page_title: 47171 Lempo
section_title: Hiisi
hierarchical_section_title: 47171 Lempo / Trinary system / Hiisi
caption_attribution_description: English: The inner binary of the 47171 Lempo triple system, as imaged by the Hubble Space Telescopes's ACS/HRC imager. The two individual components, Lempo and Hiisi, are barely resolved and are only visible as elongations. Source images have been in public domain since 2007-07-06. Proposal ID 9746 (Jean-Luc Margot).
context_page_description: 47171 Lempo, or as a binary Lempo–Hiisi, 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 trinary system's other two components, Paha and Hiisi, were discovered in 2001 and 2007, respectively, and later named after Lempo's two demon cohorts, Paha and Hiisi.
context_section_description: 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. Assuming a uniform density for all components, the mass of Hiisi itself 5.273×10¹⁸ kg.
  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.
caption: Animation of the Lempo-Hiisi system by Hubble