Potentially Hazardous Asteroids

Potentially Hazardous Asteroids (PHAs) are a specific category of near-Earth objects (NEOs) whose orbits bring them sufficiently close to Earth to pose a possible impact threat in the future. These asteroids are closely monitored by astronomical agencies due to the significant damage an impact could cause to life and infrastructure on the planet. While no PHA currently poses an immediate danger, their discovery, tracking, and study are vital components of planetary defence and space research.

Background and Classification

Asteroids are remnants from the early formation of the Solar System, composed mainly of rock, metal, and other elements. They orbit the Sun primarily in the asteroid belt between Mars and Jupiter. However, gravitational perturbations—mainly caused by Jupiter and other planets—can alter their trajectories, sending some into near-Earth orbits.
Among these near-Earth objects, a subset is designated as Potentially Hazardous Asteroids based on two key criteria defined by NASA’s Centre for Near Earth Object Studies (CNEOS) and the Minor Planet Center (MPC):

1. Minimum Orbit Intersection Distance (MOID): The asteroid’s orbit comes within 0.05 astronomical units (AU) of Earth’s orbit (about 7.5 million kilometres).
2. Size (Absolute Magnitude): The object has an absolute magnitude (H) of 22.0 or brighter, implying a diameter of approximately 140 metres or more.

Such objects are large enough to survive atmospheric entry and cause regional or even global devastation if an impact were to occur.

Characteristics of Potentially Hazardous Asteroids

PHAs vary greatly in size, composition, and orbital dynamics. Some are relatively small, measuring around 140–300 metres, while others exceed one kilometre in diameter. Their orbits may be elliptical or inclined relative to Earth’s plane, and they typically belong to one of the three main near-Earth asteroid groups:

• Amor asteroids: Approach Earth’s orbit but do not cross it.
• Apollo asteroids: Cross Earth’s orbit with semi-major axes larger than that of Earth.
• Aten asteroids: Cross Earth’s orbit but have smaller semi-major axes than Earth’s.

Many PHAs rotate rapidly and have irregular shapes, which can complicate efforts to predict their exact trajectories or potential deflection strategies.

Discovery and Monitoring

Systematic observation of PHAs began in the late 20th century with the advent of advanced telescopic surveys and digital imaging. NASA’s Near-Earth Object Observations (NEOO) Program, established in 1998, coordinates the detection, tracking, and characterisation of such bodies.
Key observatories and survey programmes include:

• LINEAR (Lincoln Near-Earth Asteroid Research) in New Mexico.
• Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) in Hawaii.
• Catalina Sky Survey (CSS) in Arizona.
• NEOWISE, a space-based infrared telescope that helps estimate asteroid sizes and compositions.

As of the 2020s, astronomers have identified over 2,300 PHAs, and new discoveries continue each year. Although most PHAs are not on a collision course with Earth, their orbits are continuously refined using radar and optical observations to ensure accurate prediction of future trajectories.

Historical Impacts and Risk Assessment

The potential hazard posed by asteroids is not purely theoretical. Geological and historical evidence confirms that asteroid impacts have profoundly influenced Earth’s evolution.

• The Chicxulub Impact (66 million years ago): A 10–12 km asteroid struck the Yucatán Peninsula, causing massive climatic disruption and contributing to the extinction of the dinosaurs.
• The Tunguska Event (1908): An object roughly 50–60 metres wide exploded over Siberia, flattening 2,000 square kilometres of forest.
• The Chelyabinsk Meteor (2013): A 20-metre asteroid exploded over Russia, releasing energy equivalent to 500 kilotonnes of TNT and injuring over 1,000 people from shock waves.

Such incidents highlight the destructive potential of even relatively small celestial objects and underscore the necessity of constant monitoring.
The Torino Scale and Palermo Technical Impact Hazard Scale are used to assess and communicate the risk posed by PHAs. The Torino Scale ranges from 0 (no hazard) to 10 (certain global catastrophe), providing a public-friendly measure of threat level.

Planetary Defence and Mitigation Strategies

Global space agencies have developed multi-tiered strategies for planetary defence, focusing on early detection, orbit prediction, and potential impact prevention.
1. Early Detection and Orbit Refinement: Continuous observation and improved computational models are essential for predicting asteroid trajectories decades in advance. Long-term tracking allows for low-cost intervention options if a hazardous orbit is identified.
2. Deflection Techniques: Various methods have been proposed to prevent an impact:

• Kinetic Impact: Striking an asteroid with a high-speed spacecraft to alter its trajectory slightly. NASA’s DART (Double Asteroid Redirection Test) mission in 2022 successfully demonstrated this by changing the orbit of the asteroid Dimorphos.
• Gravity Tractor: Using a spacecraft’s gravitational pull to slowly tug an asteroid off course.
• Laser Ablation or Solar Sails: Employing directed energy or reflected sunlight to change the asteroid’s orbit gradually.
• Nuclear Deflection: Detonating a nuclear device near the asteroid’s surface to alter its velocity—considered a last resort due to political and safety concerns.

3. Impact Preparedness and International Cooperation: The United Nations Office for Outer Space Affairs (UNOOSA) coordinates the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG) to ensure global coordination in case of a credible impact threat.

Environmental and Ecological Implications of an Impact

The environmental consequences of a large asteroid impact would be catastrophic, depending on the object’s size, composition, and impact location:

• Atmospheric Effects: Dust and aerosols could block sunlight, causing “impact winter” conditions and global cooling.
• Tsunamis: Ocean impacts could generate massive waves, inundating coastal regions.
• Wildfires and Shock Waves: Atmospheric explosions and thermal radiation could ignite large-scale fires.
• Ecosystem Collapse: Prolonged climatic changes could lead to widespread extinction events, as seen after the Cretaceous-Paleogene impact.

Even smaller impacts can cause regional devastation, making early warning systems crucial for disaster preparedness and mitigation.

Technological and Research Advances

Recent advances in astronomy and space technology have significantly enhanced humanity’s ability to detect and study PHAs:

• Infrared and Radar Observations: Provide precise data on size, rotation, and surface composition.
• Artificial Intelligence and Machine Learning: Improve the identification of new asteroids from vast datasets.
• Space Missions:
  • NASA’s OSIRIS-REx mission returned samples from asteroid Bennu in 2023, improving understanding of PHA composition and impact behaviour.
  • ESA’s Hera mission (to be launched in the mid-2020s) will follow up on DART’s impact to study crater formation and trajectory alteration.

These missions not only contribute to scientific knowledge but also enhance preparedness for future threats.

Policy, Ethics, and Public Awareness

The management of potentially hazardous asteroids involves not only scientific and technical challenges but also ethical and political considerations. Coordinated international policy is necessary to:

• Share data and resources transparently among nations.
• Develop consensus on the use of nuclear or kinetic interventions.
• Create frameworks for liability and decision-making in planetary defence.