164207 Cardea

164207 Cardea (provisional designation 2004 GU₉) is a sub-kilometer asteroid, classified as near-Earth object and potentially hazardous asteroid of the Apollo group. It is a quasi-satellite of Earth, a situation that should persist until around 2600, when it is expected to shift to a regular horseshoe orbit for a few thousand years.^[4]

164207 Cardea

Cardea photographed by the Canada–France–Hawaii Telescope in February 2007
Discovery[1]
Discovered by	LINEAR
Discovery site	Lincoln Lab's ETS
Discovery date	13 April 2004
Designations
MPC designation	(164207) Cardea
Pronunciation	/ˈkɑːrdiə/
Named after	Cardea
Alternative designations	2004 GU9
Minor planet category	NEO · Apollo
Orbital characteristics[2]
Epoch 21 November 2025 (JD 2461000.5)
Uncertainty parameter 0
Observation arc	8688 days (23.79 yr)
Aphelion	1.1372 AU (170.12 Gm)
Perihelion	0.8650 AU (129.40 Gm)
Semi-major axis	1.0011 AU (149.76 Gm)
Eccentricity	0.1359
Orbital period (sidereal)	1.0017 yr (365.87 d)
Mean anomaly	97.3298°
Mean motion	0° 59m 1.464s / day
Inclination	13.6529°
Longitude of ascending node	38.3866°
Longitude of perihelion	280.55672±0.00007°
Time of perihelion	2456145.53817±0.00006 jd
Argument of perihelion	279.3410°
Earth MOID	0.0031 AU (460,000 km)
Physical characteristics
Mean diameter	163 m[3]: 2988
Geometric albedo	0.219[3]: 2988
Absolute magnitude (H)	21.1[2]

On 14 April 2004 (with less than a 1-day observation arc), the Sentry Risk Table showed 180 virtual impactors.^[5] It was removed from the Sentry Risk Table 2 days later on 16 April 2004.^[6] As later precovery observations by Haleakala-AMOS from 2001 have been found, Cardea now has a well determined orbit with an observation arc of 24 years.^[1]

Discovery and naming

This asteroid was discovered on 13 April 2004 by the Lincoln Near-Earth Asteroid Research (LINEAR) project near Socorro, New Mexico and given the provisional designation 2004 GU₉.^[1] Following the naming of 524522 Zoozve, a quasi-satellite of Venus, Radiolab and the International Astronomical Union held a public naming campaign for this quasi-satellite from June to September 2024.^[7][8] Seven finalist names were revealed in December 2024, with the names being Bakunawa, Cardea, Ehaema, Enkidu, Ótr, Tarriaksuk, and Tecciztecatl.^[9] The winning name was Cardea, the Roman goddess of the hinge. The name was announced by the International Astronomical Union on 13 January 2025.^[10]

Orbit

Cardea orbits the Sun with a semi-major axis of 1.001 astronomical units (AU), taking 365.87 days to complete one orbit. It is classified as a near-Earth asteroid (NEA) and an Apollo asteroid;^[2] Apollo asteroids are NEAs that cross Earth's orbit but have semi-major axes above 1 AU.^[11] Its orbit is inclined by 13.653° with respect to the ecliptic plane. Along its orbit, its distance from the Sun varies from 0.865 AU at perihelion to 1.137 AU at aphelion due to its moderate^[3]: 2987 orbital eccentricity of 0.136.^[2]

Cardea is in a 1:1 mean-motion resonance with Earth, and it currently orbits in a quasi-satellite configuration.^[4]: 489 Quasi-satellites have principal libration angles ${\displaystyle \sigma =\lambda -\lambda _{p}}$^[a] that librate around 0°,^[4]: 488 appearing to distantly orbit Earth from its perspective despite directly orbiting the Sun. All Earth quasi-satellites are temporary;^[12] Cardea has been a quasi-satellite for about 600 years, and will eventually lose its status as one in about 500 years. Perturbations from Venus play a role in destabilizing Cardea from its quasi-satellite configuration even though their orbits do not cross. Currently, the libration of Cardea's ${\displaystyle \sigma }$ has an amplitude of 8–10°, with a libration period of 70 years. After exiting its quasi-satellite phase, it will enter a horseshoe configuration.^{[4]: 490–491}

Physical characteristics

Cardea has a diameter of 163 metres (535 ft) and an albedo of 0.219.^[3]: 2988

Animation of 164207 Cardea's orbit from 1600 to 2500

Relative to Sun and Earth

Around Earth

Around Sun

   Sun ·    Earth ·   Cardea

See also

• (277810) 2006 FV35, another quasi-satellite of Earth

Notes

1. Where ${\displaystyle \lambda }$ and ${\displaystyle \lambda _{p}}$ are the mean longitudes of Cardea and Earth, respectively