Planetary Dodgeball

It is often said that you miss one hundred percent of the shots you don’t take. The inverse may be considered in that you can’t catch any of the throws you can’t see. It really matters what you’re trying to catch. If what you’re catching are pitches delivered in a major league baseball game, the consequences might be losing your shot at the pennant. If the object in question is a kilometer wide asteroid, then the consequences of not seeing that might be a planetary extinction event.

On July 25, 2019, at 01:22 UTC, object 2019 OK, an asteroid between 59 and 130 meters in size passed within 72,000 kilometers, or less than a quarter of the lunar orbit radius, of earth. The nomenclature of the object indicates it was only detected in July of 2019, essentially less than a two-week warning period. Had this object impacted on land, the resultant crater would have been 764 meters wide and 164 meters deep, sufficient to obliterate the south end of Central Park in New York City. The potential energy yield would have been more than two megatons of TNT.

Objects which move within our solar system are numerous and range from pure planets to small, unnamed rocks found in the asteroid belt, Oort cloud, as comet debris or simply lone objects. Identifying even a moderately sized object in the solar system is extremely complicated. An asteroid that is two hundred meters in diameter can deliver enough energy upon impact to level a town, however, it is nearly invisible to conventional terrestrial-based telescopes. Furthermore, simply identifying the object is insufficient to determine a threat to our planet. The object must be categorized, and its trajectory determined along with a probability for coinciding with earth. These elements will determine if the object crosses the threat category into a PHO (Potentially Hazardous Object). These are the objects which pose a serious threat to the planet.

Asteroid impacts to planetary bodies are common. The most famous event on earth is the impact which is attributed to the extinction of the dinosaurs on earth, some sixty-five million years ago. There have been numerous celestial objects which have collided with our planet since then. Within modern times we have experienced events such as the legendary 1908 Tunguska blast in Siberia. In the 21st century, it was only on October 7, 2008, when a 4-meter asteroid labeled 2008 TC3 was tracked for 20 hours as it approached Earth. It impacted in Sudan and marked the first time an object was detected before it reached the atmosphere. Later, in 2013, over Chelyabinsk, Russia, a twenty-meter-wide asteroid detonated in the upper atmosphere. Although technically a Bolide (an asteroid or meteor which burns up or detonates in high atmosphere). This impact caused more than 30 million dollars in damages and injured more than 1500 people. Despite the numerous impacts, it has only been since 2008 that we can effectively track uninvited visitors to our planet. This leads to the broader question of now that we can see what’s out there, what can we do to protect ourselves?

NASA is part of the global effort for planetary defense. They established The Planetary Defense Coordination Office (PDCO) in January 2016 to manage planetary defense-related activities across NASA, and coordinate with both U.S. interagency and international efforts. Their role is to study and plan a response to the asteroid impact hazard. This network consists of an array of agencies, sensors, telescopes, monitoring systems, and models and simulations to determine the risk posed to the planet by NEOs, Near-Earth Objects. The integrated system is still in its infancy with only ten planetary defense scenarios having been conducted since 2010.

The United States defined a set of goals in its Near-Earth Object (NEO) action plan in 2018. These goals were to enhance NEO detection, characterization, and tracking capabilities, improve modeling, predictions, and information integration. This plan included the development of technologies for NEO deflection and disruption as well as an increase in international cooperation on NEO preparation. Finally, it sought to establish NEO impact emergency procedures and action protocols. This expanded the potential capabilities and systems of the nascent International Asteroid Warning Network (IAWN), established by the United Nations in 2013. At present, there are twenty signatories to this network, mostly comprised of space agencies, universities, and their associated telescope systems.

Since establishing an explicit program to look for Potentially Hazardous Asteroids (PHAs) / PHOs, more than 156 have been identified with a size greater than one kilometer. NASA’s classification officially began in 1998. The rate of discovery continues to increase as more capable systems are brought online and improved analytical techniques are employed. In 2019, more than 2433 objects were discovered via a variety of terrestrial and space based systems across the globe. The two most effective of these sensors are the Catalina Sky Survey system at the University of Arizona and the Pan-STARRS telescope at the University of Hawaii at Haleakala, Maui. At present, a total survey will take more than thirty years to complete. This presents a broad window of risk to missing a PHA / PHO which are typically greater than one hundred and forty meters in size. Understanding the risk falls to a sophisticated system called SENTRY.

Utilizing the SENTRY system, a highly automated system for rapid evaluation of PHAs/PHOs, NASA constantly evaluates possible impactors. It is supported by SCOUT, a smaller scale system for short-term, unverified objects. In turn, these systems are supported by two Hawaii-based telescopes which comprise the ATLAS (Asteroid Terrestrial-impact Last Alert System) system. ATLAS is designed to provide thirty days warning of a potential 100MT impact and one day’s warning for a 30KT “town killer” object. These systems are highly automated and feed various emergency alert systems and evaluation programs.

From the ground to the stars, the heart of earth’s space-based capability is the NEOWISE satellite, formerly WISE. NEOWISE utilizes the Wide-Field Infrared Explorer (WISE) spacecraft that surveyed the entire sky in 2010 with a cryogenically cooled 40 cm telescope and four mid-infrared array detectors. The satellite was placed into hibernation in February 2011. Then in September 2013, the WISE spacecraft was reactivated and renamed NEOWISE. Its revised mission was to detect and characterize asteroids and comets. It focused on cataloging the population of near-Earth objects that could pose an impact hazard to the Earth. (Fig.4). NEOWISE continues to survey the sky, and as of March 2021 has acquired over 1,087,000 confirmed infrared detections of approximately 38,700 different solar system objects.

Even though our planet’s vulnerability remains essentially unchanged, we have only recently begun to have any meaningful capability to identify, track and affect PHOs / PHAs. The planetary defense program uses detailed databases and algorithms to catalog, classify and assess the aspects of NEOs. Those with the most potential for destruction are given a rating on the Palermo Scale and subject to additional monitoring. The object is then further ranked by its probability for impact with earth and a timescale for this event. A complex analysis of orbital mechanics is performed to provide some type of early warning to the planet.

Once the object is identified, classified and risk assessed, scientists and policymakers attempt to determine preventative and mitigating strategies. These approaches are tested and trialed in models and simulations, culminating in an annual exercise scenario. These scenarios are called Hypothetical Impact Scenarios and they are carefully scripted to identify gaps in detection, prevention, and mitigation programs. To date, no scenario has been successful in preventing the impact on earth of even the smallest object.

NASA has identified a system for detecting, classifying, and monitoring PHAs greater than 140 meters, the threshold for significant physical damage to the earth. The NEO (Near-Earth Object) surveillance mission concept objectives include finding 65% of undiscovered PHAs greater than 140m in 5 years with a goal of 90% in 10 years. The sizes of the objects would be derived from IR (Infrared) signatures. The system would then compute the cumulative chance of impact over the next century for PHAs more than 50m and comets. Data would be sent daily to the Minor Planet Center. In the summer of 2021, NASA plans to launch the Double Asteroid Redirection Test (DART). This will be a demonstrator for asteroid deflection technology. It is the first mission directly related to planetary defense. DART will impact the asteroid Dimorphos in the Fall of 2022. The spacecraft will attempt to change its orbit in space, this will be critical to mitigation of a PHA/PHO.

These systems are in their infancy and rely upon an imperfect and rapidly changing set of data. All this data is equally embedded within the challenging environment of orbital bodies and planetary mechanics. Sensor systems will need to be upgraded, deployed with substantial long-term observation capability, extreme resolution, and rapid response capabilities. The teams working on these issues span the full gamut of astronomers, space scientists, systems engineers, and other technical professionals focused upon being ready when that big pitch comes in from outer space. The earth will only have one chance to hit it out of the planetary park.

References

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NASA. (2021, April 27). Planetary Defense Coordination Office — Overview. Retrieved from Planetary Defense Coordination Office: https://www.nasa.gov/planetarydefense/overview

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Paul K. Martin, NASA Inspector General. (SEPTEMBER 15, 2014). NASA’s Efforts to Identify Near-Earth Objects and Mitigate Hazards. Washington, DC: NASA Inspector General, Office of Audits.