Editing Jupiter trojan (section)

== Physical properties ==
[[File:624Hektor-LB1-mag15.jpg|thumb|right|Trojan [[624 Hektor]] (indicated) is similar in [[apparent magnitude|brightness]] to [[dwarf planet]] [[Pluto]].]]

Jupiter trojans are dark bodies of irregular shape. Their [[geometric albedo]]s generally vary between 3 and 10%.<ref name=Fernandes2003/> The average value is 0.056&nbsp;±&nbsp;0.003 for the objects larger than 57&nbsp;km,<ref name=Jewitt2004/> and 0.121&nbsp;±&nbsp;0.003 (R-band) for those smaller than 25&nbsp;km.<ref name=Fernandes2009/> The asteroid [[4709 Ennomos]] has the highest albedo (0.18) of all known Jupiter trojans.<ref name=Fernandes2003/> Little is known about the masses, chemical composition, rotation or other physical properties of the Jupiter trojans.<ref name=Jewitt2004/>

=== Rotation ===
The rotational properties of Jupiter trojans are not well known. Analysis of the rotational [[light curve]]s of 72 Jupiter trojans gave an average rotational period of about 11.2&nbsp;hours, whereas the average period of the control sample of asteroids in the asteroid belt was 10.6&nbsp;hours.<ref name=Barucci2002/> The distribution of the rotational periods of Jupiter trojans appeared to be well approximated by a [[Maxwell distribution|Maxwellian function]],<ref group="Note">The Maxwellian function is <math>F=\begin{smallmatrix}\frac{1}{\sqrt{2\pi}\sigma}P^2\exp(-(P-P_0)^2/\sigma^2)\end{smallmatrix}</math>, where <math>P_0</math> is the average rotational period, <math>\sigma</math> is the [[Statistical dispersion|dispersion]] of periods.</ref> whereas the distribution for main-belt asteroids was found to be non-Maxwellian, with a deficit of periods in the range 8–10&nbsp;hours.<ref name=Barucci2002/> The Maxwellian distribution of the rotational periods of Jupiter trojans may indicate that they have undergone a stronger collisional evolution compared to the asteroid belt.<ref name=Barucci2002/>

In 2008 a team from [[Calvin College]] examined the [[light curve]]s of a debiased sample of ten Jupiter trojans, and found a [[median]] spin period of 18.9 hours. This value was significantly higher than that for main-belt asteroids of similar size (11.5 hours). The difference could mean that the Jupiter trojans possess a lower average density, which may imply that they formed in the [[Kuiper belt]] (see below).<ref>{{cite journal|last1=Molnar|first1=Lawrence A.|last2=Haegert|first2=Melissa J.|last3=Hoogeboom|first3=Kathleen M.|date=April 2008|title=Lightcurve Analysis of an Unbiased Sample of Trojan Asteroids|journal=The Minor Planet Bulletin|publisher=Association of Lunar and Planetary Observers|volume=35|issue=2|pages=82–84|oclc=85447686|bibcode=2008MPBu...35...82M}}</ref>

=== Composition ===
[[Spectroscopy|Spectroscopically]], the Jupiter trojans mostly are [[D-type asteroid]]s, which predominate in the outer regions of the asteroid belt.<ref name=Jewitt2004/> A small number are classified as [[P-type asteroid|P]] or [[C-type asteroid]]s.<ref name=Barucci2002/> Their spectra are red (meaning that they reflect more light at longer wavelengths) or neutral and featureless.<ref name=Fernandes2003/> No firm evidence of water, organics or other chemical compounds has been obtained {{as of|2007|lc=on}}. [[4709 Ennomos]] has an albedo slightly higher than the Jupiter-trojan average, which may indicate the presence of water ice. Some other Jupiter Trojans, such as [[911 Agamemnon]] and [[617 Patroclus]], have shown very weak absorptions at 1.7 and 2.3&nbsp;μm, which might indicate the presence of organics.<ref>{{cite journal|title=Spectroscopic Search for Water Ice on Jovian Trojan Asteroids|last1=Yang|first1=Bin|last2=Jewitt|first2=David|date=2007|journal=The Astronomical Journal|volume=134|issue=1|pages=223–228|doi=10.1086/518368|url=http://www.iop.org/EJ/abstract/1538-3881/134/1/223/|access-date=19 January 2009|bibcode=2007AJ....134..223Y|doi-access=free}}</ref> The Jupiter trojans' spectra are similar to those of the [[Moons of Jupiter#Irregular satellites|irregular moons of Jupiter]] and, to a certain extent, [[comet nuclei]], though Jupiter trojans are spectrally very different from the redder Kuiper belt objects.<ref name=Yoshida2005/><ref name=Jewitt2004/> A Jupiter trojan's spectrum can be matched to a mixture of water ice, a large amount of carbon-rich material ([[charcoal]]),<ref name=Jewitt2004/> and possibly [[magnesium]]-rich [[silicate]]s.<ref name=Barucci2002/> The composition of the Jupiter trojan population appears to be markedly uniform, with little or no differentiation between the two swarms.<ref>{{cite journal|title=The surface composition of Jupiter trojans: Visible and near-infrared survey of dynamical families|author=Dotto, E.|author2=Fornasier, S.|author3=Barucci, M. A.|journal=Icarus|volume=183|issue=2|date=August 2006|pages=420–434|doi=10.1016/j.icarus.2006.02.012|bibcode=2006Icar..183..420D|display-authors=etal}}</ref>

A team from the [[Keck Observatory]] in Hawaii announced in 2006 that it had measured the density of the binary Jupiter trojan [[617 Patroclus]] as being less than that of water ice (0.8&nbsp;g/cm<sup>3</sup>), suggesting that the pair, and possibly many other Trojan objects, more closely resemble [[comet]]s or Kuiper belt objects in composition—water ice with a layer of dust—than they do the main-belt asteroids.<ref name=Marchis2006/> Countering this argument, the density of Hektor as determined from its rotational lightcurve (2.480&nbsp;g/cm<sup>3</sup>) is significantly higher than that of 617 Patroclus.<ref name=Lacerda2007/> Such a difference in densities suggests that density may not be a good indicator of asteroid origin.<ref name=Lacerda2007/>