47171 Lempo
Lempo–Hiisi and their outer companion Paha, imaged with the Hubble Space Telescope in 2001. Lempo and Hiisi are unresolved.
Discovered byEric P. Rubenstein
Louis-Gregory Strolger
Discovery siteKitt Peak National Obs.
Discovery date1 October 1999
(47171) Lempo
Pronunciation/ˈlɛmp/[citation needed]
Named after
Lempo (Finnish mythology)[2]
1999 TC36
TNO · plutino[3] · distant[2] · trinary[4]
Orbital characteristics[1]
Epoch 17 December 2020 (JD 2459200.5)
Uncertainty parameter 1
Observation arc46.58 yr (17,013 days)
Earliest precovery date18 June 1974
Aphelion48.397 AU
Perihelion30.542 AU
39.470 AU
247.97 yr (90,572 days)
0° 0m 14.309s / day
Known satellites2
Physical characteristics
Mean diameter
Mass(12.75±0.06)×1018 kg
(overall system)[4]
(14.20±0.05)×1018 kg
(without Paha)[4]
6.71×1018 kg
Mean density
RR (very red)[7][8]
4.8 (assumed)[1]

47171 Lempo, or as a binary (47171) Lempo–Hiisi (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 on 1 October 1999, by American astronomers Eric Rubenstein and Louis-Gregory Strolger during an observing run at Kitt Peak National Observatory in Arizona, United States.[2][11] Rubenstein was searching images taken by Strolger as part of their Nearby Galaxies Supernova Search project. It is classified as a plutino with a 2:3 mean-motion resonance with Neptune and is among the brighter TNOs. It reached perihelion in July 2015. This minor planet was named after Lempo from Finnish mythology.[2]

The trinary system's other two components, Paha /ˈpɑːhɑː/ and Hiisi /ˈhsi/, were discovered in 2001 and 2007, respectively, and later named after Lempo's two demon cohorts, Paha and Hiisi.[12]



Lempo was discovered with the 0.9-meter WIYN telescope (left) at Kitt Peak

The Lempo system was discovered on 1 October 1999 by American astronomers Eric Rubenstein and Louis-Gregory Strolger during an observing run for their Nearby Galaxies Supernova Search (NGSS) project at the Kitt Peak National Observatory in Arizona. Initiated in 1998 as part of Strolger's doctoral thesis, the NGSS project was a three-year-long CCD-based survey of galaxies along the celestial equator to search for nearby, low-redshift supernovae. The Kitt Peak Observatory's WIYN 0.9-meter telescope was used for wide-field imaging of this region, which coincided with the ecliptic plane where Kuiper belt objects (KBOs) including Lempo were likely to appear.[13] Rubenstein identified Lempo as a relatively bright, slow-moving object in the constellation Cetus on images taken by Strolger on 1 October 1999.[11][a] At an apparent magnitude of 20, its exceptional brightness for a suspected KBO warranted follow-up observations to confirm the object.[13][14]

Lempo was observed by Rubenstein and Strolger for three consecutive days after its discovery. The object was also found in images taken by Strolger on 30 September 1999, one day prior to its discovery. The discovery was then announced by the Minor Planet Center on 21 December 1999 and the object was given the provisional designation 1999 TC36.[11] The provisional designation indicates that Lempo was the 903rd minor planet discovered in the first half of October 1999.[b] By 2002, additional observations have extended Lempo's observation arc to over two years, sufficient to determine an accurate orbit.[2] Lempo was consequently given the permanent minor planet number 47171 by the Minor Planet Center on 21 September 2002.[16] As of 2021, more than 500 total observations of Lempo over an observation arc of over 46 years have been documented.[2] The earliest known precovery observations of Lempo have been found in photographic plates of the Siding Spring Observatory's Digitized Sky Survey from June 1974 and May and September 1976.[17]


The largest primary component of the triple system is named after Lempo from Finnish mythology.[c] Originally worshiped as the god of love and fertility, he was later depicted as a devil after Christianity came to Finland. Lempo brought down the hero Väinämöinen with the help of his two demon cohorts Hiisi and Paha, whose names denominate the smaller inner and outer components, respectively.[2] The names were chosen on behalf of astronomer Bryan J. Holler.[14] The official naming citation was published by the Minor Planet Center on 5 October 2017.[12]

Trinary system

Size comparison of the Lempo system's components
Orbit diagram of the Lempo triple system


Lempo is a hierarchical triple (or trinary) system consisting of a central primary, which is itself a binary system of two similarly-sized components (Lempo and Hiisi), and a small satellite on a wide and eccentric circumbinary orbit (Paha). The structure of the hierarchy is discerned by denoting the apparent Lempo–Hiisi primary with the letter A and the smaller, outer companion Paha with the letter B; the individual primary components Lempo and Hiisi are distinguished as A1 and A2, respectively.[4] The three components ordered from largest to smallest are Lempo, Hiisi, and Paha.[18]

Assuming spherical shapes with a uniform bulk density for all components, the system mass estimated based on the motion of Paha is (12.75±0.06)×1018 kg.[4][6] The orbital motion of the Lempo–Hiisi components gives somewhat a higher estimated mass of (14.20±0.05)×1018 kg. This discrepancy is probably related to unaccounted gravitational interactions of the components in a complex triple system.[4]

Lempo is one of the only three known trans-Neptunian multiple systems with more than two components; the other two are the dwarf planets Pluto and Haumea.[4] The binary Kuiper belt object 385446 Manwë is suspected to have once been a hierarchical triple system similar to Lempo, but the orbit of its inner binary evolved by tides and became a contact binary.[19]


Hubble images of the Lempo–Hiisi system, with the two components marked "1" and "2"
Comparison of mean separation distances and diameters of trans-Neptunian close binaries including Lempo–Hiisi

Hiisi, officially designated (47171) Lempo II Hiisi,[20] is the inner, second-largest component of the Lempo triple system, discovered last among the three. Together with the primary component Lempo, it forms the central binary Lempo–Hiisi which the outer component Paha revolves around. The existence of a third, inner component (or second companion) in the Lempo system was first hypothesized in 2006 by John Stansberry and collaborators, who noted that the primary seemed to have an unusually low density.[21] Further evidence to the existence of an inner component was posited by Seth Jacobson and Jean-Luc Margot in October 2007, who noticed a distinct elongation of the primary in Hubble images.[22] The binarity of the Lempo primary was eventually confirmed in a more extensive analysis of Hubble images by Susan Benecchi, Keith Noll, Will Grundy and Hal Levison in 2009.[4][2]

Due to complex discovery circumstances involving different independent groups of researchers,[23] Hiisi did not have a formal provisional designation signifying the year of its first observation or discovery.[20] Instead, it was unofficially designated "component A2" in scientific literature for being the smaller component of the central Lempo–Hiisi binary.[4] It eventually received its permanent satellite designation and name while the larger, first component A1 maintained the name Lempo on 5 October 2017.[12]

The separation between the two components is only about half the diffraction limit of Hubble, making it impossible to fully resolve the system. Instead, it appears elongated in Hubble images, revealing its binary nature.[4] This central pair has a semi-major axis of around 867 km and a period of about 1.9 days.[4] Assuming equal albedos of about 0.079, Lempo and Hiisi are approximately 272+17
and 251+16
in diameter, respectively.[5] Assuming a uniform density for all components, the mass of Hiisi itself 5.273×1018 kg.[6]


The Lempo system imaged by Hubble from 2003–2006

Paha, officially designated (47171) Lempo I Paha,[2] is the smaller, outer component of the Lempo triple system. It was discovered on 8 December 2001 by astronomers Chadwick Trujillo and Michael Brown using the Hubble Space Telescope's Space Telescope Imaging Spectrograph to survey for binary trans-Neptunian objects.[24] The discovery was reported in an IAU Circular notice published by the International Astronomical Union 10 January 2002.[25] The confirmation of Paha in archival 4 October 2001 observations from the Lick Observatory's Shane telescope adaptive optics system was reported in a follow-up IAU Circular published on 24 January 2002.[26][27]

Paha previously had the temporary provisional designation S/2001 (1999 TC36) 1 before it was changed to S/2001 (47171) 1 after Lempo was numbered.[28] Being the smaller, outer component on a circumbinary orbit around the central Lempo–Hiisi binary, it was sometimes designated "component B" in scientific literature.[4] It received its permanent satellite designation and name alongside Lempo and Hiisi on 5 October 2017.[12]

In unfiltered visual wavelengths, Paha appears 2.2 magnitudes dimmer than the primary on average, corresponding to an individual apparent magnitude of 22.6.[29] The satellite has an estimated diameter of 132+8
[5] and a semi-major axis of 7411±12 km, orbiting its primary in 50.302±0.001 d.[4] It is estimated to only have a mass of about 7.67×1017 kg.[6]

System dynamics

The orbital dynamics of the Lempo system are highly complex and could not be modelled with solely Keplerian dynamics.[4][6] Many crucial parameters such as initial spin states and shapes of the individual components are unknown and thus could not adequately model the dynamics of the Lempo system as a three-body problem without leading to significantly chaotic behavior. In a 2018 dynamical study, Alexandre Correia found that simulated models using realistically assumed spin states and shapes failed to explain the presently eccentric mutual orbit of the inner Lempo–Hiisi binary, even with the inclusion of eccentricity-damping tidal forces. Correia concluded that the present orbits, spin states and shapes of all components of the Lempo system needed to be remeasured to a greater precision before a more sophisticated model could be developed.[6]


There exist two main hypotheses on how this triple system 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 Lempo and Hiisi favor the latter hypothesis.[4]

Physical characteristics

The combined observations by the infrared Spitzer Space Telescope,[21] Herschel Space Telescope[5] and the Hubble Space Telescope (HST) make it possible to estimate the sizes of the system's components and consequently provide the range of possible values for the objects' bulk density.[4] 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.[21] 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.[4] 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.[5]

Lempo has a very red spectral slope in visible light[30] 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.[31]


The Lempo system has been considered for future exploration due to its unusual configuration.[32] 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.[33]

See also


  1. ^ The celestial coordinates of Lempo at the time of discovery were 23h 57m 49.00s −09° 31′ 52.1″.[11] See Cetus for constellation coordinates.
  2. ^ In the convention for minor planet provisional designations, the first letter represents the half-month of the year of discovery while the second letter and numbers indicate the order of discovery within that half-month. In the case for 1999 TC36, the first letter 'T' corresponds to the first half-month of October 1999 while the succeeding letter 'C' indicates that it is the 3rd object discovered on the 37th cycle of discoveries (with 36 cycles completed). Each completed cycle consists of 25 letters representing discoveries, therefore 3 + (36 completed cycles × 25 letters) = 903.[15]
  3. ^ The primary's namesake is also used to refer to the entire triple system.[6]


  1. ^ a b c "JPL Small-Body Database Browser: 47171 Lempo (1999 TC36)" (2021-01-15 last obs.). Jet Propulsion Laboratory. 4 April 2021.
  2. ^ a b c d e f g h i "(47171) Lempo = 1999 TC36". Minor Planet Center. International Astronomical Union. Retrieved 9 October 2017.
  3. ^ Buie, M. W. "Orbit Fit and Astrometric record for 47171". Southwest Research Institute. Retrieved 23 October 2020.
  4. ^ a b c d e f g h i j k l m n o p q Benecchi, S. D.; Noll, K. S.; Grundy, W. M.; Levison, H. F. (June 2010). "(47171) 1999 TC36, A transneptunian Triple". Icarus. 207 (2): 978–991. arXiv:0912.2074. Bibcode:2010Icar..207..978B. doi:10.1016/j.icarus.2009.12.017. S2CID 118430134.
  5. ^ a b c d e f g h i Mommert, M.; Harris, A. W.; Kiss, C.; Pál, A.; Santos-Sanz, P.; Stansberry, J.; et al. (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): 17. arXiv:1202.3657. Bibcode:2012A&A...541A..93M. doi:10.1051/0004-6361/201118562.
  6. ^ a b c d e f g Correia, Alexandre C. M. (May 2018). "Chaotic dynamics in the (47171) Lempo triple system". Icarus. 305: 250–261. arXiv:1710.08401. Bibcode:2018Icar..305..250C. doi:10.1016/j.icarus.2018.01.008. S2CID 119305475.
  7. ^ Cruikshank, D. P.; Barucci, M. A.; Emery, J. P.; Fernández, Y. R.; Grundy, W. M.; Noll, K. S.; Stansberry, J. A. (2007). "Physical Properties of Transneptunian Objects" (PDF). Protostars and Planets V. University of Arizona Press. pp. 879–893. Bibcode:2007prpl.conf..879C. ISBN 978-0-8165-2755-7.
  8. ^ Fulchignoni, Marcello; Belskaya, Irina; Barucci, Maria Antonietta; De Sanctis, Maria Cristina; Doressoundiram, Alain (2008). "Transneptunian Object Taxonomy" (PDF). The Solar System Beyond Neptune. University of Arizona Press. pp. 181–192. Bibcode:2008ssbn.book..181F. ISBN 978-0-8165-2755-7.
  9. ^ a b c Hainaut, O. R.; Boehnhardt, H.; Protopapa, S. (October 2012). "Colours of minor bodies in the outer solar system. II. A statistical analysis revisited" (PDF). Astronomy & Astrophysics. 546: 20. arXiv:1209.1896. Bibcode:2012A&A...546A.115H. doi:10.1051/0004-6361/201219566. S2CID 54776793.
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  11. ^ a b c d Marsden, Brian G. (21 December 1999). "MPEC 1999-Y19 : 1999 RV214, 1999 TC36, 1999 XX143, 1999 XY143". Minor Planet Electronic Circular. Minor Planet Center. Retrieved 8 May 2015.
  12. ^ a b c d "M.P.C. 106502" (PDF). Minor Planet Circular. Minor Planet Center. 5 October 2017. p. 926. Retrieved 9 October 2017.
  13. ^ a b Strolger, Louis-Gregory (August 2003). The Nearby Galaxies Supernova Search project: The rate of supernovae in the local universe (PhD thesis). University of Michigan. p. 22. Bibcode:2003PhDT........14S. ISBN 9780496274130. Retrieved 22 October 2020.
  14. ^ a b Strolger, Louis-Gregory (2020). "Lou Strolger's Page – An Old Discovery Gets a New Name". Space Telescope Science Institute. Retrieved 22 October 2020.
  15. ^ "How Are Minor Planets Named?". Minor Planet Center. International Astronomical Union. Retrieved 22 October 2020.
  16. ^ "M.P.C. 46628" (PDF). Minor Planet Circular. Minor Planet Center. 21 September 2002. p. 160. Retrieved 22 October 2020.
  17. ^ Lowe, Andrew. "(47171) 1999 TC36 Precovery Images". Retrieved 22 October 2020.
  18. ^ Johnston, Wm. Robert (8 October 2017). "(47171) Lempo, Paha, and Hiisi". Asteroids with Satellites Database. Johnston's Archive. Retrieved 21 December 2017.
  19. ^ Brunini, Adrián; López, María Cristina (December 2020). "The origin of (47171) Lempo-like Kuiper belt triple systems during binary-binary interactions". Monthly Notices of the Royal Astronomical Society. 499 (3): 4206–4212. Bibcode:2020MNRAS.499.4206B. doi:10.1093/mnras/staa3105.
  20. ^ a b Johnston, Wm. Robert (14 March 2021). "Asteroids/TNOs with satellites: designation data". Asteroids with Satellites Database. Johnston's Archive. Retrieved 7 April 2021.
  21. ^ a b c Stansberry, J. A.; Grundy, W. M.; Margot, J. L.; Cruikshank, D. P.; Emery, J. P.; Rieke, G. H.; Trilling, D. E (May 2006). "The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36". The Astronomical Journal. 643 (1): 556–566. arXiv:astro-ph/0602316. Bibcode:2006ApJ...643..556S. doi:10.1086/502674. S2CID 18146599.
  22. ^ Jacobson, Seth; Margot, J. L. (October 2007). Colors of TNO Binaries and Evidence for a Triple System from HST Observations. 39th Division for Planetary Sciences Meeting. Vol. 39. American Astronomical Society. p. 519. Bibcode:2007DPS....39.5211J. 52.11.
  23. ^ Johnston, Wm. Robert (25 December 2019). "Combined Listing of Reported Asteroid/TNO Companions by Date Reported". Asteroids with Satellites Database. Johnston's Archive. Retrieved 7 April 2021.
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  25. ^ Green, Daniel W. E. (10 January 2002). "IAUC 7787: 1999 TC_36; 2001is, 2001it". Central Bureau for Astronomical Telegrams (1). International Astronomical Union. Bibcode:2002IAUC.7787....1T. Retrieved 6 December 2008.
  26. ^ Green, Daniel W. E. (24 January 2002). "IAUC 7807: (26308) 1998 SM_165; 1999 TC_36; C/1996 Y2". Central Bureau for Astronomical Telegrams (2). International Astronomical Union. Bibcode:2002IAUC.7807....2M. Retrieved 7 April 2021.
  27. ^ Marchis, F.; Berthier, J. (12 March 2003). "First Observation of 1999 TC36". Department of Astronomy. University of California, Berkeley. Archived from the original on 4 June 2003. Retrieved 7 April 2021.
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  29. ^ Grundy, Will (21 November 2020). "Lempo (47171 1999 TC36)". Lowell Observatory. Retrieved 7 April 2021.
  30. ^ Doressoundiram, A.; Peixinho, N.; Moullet, A.; Fornasier, S.; Barucci, M. A.; Beuzit, J.-L.; Veillet, C. (December 2007). "The Meudon Multicolor Survey (2MS) of Centaurs and Trans-Neptunian Objects: From Visible to Infrared Colors". The Astronomical Journal. 134 (6): 2186–2199. Bibcode:2007AJ....134.2186D. doi:10.1086/522783. S2CID 122936619.
  31. ^ Protopapa, S.; Alvarez-Candal, A.; Barucci, M. A.; Tozzi, G. P.; Fornasier, S.; Delsanti, A.; Merlin, F. (July 2009). "ESO large program about transneptunian objects: surface variations on (47171) 1999 TC36". Astronomy & Astrophysics. 501 (1): 375–380. Bibcode:2009A&A...501..375P. doi:10.1051/0004-6361/200810572. S2CID 123037033.
  32. ^ Holler, Bryan; Bannister, Michele T.; Singer, Kelsi N.; Stern, S. Alan; Benecchi, Susan D.; Dalle Ore, Cristina M.; et al. (May 2021). Prospects for Future Exploration of the Trans-Neptunian Region. Bulletin of the American Astronomical Society (Planetary Science and Astrobiology Decadal Survey 2023-2032 white paper). Vol. 53. Bibcode:2021BAAS...53d.228H. doi:10.3847/25c2cfeb.5950ca1c. 228.
  33. ^ Stern, Alan; Binzel, Rick; Levison, Hal; Lopes, Rosaly; Millis, Bob; Moore, Jeff. "New Horizons 2" (PDF). Outer Planets Assessment Group. Lunar and Planetary Institute. Retrieved 13 May 2012.