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:
ascending node Ω:
longitude of perihelion ω:
mean anomaly M:
time of perihelion Tp:
absolute magnitude H:
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.
|2013 Jan 01||37.03||29.5238||30.0751|
|2014 Jan 01||36.11||23.7139||24.2690|
|2015 Jan 01||34.94||17.8341||18.3953|
|2016 Jan 01||33.27||11.8274||12.4012|
|2017 Jan 17||30.04||5.1971||5.7660||crossed Jupiterís orbit inbound|
|2017 Aug 10||24.51||0.9877||1.3230||crossed Earthís orbit inbound|
|2017 Sep 09||21.05||0.2553||1.1924||closest approach to Sun|
|2017 Oct 01||23.81||0.7513||0.4991|
|2017 Oct 11||21.48||1.0140||0.2073||crossed Earthís orbit outbound|
|2017 Oct 14||20.02||1.1086||0.1616||closest approach to Earth|
|2017 Oct 17||19.63||1.1834||0.1928||brightest as seen from Earth|
|2017 Oct 19||19.70||1.2142||0.2198||discovery|
|2017 Nov 01||23.18||1.5259||0.6347|
|2017 Dec 01||26.17||2.1981||1.7395|
|2018 Jan 01||27.66||2.8484||2.9030|
|2018 Feb 01||28.55||3.4699||3.9914|
|2018 Mar 01||29.05||4.0140||4.8383|
|2018 Apr 01||29.42||4.6019||5.5691|
|2018 May 04||30.02||5.2150||6.0805||crosses Jupiterís orbit outbound|
|2018 Jul 01||30.71||6.2676||6.4152|
|2018 Oct 01||31.07||7.8920||6.9546|
|2019 Jan 01||32.32||9.4782||9.5054|
|2020 Jan 01||34.35||15.5664||15.5875|
|2021 Jan 01||35.69||21.5009||21.5334|
|2022 Jan 01||36.69||27.3320||27.3600|
|2025 Jan 01||38.75||44.5763||44.6077|
|2030 Jan 01||40.84||72.9270||72.9538|
|2035 Jan 01||42.23||101.0897||101.1119|
|2040 Jan 01||43.28||129.1365||129.1542|
|2045 Jan 01||44.12||157.1565||157.1858|
|2050 Jan 01||44.83||185.1110||185.1357|
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.
to Jupiter (AU)
|1I/ 'Oumuamua||0.2553||1.199236±0.000164||1.196488±0.000164||1.200366±0.000167||4.813||never bound||Dybczynski and Krolikowska (2017)|
|C/1980 E1 Bowell||3.3639||1.057256±0.000011||0.999831±0.000012||1.053734±0.000009||0.228||ejected at 3.76 km/s||SSDP (2013)|
|C/1997 P2 Spacewatch||4.2625||1.028253±0.000125||1.000059±0.000058||1.009822±0.000132||0.652||ejected at 1.43 km/s||SSDP (2013)|
|C/2017 U4 PANSTARRS||7.7122||1.009522±0.047301||5.635||JPL (2017)|
|C/2009 UG89 Lemmon||3.9312||1.008057±0.000002||0.999355±0.000002||1.005629±0.000002||0.499||ejected at 1.13 km/s||SSDP (2013)|
|C/2002 A3 LINEAR||5.1514||1.007913±0.000009||0.999893±0.000009||0.968222±0.000010||0.502||bound with 2,100 yr period||SSDP (2013)|
|C/1999 U4 Catalina-Skiff||4.9141||1.007662±0.000003||0.999845±0.000003||1.001440±0.000003||2.161||ejected at 0.51 km/s||SSDP (2013)|
|C/1999 S2 McNaught-Watson||6.4662||1.007255±0.000025||0.999635±0.000025||1.002076±0.000024||3.302||ejected at 0.53 km/s||SSDP (2013)|
|C/2014 W3 PANSTARRS||6.0637||1.006939±0.000140||1.797||JPL (2017)|
|C/2003 A2 Gleason||11.4264||1.006932±0.000029||0.999527±0.000029||0.998467±0.000029||6.191||bound with 640,000 yr period||SSDP (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.
'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
|2017 Oct 14||Catalina Sky Survey, AZ, USA||A|
|2017 Oct 17||Catalina Sky Survey, AZ, USA||A|
|2017 Oct 18-19||Pan-STARRS 1, Haleakala, HI, USA (discovery)||A|
|2017 Oct 19||San Marcello Pistoiese, Tuscany, Italy||A|
|2017 Oct 19||ESA Optical Ground Station, Tenerife, Spain||A|
|2017 Oct 19||Klet Observatory, South Bohemia, Czech Republic||A|
|2017 Oct 20||Steward Observatory, Mt. Lemmon Station, AZ, USA||A|
|2017 Oct 21||LPL/Spacewatch II, Kitt Peak, AZ, USA||A|
|2017 Oct 21||Tenagra II Observatory, AZ, USA||A|
|2017 Oct 22||iTelescope Observatory, Mayhill, NM, USA||A|
|2017 Oct 22||iTelescope Observatory, Siding Spring, NSW, Australia||A|
|2017 Oct 22||Canada-France-Hawaii Telescope, Mauna Kea, HI, USA||P|
|2017 Oct 23||Farpoint Observatory, KS, USA||A|
|2017 Oct 24-25||Mt. Lemmon Survey, AZ, USA||A|
|2017 Oct 25||San Marcello Pistoiese, Tuscany, Italy||A|
|2017 Oct 25||Big Water Observatory, UT, USA||A|
|2017 Oct 25||Palomar Mountain Observatory, CA, USA||P|
|2017 Oct 25||William Herschel Telescope, La Palma, Spain||P|
|2017 Oct 25-26||La Palma Observatory, La Palma, Spain||A|
|2017 Oct 25-26||Nordic Optical Telescope, La Palma, Spain||P|
|2017 Oct 25-27||Very Large Telescope, European Southern Observatory, Paranal, Chile||P|
|2017 Oct 25-27||Tenagra II Observatory, AZ, USA||A|
|2017 Oct 26||Catalina Sky Survey, AZ, USA||A|
|2017 Oct 26||LPL/Spacewatch II, Kitt Peak, AZ, USA||A|
|2017 Oct 26||Schiaparelli Observatory, Varese, Italy||A|
|2017 Oct 26||Hale Telescope, Palomar Mountain Observatory, CA, USA||A/P|
|2017 Oct 26-27||Sutherland-Las Cumbres Observatory, South Africa||A|
|2017 Oct 26-27||Gemini South Telescope, Cerro Pachon, Chile||P|
|2017 Oct 27||Cordell-Lorenz Observatory, Sewanee, TN, USA||A|
|2017 Oct 27||Canada-France-Hawaii Telescope, Mauna Kea, HI, USA||P|
|2017 Oct 27||United Kingdom Infrared Telescope, Mauna Kea, HI, USA||P|
|2017 Oct 27||Keck 2 Telescope, Mauna Kea, HI, USA||P|
|2017 Oct 27-28||Great Shefford Observatory, West Berkshire, England, UK||A|
|2017 Oct 28-30||Magdalena Ridge Observatory, Socorro, NM, USA||A|
|2017 Oct 29||Apache Point Observatory, Sunspot, NM, USA||A/P|
|2017 Oct 29||Gemini North Telescope, Mauna Kea, HI, USA||P|
|2017 Oct 29||William Herschel Telescope, La Palma, Spain||P|
|2017 Oct 29-30||Nordic Optical Telescope, La Palma, Spain||P|
|2017 Oct 30||Discovery Channel Telescope, Lowell Observatory, AZ, USA||P|
|2017 Nov 9-10||Magdalena Ridge Observatory, Socorro, NM, USA||A|
|2017 Nov 12||Magdalena Ridge Observatory, Socorro, NM, USA||A|
|2017 Nov 17||Magdalena Ridge Observatory, Socorro, NM, USA||A|
|2017 Nov 23-~26||Allen Telescope Array, Hat Creek, CA, USA||R|
|2017 Nov ~23||Spitzer Space Telescope||T|
|2017 Nov ~23||Hubble Space Telescope||?|
|2017 Dec 13-14||Green Bank Radio Telescope, WV, USA||R|
|2017 Dec 16||Allen Telescope Array, Hat Creek, CA, USA||R|
|2017 Dec 18-20||Green Bank Radio Telescope, WV, USA||R|
|2017 Dec 22-23||Green Bank Radio Telescope, WV, USA||R|
|2017 Dec 30||Green Bank Radio Telescope, WV, USA||R|
|2018 Jan||Hubble Space Telescope (requested)||A|
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), but no results have yet been reported (which is unsurprising as these data would take longer to analyze). Additional time on the Hubble Space Telescope was requested for January 2018 for orbit tracking of '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-V||0.70 ± 0.06||Jewitt et al. (2017)|
|V-R||0.45 ± 0.05||Jewitt et al. (2017)|
|g-r||0.47 ± 0.04||Bannister et al. (2017)|
|g-r||0.63 ± 0.31||Bannister et al. (2017)|
|g-r||0.41 ± 0.24||Bolin et al. (2017)|
|g-r||0.84 ± 0.05||Meech et al. (2017)|
|g-r||0.60 ± 0.23||Ye et al. (2017)|
|g-Y||1.60 ± 0.20||Meech et al. (2017)|
|r-i||0.36 ± 0.16||Bannister et al. (2017)|
|r-i||0.23 ± 0.25||Bolin et al. (2017)|
|r-i||0.29||Meech et al. (2017)|
|r-J||1.20 ± 0.11||Bannister et al. (2017)|
|r-z||0.41||Meech 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).
|8.1||1.8||5.3||Bannister et al. (2017)|
|8.14 ± 0.02||2.11 ± 0.53||6.91 ± 3.41||Bolin et al. (2017)|
|8.14 ± 0.02||1.54 ± 0.1||4.13 ± 0.48||Bolin et al. (2017)|
|7.5483 ± 0.0073||>2.5||10||Drahus et al. (2017)|
|6.831||[≥1.75]||≥5||Fraser et al. (2017)|
|8.256||2.0 ± 0.2||[6.3±1.2]||Jewitt et al. (2017)|
|>3-5||≥1.2||≥3||Knight et al. (2017)|
|7.34||[2.5±0.1]||10 ± 1||Meech 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.
|dimensions (m) for|
axial ratio 1:5
|dimensions (m) for|
axial ratio 1:10
|0.01||510||1140 x 230||1600 x 160|
|0.04||260||570 x 110||810 x 81|
|0.10||160||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).
|diameter (km)||dynamical type|
|(80636) 2000 AV214||100?||3.0||15.8||~1.4||main belt|
|(44530) Horakova||160?||2.68||11.8||6.9||main belt|
|(367248) 2007 MK13||5.286||2.2||7.6||0.39||Apollo|
|(163899) 2003 SD220||285||2.2||7.6||0.80||Aten|
|(488515) 2001 FE90||0.478||2.13||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||0.611||2.05||6.6||0.45||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.
|diameter (m)||albedo||dynamical type|
|(367248) 2007 MK13||5.286||2.2||7.6||390||0.12||Apollo, PHA|
|(488515) 2001 FE90||0.478||2.13||7.1||~250||?||Apollo, PHA|
|(436724) 2011 UW158||0.611||2.05||6.6||450||0.10||Apollo, PHA|
|(416186) 2002 TD60||2.851||2.0||6.3||230||0.50||Amor|
|(469896) 2005 WC1||2.582||2.0||6.3||400||0.06||Apollo, PHA|
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 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).
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).
--articles and preprints on 'Oumuamua (arXiv posting date in parentheses)
--other items on 'Oumuamua
--Minor Planet Center releases on 'Oumuamua
--background articles and data sets
Comments? Questions? Corrections? Contact me.
Last modified 19 January 2018.
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