Lempo and its outer companion Paha imaged with the Hubble Space Telescope in 2001. Lempo's inner companion Hiisi is not resolved in this image.
|Discovered by||E. P. Rubenstein|
|Discovery site||Kitt Peak National Obs.|
|Discovery date||1 October 1999|
|MPC designation||(47171) Lempo|
|Lempo (Finnish mythology) |
|TNO  · plutino |
|Orbital characteristics |
|Epoch 4 September 2017 (JD 2458000.5)|
|Uncertainty parameter 2|
|Observation arc||42.53 yr (15,534 days)|
|Earliest precovery date||18 June 1974|
|245.48 yr (89,662 days)|
|0° 0m 14.4s / day|
|Known satellites||Paha · S/2001 (47171) 1|
Hiisi · S/2007 (47171) 1
|393.1 km (overall system)|
272 km (Lempo only)
251 km (Hiisi only)
132 km (Paha only)
|B–V = 1.00±0.13|
V−R = 0.70±0.03
|5.41±0.10 · 5.0|
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 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. 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, and, currently only being 30.5 AU from the Sun, is among the brighter TNOs. It reached perihelion in July 2015. This minor planet was named after Lempo from Finnish mythology.
The combined observations by the infrared Spitzer Space Telescope, Herschel Space Telescope 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. 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. 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
−0.211 g/cm3 confirming the earlier conclusion that the object is probably a rubble pile. The density was revised up to 0.64+0.15
−0.11 g/cm3 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.
Lempo has a very red spectral slope in visible light 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.
On 8 December 2001, observations by Chadwick Trujillo and Michael Brown using the Hubble Space Telescope, revealed the presence of a minor-planet moon, provisional designation S/2001 (47171) 1, later named Paha. The discovery was announced on 10 January 2002. The satellite has an estimated diameter of 132+8
−9 km and a semi-major axis of 7411±12 km, orbiting its primary in 50.302±0.001 d. It is estimated to only have a mass of about 7.5×1017 kg.
In 2007, analysis of Hubble images revealed that the primary is itself a binary system composed of two similarly sized components. While the first component (A1) maintained the name Lempo, the second, new component (A2), provisionally designated S/2007 (47171) 1, was later named Hiisi.
This central pair has a semi-major axis of around 867 km and a period of about 1.9 days. Assuming equal albedos of about 0.079, Lempo and Hiisi are approximately 272+17
−19 km and 251+16
−17 km in diameter, respectively. The earlier discovered satellite Paha orbits the barycenter of the Lempo–Hiisi system.
The system mass estimated based on the motion of Paha is (12.75±0.06)×1018 kg. The orbital motion of the Lempo–Hiisi 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.
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.
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.
This minor planet was named after Lempo from Finnish mythology. 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. The official naming citation was published by the Minor Planet Center on 5 October 2017 (M.P.C. 106502).