ImpactEarth: Comets - Potential Threats to Earth (1)

Introduction

The first half of the course focused on the frequency and potential dangers of asteroid impacts on Earth.
Graphs including “Global Catastrophe” indicate serious concern regarding these events.

Asteroid 2012TC4, approximately 30m in diameter, passed close to Earth on October 12, 2017, at a distance of 27,300 miles and may return in 2050, with a potential impact in 2079.
Near Earth Asteroids (NEAs) are concerning due to their proximity to Earth’s orbit.
Many projects are dedicated to finding, tracking, and predicting the movements of NEAs.

Comets were known long before asteroids due to their brightness when near Earth.
Chinese astronomers observed comets as early as 2467 BCE.
Approximately 86 comets per century are visible to the naked eye, about 1 per year.
Examples of comets include Comet 2020/F3 (NEOWise), Comet Leonard (December 2021), Comet C22 E3 (last February), and Comet C/2025 F2 (SWAN).
Halley’s Comet is one of the most famous, with its passage in 1066 depicted in the Bayeux Tapestry.
Edmond Halley proposed in the early 18th century that Halley's Comet returns every 75 years.

Most comets reside far from the Sun in structures like the Kuiper Belt, located beyond Neptune's orbit.
The accretion processes that form planets slow down with distance from the Sun.
Models suggested that material within the orbits of giant planets would concentrate just beyond Neptune.
On January 1, 2019, the New Horizons probe flew past Kuiper Belt Object (KBO) 2014 MU69, renamed “Arrokoth”.
Arrokoth appears reddish due to the presence of tholins (solid methane) on its surface.

Comets have two primary populations: those with low orbital inclinations in the ecliptic plane and those with random inclinations.
Long-period comets originate from the Oort Cloud, a spherical distribution extending almost halfway to the nearest star (almost 2 light years).
Oort Cloud comets were likely scattered to their distant orbits by encounters with giant planets.

In 1992, Comet Shoemaker-Levy 9 (SL9) passed close to Jupiter and was torn apart by tidal forces.
The debris collided with Jupiter, releasing more energy than the combined nuclear arsenals of Earth.

Comets consist of volatile materials like water, ammonia, and methane ice.
Evaporation of these ices causes the bright tails seen as comets approach the Sun.
Spacecraft images of comets like Halley’s Comet reveal an asteroid-like nucleus with jets of evaporating material.

The gas and dust in cometary tails interact differently with sunlight.
Gas is pushed directly away from the Sun, while heavier dust particles remain in the comet's orbit.