NearEarth object

Near-Earth objects are small bodies in the Solar System whose orbits bring them into relative proximity with Earth. These objects—mostly asteroids and a small fraction of comets—are significant both for scientific study and for planetary-defence planning. Their orbits around the Sun occasionally enable close approaches to Earth, and in rare cases, impacts that may influence geological or biological history.

Definition and Classification

A near-Earth object is formally defined as any small Solar System body whose orbit brings it to within 1.3 astronomical units of the Sun. This definition is based on orbital parameters rather than the object’s current location, allowing bodies that are far from Earth at a given time to be categorised as NEOs if their orbital paths meet the criteria.
NEOs include two principal groups:

• Near-Earth asteroids, rocky remnants from the early Solar System.
• Near-Earth comets, usually short-period comets with orbital periods under 200 years.

A particularly important subset is the potentially hazardous object, comprising bodies whose orbits intersect that of Earth and whose estimated diameter exceeds 140 metres. These are classified using measurements such as the minimum orbit intersection distance and absolute magnitude, providing an indication of possible impact consequences.

Population and Characteristics

Tens of thousands of near-Earth asteroids have been identified, alongside more than one hundred known near-Earth comets. Many NEOs follow complex orbital paths that are continually influenced by gravitational interactions, especially with Earth. Some temporarily enter Earth-centred orbits before resuming solar orbits.
Although most NEOs are relatively small, even bodies only a few tens of metres in diameter can cause substantial regional damage if they enter the atmosphere. Larger objects may reach the surface with enough energy to form impact craters on land or generate large tsunamis at sea. Scientific consensus recognises that major impacts in Earth’s past have played a role in shaping its environmental and biological evolution.

Detection and Monitoring

Interest in NEOs increased in the late twentieth century due to heightened awareness of their potential hazard. International monitoring efforts led to the establishment of large-scale surveys under the collective label Spaceguard. These programmes use ground-based telescopes and increasingly sensitive detection technologies to catalogue objects of varying sizes.
The initial target of identifying 90 per cent of NEOs of one kilometre or more in diameter—a class capable of causing global devastation—was achieved in 2011. Survey goals have since expanded to detect smaller bodies that might cause significant local or regional impacts.
To communicate risks, two scales are commonly used:

• The Torino scale, providing a simple categorisation of impact probability and damage potential.
• The Palermo scale, combining event probability and expected damage relative to background impact risk.

Close Approaches and Impact Events

Throughout history, comets were the first NEOs observed. Their extraterrestrial origin was confirmed in the sixteenth century, and by the eighteenth century astronomers could predict their returns. The nineteenth century saw recognition that meteoroid streams such as the Leonids were linked to cometary debris.
The first identified near-Earth asteroid, Eros, was discovered in 1898. Subsequent discoveries and improvements in observational techniques have led to a rapidly increasing catalogue of known NEOs. As surveys improved, astronomers began to identify objects passing closer to Earth than the Moon’s orbit. Numerous close approaches are now detected each year, including instances of very small bodies grazing Earth’s atmosphere or passing only a few thousand kilometres from the surface.
Radar observations supplement optical tracking and can determine shape, rotation and surface features during close approaches. Some objects are accompanied by small moons, adding further complexity to their behaviour.

Space Missions and Exploration

NEOs have played an increasingly important role in planetary science due to their accessibility. Several near-Earth comets and asteroids have been visited by spacecraft, providing detailed information about their structure and composition. Sample-return missions have successfully collected and returned material to Earth, offering insights into the early Solar System. Experiments have also demonstrated techniques for altering the trajectory of small bodies, providing foundational knowledge for impact-mitigation strategies.

Mitigation and Future Prospects

Deflection technologies, such as kinetic impactors or gravitational tractors, have been studied as potential means of reducing impact risk. Although such methods are technically challenging, successful demonstrations have shown that changing the course of a small object is possible in principle.
The relatively low surface gravity of many NEOs and their proximity to Earth have led to proposals for commercial resource extraction. While several conceptual studies have been produced, few have progressed beyond early development.
Near-Earth objects remain a focus of international cooperation, scientific study and public interest. Their dual role as both potential hazards and sources of scientific knowledge underscores their importance in contemporary planetary science and long-term strategies for safeguarding Earth.