2005 YU55: The 400m Asteroid That Grazed Earth

Trajectory of asteroid 2005 YU55 (November 2011) — In 2011, the 400-meter colossus asteroid 2005 YU55 passed between Earth and the Moon at just 324,600 km from us, a record distance for an object of this size, scrutinized in detail by ground-based radars.
Image source: astronoo.com (new window) — AI-generated image, public domain.

Scientific summary

Astronoo's article recounts the close approach of the asteroid 2005 YU55, a C-type near-Earth object measuring 400 meters in diameter. On November 8, 2011, it passed just 324,600 km from Earth, closer than the Earth-Moon distance. This event enabled an unprecedented radar observation campaign, conducted by NASA (Goldstone and Arecibo), which revealed a nearly spherical shape, a slow rotation of 18 hours, and a primitive carbonaceous composition. Orbital data and future trajectory modeling, including until 2136, confirm that no collision is to be feared. This approach underscores the importance of planetary surveillance programs for the early detection of potentially hazardous objects.

Why did the approach of asteroid 2005 YU55 in 2011 mark astronomers?

This article is dedicated to the asteroid 2005 YU55, a celestial object about 400 meters in diameter that passed at an exceptionally close distance from Earth on November 8, 2011. But what made this flyby so remarkable for the scientific community? The answer lies in the rarity of such an event: it is one of the largest asteroids to have come this close to our planet since radar observations enabled precise tracking of these bodies. Its distance of 324,600 km was less than that separating us from the Moon, offering astronomers a unique opportunity to study a C-type (carbon-rich) near-Earth object with unprecedented resolution. This flyby not only allowed characterization of its shape, rotation, and composition, but also highlighted the effectiveness of planetary defense systems and the importance of maintaining them to anticipate potential future threats. The article explores in detail the orbital data, the asteroid's composition, and prospects for planetary defense.

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What are near-Earth asteroids?

A near-Earth asteroid is an object whose orbit approaches within 1.3 astronomical units (AU) of the Sun, i.e., within 195 million kilometers. This means it could potentially approach Earth dangerously. Near-Earth objects are divided into orbital subclasses (Apollo, Amor, Atira, Aten) according to the shape and location of their orbit, but their composition is a separate classification.

Classification of near-Earth asteroids (NEA) by orbital type
Type	Semi-major axis (a)	Perihelion (q)	Aphelion (Q)	Main characteristic
Atira (or Apohele)	<1 AU	>0.983 AU	<1 AU	Orbits entirely inside Earth's orbit
Aten	<1 AU	<1.017 AU	>1 AU	Mostly inner orbits, but cross Earth's orbit
Apollo	>1 AU	<1.017 AU	>1 AU	Mostly outer orbits, also cross Earth's orbit
Amor	>1 AU	between 1.017 and 1.3 AU	-	Approach Earth's orbit without crossing it

2005 YU55: When a 400m asteroid grazes Earth at 324,600 km

An exceptional flyby

On November 8, 2011, the asteroid 2005 YU55 passed just 324,600 km from Earth, a distance less than that separating our planet from the Moon (384,400 km). This 400-meter-diameter object, belonging to the class of C-type (dark, carbon-rich) near-Earth asteroids, represents one of the largest celestial objects to have come this close to Earth since radar observations began tracking these bodies. This event, described as a flyby, posed no immediate danger but enabled an exceptional observation campaign conducted by NASA, particularly via the Goldstone and Arecibo radar antennas.

Physical characteristics and composition

2005 YU55 is a dark asteroid, with an albedo of only 0.06, typical of carbon-rich objects (spectral type C). Its shape is nearly spherical, and its rotation is relatively slow with a period of 18 hours. According to radar data, it is composed of primitive carbonaceous materials, similar to those found in some chondritic meteorites. Spectral analysis indicates a mixed rocky composition, potentially rich in volatile compounds, making it an excellent candidate for a future robotic or crewed exploration mission.

2005 YU55: Physical, orbital, and energetic characteristics

The asteroid 2005 YU55 has physical and orbital parameters that make it a particularly studied object by astronomers. Classified as a potentially hazardous asteroid (PHA) due to its size and proximity to Earth's orbit, it nevertheless follows a perfectly well-understood trajectory for the coming decades. The table below summarizes all its essential characteristics, from its dimensions to its potential impact energy, including the key elements of its orbit around the Sun.

Summary table of physical and orbital parameters

The asteroid 2005 YU55 is classified as a PHA (Potentially Hazardous Asteroid), but orbital ephemerides computed by CNEOS/NASA show no significant probability of collision at least until 2136. Its extremely low inclination (0.4° relative to the ecliptic plane) means its orbit is almost perfectly aligned with Earth's, facilitating trajectory calculations and radar observations. Like any object in orbit, 2005 YU55 crosses the ecliptic plane twice per revolution — at the ascending and descending nodes — key passages for impact risk calculations.

Complete characteristics of asteroid 2005 YU55
Parameter	Value	Comment
Diameter	≈ 400 m	Size comparable to a skyscraper
Estimated mass	~6 to 8 × 10¹⁰ kg	For an average density of 2,500 to 3,000 kg/m³
Estimated impact velocity	~12 to 20 km/s	Typical speed: 17 km/s
Orbit type	Apollo	NEA with semi-major axis >1 AU, perihelion <1.017 AU
Semi-major axis	1.142 AU	Approximately 171 million kilometers
Eccentricity	0.429	Moderately elliptical orbit
Perihelion	0.847 AU	Minimum distance to the Sun (≈ 127 million km)
Aphelion	1.437 AU	Maximum distance to the Sun (≈ 215 million km)
Inclination	0.4°	Very low, almost in the ecliptic plane
MOID (Earth)	~0.0022 AU	≈ 330,000 km (less than Earth-Moon distance)
Impact energy	≈ 2,400 megatons of TNT	Tens of thousands of times Hiroshima
Collision probability	Once every 80,000 to 150,000 years	Order of magnitude for an object of this size

2005 YU55: Surveillance and modeling

Radar observations and topography

The approach of 2005 YU55 enabled high-resolution radar measurements, allowing reconstruction of its topography and estimation of its surface properties. The combination of radar technologies, space telescopes, and orbit calculations now ensures effective planetary surveillance.

Challenges for planetary defense

Although 2005 YU55 continued on its course without consequence, it reminds us of the vulnerability of our planet to wandering bodies from the cosmos. Nevertheless, this approach highlighted the need for space surveillance programs such as NEO-WISE or Pan-STARRS, in order to anticipate potential impacts from objects of similar sizes.

References

CNEOS - Center for Near Earth Object Studies (NASA/JPL) (new window)
NASA/JPL - Goldstone Radar Observations of 2005 YU55 (technical archive) (new window)
NASA/JPL - News: radar images of 2005 YU55 (2011) (new window)
arXiv preprint:1111.3945 (new window) — The properties of the distribution of Gaussian packets on a spatial network (math-physics, not related to the asteroid, kept for reference).

FAQ: Everything you need to know about asteroid 2005 YU55

What is asteroid 2005 YU55?

2005 YU55 is a near-Earth asteroid (NEA) of type C, rich in carbon, about 400 meters in diameter. It was discovered in 2005 by Robert McMillan during the Spacewatch program. It is classified as a potentially hazardous asteroid (PHA) due to its size and orbit that crosses Earth's, but calculations show it poses no collision risk at least until 2136.

How far was 2005 YU55 from us during its 2011 pass?

On November 8, 2011, the asteroid passed within 324,600 km of Earth. This distance is less than the Earth-Moon distance (384,400 km), making it one of the largest objects to have come so close to our planet.

Why was this flyby so important to scientists?

This close passage offered a unique opportunity to observe an asteroid of this size with great precision. The Goldstone and Arecibo radars allowed mapping of its surface, determination of its nearly spherical shape, its slow rotation period (18 hours), and its carbonaceous composition. These data are valuable for understanding the nature of primitive objects in the solar system and for preparing potential deflection or exploration missions.

Could 2005 YU55 collide with Earth?

No, according to current orbital ephemerides computed by CNEOS (NASA), there is no significant probability of collision with Earth at least until 2136. Its trajectory is perfectly known and poses no long-term risk to our planet.

What would happen if an asteroid of this size hit Earth?

An object 400 meters in diameter like 2005 YU55 would release an impact energy estimated at about 2,400 megatons of TNT, equivalent to tens of thousands of times the Hiroshima bomb. Such an impact would cause massive destruction on a regional scale, tsunamis in case of an oceanic impact, and global climate disruptions due to dust injected into the atmosphere.

How do astronomers monitor near-Earth asteroids?

Astronomers use a network of ground-based and space telescopes, such as Pan-STARRS, the Catalina Sky Survey program, and the NEO-WISE space mission. Radar observations, such as those conducted with Goldstone and Arecibo, are also essential for refining orbits and characterizing the surface of objects. Agencies like NASA and ESA coordinate these efforts as part of planetary defense.