 Lempo–Hiisi and their outer companion Paha, imaged with theHubble Space Telescope in 2001. Lempo and Hiisi are unresolved. | |
| Discovery[1] | |
|---|---|
| Discovered by | Eric P. Rubenstein Louis-Gregory Strolger |
| Discovery site | Kitt Peak National Obs. |
| Discovery date | 1 October 1999 |
| Designations | |
| (47171) Lempo | |
| Pronunciation | /ˈlɛmpoʊ/[citation needed] |
Named after | Lempo(Finnish mythology)[2] |
| 1999 TC36 | |
| TNO · plutino[3] · distant[2] · triple[4] | |
| Orbital characteristics[1] | |
| Epoch 17 December 2020 (JD 2459200.5) | |
| Uncertainty parameter 1 | |
| Observation arc | 46.58 yr (17,013 days) |
| Earliestprecovery date | 18 June 1974 |
| Aphelion | 48.397AU |
| Perihelion | 30.542 AU |
| 39.470 AU | |
| Eccentricity | 0.22618 |
| 247.97yr (90,572 days) | |
| 8.547° | |
| 0° 0m 14.309s / day | |
| Inclination | 8.4233° |
| 97.020° | |
| 294.424° | |
| Knownsatellites | 2 |
| Physical characteristics | |
| 272+17 −19 km (primary)[5] | |
| Mass | (12.75±0.06)×1018 kg (overall system)[4] (14.20±0.05)×1018 kg (without Paha)[4] 6.71×1018 kg (primary)[6] |
Meandensity | 0.64+0.15 −0.11 g/cm3 (system)[5] |
| 0.079+0.013 −0.011 (system)[5] | |
| RR (very red)[7][8] B–V=1.029±0.047[9] V−R=0.693±0.032[9] V−I=1.270±0.050[9] | |
| 19.9[10] | |
| 5.41±0.10[5] 4.8 (assumed)[1] | |
47171 Lempo, or as a binary(47171) Lempo–Hiisi (provisional designation1999 TC36), is a tripletrans-Neptunian object in theKuiper belt, located in the outermost regions of theSolar System. It was discovered on 1 October 1999, by American astronomersEric Rubenstein andLouis-Gregory Strolger during an observing run atKitt Peak National Observatory inArizona, United States.[2][11] Rubenstein was searching images taken by Strolger as part of their Nearby Galaxies Supernova Search project. It is classified as aplutino with a 2:3mean-motion resonance withNeptune and is among the brighter TNOs. It reachedperihelion in July 2015. This minor planet was named afterLempo from Finnish mythology.[2]
The system's other two components,Paha/ˈpɑːhɑː/ andHiisi/ˈhiːsi/, were discovered in 2001 and 2007, respectively, and later named after Lempo's two demon cohorts, Paha andHiisi.[12]
The Lempo system was discovered on 1 October 1999 by American astronomersEric Rubenstein andLouis-Gregory Strolger during an observing run for their Nearby Galaxies Supernova Search (NGSS) project at theKitt Peak National Observatory inArizona. Initiated in 1998 as part of Strolger'sdoctoral thesis, the NGSS project was a three-year-longCCD-based survey ofgalaxies along thecelestial equator to search for nearby, low-redshiftsupernovae. The Kitt Peak Observatory's WIYN 0.9-meter telescope was used for wide-field imaging of this region, which coincided with theecliptic plane whereKuiper belt objects (KBOs) including Lempo were likely to appear.[13] Rubenstein identified Lempo as a relatively bright, slow-moving object in theconstellationCetus on images taken by Strolger on 1 October 1999.[11][a] At anapparent 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 theMinor Planet Center on 21 December 1999 and the object was given theprovisional designation1999 TC36.[11] The provisional designation indicates that Lempo was the 903rdminor planet discovered in the first half of October 1999.[b] By 2002, additional observations have extended Lempo'sobservation arc to over two years, sufficient to determine an accurate orbit.[2] Lempo was consequently given the permanentminor planet number 47171 by the Minor Planet Center on 21 September 2002.[16] As of 2021[update], more than 500 total observations of Lempo over an observation arc of over 46 years have been documented.[2] The earliest knownprecovery observations of Lempo have been found inphotographic plates of theSiding Spring Observatory'sDigitized Sky Survey from June 1974 and May and September 1976.[17]
The largest primary component of the triple system is named afterLempo fromFinnish mythology.[c] Originally worshiped as the god of love and fertility, he was later depicted as adevil afterChristianity came to Finland. Lempo brought down the heroVäinämöinen with the help of his two demon cohortsHiisi 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]
Lempo is a hierarchicaltriple system consisting of a central primary, which is itself abinary system of two similarly-sized components (Lempo and Hiisi), and a smallsatellite on a wide andeccentriccircumbinary orbit (Paha). The structure of the hierarchy is discerned by denoting the apparent Lempo–Hiisi primary with the letterA and the smaller, outer companion Paha with the letterB; the individual primary components Lempo and Hiisi are distinguished asA1 andA2, respectively.[4] The three components ordered from largest to smallest are Lempo, Hiisi, and Paha.[18]
Assuming spherical shapes with a uniformbulk 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 thedwarf planetsPluto andHaumea.[4]Quaoar may also have more then two components, but this is currently unconfirmed.[19] The binaryKuiper belt object385446 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 acontact binary.[20]
Hiisi, formally designated(47171) Lempo II,[21] 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.[22] Further evidence to the existence of an inner component was posited by Seth Jacobson andJean-Luc Margot in October 2007, who noticed a distinct elongation of the primary in Hubble images.[23] 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,[24] Hiisi did not have a formal provisional designation signifying the year of its first observation or discovery.[21] 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 permanentsatellite 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 thediffraction 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 equalalbedos of about 0.079,Lempo andHiisi are approximately272+17
−19 km and251+16
−17 km in diameter, respectively.[5] Assuming a uniform density for all components, the mass of Hiisi itself5.273×1018 kg.[6]
Paha, formally designated(47171) Lempo I,[2] is the smaller, outer component of the Lempo triple system. It was discovered on 8 December 2001 by astronomersChadwick Trujillo andMichael Brown using theHubble Space Telescope'sSpace Telescope Imaging Spectrograph to survey for binary trans-Neptunian objects.[25] The discovery was reported in anIAU Circular notice published by theInternational Astronomical Union 10 January 2002.[26] The confirmation of Paha in archival 4 October 2001 observations from theLick Observatory'sShane telescopeadaptive optics system was reported in a follow-upIAU Circular published on 24 January 2002.[27][28]
Paha previously had the temporaryprovisional designationS/2001 (1999 TC36) 1 before it was changed to S/2001 (47171) 1 after Lempo was numbered.[29] 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 permanentsatellite 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 individualapparent magnitude of 22.6.[30] The satellite has an estimated diameter of132+8
−9 km[5] and asemi-major axis of7411±12 km, orbiting its primary in50.302±0.001 d.[4] It is estimated to only have a mass of about7.67×1017 kg.[6]
The orbital dynamics of the Lempo system are highly complex and could not be modelled with solelyKeplerian 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 athree-body problem without leading to significantlychaotic 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-dampingtidal 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 mainhypotheses on how this triple system formed. The first one is a giant collision and subsequent reaccretion in thedisc. 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]
The combined observations by the infraredSpitzer Space Telescope,[22]Herschel Space Telescope[5] and theHubble 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' bulkdensity.[4] The single-body diameter (effective system size) ofLempo is currently estimated at393.1+25.2
−26.8 km.[5]
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 highporosity of 50–75%, assuming an equal mixture of rock and ice.[22] 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 of0.532+0.317
−0.211 g/cm3 confirming the earlier conclusion that the object is probably arubble pile.[4] The density was revised up to0.64+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.[5]
Lempo has a very redspectral slope invisible light[31] and a flat spectrum innear infrared. There is also a weak absorption feature near the wavelength of 2 μm, probably caused by waterice. The best model reproducing the near infrared spectrum includestholins, crystalline water ice, andserpentine as surface materials. These results are for the integrated spectrum of all three components of the system.[32]
The Lempo system has been considered for future exploration due to its unusual configuration.[33] Lempo was suggested as a target forNew Horizons 2, a proposed twin of its namesake that would fly by Jupiter, Uranus, and up to four KBOs.[34]
