5261 Eureka

{{Short description|Trojan asteroid of Mars}}

{{Use dmy dates|date=October 2019}}

{{Infobox planet

| minorplanet = yes

| background = #FA8072

| name = 5261 Eureka

| discovery_ref =

| discoverer = David H. Levy and Henry Holt

| discovered = 20 June 1990

| alt_names = 1990 MB

| named_after = Eureka

| pronounced = {{IPAc-en|j|ʊ|ˈ|r|iː|k|ə}} {{respell|yuurr|EE|kə}}{{OED|Eureka}}

| mp_category = {{mpcat|Martian L5|125}}

| orbit_ref =

| epoch = 13 January 2016 (JD 2457400.5)

| aphelion = {{Convert|1.6222|AU|Gm|abbr=on|lk=on}}

| perihelion = {{Convert|1.4249|AU|Gm|abbr=on}}

| semimajor = {{Convert|1.5236|AU|Gm|abbr=on}}

| eccentricity = 0.064766

| period = 1.88 yr (686.89 d)

| inclination = 20.280°

| asc_node = 245.057°

| arg_peri = 95.456°

| mean_anomaly = 145.29°

| avg_speed = 24.11 km/s

| satellites = 1

| dimensions = 1.3 km
~2–4 km^{[https://web.archive.org/web/20010302182040/http://neo.jpl.nasa.gov/glossary/h.html H]}

| abs_magnitude = 16.0, 16.1

| single_temperature = ~250 K

| spectral_type = S(I) (Gaffey)

| albedo = 0.39

| mean_motion = {{Deg2DMS|0.52410|sup=ms}} / day

| rotation = {{Convert|2.6902|h|d|abbr=on|lk=on}}

| observation_arc = 13267 days (36.32 yr)

| uncertainty = 0

| moid = {{Convert|0.497052|AU|Gm|abbr=on}}

| jupiter_moid = {{Convert|3.52162|AU|Gm|abbr=on}}

| tisserand = 4.428

}}

5261 Eureka is the first Mars trojan discovered.{{Cite web|title=List of Martian Trojans|access-date=2011-02-26|url=https://www.minorplanetcenter.net/iau/lists/MarsTrojans.html}} It was discovered by David H. Levy and Henry Holt at Palomar Observatory on 20 June 1990. It trails Mars (at the {{L5|pt=yes}}) at a distance varying by only 0.3 AU during each revolution (with a secular trend superimposed, changing the distance from 1.5–1.8 AU around 1850 to 1.3–1.6 AU around 2400). Minimum distances from Earth, Venus, and Jupiter, are 0.5, 0.8, and 3.5 AU, respectively.

Long-term numerical integration shows that the orbit is stable. Kimmo A. Innanen and Seppo Mikkola note that "contrary to intuition, there is clear empirical evidence for the stability of motion around the {{L4}} and {{L5}} points of all the terrestrial planets over a timeframe of several million years".

Since the discovery of 5261 Eureka, the Minor Planet Center has recognized three other asteroids as Martian trojans: {{mpl|1999 UJ|7}} at the {{L4}} point, {{mpl|1998 VF|31}} at the {{L5}} point, and {{mpl|2007 NS|2}}, also at the {{L5}} point.[https://www.minorplanetcenter.net/mpec/K07/K07O09.html MPEC 2007-O09 : 2007 NS2] At least five other asteroids in near-1:1 resonances with Mars have been discovered, but they do not exhibit trojan behavior. They are {{mpl|2001 FR|127}}, {{mpl|2001 FG|24}}, (36017) 1999 ND₄₃, {{mpl|1998 QH|56}} and (152704) 1998 SD₄. Due to close orbital similarities, most of the other, smaller, members of the L₅ group are hypothesized to be fragments of 5261 Eureka that were detached after it was spun up by the YORP effect (consistent with its rotational period of 2.69 h).{{cite journal|last1= Lovett|first1= R.|title= Sun's light touch explains asteroids flying in formation behind Mars|journal= Science|date= 2017-10-20|doi= 10.1126/science.aar2794}}

The infrared spectrum for 5261 Eureka is typical for an A-type asteroid, but the visual spectrum is consistent with an evolved form of achondrite called an angrite. A-class asteroids are tinted red in hue, with a moderate albedo. The asteroid is located deep within a stable Lagrangian zone of Mars, which is considered indicative of a primordial origin—meaning the asteroid has most likely been in this orbit for much of the history of the Solar System.{{citation needed|date=December 2020}}