Horseshoe orbit

In celestial mechanics, a horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body. The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape as viewed from the larger object in a rotating reference frame.

A complex horseshoe orbit (the vertical looping is due to inclination of the smaller body's orbit to that of the Earth, and would be absent if both orbited in the same plane)
   Sun ·    Earth ·    (419624) 2010 SO16

The loop is not closed but will drift forward or backward slightly each time, so that the point it circles will appear to move smoothly along the larger body's orbit over a long period of time. When the object approaches the larger body closely at either end of its trajectory, its apparent direction changes. Over an entire cycle the center traces the outline of a horseshoe, with the larger body between the 'horns'.

Asteroids in horseshoe orbits with respect to Earth include 54509 YORP, 2002 AA29, 2010 SO16, 2015 SO2 and possibly 2001 GO2. A broader definition includes 3753 Cruithne, which can be said to be in a compound and/or transition orbit,[1] or (85770) 1998 UP1 and 2003 YN107. By 2016, 12 horseshoe librators of Earth have been discovered.[2]

Saturn's moons Epimetheus and Janus occupy horseshoe orbits with respect to each other (in their case, there is no repeated looping: each one traces a full horseshoe with respect to the other).

Explanation of horseshoe orbital cycle

Background

The following explanation relates to an asteroid which is in such an orbit around the Sun, and is also affected by the Earth.

The asteroid is in almost the same solar orbit as Earth. Both take approximately one year to orbit the Sun.

It is also necessary to grasp two rules of orbit dynamics:

  1. A body closer to the Sun completes an orbit more quickly than a body farther away.
  2. If a body accelerates along its orbit, its orbit moves outwards from the Sun. If it decelerates, the orbital radius decreases.

The horseshoe orbit arises because the gravitational attraction of the Earth changes the shape of the elliptical orbit of the asteroid. The shape changes are very small but result in significant changes relative to the Earth.

The horseshoe becomes apparent only when mapping the movement of the asteroid relative to both the Sun and the Earth. The asteroid always orbits the Sun in the same direction. However, it goes through a cycle of catching up with the Earth and falling behind, so that its movement relative to both the Sun and the Earth traces a shape like the outline of a horseshoe.

Stages of the orbit

Figure 1. Plan showing possible orbits along gravitational contours. In this image, the Earth (and the whole image with it) is rotating counterclockwise around the Sun.
Figure 2. Thin horseshoe orbit

Starting at point A, on the inner ring between L5 and Earth, the satellite is orbiting faster than the Earth and is on its way toward passing between the Earth and the Sun. But Earth's gravity exerts an outward accelerating force, pulling the satellite into a higher orbit which (per Kepler's third law) decreases its angular speed.

When the satellite gets to point B, it is traveling at the same speed as Earth. Earth's gravity is still accelerating the satellite along the orbital path, and continues to pull the satellite into a higher orbit. Eventually, at Point C, the satellite reaches a high and slow enough orbit such that it starts to lag behind Earth. It then spends the next century or more appearing to drift 'backwards' around the orbit when viewed relative to the Earth. Its orbit around the Sun still takes only slightly more than one Earth year. Given enough time, the Earth and the satellite will be on opposite sides of the Sun.

Eventually the satellite comes around to point D where Earth's gravity is now reducing the satellite's orbital velocity. This causes it to fall into a lower orbit, which actually increases the angular speed of the satellite around the Sun. This continues until point E where the satellite's orbit is now lower and faster than Earth's orbit, and it begins moving out ahead of Earth. Over the next few centuries it completes its journey back to point A.

On the longer term, asteroids can transfer between horseshoe orbits and quasi-satellite orbits. Quasi-satellites aren't gravitationally bound to their planet, but appear to circle it in a retrograde direction as they circle the Sun with the same orbital period as the planet. By 2016, orbital calculations showed that four of Earth's horseshoe librators and all five of its then known quasi-satellites repeatedly transfer between horseshoe and quasi-satellite orbits.[3]

Energy viewpoint

A somewhat different, but equivalent, view of the situation may be noted by considering conservation of energy. It is a theorem of classical mechanics that a body moving in a time-independent potential field will have its total energy, E = T + V, conserved, where E is total energy, T is kinetic energy (always non-negative) and V is potential energy, which is negative. It is apparent then, since V = -GM/R near a gravitating body of mass M and orbital radius R, that seen from a stationary frame, V will be increasing for the region behind M, and decreasing for the region in front of it. However, orbits with lower total energy have shorter periods, and so a body moving slowly on the forward side of a planet will lose energy, fall into a shorter-period orbit, and thus slowly move away, or be "repelled" from it. Bodies moving slowly on the trailing side of the planet will gain energy, rise to a higher, slower, orbit, and thereby fall behind, similarly repelled. Thus a small body can move back and forth between a leading and a trailing position, never approaching too close to the planet that dominates the region.

Tadpole orbit

An example of a tadpole orbit
   Sun ·    Earth ·    2010 TK7
See also Trojan (astronomy).

Figure 1 above shows shorter orbits around the Lagrangian points L4 and L5 (e.g. the lines close to the blue triangles). These are called tadpole orbits and can be explained in a similar way, except that the asteroid's distance from the Earth does not oscillate as far as the L3 point on the other side of the Sun. As it moves closer to or farther from the Earth, the changing pull of Earth's gravitational field causes it to accelerate or decelerate, causing a change in its orbit known as libration.

An example of a body in a tadpole orbit is Polydeuces, a small moon of Saturn which librates around the trailing L5 point relative to a larger moon, Dione. In relation to the orbit of Earth, the 300-metre-diameter (980-foot) asteroid 2010 TK7 is in a tadpole orbit around the leading L4 point. 2020 VT1 follows a temporary horseshoe orbit with respect to Mars.[4]

Known and suspected companions of Earth
Name Eccentricity Diameter
(m)
Discoverer Year of Discovery Type Current Type
Moon0.0551737400??Natural satelliteNatural satellite
1913 Great Meteor Procession???9 February 1913Possible Temporary satelliteDestroyed
3753 Cruithne0.5155000Duncan Waldron10 October 1986Quasi-satelliteHorseshoe orbit
1991 VG0.0535–12Spacewatch6 November 1991Temporary satelliteApollo asteroid
(85770) 1998 UP10.345210–470Lincoln Lab's ETS18 October 1998Horseshoe orbitHorseshoe orbit
54509 YORP0.230124Lincoln Lab's ETS3 August 2000Horseshoe orbitHorseshoe orbit
2001 GO20.16835–85Lincoln Lab's ETS13 April 2001Possible Horseshoe orbitPossible Horseshoe orbit
2002 AA290.01320–100LINEAR9 January 2002Quasi-satelliteHorseshoe orbit
2003 YN1070.01410–30LINEAR20 December 2003Quasi-satelliteHorseshoe orbit
(164207) 2004 GU90.136160–360LINEAR13 April 2004Quasi-satelliteQuasi-satellite
(277810) 2006 FV350.377140–320Spacewatch29 March 2006Quasi-satelliteQuasi-satellite
2006 JY260.0836–13Catalina Sky Survey6 May 2006Horseshoe orbitHorseshoe orbit
2006 RH1200.0242–3Catalina Sky Survey14 September 2006Temporary satelliteApollo asteroid
(419624) 2010 SO160.075357WISE17 September 2010Horseshoe orbitHorseshoe orbit
2010 TK70.191150–500WISE1 October 2010Earth trojanEarth trojan
2013 BS450.08320–40Spacewatch20 January 2010Horseshoe orbitHorseshoe orbit
2013 LX280.452130–300Pan-STARRS12 June 2013Quasi-satellite temporaryQuasi-satellite temporary
2014 OL3390.46170–160EURONEAR29 July 2014Quasi-satellite temporaryQuasi-satellite temporary
2015 SO20.10850–110Črni Vrh Observatory21 September 2015Quasi-satelliteHorseshoe orbit temporary
2015 XX1690.1849–22Mount Lemmon Survey9 December 2015Horseshoe orbit temporaryHorseshoe orbit temporary
2015 YA0.2799–22Catalina Sky Survey16 December 2015Horseshoe orbit temporaryHorseshoe orbit temporary
2015 YQ10.4047–16Mount Lemmon Survey19 December 2015Horseshoe orbit temporaryHorseshoe orbit temporary
469219 Kamoʻoalewa0.10440-100Pan-STARRS27 April 2016Quasi-satellite stableQuasi-satellite stable
DN16082203???22 August 2016Possible Temporary satelliteDestroyed
2020 CD30.0171–6Mount Lemmon Survey15 February 2020Temporary satelliteTemporary satellite
2020 PN10.12710–50ATLAS-HKO12 August 2020Horseshoe orbit temporaryHorseshoe orbit temporary
2020 PP10.07410–20Pan-STARRS12 August 2020Quasi-satellite stableQuasi-satellite stable
2020 VT10.16770–150Pan-STARRS10 November 2020Horseshoe orbit temporaryHorseshoe orbit temporary
2020 XL50.387250–550Pan-STARRS12 December 2020Earth trojan (suspected)Earth trojan

See also

References

  1. Christou, Apostolos A.; Asher, David J. (2011). "A long-lived horseshoe companion to the Earth". Monthly Notices of the Royal Astronomical Society. 414 (4): 2965–2969. arXiv:1104.0036. Bibcode:2011MNRAS.414.2965C. doi:10.1111/j.1365-2966.2011.18595.x. S2CID 13832179.
  2. de la Fuente Marcos, C.; de la Fuente Marcos, R. (April 2016). "A trio of horseshoes: past, present and future dynamical evolution of Earth co-orbital asteroids 2015 XX169, 2015 YA and 2015 YQ1". Astrophysics and Space Science. 361 (4): 121–133. arXiv:1603.02415. Bibcode:2016Ap&SS.361..121D. doi:10.1007/s10509-016-2711-6. S2CID 119222384.
  3. de la Fuente Marcos, C.; de la Fuente Marcos, R. (November 11, 2016). "Asteroid (469219) (469219) 2016 HO3, the smallest and closest Earth quasi-satellite". Monthly Notices of the Royal Astronomical Society. 462 (4): 3441–3456. arXiv:1608.01518. Bibcode:2016MNRAS.462.3441D. doi:10.1093/mnras/stw1972. S2CID 118580771.
  4. de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (March 2021). "Using Mars co-orbitals to estimate the importance of rotation-induced YORP break-up events in Earth co-orbital space". Monthly Notices of the Royal Astronomical Society. 501 (4): 6007–6025. arXiv:2101.02563. Bibcode:2021MNRAS.501.6007D. doi:10.1093/mnras/stab062.
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