Echidna (moon)

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(42355) Typhon I (Echidna)
Provisional or systematic name S / 2006 (42355) 1
Central body (42355) Typhon
Properties of the orbit
Major semi-axis 1580 ± 20 km
Periapsis 779 km
Apoapsis 2381 km
eccentricity 0.507 ± 0.009
Orbit inclination 42 ° ± 2 °°
Orbital time 18.982 ± 0.001 d
Mean orbital velocity 0.0031 km / s
Physical Properties
Apparent brightness 6.35 mag
Medium diameter km
Medium density ≈ g / cm 3
Acceleration of gravity on the surface ≈ 0 m / s 2
Escape speed ≈ 0 m / s
discovery
Explorer

Keith S. Noll
Harold F. Levison
William M. Grundy
Denise C. Stephens

Date of discovery January 20, 2006
Remarks First centaur moon discovered.

Echidna is a moon of the SDO - Centaur asteroid (42355) Typhon . Its mean diameter is 89 kilometers . This is just over half the diameter of Typhon. Echidna is the first moon to be discovered by an asteroid belonging to the centaur group .

Discovery and naming

Echidna was discovered on January 20, 2006 by Keith S. Noll, Harold F. Levison, Will M. Grundy, and Denise C. Stephens using the high-resolution camera of the Hubble Space Telescope . Echidna was found at 0.11 ± 0.01 arc seconds from the Typhon discovered four years earlier, with a difference in apparent magnitude of 1.30 ± 0.06. The moon was discovered as part of a program to determine the frequency of multiple systems. Since the moon was found after only 8 examined centaurs, this indicates that multiple systems between the orbits of the giant planets can , contrary to expectations, be stable and thus discoveries of comets' companions are possible.

The discovery was announced on February 15, 2006; the companion was given the provisional designation S / 2006 (42355) 1 . On November 23, 2006, together with the systems Logos / Zoe and Ceto / Phorcys , the moon was officially named by the International Astronomical Union (IAU) after Echidna , who in Greek mythology was the wife of Typhon and mother of numerous well-known monsters such as Cerberus , Hydra , the Chimera or the Sphinx . Half nymph, half snake, the immortal Echidna lived in a cave called Arima.

Track properties

Echidna orbits Typhon in a progressive , highly elliptical orbit between 779 and 2381 km from its center (major orbit half-axis 1580 km or approx. 19.5 Typhon or 35.5 Echidna radii). This results in an average distance between the two surfaces of about 1454.5 km, assuming that both bodies are round. Since both revolve around the common center of gravity , the system is to be understood as a double asteroid system. The orbit eccentricity is 0.507, the orbit is inclined 42.2 ° with respect to the Typhon equator . 18.982 Echidna and Typhon orbit each other in 18 days, 23 hours and 34.1 minutes, which corresponds to about 4,517.2 orbits in a Typhon year (around 234.76 earth years).

Physical Properties

Echidna has a diameter of 89 km (54.9% of the central body), based on the assumed equal reflectivity of 5% corresponding to the typhon. The surface is therefore extremely dark. Unlike most asteroids, the two seem to consist primarily of water ice instead of rock , which suggests that they are formed at a greater distance from the sun . The exceptionally low density of 0.44 (+ 0.44 / -0.17) g / cm 3 - which is lower than water - is more like a cometary composition. Another possibility would be that Typhon and Echidna are porous inside and belong to the Rubble Piles .

Provisions of the diameter for echidna
year Dimensions km source
2008 84.0 +16.0−18.0 Grundy et al. a.
2011 61.5 Grundy et al. a.
2012 89.0 ± 6.0 Santos – Sanz et al. a.
2012 77.0 ± 16.0 Stansberry et al. a.
2014 <42.0 Thirouin et al. a.
The most precise determination is marked in bold .

exploration

Since its discovery in 2006, Echidna has been observed through the Hubble Space Telescope as well as through terrestrial telescopes and its orbital elements have been determined.

The common theories of comet formation see the centaurs of the outer giant planets Uranus and Neptune as the forerunners of the short-period comets. The discovery of a satellite in this asteroid class therefore raises the question of possible satellites of comets. If the frequency of multiple systems among the centaurs should actually be greater than previously assumed, a significant proportion of multiple systems can also be expected among the (short-period) comets - however, no “cometary moon” has yet been discovered; the actual frequency of multiple systems can, however, only be determined through further theoretical analyzes and an enlarged database through more precise observation of further centaurs.

See also

Web links

Individual evidence

  1. ↑ W. Grundy et al .: (42355) Typhon-Echidna: Scheduling observations for binary orbit determination (April 2008)
  2. ^ W. Grundy et al .: Five New and Three Improved Mutual Orbits of Transneptunian Binaries (March 2011)
  3. ↑ P. Santos – Sanz et al .: "TNOs are Cool": A Survey of the Transneptunian Region IV. Size / albedo characterization of 15 scattered disk and detached objects observed with Herschel Space Observatory-PACS (February 2012)
  4. ↑ J. Stansberry et al .: Physical properties of trans-neptunian binaries (120347) Salacia-Actaea and (42355) Typhon-Echidna (June 2012)
  5. ↑ A. Thirouin include: Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt (July 2014)