/sentry/intro.html
The International Astronomical Union: Minor Planet Center: https://www.minorplanetcenter.net/