Orbits of Near-Earth Asteroids: When Asteroids Brush Past Earth

Near-Earth Asteroids

Near-Earth asteroids, or NEOs (Near-Earth Objects), are celestial bodies whose orbits cross or come close to Earth's orbit. Their orbit is characterized by a perigee of less than 1.3 astronomical units (AU), or 195,000,000 km. These objects are mainly divided into four families based on their orbital parameters: Amors (orbit outside Earth's orbit), Apollos and Atens (orbits crossing Earth's orbit), and Atiras (entirely within Earth's orbit).

Near-Earth Asteroids: The Four Families

The Amors are asteroids near Earth but whose orbit remains outside that of our planet. Their perihelion (closest point to the Sun) is between 1.017 AU (minimum Earth-Sun distance) and 1.3 AU. Although they do not directly cross Earth's orbit, they are considered near objects (NEO) because they can be disturbed by the inner planets and eventually become effective Earth-crossers. Examples: 1221 Amor or 433 Eros.

The Apollos constitute the most numerous family of Earth-crossers. They have a semi-major axis greater than 1 AU and an orbit that crosses that of Earth. Their perihelion is less than 1.017 AU, meaning they pass inside Earth's orbit. Their orbital eccentricity is often high, making them sensitive to gravitational perturbations. Famous example: 1862 Apollo, which gave its name to the family.

The Atens have an orbital behavior opposite to that of the Apollos. Their semi-major axis is less than 1 AU but their aphelion exceeds 1 AU, which also leads them to cross Earth's orbit. Their passages are more frequent near Earth because their orbital period is less than one year. Due to their short period, they represent a class of strategic interest for space missions. Example: 2062 Aten.

The Atiras (sometimes called Apohele asteroids or IEO for Inner Earth Object) are the rarest and most difficult to detect. Their orbit is entirely contained within that of Earth, with an aphelion less than 0.983 AU. These objects do not currently interact with Earth, but their proximity to the Sun makes their observation delicate from the ground. They are of growing interest in space surveillance. Example: 163693 Atira.

Asteroids: Chaotic and Unstable Orbits

The orbits of near-Earth asteroids are often highly elliptical, sometimes inclined, and sensitive to gravitational perturbations, particularly by giant planets like Jupiter. These interactions gradually modify their trajectory over time, a phenomenon modeled by Gauss's equations and the numerical integration of the equations of motion. An asteroid whose minimum orbit intersection distance (MOID) with Earth is less than 0.05 AU (7,500,000 km) is classified as a PHA (Potentially Hazardous Asteroid).

Why is an asteroid within 0.05 AU considered potentially hazardous?

The classification of a PHA (Potentially Hazardous Asteroid) is based on geometric, energetic, and dynamic criteria, regardless of its current distance. The threshold of 0.05 AU (or 7,479,894 km) corresponds to a minimum distance of intersection with Earth's orbit (MOID) low enough to represent a potential long-term threat. This value does not indicate an immediate danger but an orbital configuration a priori favorable to a future collision if other dynamic conditions are met.

This geometric criterion reflects the possibility that an asteroid will actually cross Earth's orbit at some point, under the influence of gravitational perturbations (notably from Jupiter or Mars) or non-gravitational effects such as the Yarkovsky effect. Thus, even if the asteroid is currently very far away, a spatio-temporal alignment between Earth and the asteroid could occur in the future.

An asteroid ≥140 m in diameter, crossing this 0.05 AU limit, has a potential impact kinetic energy on the order of 10¹⁷ J, or about 100 megatons of TNT. This is equivalent to more than 7,000 times the Hiroshima bomb. At a typical relative speed of 20 km/s, such an object would take only 4.3 days to travel 7.5 million kilometers. The short warning time justifies constant monitoring.

Finally, the orbits of near-Earth asteroids are chaotic over the long term. An initial MOID of 0.049 AU can, under the effect of orbital resonances or successive perturbations, quickly evolve into a MOID smaller than Earth's radius. This instability justifies the use of the 0.05 AU threshold as a scientific precautionary barrier. A PHA is therefore an object whose current orbital characteristics make it potentially dangerous in the coming decades or centuries.

Observation and Prediction of Trajectories

Thanks to programs like CNEOS (Center for Near Earth Object Studies), the orbits of near-Earth asteroids are monitored with precision. The calculation of their orbit is based on astrometric observation and the resolution of the perturbed Kepler equations:

$$ r(t) = \frac{a(1 - e^2)}{1 + e \cos(\theta)} $$

where \( a \) is the semi-major axis, \( e \) is the eccentricity, and \( \theta \) is the true anomaly. This model is then corrected to include perturbations and non-gravitational effects such as the Yarkovsky effect.

When Asteroids Brush Past Earth: Concerning Approaches

Each year, several dozen asteroids come close to Earth at distances less than that of the Moon. These events, called close approaches, are closely monitored by tracking centers such as NASA's CNEOS. An asteroid flyby is defined by an extremely low MOID and a temporal conjunction with Earth's orbit. If the object is large or passes within a few tens of thousands of kilometers, the situation becomes critical.

The case of asteroid 2020 QG is emblematic. This small object, about 5 to 10 meters in diameter, passed only 2,950 km from Earth's surface on August 16, 2020. This is the closest flyby ever observed for a non-impacting asteroid. It was detected after its passage, highlighting the limits of our detection system, particularly for low-albedo objects approaching from the direction of the Sun.

Another notable case is that of 2004 FU162, a 6-meter asteroid detected just a few hours before it passed 6,500 km from Earth on March 31, 2004. At this distance, Earth's gravity significantly altered its orbit. These gravitational perturbations can turn a benign passage into a concerning future trajectory.

Finally, the passage of asteroid 2023 BU on January 26, 2023 is a spectacular example. This 3 to 5-meter object brushed past Earth at an altitude of 3,600 km above South America. This extremely close flyby occurred within the orbit of geostationary satellites. Although too small to cause damage on the ground, 2023 BU could have disrupted or hit a strategic satellite. This event highlights the importance of a global short-range detection network.

Planetary Defense: Are We Ready to Stop an Asteroid?

To anticipate risks, missions like DART (NASA, 2022) aim to test asteroid deflection.

A real but rare threat

The probability that a large asteroid will collide with Earth in the short term remains extremely low. As the consequences of an impact would be catastrophic, scientists are scanning the sky for near-Earth objects (NEOs: Near-Earth Objects).

Identifying the threat: NEO surveillance

The ESA's NEOCC and NASA's CNEOS programs track more than 30,000 objects near Earth. The Large Synoptic Survey Telescope (LSST) of Vera Rubin (1928-2016), active since 2025, promises to map the night sky even more effectively. Thanks to the method of Keplerian orbital elements, each NEO is tracked with increasing precision. However, small objects a few tens of meters in size remain the most difficult to detect, although they are large enough to cause regional damage (such as the Tunguska event in 1908).

DART: the first deflection mission

On September 26, 2022, NASA conducted a full-scale test: the DART (Double Asteroid Redirection Test) mission. The goal was to alter the trajectory of Dimorphos, a 160 m asteroid orbiting Didymos, by impacting it at over 6 km/s. Result: Dimorphos' orbit was shortened by 33 minutes, experimental proof that we can alter an asteroid's trajectory through kinetic impact.

This mission is based on a simple but demanding principle: the conservation of momentum. A high-speed impact transfers enough momentum to slightly alter the orbit of a celestial body. Even a small change, applied early enough, can be enough to avoid a collision with Earth years later.

Defense Strategies

• Kinetic impact: altering the trajectory of an asteroid through kinetic impact.
• The gravity tractor: a massive spacecraft positions itself near the asteroid and uses its own gravity to slowly pull the object off its trajectory.
• Directed energy propulsion: high-power lasers could vaporize part of the asteroid's surface, creating a jet of material that acts as reverse thrust.
• Nuclear explosion: as a last resort, a nearby nuclear explosion (not in contact) could fragment or deflect the NEO. This method is controversial, as it can generate multiple fragments, potentially still dangerous.

Are we ready?

Technologically, the DART mission has shown that we can detect, track, and impact an asteroid. However, several limitations persist:

• It takes years of advance notice to effectively act on the trajectory of a threatening object.
• Small and fast objects are difficult to detect in time.
• There is not yet a coordinated international response system. Collaboration between NASA, ESA, and other agencies is crucial.

Faced with such an unpredictable cosmic danger, we are not yet ready, but each advance, each mission, brings us closer to the ability to protect our planet. This global technological and organizational challenge must be met.