47171 Lempo

Lore & Background

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 project at Kitt Peak National Observatory. Rubenstein identified Lempo as a relatively bright, slow-moving object in the constellation Cetus. At an apparent magnitude of 20, its exceptional brightness for a suspected KBO warranted follow-up observations. The discovery was announced by the Minor Planet Center on 21 December 1999, and the object received the permanent minor planet number 47171 on 21 September 2002. The earliest known precovery observations date from June 1974 and May and September 1976 from Siding Spring Observatory's Digitized Sky Survey.

The system's three components are named after figures from Finnish mythology: Lempo (the largest), Hiisi, and Paha. Lempo was originally worshiped as the god of love and fertility, later depicted as a devil after Christianity came to Finland. The names were chosen on behalf of astronomer Bryan J. Holler, and the official naming citation was published by the Minor Planet Center on 5 October 2017. Hiisi shares its name with the exoplanet HAT-P-38b.

The system is a hierarchical triple: a central binary of two similarly-sized components (Lempo and Hiisi) orbited by a small satellite (Paha) on a wide, eccentric circumbinary orbit. The separation between Lempo and Hiisi is only about half the diffraction limit of Hubble, making them appear elongated in images. Assuming equal albedos of about 0.079, Lempo and Hiisi are approximately 272+17−19 km and 251+16−17 km in diameter, respectively. Paha has an estimated diameter of 132+8−9 km and orbits at a semi-major axis of 7411±12 km with a period of 50.302±0.001 days.

Reader's Guide

47171 Lempo holds significance as one of only three confirmed trans-Neptunian multiple systems with more than two components, alongside Pluto and Haumea. Its triple nature provides a rare laboratory for studying complex orbital dynamics in the outer Solar System. The system's orbital dynamics are highly complex and could not be modeled with solely Keplerian dynamics; many crucial parameters such as initial spin states and shapes are unknown, leading to significantly chaotic behavior in three-body simulations. A 2018 dynamical study by Alexandre Correia found that simulated models using realistically assumed spin states and shapes failed to explain the present eccentric mutual orbit of the inner Lempo–Hiisi binary, even with the inclusion of eccentricity-damping tidal forces. The system's mass estimates show a discrepancy: (12.75±0.06)×10^18 kg based on Paha's motion versus (14.20±0.05)×10^18 kg from the inner binary's motion, likely due to unaccounted gravitational interactions. The binary Kuiper belt object 385446 Manwë is suspected to have once been a similar hierarchical triple system, and 58534 Logos is thought to be a contact binary triple system, suggesting that such configurations may be more common than currently observed.

Did You Know?

• Lempo is one of only three known trans-Neptunian multiple systems with more than two confirmed components, the others being Pluto and Haumea.
• The system's three components are named after Lempo, Hiisi, and Paha from Finnish mythology.
• The inner binary Lempo–Hiisi has a separation of only about half the diffraction limit of the Hubble Space Telescope, making it impossible to fully resolve.
• The earliest precovery observations of Lempo date from photographic plates taken in June 1974 at Siding Spring Observatory.