Trojan (celestial body)

In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, a trojan is a small <a href="/facts/Celestial_body/BCHLa0Q2">celestial body</a> (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its <a href="/facts/Lagrangian_point/XCAN4o3P">Lagrangian points</a> <a href="/facts/Lagrange_point/XCAN4o3P">L4</a> and <a href="/facts/Lagrange_point/XCAN4o3P">L5</a>. Trojans can share the orbits of <a href="/facts/Planet/rbPjE6Bx">planets</a> or of large <a href="/facts/Natural_satellite/AE4KQKbq">moons</a>.
Trojans are one type of <a href="/facts/Co-orbital_configuration/q7jR91z6">co-orbital object</a>. In this arrangement, a star and a planet orbit about their common <a href="/facts/Barycenter/Fw9renao">barycenter</a>, which is close to the center of the star because it is usually much more massive than the orbiting planet. In turn, a much smaller mass than both the star and the planet, located at one of the Lagrangian points of the star–planet system, is subject to a combined gravitational force that acts through this barycenter. Hence the smallest object orbits around the barycenter with the same <a href="/facts/Orbital_period/94f56uh2">orbital period</a> as the planet, and the arrangement can remain stable over time.
In the Solar System, most known trojans share the <a href="/facts/Jupiter_trojan/KN9dgpUr">orbit of Jupiter</a>. They are divided into the <a href="/facts/Greek_camp/grMtpT2b">Greek camp</a> at L4 (ahead of Jupiter) and the <a href="/facts/Trojan_camp/RkLNwNWK">Trojan camp</a> at L5 (trailing Jupiter). More than a million Jupiter trojans larger than one kilometer are thought to exist, of which more than 7,000 are currently catalogued. In other planetary orbits only nine <a href="/facts/Mars_trojan/CvWoFsWo">Mars trojans</a>, 31 <a href="/facts/Neptune_trojan/CaLk7pNv">Neptune trojans</a>, two <a href="/facts/Uranus_trojan/30ADP7mW">Uranus trojans</a>, two <a href="/facts/Earth_trojan/tFbTDRWF">Earth trojans</a>, and one <a href="/facts/2019_UO14/pQwCnENT">Saturn trojan</a> have been found to date. A temporary <a href="/facts/2013_ND15/AkxFfbts">Venus trojan</a> is also known. Numerical orbital dynamics stability simulations indicate that Saturn probably does not have any primordial trojans.
The same arrangement can appear when the primary object is a planet and the secondary is one of its moons, whereby much smaller <a href="/facts/Trojan_moon/q7jR91z6">trojan moons</a> can share its orbit. All known trojan moons are part of the <a href="/facts/Moons_of_Saturn/HA64rVRw">Saturn system</a>. <a href="/facts/Telesto_(moon)/ehthK9sX">Telesto</a> and <a href="/facts/Calypso_(moon)/KltsKvAp">Calypso</a> are trojans of <a href="/facts/Tethys_(moon)/UWCPXvaN">Tethys</a>, and <a href="/facts/Helene_(moon)/Jh1ykJ3R">Helene</a> and <a href="/facts/Polydeuces_(moon)/IlHxTuhL">Polydeuces</a> of <a href="/facts/Dione_(moon)/uDt90r5R">Dione</a>.

Trojan (celestial body) open-in-new

Trojan (celestial body)