Asteroids | Science for ACT PDF Download

Introduction

• Asteroids represent the rocky remnants leftover from the formation of the solar system and its planets approximately 4.6 billion years ago.
• The primary source of asteroids is the main asteroid belt situated between Mars and Jupiter, as indicated by NASA. The current count of asteroids by NASA exceeds 1 million.
• Asteroids follow elliptical orbits around the sun, often exhibiting erratic rotations and tumbling movements through space. Many larger asteroids possess one or more smaller companion moons. For instance, Didymos, a half-mile (780 meters) wide asteroid, is orbited by the moonlet Dimorphos, measuring just 525 feet (160 m) across.
• Referred to as "minor planets," asteroids vary greatly in size, ranging from the largest known example, Vesta, with a diameter of approximately 326 miles (525 kilometers), to bodies less than 33 feet (10 meters) across.
• Recently, Vesta claimed the title of the largest asteroid, displacing Ceres, now classified as a dwarf planet by NASA. Ceres holds the distinction of being the largest object in the main asteroid belt, while Vesta ranks as the second largest.
• In addition to a range of sizes, asteroids also exhibit various shapes, from near spheres to irregular double-lobed peanut-shaped formations like Itokawa. The surfaces of most asteroids are marked with impact craters resulting from collisions with other celestial bodies.
• Although the majority of asteroids reside within the asteroid belt, the significant gravitational influence of Jupiter can propel them in random trajectories, including into the inner solar system and towards Earth. However, NASA's Planetary Defense Coordination Office is actively monitoring near-Earth objects (NEOs), including asteroids, to evaluate potential impact hazards and assist in devising strategies for responding to any impending threats.

What constitutes an asteroid?

• According to NASA's definitions, an asteroid is described as "A relatively small, inactive, rocky body that orbits the sun." Conversely, a comet is characterized as a "relatively small object that can exhibit activity, with its ices vaporizing in sunlight to form an atmosphere (coma) consisting of dust and gas, and occasionally, a tail composed of dust and/or gas."
• Furthermore, NASA defines a meteorite as a "meteoroid that survives its passage through the Earth's atmosphere and lands on the Earth's surface," while a meteor is delineated as a "light phenomenon resulting from the vaporization of a meteoroid upon entering the Earth's atmosphere; colloquially known as a shooting star."

What constitutes an asteroid?

• According to NASA's definitions, an asteroid is described as "A relatively small, inactive, rocky body that orbits the sun." Conversely, a comet is characterized as a "relatively small object that can exhibit activity, with its ices vaporizing in sunlight to form an atmosphere (coma) consisting of dust and gas, and occasionally, a tail composed of dust and/or gas."
• Furthermore, NASA defines a meteorite as a "meteoroid that survives its passage through the Earth's atmosphere and lands on the Earth's surface," while a meteor is delineated as a "light phenomenon resulting from the vaporization of a meteoroid upon entering the Earth's atmosphere; colloquially known as a shooting star."
• During the tumultuous early stages of the solar system, celestial materials underwent repeated collisions, with minuscule grains aggregating to form small rocks, which in turn coalesced to create larger rocks and eventually planetesimals—entities that failed to attain sufficient mass to evolve into planets. Subsequent collisions fragmented these planetesimals, giving rise to the asteroids observable today.
• "All of these events occurred approximately 4.5 billion years ago, yet the solar system has retained its dynamic nature throughout subsequent epochs," commented Jourdan. "Over the next several billion years leading up to the present, certain asteroids collided with one another, leading to their destruction, while the resultant debris reassembled to form what we now identify as rubble pile asteroids."
• Consequently, asteroids can exhibit variability in their composition and structure. While some asteroids consist of a single, cohesive monolithic body, others, such as Bennu, represent conglomerates of smaller bodies loosely bound together by gravitational forces, essentially forming floating rubble piles.

Asteroids are categorized into three main composition classes:

• C-type, also known as 'chondrite', primarily composed of clay and silicate.
• S-type, referred to as 'stony', comprised of silicate and nickel-iron.
• M-type, known as 'metallic', predominantly consisting of nickel-iron.

The variances in asteroid composition are linked to the distance from the Sun at which the asteroid originated. Proximity to the Sun and subsequent exposure to high temperatures post-formation resulted in asteroid melting, causing the iron to descend towards the core while forcing basaltic (volcanic) lava to surface.

How frequently do asteroids collide with Earth?

• Asteroids of sufficient size to cause ground-level damage strike Earth approximately once per century. As asteroid size increases, the frequency of impacts diminishes. 
• At the lower end of the size spectrum, desk-sized asteroids impact Earth about once a month, producing spectacular fireballs as they disintegrate in the atmosphere. Larger asteroids, however, strike with progressively less frequency.

What distinguishes asteroids, meteorites, and comets?

• Asteroids are rocky or dusty bodies orbiting the Sun. Meteorites are remnants left on the ground after asteroids disintegrate in the atmosphere. Most asteroids are fragile, disintegrating upon atmospheric entry and showering the ground with meteorites.
• Comets, like asteroids, orbit the Sun, but they also contain ices that sublimate when near the Sun, forming a gas and dust atmosphere and tail.

How do we detect potentially hazardous asteroids?

• Upon discovery, asteroids are closely monitored using Earth-based telescopes. Tracking data is then input into orbit determination software to calculate the asteroid's trajectory and predict its future path. 
• With extensive tracking data spanning days, weeks, months, or even years, the orbits of most asteroids can be accurately predicted, allowing for early detection of any potential impact hazards. NASA's Jet Propulsion Laboratory maintains the Sentry Risk table, listing asteroids with even a remote possibility of impacting Earth over the next century.

What are Near Earth Objects (NEOs) and Potentially Hazardous Asteroids (PHAs)?

• Asteroids are further categorized based on their size and their proximity to Earth. According to the Centre for Near Earth Object Studies (CNEOS), the size of asteroids is estimated using their brightness, known as "absolute magnitude," and their albedo, which measures how effectively they reflect sunlight back into space.
• "A Near-Earth Object (NEO) is defined as an asteroid or comet whose orbit brings it into the Earth's vicinity within the Solar System," stated Chodas. While most asteroids follow relatively circular orbits within the asteroid belt between Mars and Jupiter, NEOs orbit closer to the Sun, often intersecting with Earth's orbit.
• In terms of orbital characteristics, NASA classifies NEOs as objects that approach Earth at distances less than 1.3 astronomical units (AU), with 1 AU representing the average distance between Earth and the Sun. 

The majority of NEOs fall under the category of "Near Earth Asteroids" (NEA), which are further subdivided into three groups:

• Athenas: NEAs that intersect Earth's orbit with a semi-major axis smaller than Earth's orbit. They are named after 2062 Aten.
• Apollos: NEAs whose orbits cross Earth's orbit, but with a semi-major axis larger than Earth's orbit. They are named after 1862 Apollo.
• Amors: Earth-approaching NEAs with orbits outside Earth's orbit but inside the orbit of Mars.

Additionally, there exists a subset of asteroids known as Potentially Hazardous Objects (PHOs). These are asteroids large enough to pose a threat if they were to collide with Earth, typically exceeding 140 meters (about 500 feet) in diameter and approaching within approximately 5 million miles of Earth's orbit. However, proximity alone does not guarantee an impact, as the relative motions of Earth and the PHO around the Sun may prevent a collision in the near future.

Do asteroids frequently collide with Earth, and what are the consequences?

While asteroid impacts were more frequent in the early solar system, occurring between 7.8 million and 11.7 million years ago, today they are relatively rare. Nevertheless, occasional large and devastating impacts occur, such as the Chicxulub impactor event, believed to have occurred 6.5 million years ago, which led to the extinction of 75% of Earth's species, including the dinosaurs. However, impacts from such large asteroids are infrequent, and currently, there are no known asteroids large enough to cause a significant threat to Earth in the foreseeable future.

Can Earth be saved from an asteroid impact?

Efforts to detect and potentially deflect asteroids are ongoing. While numerous concepts exist, the Double Asteroid Redirection Test (DART) mission represents a practical testing of asteroid deflection techniques. By altering the trajectory of a small asteroid using a kinetic impactor, such as DART, humanity may have the capability to divert asteroids on a collision course with Earth, provided sufficient advance notice is available. Continued study of asteroids and meteorites is essential for developing effective deflection strategies tailored to the characteristics of each asteroid type.