The combined observations by the infrared Spitzer Space Telescope, Herschel Space Telescope and the Hubble Space Telescope 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 of Lempo is currently estimated at. The very low estimated density of 0.3–0.8 g/cm3 obtained in 2006 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 confirming the earlier conclusion that the object is probably a rubble pile. The density was revised up to 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.
Trinary system
Lempo is a triple systemconsisting of a central primary, which is itself a binary, Lempo and Hiisi, and a small satellite, Paha.
In 2007, analysis of Hubble images revealed that the primary is itself a binary systemcomposed of two similarly sized components. While the first component maintained the name Lempo, the second, new component, provisionally designated, 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 and 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. The orbital motion of the Lempo–Hiisi components gives somewhat a higher estimated mass of. 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. 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.
Origin
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.
Exploration
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.
Naming
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.
