Hiisi (moon)

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(47171) Lempo II (Hiisi)
Provisional or systematic name S / 2007 (47171) 1
Central body (47171) Lempo
Properties of the orbit
Major semi-axis 867 ± 11 km
Periapsis unknown
Apoapsis unknown
Orbital time 1.9070 ± 0.0001 d
Physical Properties
Albedo 0.079 + 00.013- 00.011
Medium diameter 251.0 + 016.0- 017.0 km
Dimensions ~ 6 - 7 · 10 19 kg
surface 198,000 km 2
Medium density 0.64 + 00.15- 00.11g / cm 3
Surface temperature 44–45 (−229 to −228 ° C) K
discovery
Explorer

?
( Hubble Space Telescope )

Date of discovery October 2007
Remarks Second largest component of the (47171) Lempo triple system

Hiisi (systematic name S / 2007 (47171) 1 ) is - depending on the definition - the component that is only 7.6% smaller or the larger, inner moon of Plutino (47171) Lempo . The discovery of this companion led to the first known multiple asteroid system in the Kuiper Belt . As a special feature, the total of three components are comparable in size.

Discovery and naming

Hiisi was discovered by a team of astronomers in October 2007 through examination of images of the system taken with the Hubble Space Telescope . The discovery was announced in October 2007; the moon was given the provisional designation S / 2007 (47171) 1 . The discovery was confirmed in 2009.

On October 5, 2017, the Minor Planet Center announced the name Hiisi . Hiisi is - like Paha - one of the helpers of the god Lempo from Finnish mythology . The other two components of the system have been named after Lempo and Paha.

Since the discovery of the companion, the term "(47171) 1999 TC was 36 " officially for the largest component of the system (A 1 used), while for the smaller component, the designation S / 2007 (47171) 1 and for the more distant moon S / 2001 (47171) 1 applied. But the name is often used for the whole system. Unofficially, the companion is therefore sometimes referred to as "(47171) 1999 TC 36 A 2 " or simply "A 2 " .

The Lempo system has been observed through several space-based and earth-based telescopes. In December 2017, there were a total of 331 observations over a period of 43 years.

properties

Orbit

Hiisi and Lempo orbit one another around the common center of gravity in an orbit at a distance of 867 km (about 6.3 A 1 and 6.9 A 2 radii) from the centers of the two bodies; this results in an average distance between the surfaces of the two bodies of about 606 km.

The orbital eccentricity and inclination are not yet determined.

The orbital time of Hiisi is 1 day 21 hours and 45.8 minutes. Its orbit is well within the Hill radius of 590,000 km.

rotation

Hiisi's rotation period has not yet been determined with certainty. It may also be 21 hours and 45.8 minutes for 1 day. In this case the orbital time would be the same as the rotation time, and since the larger component Lempo would have to behave in the same way, it would be a doubly bound rotation . This means that both bodies always face the same side. According to the Pluto / Charon system , it would be the first time that a further double-bound rotation in the Kuiper belt could be detected. In the main belt , this is about at (90) Antiope A and B the case. The prerequisite for this is a similar mass and a narrow orbit of the two bodies. In this case, like the main body, the companion would perform 47,022.4 self- rotations in a (47171) Lempo year .

size

The most precise determination of the diameter ( geometric mean ) Hiisis is 251 km. The companion thus has 92.7% of the diameter of the main body. Since the third component of the Paha system has roughly half the diameter of the two main components, the size ratio is roughly 2: 2: 1, which is unique in the solar system.

Based on an average diameter of 251 km, the surface area is about 198,000 km², which is just about the size of Kyrgyzstan .

Provisions of the diameter for hiisi
year Dimensions km source
2009 258.0 + 041.0- 035.0 Benecchi et al. a.
2010 265.0 + 041.0- 035.0 Benecchi et al. a.
2012 251.0 + 016.0- 017.0 Mommert et al. a.
The most precise determination is marked in bold .

internal structure

The unusually low mean density of 0.64 g / cm³ is an indication that it is not a compact body, but that the object should be a rubble pile , an accumulation of dust and rocks, which is interspersed with cavities. The porosity is estimated to be up to 68%.

Hiisi's mass should be around 6 - 7 · 10 19 ; the total mass of the system is about 1.3 ∙ 10 19 .

The mean surface temperature is 44–45  K (−229 to −228 ° C).

Emergence

So far, there are two hypotheses about the origin of the triple system. Either it came about as a result of a major collision, after which the entire system was re-formed from the accretion disk that had arisen, or the third component was gravitationally captured by the already existing double asteroid. The similar sizes and masses of the A 1 and A 2 components favor the latter hypothesis.

See also

Web links

Individual evidence

  1. ^ Seth Jacobson, Jean-Luc Margot: Colors of TNO Binaries and Evidence for a Triple System from HST Observations (October 2007). Retrieved September 17, 2017 .
  2. ↑ MPC / MPO / MPS archives . Minor Planet Center. Retrieved December 15, 2017.
  3. ↑ JPL: 47171 Lempo (1999 TC 36 ) at the JPL. Retrieved December 14, 2017 .
  4. ↑ SD Benecchi et al .: (47171) 1999 TC36, A Transneptunian Triple (September 2009) (PDF). Retrieved September 17, 2017 .
  5. ↑ SD Benecchi et al .: (47171) 1999 TC 36 , A transneptunian triple (June 2010). Retrieved September 17, 2017 .
  6. ↑ M. Mommert, AW Harris, C. Kiss, A. Pál, P. Santos-Sanz, J. Stansberry, A. Delsanti, E. Vilenius, TG Müller, N. Peixinho, E. Lellouch, N. Szalai, F. Henry, R. Duffard, S. Fornasier, P. Hartogh, M. Mueller, JL Ortiz, S. Protopapa, M. Rengel, A. Thirouin: “TNOs are Cool”: A survey of the trans-Neptunian region. V. Physical characterization of 18 Plutinos using Herschel-PACS observations . In: Astronomy and Astrophysics . 541, no.A93 , May 4, 2012. arxiv : 1202.3657 . bibcode : 2012A & A ... 541A..93M . doi : 10.1051 / 0004-6361 / 201118562 .