As of this writing, three objects have been discovered that have passed through our solar system coming from interstellar space.
1I/’Oumuamua Discovery Date: 2017-10-19 Perihelion Date: 2017-09-09 (40 days after perihelion) Perihelion Distance: 0.26 AU Incoming and Outgoing Speed: 26.4 km/s Dimensions: approximately 115 m × 111 m × 19 m, or 230 m × 35m × 35m Object Type: asteroid?
2I/Borisov Discovery Date: 2019-08-29 Perihelion Date: 2019-12-08 (101 days before perihelion) Perihelion Distance: 2.01 AU Incoming and Outgoing Speed: 32.3 km/s Diameter: 400 m – 1000 m Object Type: comet
3I/Atlas Discovery Date: 2025-07-01 Perihelion Date: 2025-10-29 (120 days before perihelion) Perihelion Distance: 1.36 AU Incoming and Outgoing Speed: 58.0 km/s Diameter: 320 m – 5,600 m (most likely 520 m – 748 m) Object Type: comet
Here are charts showing the direction from which each object entered our solar system, and the direction towards which each object is leaving our solar system.
1I/’Oumuamua came in from the direction of the constellation Lyra1I/’Oumuamua is exiting in a direction inside the Square of Pegasus2I/Borisov came in from the direction of the constellation Cassiopeia2I/Borisov is exiting in the direction of the southern constellation Telescopium3I/Atlas came in from the direction of the constellation Sagittarius3I/Atlas is exiting in the direction of the constellation Gemini
It is perhaps not surprising that 1I/’Oumuamua had the greatest deflection angle of the three interstellar objects. It came quite close to the Sun (0.26 AU, well inside the orbit of Mercury) and had the lowest incoming speed (26.4 km/s).
3I/Atlas, on the other hand, had the greatest incoming speed by far (58.0 km/s), so it was deflected by only 18.7° from a straight-line trajectory (angular separation 180° and deflection angle 0°).
Of the 793,918 asteroids and trans-Neptunian objects (TNOs) currently catalogued, only 98 are in retrograde orbits around the Sun. That’s just 0.01%.
By “retrograde” we mean that the object orbits the Sun in the opposite sense of all the major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. From a vantage point above the north pole of the Earth, all of the major planets orbit in a counterclockwise direction around the Sun.
But a retrograde object would be seen to orbit in a clockwise direction around the Sun, as is shown in the animation below for Jupiter retrograde co-orbital asteroid 514107 (2015 BZ509), with respect to Jupiter and its two “clouds” of trojan asteroids.
Of these 98 retrograde objects, only 14 have orbits well-enough determined to have received a minor planet number, and only one has yet received an official name (20461 Dioretsa).
Semimajor Axis (a) between…
Number of Retrograde Minor Planets
Mars – Jupiter
3
Jupiter – Saturn*
20
Saturn – Uranus*
15
Uranus – Neptune*
20
TNOs
40
*asteroids between the orbits of Jupiter and Neptune are often referred to as centaurs
At least some of these objects may be captured interstellar objects.
Let’s now take a look at some of these 98 retrograde objects in greater detail.
20461 Dioretsa The first retrograde asteroid to be discovered was 20461 Dioretsa, in 1999. The only named retrograde asteroid to date, Dioretsa is an anadrome of the word “asteroid”. It is a centaur in a highly eccentric orbit (0.90), ranging between the orbits of Mars and Jupiter out to beyond the orbit of Neptune. Objects in cometlike orbits that show no evidence of cometary activity are often referred to as damocloids. Dioretsa is both a centaur and a damocloid. Its orbital inclination (relative to the ecliptic) is 160°, which is a 20° tilt from an anti-ecliptic orbit. It takes nearly 117 years to orbit the Sun once. It is a dark object with a reflectivity only around 3% and is estimated to be about 9 miles across.
2010 EQ169 This retrograde asteroid holds the distinction (at least temporarily) of being the most highly-inclined main-belt asteroid (91.6°), relative to the ecliptic plane. It is also the retrograde asteroid with the smallest semimajor axis (2.05 AU) and lowest orbital eccentricity (0.10). Unfortunately, it was discovered after the fact by analyzing past data from the Wide-field Infrared Survey Explorer (WISE) space telescope, and has not been seen since. We have only a three-day arc of 17 astrometric observations of 2010 EQ169 between March 7-9, 2010 from which to determine its orbit. Nominally, 2010 EQ169 orbits the Sun at nearly a right angle to the ecliptic plane once every 2.9 years, between the orbits of Mars and Jupiter. However, our knowledge of its orbit is extremely uncertain, as shown below, and it has been lost. Our only hope will be to back-calculate the positions of future asteroids discovered to these dates to see if it matches the WISE positions.
Element
Value
1σ Uncertainty
Inclination (i)
91.606°
18.177°
Semimajor Axis (a)
2.0518 AU
2.2176
Orbital Eccentricity (e)
0.10153
0.90213
Orbital Period (P)
2.94y
4.765
2013 BL76 This retrograde TNO has the largest known semi-major axis of any of the retrograde non-cometary objects: 966.4274 ± 2.2149 AU. In a highly eccentric cometlike orbit (e = 0.99135), its perihelion is in the realm of the centaurs between the orbits of Jupiter and Saturn (8.35 AU), and its aphelion is way out around 1,924 AU. It takes about 30,000 years to orbit the Sun. Its orbit is inclined 98.6° with respect to the ecliptic.
2013 LA2 This retrograde centaur is in an orbit closest to the ecliptic plane (i = 175.2°), tilted 4.8° with respect to the ecliptic. It orbits the Sun about once every 21 years between the orbits of Mars and Uranus.
2017 UX51 The distinction for this retrograde TNO is that it has the highest orbital eccentricity of any non-cometary solar system object (e = 0.9967). Or is it an old inactive comet? 2017 UX51 orbits the Sun every 7,419 ± 2,883 years as close in as between the orbits of Earth and Mars (perihelion q = 1.24 AU)—classifying it as an Amor object—out to far beyond the orbit of Neptune (aphelion Q = 759.54 ± 196.77 AU). Its orbital inclination is 108.2°.
343158 (2009 HC82) An Apollo asteroid, 343158 is the only known retrograde near-Earth asteroid (NEA), with an orbital inclination of 154.4°. It orbits the Sun every 4.0 years, between 0.49 AU (almost as close in as the aphelion of Mercury) out to 4.57 AU (between the orbits of Mars and Jupiter).
References Conover, E., 2017. Science News, 191, 9, 5.
