1I 'Oumuamua (2017 U1): The first interstellar object within our solar system

by Wm. Robert Johnston
last updated 8 July 2019

Discovery and designation

An object on a hyperbolic trajectory through the inner solar system was discovered by the Pan-STARRS survey on 19 October 2017 and given a cometary designation C/2017 U1 (PANSTARRS). It was subsequently identified in images from 14 October by the Catalina Sky Survey. On 25 October it was assigned an asteroidal designation A/2017 U1 based on the lack of observed cometary activity. On 6 November the Minor Planet Center assigned it the designation 1I as the first object in a new designation scheme for interstellar objects, and they also announced approval of the name 'Oumuamua suggested by the Pan-STARRS team.

Orbit and ephemeris (JPL, 2017)

perihelion distance q:
eccentricity e:
inclination i:
ascending node Ω:
longitude of perihelion ω:
mean anomaly M:
time of perihelion Tp:
absolute magnitude H:
data arc:
0.255343 ± 0.000067 AU
1.199512 ± 0.000181
122.6872 ± 0.0063°
24.5992 ± 0.0003°
241.7030 ± 0.0123°
36.4253 ± 0.0342°
2017 Sep 09.4905 ± 0.0015
2017 Nov 2.0
22.08 ± 0.45
34 days (2017 Oct 14--Nov 17), 121 obs.

Orbit and ephemeris (JPL, 2018)

perihelion distance q:
eccentricity e:
inclination i:
ascending node Ω:
longitude of perihelion ω:
mean anomaly M:
time of perihelion Tp:
nongravitational parameter A1:
nongravitational parameter A2:
nongravitational parameter A3:
absolute magnitude H:
data arc:
0.2559116 ± 0.0000067 AU
1.201134 ± 0.000021
122.74171 ± 0.00029°
24.59691 ± 0.00025°
241.8105 ± 0.0012°
51.1576 ± 0.0061°
2017 Sep 09.50732 ± 0.00026
2017 Nov 23.0
(2.79 ± 0.36) x 10-7
(1.44 ± 2.45) x 10-8
(1.57 ± 2.25) x 10-8
22.08 ± 0.45
80 days (2017 Oct 14-2018 Jan 02), 207 obs.

datemagnitudedistance to
Sun (AU)
distance to
Earth (AU)
2013 Jan 0137.0329.57730.129 
2014 Jan 0136.1223.75424.311 
2015 Jan 0134.9417.86218.425 
2016 Jan 0133.2711.84412.419 
2017 Jan 1730.055.2025.771crossed Jupiterís orbit inbound
2017 Aug 1024.510.9881.325crossed Earthís orbit inbound
2017 Sep 0920.990.2571.204closest approach to Sun
2017 Oct 0123.810.7510.499 
2017 Oct 1121.481.0140.207crossed Earthís orbit outbound
2017 Oct 1519.961.1150.162closest approach to Earth
2017 Oct 1819.631.1900.198brightest as seen from Earth
2017 Oct 1919.701.2140.220discovery
2017 Nov 0123.191.5260.635 
2017 Dec 0126.172.1991.741 
2018 Jan 0127.662.8502.905 
2018 Feb 0128.553.4733.995 
2018 Mar 0129.054.0184.842 
2018 Apr 0129.424.6075.574 
2018 May 0430.025.2216.087crossed Jupiterís orbit outbound
2018 Jul 0130.726.2766.423 
2018 Oct 0131.077.9046.966 
2019 Jan 0132.339.4949.521 
2020 Jan 0134.3615.59715.618 
2021 Jan 0135.7021.54721.580 
2022 Jan 0136.7027.39427.423 
2025 Jan 0138.7644.68844.720 
2030 Jan 0140.8573.12373.150 
2035 Jan 0142.25101.371101.393 
2040 Jan 0143.29129.503129.521 
2045 Jan 0144.14157.609157.639 
2050 Jan 0144.84185.650185.675 

Confirmation as interstellar in origin

The MPC's initial automated calculations of the orbit of 2017 U1 assumed the orbit was elliptical or parabolic, resulting in the rejection of multiple reported observations. Not until 22 October was the orbit successfully identified as highly hyperbolic with an orbital eccentricity of 1.188 ± 0.016, exceeding the highest previously determined eccentricity for a solar system object. The eccentricity was later revised to 1.19951 ± 0.00018.

The figure below shows eccentricity vs. perihelion distance for 3,987 solar system comets (those known through December 2017) plus 'Oumuamua, showing it to be an outlier. If there were no influence from the gravitation of the planets, a comet arriving in the inner solar system from the Oort cloud would appear to have a nearly parabolic orbit, i.e. an orbital eccentricity of 1.000. The gravity of the planets (mostly Jupiter) will perturb a comet's orbit during passage through the inner solar system, often producing a slightly hyperbolic orbit. For example, a simplistic treatment implies that a comet with an eccentricity of 1.000 when beyond the jovian planets could have an heliocentric eccentricity of ~1.0027 in the inner solar system. Full modeling of the influence of the planets and non-gravitational effects (outgassing from the comet) shows case-by-case results may be larger.

For 6,248 comets observed through December 2017, 347 had measured orbital eccentricities larger than 1.000. Only 53 had eccentricities larger than 1.0027, the largest for C/1980 E1 Bowell with e=1.057. The SSDP group has tracked the dynamics of hyperbolic comets observed through 2012. For 34 comets with eccentricity greater than 1.0027, all but two were clearly bound to the solar system before passing through the inner solar system with e<1.000. The remaining two had pre-encounter eccentricities of ~1.0001--this difference from parabolic is comparable to estimated errors and equates to a velocity of only ~0.1 km/s when previously distant from the Sun. However, 18 of these comets were clearly unbound on their exit from the solar system, with hyperbolic eccentricities. In the case of C/1980 E1 Bowell, it passed 0.228 AU from Jupiter before perihelion which resulted in its being ejected from the solar system with what will be a final velocity relative to the Sun of 3.8 km/s (currently, in Dec. 2017, it is 64 AU from the Sun receeding at 6 km/s). The table below summarizes data for the 10 highest eccentricity objects observed.

comet/object designation perihelion
distance (AU)
near perihelion
minimum distance
to Jupiter (AU)
fate with
final velocity
1I/ 'Oumuamua 0.25531.199236±0.0001641.196488±0.0001641.200366±0.0001674.813never bound Dybczynski and Krolikowska (2017)
C/1980 E1 Bowell 3.36391.057256±0.0000110.999831±0.0000121.053734±0.0000090.228ejected at 3.76 km/s SSDP (2013)
C/1997 P2 Spacewatch 4.26251.028253±0.0001251.000059±0.0000581.009822±0.0001320.652ejected at 1.43 km/s SSDP (2013)
C/2017 U4 PANSTARRS 7.71221.009522±0.047301    5.635  JPL (2017)
C/2009 UG89 Lemmon 3.93121.008057±0.0000020.999355±0.0000021.005629±0.0000020.499ejected at 1.13 km/s SSDP (2013)
C/2002 A3 LINEAR 5.15141.007913±0.0000090.999893±0.0000090.968222±0.0000100.502bound with 2,100 yr period SSDP (2013)
C/1999 U4 Catalina-Skiff 4.91411.007662±0.0000030.999845±0.0000031.001440±0.0000032.161ejected at 0.51 km/s SSDP (2013)
C/1999 S2 McNaught-Watson 6.46621.007255±0.0000250.999635±0.0000251.002076±0.0000243.302ejected at 0.53 km/s SSDP (2013)
C/2014 W3 PANSTARRS 6.06371.006939±0.000140    1.797  JPL (2017)
C/2003 A2 Gleason 11.42641.006932±0.0000290.999527±0.0000290.998467±0.0000296.191bound with 640,000 yr periodSSDP (2013)

The figure below shows, for 496 hyperbolic and near-parabolic comets, the change in velocity resulting from passage through the inner solar system versus the original (pre-passage) velocity. Velocities are relative to the Sun and computed for 250 AU before and after passage. The sharp cutoff for pre-encounter velocities at ~2.9 km/s argues that all of these objects were bound to the solar system (within error bars). Many of these objects (likely over 100) were perturbed sufficient during passage through the planetary region that they became unbound. 'Oumuamua, however, was clearly never bound as its velocity relative to the Sun is far greater than what could result from a planetary encounter.

In early 2018 discovery was reported of two hyperbolic asteroids, A/2017 U7 and A/2018 C2 (JPL, 2018). These objects are only slightly hyperbolic (eccentricities of 1.00148 and 1.00177, respectively); both were bound to the solar system prior to entering the planetary region. A/2018 C2 will be marginally bound after departing the planetary region. While it initially appeared that A/2017 U7 would be slightly unbound, it was later reported that it will still be bound [Hui, 2018]. A/2017 U7 will not cross within the orbit of Jupiter, reaching perihelion in Sept. 2019 at 6.4 AU, while A/2018 C2 will pass 1.96 AU from the Sun in June 2018. Both objects have orbits typical of comets, and 2018 C2 later showed cometary activity [Hui, 2018]. For an albedo of 0.04, these objects would be 50 km and 6 km in size, respectively.

In 2018 Namouni and Morais (2018) concluded that asteroid (514107) 2015 BZ509 was of interstellar origin based on integration of its orbit over long integration time periods. 2015 BZ509 is coorbital with Jupiter but with a retrograde orbit--semimajor axis of 5.140 AU, eccentricity of 0.3807, and inclination of 163.0°. The conclusion is based on finding that the current orbit is stable for multi-billion year timescales combined with the understanding that "planet formation models cannot produce such a primordial large inclination orbit." Currently, interstellar origin of (514107) 2015 BZ509 is at least in part model dependent.

'Oumuamua's pre-encounter velocity with respect to the solar system was 26.1 km/s, arriving roughly from the direction of Vega. Its outgoing direction will be in the constellation Pegasus at a final velocity of 26.4 km/s. The sky map below shows the direction 'Oumuamua arrived from on the right (green dot), its original direction prior to encountering the solar system on the left (magenta dot), and its final outbound direction after encountering the solar system in the center (green dot).

The space velocity of 'Oumuamua is similar to the local standard of rest, hinting that it could have been ejected from a relatively young local system. Modeling of the past dynamics of 'Oumuamua, however, does not suggest a particular neighboring system as its origin, since any identifiable past encounters are at velocities too high to suggest ejection. For example, Portegies Zwart et al. (2017) identify an encounter 1.3 Myr ago at a distance of 0.52 light years from TYC4742-1027-1 (a ~K-type star currently 440 light years from the Sun) but at a relative velocity of 103 km/s. Alternately, Dybczynski and Krolikowska (2017) identify low velocity encounters but at large distances, e.g. one with UCAC4 535-065571 at 5.2 km/s 2.1 million years ago but at a distance of 4.65 light years. They also identify a pass 0.14 light years from HIP 3757 118,000 years ago with a relative velocity of 185 km/s. Note that all of these encounter estimates are dependent on the accuracy of our knowledge of local stellar motions and distances as well as accuracy of 'Oumuamua's past trajectory.

Catalog of observations

(R)adio, or
2017 Oct 14Catalina Sky Survey, AZ, USAA
2017 Oct 17Catalina Sky Survey, AZ, USAA
2017 Oct 18-19Pan-STARRS 1, Haleakala, HI, USA (discovery)A
2017 Oct 19San Marcello Pistoiese, Tuscany, ItalyA
2017 Oct 19ESA Optical Ground Station, Tenerife, SpainA
2017 Oct 19Klet Observatory, South Bohemia, Czech RepublicA
2017 Oct 20Steward Observatory, Mt. Lemmon Station, AZ, USAA
2017 Oct 21LPL/Spacewatch II, Kitt Peak, AZ, USAA
2017 Oct 21Tenagra II Observatory, AZ, USAA
2017 Oct 22iTelescope Observatory, Mayhill, NM, USAA
2017 Oct 22iTelescope Observatory, Siding Spring, NSW, AustraliaA
2017 Oct 22Canada-France-Hawaii Telescope, Mauna Kea, HI, USAP
2017 Oct 23Farpoint Observatory, KS, USAA
2017 Oct 24-25Mt. Lemmon Survey, AZ, USAA
2017 Oct 25San Marcello Pistoiese, Tuscany, ItalyA
2017 Oct 25Big Water Observatory, UT, USAA
2017 Oct 25Palomar Mountain Observatory, CA, USAP
2017 Oct 25William Herschel Telescope, La Palma, SpainP
2017 Oct 25-26La Palma Observatory, La Palma, SpainA
2017 Oct 25-26Nordic Optical Telescope, La Palma, SpainP
2017 Oct 25-27Very Large Telescope, European Southern Observatory, Paranal, ChileP
2017 Oct 25-27Tenagra II Observatory, AZ, USAA
2017 Oct 26Catalina Sky Survey, AZ, USAA
2017 Oct 26LPL/Spacewatch II, Kitt Peak, AZ, USAA
2017 Oct 26Schiaparelli Observatory, Varese, ItalyA
2017 Oct 26Hale Telescope, Palomar Mountain Observatory, CA, USAA/P
2017 Oct 26-27Sutherland-Las Cumbres Observatory, South AfricaA
2017 Oct 26-27Gemini South Telescope, Cerro Pachon, ChileP
2017 Oct 27Cordell-Lorenz Observatory, Sewanee, TN, USAA
2017 Oct 27Canada-France-Hawaii Telescope, Mauna Kea, HI, USAP
2017 Oct 27United Kingdom Infrared Telescope, Mauna Kea, HI, USAP
2017 Oct 27Keck 2 Telescope, Mauna Kea, HI, USAP
2017 Oct 27-28Great Shefford Observatory, West Berkshire, England, UKA
2017 Oct 28-30Magdalena Ridge Observatory, Socorro, NM, USAA
2017 Oct 29Apache Point Observatory, Sunspot, NM, USAA/P
2017 Oct 29Gemini North Telescope, Mauna Kea, HI, USAP
2017 Oct 29William Herschel Telescope, La Palma, SpainP
2017 Oct 29-30Nordic Optical Telescope, La Palma, SpainP
2017 Oct 30Discovery Channel Telescope, Lowell Observatory, AZ, USAP
2017 Nov 9-10Magdalena Ridge Observatory, Socorro, NM, USAA
2017 Nov 12Magdalena Ridge Observatory, Socorro, NM, USAA
2017 Nov 17Magdalena Ridge Observatory, Socorro, NM, USAA
2017 Nov 21Spitzer Space TelescopeT
2017 Nov 23-Dec 5Allen Telescope Array, Hat Creek, CA, USAR
2017 Nov 21-22Magellan Telescope, Las Campanas Observatory, ChileP
2017 Nov 21-22Hubble Space TelescopeA
2017 Dec 12Hubble Space TelescopeA
2017 Dec 13-14Green Bank Radio Telescope, WV, USAR
2017 Dec 16Allen Telescope Array, Hat Creek, CA, USAR
2017 Dec 18-20Green Bank Radio Telescope, WV, USAR
2017 Dec 22-23Green Bank Radio Telescope, WV, USAR
2017 Dec 30Green Bank Radio Telescope, WV, USAR
2018 Jan 2Hubble Space TelescopeA

The map below shows the locations of facilities (red dots) that have observed 'Oumuamua. Observing time was also obtained on both the Hubble Space Telescope and the Spitzer Space Telescope in late November (NASA, 2017), and additional time on the Hubble Space Telescope in December and January. Astrometric results from the Hubble Space Telescope observations helped constrain 'Oumuamua's orbit (Micheli et al., 2018). No results have yet been reported from the Spizter observations--this is unsurprising as these data would take longer to analyze, though it could also result from a failure to detect 'Oumuamua.

Color and lack of cometary activity

Observations from the Canada-France-Hawaii Telescope on 22 October of 'Oumuamua when the object was about 1.25 AU from the Sun showed no cometary coma, with similar subsequent results from other telescopes (Meech et al., 2017). Using these results to set an upper limit on dust production gives a limit of 1.7 grams/sec, 7-8 orders of magnitude less than typical long-period comets (Meech et al., 2017). Meech et al. (2017) also report "The immediate area surrounding the object was searched for faint companions with similar motion but none were found." Fitzsimmons et al. (2017) suggest based on thermal modeling that ices in the interior of the object could have been sufficiently insulated during the object's perihelion passage to not produce outgassing, though this conclusion only permits (does not require) the possibility of a cometary composition given certain assumptions.

Somewhat contradictory conclusions have been reported regarding 'Oumuamua's color. Observations generally indicate it is reddish, but not as red as trans-Neptunian objects. Reported colors in standard channels are summarized below; these results vary by about 50% in spectral slope (i.e., "redness" of spectra). The table and figure below summarize these results.

B-V0.70 ± 0.06Jewitt et al. (2017)
V-R0.45 ± 0.05Jewitt et al. (2017)
g-r0.47 ± 0.04Bannister et al. (2017)
g-r0.63 ± 0.31Bannister et al. (2017)
g-r0.41 ± 0.24Bolin et al. (2017)
g-r0.47Masiero (2017)
g-r0.84 ± 0.05Meech et al. (2017)
g-r0.60 ± 0.23Ye et al. (2017)
g-Y1.60 ± 0.20Meech et al. (2017)
r-i0.36 ± 0.16Bannister et al. (2017)
r-i0.23 ± 0.25Bolin et al. (2017)
r-i0.29Meech et al. (2017)
r-J1.20 ± 0.11Bannister et al. (2017)
r-z0.58Masiero (2017)
r-z0.41Meech et al. (2017)

Fraser et al. (2017) and Fitzsimmons et al. (2017) suggest that the varying results can be explained by the rotational tumbling if one region of 'Oumuamua is redder than the remaining surface. Fitzsimmons et al. (2017) report higher resolution spectra, finding that the colors are more consistent with Jupiter trojan asteroids than with typical outer solar system objects. Within the limitations of their spectra resolution, they found no specific absorption features attributable to ices or rocky minerals.

Lightcurve and size

Observations of 'Oumuamua's rotational lightcurve showed it to have a very extreme lightcurve variation, implying a highly elogated shape. Reported lightcurve results are summarized in the table below (derived values in brackets).

period (hr)
implied axial
ratio a/c
reference et al. (2017)
8.14 ± 0.022.11 ± 0.536.91 ± 3.41Bolin et al. (2017)
8.14 ± 0.021.54 ± 0.14.13 ± 0.48Bolin et al. (2017)
7.5483 ± 0.0073>2.510Drahus et al. (2017)
6.831[≥1.75]≥5Fraser et al. (2017)
8.2562.0 ± 0.2[6.3±1.2]Jewitt et al. (2017)
>3-5≥1.2≥3Knight et al. (2017)
7.34[2.5±0.1]10 ± 1Meech et al. (2017)

Several of the published results are for partial rotation observations only, so they only set a lower bound to the magnitude variation. Fraser et al. (2017) suggest that the Meech et al. (2017) could overestimate the true lightcurve amplitude as these observations were at a non-zero phase angle. Alternately, any observations that were not viewing from the plane perpendicular to 'Oumuamua's rotation axis could underestimate the true maximum lightcurve amplitude. Thus it is plausible to conclude that 'Oumuamua's axial ratio (a/c) is between 5:1 and 10:1.

The inconsistency of the rotation period results is noteworthy, and in fact is irreconcilable unless the object is tumbling as concluded by Fraser et al. (2017) and Drahus et al. (2017).

'Oumuamua is not resolved in any telescopic observations, so estimates of its size depend on assumptions of albedo. If its albedo is assumed to be similar to small bodies in our solar system with similar colors, then an albedo of 0.04-0.1 is implied. Sizes and dimensions for 'Oumuamua for different assumed values of albedo and axial ratio are given in the table below.

diameter (m)
dimensions (m) for
axial ratio 1:5
dimensions (m) for
axial ratio 1:10
0.01 5101140 x 2301600 x 160
0.04260 570 x 110 810 x 81
0.10160 360 x 72 510 x 51
0.30 93 210 x 42 290 x 29
0.90 54 120 x 24 170 x 17

Context of extreme lightcurve

The 1:7 to 1:10 axial ratio implied by 'Oumuamua's lightcurve is an extreme outlier among solar system objects. Of 18,462 asteroids with observed lightcurves, only a few show lightcurve variations this extreme. The asteroids with the most extreme lightcurve variations are listed in the following table (lightcurve data from Warner et al., 2017).

period (h)
a/c ratio
diameter (km)dynamical type
(80636) 2000 AV214100? 3.0 15.8~1.4 main belt
(44530) Horakova 160? 2.6811.8 6.9 main belt
2010 XM56 2.35 2.3 8.3~0.028Apollo
(1865) Cerberus 6.8042.3 8.3 1.61 Apollo
2009 UU1 0.1242.2 7.6~0.034Apollo
(367248) 2007 MK13 5.2862.2 7.6 0.39 Apollo
(163899) 2003 SD220285 2.2 7.6 0.80 Aten
(488515) 2001 FE90 0.4782.13 7.1~0.25 Apollo
(263670) 2008 GQ114 5.03 2.11 7.0~0.85 main belt
(98496) 2000 VT3 28.24?2.1 6.9~3.4 main belt
(44881) 1999 UJ51 20? 2.07 6.7~1.3 main belt
(436724) 2011 UW158 0.6112.05 6.6 0.45 Apollo
(1620) Geographos 5.2222.03 6.5 1.8 Apollo

The figure below shows the axial ratio implied by lightcurves variations for all asteroids with measured lightcurves (lightcurve and diameter data from Warner et al., 2017). This assumes observed lightcurve variations are due to irregular shape and not due to brightness variations on the surface. For many of the objects larger than 400 km in diameter, such brightness variations are more important than non-spherical shape in producing lightcurve variations. The red region shows where 'Oumuamua would be placed.

While axial ratios as extreme as 'Oumuamua are very rare among all asteroids, they are slightly less rare for objects in the size range estimated for 'Oumuamua. The table below lists information on asteroids with measured lightcurves implying axial ratios of 5:1 or greater and with measured or estimated diameters from 55 to 450 m (lightcurve data from Warner et al., 2017; diameter data from multiple sources or estimated based on an albedo of 0.16). PHA indicates potentially hazardous asteroid. These 10 asteroids are among 528 with measured lightcurves in this size range, i.e. 1.9% of those in this size range have lightcurves implying at least a 5:1 axial ratio.

period (h)
a/c ratio
diameter (m)albedodynamical type
(367248) 2007 MK13 5.2862.2 7.6 3900.12Apollo, PHA
(488515) 2001 FE90 0.4782.137.1~250 ? Apollo, PHA
(436724) 2011 UW1580.6112.056.6 4500.10Apollo, PHA
1995 HM 1.62 2.0 6.3 ~80 ? Amor
(416186) 2002 TD60 2.8512.0 6.3 2300.50Amor
(469896) 2005 WC1 2.5822.0 6.3 4000.06Apollo, PHA
2008 UP1009.0101.956.0 ~55 ? Apollo
2007 RQ12 0.23?1.9 5.8 ~63 ? Apollo
2010 NR1 0.89 1.8 5.2~140 ? Amor
2011 HS 6 1.8 5.2~200 ? Apollo

The following four figures are based on data from Warner et al. (2017). The first figure shows the cumulative fraction of solar system objects with apparent axial ratios exceeding a given value (assuming axial ratio is the sole factor in observed lightcurve variations). "Anthropogenic items" refers to man-made objects orbiting the Sun, mostly space probes or spent rocket upper stages.

The next figure shows histograms of percentage of asteroids in various size ranges as a function of apparent axial ratios. Asteroids larger than 100 km tend to have low axial ratios as they are large enough for self-gravitation to force a shape in hydrostatic equilibrium. Extreme shapes are more common at smaller sizes, but axial ratios more extreme than 1:5 remain uncommon. As most smaller objects are either collisional fragments or rubble piles, this is consistent with expectations that extreme shapes are hard to produce as either fragments or rubble piles.

The figure below shows the fraction of asteroids as a function of diameter and apparent axial ratios. Percentages are within each diameter bin; each bin includes about 1,000 asteroids with the exception of the three largest and three smallest size bins which have smaller samples. The black box indicates the size and axial ratio ranges estimated for 'Oumuamua. For asteroids the size of 'Oumuamua, of order 1% have axial ratios of 5:1 or greater; none have axial ratios greater than 8:1.

The figure below shows apparent axial ratio vs. apparent size for asteroids smaller than 1 km in diamter and for ~150 manmade objects in solar orbit (axial ratios for manmade objects are illustrative, as their lightcurves are more complex). The red region shows the best estimates for 'Oumuamua.

Anomalous characteristics and implications for origin

While small objects of interstellar origin are not unexpected, several of 'Oumuamua's characteristics are outliers relative to expectations based on observations of our solar system and expectations based on prevailing notions of planetary system formation and evolution. In summary:

The apparent lack of volatiles is surprising because most planetary system debris ejected into interstellar space is expected to be cometary material. This is because (1) stellar elemental abundances imply that most planetesimal debris is icy/volatile and (2) cometary material would tend to reside at greater distances from the central star(s) and hence be easier to become gravitationally unbound. However, this is in the context of existing assumptions regarding planetary system formation. Jackson et al. (2017) suggest that binary star systems may eject as much or more rocky planetesimals as icy ones.

The extreme axial ratio is shown to be an outlier among solar system small bodies--1 in 100 to 1 in 10,000. Objects larger than 200 km to 600 km in size (depending on composition) will tend to be in hydrostatic equilibrium due to self-gravitation. They may be ellipsoidal for fast rotating bodies, but with faster rotation they will tend to fission into binaries before reaching extreme shapes. Smaller objects have shapes potentially driven by several factors:

The fact that 'Oumuamua is tumbling strongly suggests it is a rigid body with high internal strength, as a body with low internal strength (a rubble pile) would rapidly dampen any tumbling via internal reconfiguration until it settles in a single principal axis rotation state (Fraser et al., 2017). The alternate explanation is that 'Oumuamua was recently perturbed into tumbling rotation, though this seems unlikely given that it appears to have been travelling in interstellar space for a long time.

The surface inhomogeniety (local reddening) postulated for 'Oumuamua (Fraser et al., 2017) contradicts the lack of differentiation expected for small bodies (and is also a rare feature for small asteroids in our solar system). Taken with the extreme shape that suggests the possibility of internal strength that is not uniform in all directions, one explanation could be that 'Oumuamua is a fragment of a planetary-class object. This has been suggested by Cuk (2017), Hansen and Zuckerman (2017), and Jackson et al. (2017), who discuss various possibilities involving tidal disruption of a planet followed by ejection from the system of origin of some of the resulting fragments.

Prospects for further exploration

The SETI project observed 'Oumuamua for possible radio emissions in November and December using the Allen Telescope Array and the Green Bank Telescope; results were negative (Beatty, 2017; Enriquez et al., 2018; Harp et al., 2018; Tingya et al., 2018).

It would be extremely difficult and expensive to send a robotic spaceprobe at sufficient speed to catch up with 'Oumuamua--but not impossible given current technology (Hein et al., 2017).

A proposal has been submitted for use of the Spitzer Space Telescope to observe any future interstellar asteroids (Trilling et al., 2018).


--articles and preprints on 'Oumuamua (from newest to oldest; arXiv posting date in parentheses)

--other items on 'Oumuamua (from newest to oldest)

--Minor Planet Center releases on 'Oumuamua

--background articles and data sets

Comments? Questions? Corrections? Contact me.
Last modified 8 July 2019.
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