47171 Lempo

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47171 Lempo
Lempo seen as a binary using HST in 2001
Discovered by E. P. Rubenstein
L.-G. Strolger
Discovery site Kitt Peak National Obs.
Discovery date 1 October 1999
MPC designation (47171) Lempo
Named after
Lempo (Finnish mythology)[1]
1999 TC36
TNO · Plutino[2][3]
Orbital characteristics[6]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 2
Observation arc 42.53 yr (15,534 days)
Aphelion 47.857 AU
Perihelion 30.552 AU
39.205 AU
Eccentricity 0.2207
245.48 yr (89,662 days)
0° 0m 14.4s / day
Inclination 8.4197°
Known satellites S/2001 (47171) 1:
(0.746±0.06)×1018 kg[5]
Physical characteristics

Assuming equal albedos



[4](effective system diameter)
Mass (12.75±0.06)×1018 kg (A1+A2+B)[5]
(14.20±0.05)×1018 kg (A1+A2)[5]
Mean density
Equatorial surface gravity
0.039–0.020 m/s2
Equatorial escape velocity
0.117–0.096 km/s
6.21 h (0.259 d)
Temperature ≈45–44 K
B−V= 1.00±0.13
V−R= 0.70±0.03[4]
5.41±0.10,[4] 5.0[6]

47171 Lempo, provisional designation 1999 TC36, is a trans-Neptunian object and trinary system from the Kuiper belt, located in the outermost regions of the Solar System. It was discovered in 1999,[8] by Eric Rubenstein and Louis-Gregory Strolger during an observing run at Kitt Peak National Observatory in Arizona, United States. Rubenstein was searching images taken by Strolger as part of the Low-Z Supernova Search program, it is classified as a plutino with a 2:3 mean-motion resonance with Neptune,[2] and, currently only being 30.5 AU from the Sun,[7] is among the brighter TNOs. It reached perihelion in July 2015, this minor planet was named after Lempo from Finnish mythology.[1]

Physical characteristics[edit]

Lempo is a triple system consisting of a central primary, which is itself a binary, and a small moon (component B).[5] The combined observations by the infrared Spitzer Space Telescope,[9] Herschel Space Telescope[4] and the Hubble Space Telescope (HST) make it possible to estimate the sizes of the components and consequently provide the range of possible values for the objects’ bulk density.[5] The single-body diameter (effective system size) of Lempo is currently estimated at 393.1+25.2

The very low estimated density of 0.3–0.8 g/cm3 obtained in 2006 (when the system was thought to be a binary) would require an unusually high porosity of 50–75%, assuming an equal mixture of rock and ice.[9] The direct measurement of visible fluxes of all three components of the system in 2009 by the HST has resulted in an improved average density of 0.532+0.317
confirming the earlier conclusion that the object is probably a rubble pile.[5] The density was revised up to 0.64+0.15
in 2012 when new information from the Herschel became available. For a bulk density in the range 1–2 g/cm3 the porosity is in the range 36–68%, again confirming that the object is a rubble pile.[4]

Lempo has a very red spectral slope in visible light[10] and a flat spectrum in near infrared. There is also a weak absorption feature near the wavelength of 2 μm, probably caused by water ice. The best model reproducing the near infrared spectrum includes tholins, crystalline water ice, and serpentine as surface materials, these results are for the integrated spectrum of all three components of the system.[11]


In 2009, analysis of Hubble images revealed that the primary is itself composed of two similar-sized components,[5] the Component A1 is called Lempo, while Component A2 is called Hiisi. This central pair has a semi-major axis of around 867 km and a period of about 1.9 days.[5] Assuming equal albedos of about 0.079, the primary components are approximately A1=272+17
and A2=251+16
in diameter.[4] The component B orbits the barycenter of the A1+A2 system. The system mass estimated from the motion of the component B is (12.75±0.06)×1018 kg.[5] The orbital motion of the A1 and A2 components gives somewhat a higher estimated mass of (14.20±0.05)×1018 kg. The discrepancy is probably related to unaccounted gravitational interactions of the components in a complex triple system.[5]


The moon, called Paha, discovered from 8 December 2001 observations by Chadwick A. Trujillo and Michael Brown using the Hubble Space Telescope and announced on 10 January 2002,[12] has an estimated diameter of 132+8
[4] and a semi-major axis of 7411±12 km, orbiting its primary in 50.302±0.001 d.[5] The moon is estimated to only have a mass of about 7.5×1017 kg.[5]


There exist two main hypotheses on how the triple system of Lempo formed, the first one is a giant collision and subsequent reaccretion in the disc. The second one is gravitational capture of a third object by a preexisting binary, the similar sizes of A1 and A2 components favor the latter hypothesis.[5]


Lempo was suggested as a target for New Horizons 2, a proposed twin of its namesake that would fly by Jupiter, Uranus, and up to four KBOs.[13]


This minor planet was named after Lempo from Finnish mythology. Originally worshiped as the god of love and fertility, he was later depicted as devil, after Christianity came to Finland. Lempo brought down the hero Väinämöinen with the help of his two demon cohorts Hiisi and Paha,[1] the official naming citation was published by the Minor Planet Center on 5 October 2017 (M.P.C. 106502).[14]


  1. ^ a b c "47171 Lempo (1999 TC36)". Minor Planet Center. Retrieved 9 October 2017. 
  2. ^ a b "MPEC 2009-V05 :Distant Minor Planets (2009 NOV. 15.0 TT)". IAU Minor Planet Center. 2009-11-03. Retrieved 2009-12-10. 
  3. ^ Marc W. Buie (2005-08-31). "Orbit Fit and Astrometric record for 47171". SwRI (Space Science Department). Retrieved 2008-07-24. 
  4. ^ a b c d e f g h i j k Mommert, Michael; Harris, A. W.; Kiss, C.; Pál, A.; Santos-Sanz, P.; Stansberry, J.; Delsanti, A.; Vilenius, E.; Müller, T. G.; Peixinho, N.; Lellouch, E.; Szalai, N.; Henry, F.; Duffard, R.; Fornasier, S.; Hartogh, P.; Mueller, M.; Ortiz, J. L.; Protopapa, S.; Rengel, M.; Thirouin, A. (May 2012). "TNOs are cool: A survey of the trans-Neptunian region—V. Physical characterization of 18 Plutinos using Herschel-PACS observations". Astronomy & Astrophysics. 541: A93. arXiv:1202.3657Freely accessible. Bibcode:2012A&A...541A..93M. doi:10.1051/0004-6361/201118562. 
  5. ^ a b c d e f g h i j k l m Benecchi, S.D; Noll, K. S.; Grundy, W. M.; Levison, H. F. (2010). "(47171) 1999 TC36, A Transneptunian Triple". Icarus. 207 (2): 978–991. arXiv:0912.2074Freely accessible. Bibcode:2010Icar..207..978B. doi:10.1016/j.icarus.2009.12.017. 
  6. ^ a b "JPL Small-Body Database Browser: 47171 Lempo (1999 TC36)" (2016-12-28 last obs.). Jet Propulsion Laboratory. Retrieved 9 October 2017. 
  7. ^ a b "AstDys (47171) 1999TC36 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2009-12-07. 
  8. ^ "MPEC 1999-Y19 :Distant Minor Planets (1999 DEC. 21.8 UT)". IAU Minor Planet Center. 1999-12-21. Retrieved 2015-05-08. 
  9. ^ a b J. Stansberry; W. Grundy; J-L. Margot; D. Cruikshank; J. Emery; G. Rieke; D. Trilling (2006). "The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36". The Astrophysical Journal. 643 (1): 556–566. arXiv:astro-ph/0602316Freely accessible. Bibcode:2006ApJ...643..556S. doi:10.1086/502674. 
  10. ^ Doressoundiram, A.; Peixinho, N.; Moullet, A.; Fornasier, S.; Barucci, M. A.; Beuzit, J. -L.; Veillet, C. (2007). "The Meudon Multicolor Survey (2MS) of Centaurs and Trans-Neptunian Objects: From Visible to Infrared Colors". The Astronomical Journal. 134 (6): 2186. Bibcode:2007AJ....134.2186D. doi:10.1086/522783. 
  11. ^ Protopapa, S.; Alvarez-Candal, A.; et al. (2009). "ESO large program about transneptunian objects: surface variations on (47171) 1999 TC36". Astronomy and Astrophysics. 501 (1): 375–380. Bibcode:2009A&A...501..375P. doi:10.1051/0004-6361/200810572. 
  12. ^ "IAU Circular No. 7787". International Astronomical Union. Retrieved 2008-12-06. 
  13. ^ Stern, Alan; et al. "New Horizons 2" (pdf). NASA (Outer Planets Assessment Group). Retrieved 13 May 2012.  parent
  14. ^ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 9 October 2017. 

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