Deemo Chen, grade 11 student. Barstow School – Ningbo Campus.


1I/2017 U1 (‘Oumuamua), a recently discovered asteroid in a hyperbolic orbit, is likely the first macroscopic object of extrasolar origin identified in the solar system. This paper mainly gives reasons why ‘Oumuamua should be a comet, by explaining three unusual behaviors of ‘Oumuamua which are unnormal ratio, strange track eccentricity, and Peculiar Acceleration; disproving four main hypothesizes to say ʻOumuamua is not a comet, then provide an evidence of reflectivity of surface of ʻOumuamua to prove ʻOumuamua in a regular interval of comet group. Thus, ʻOumuamua is most likely to be a comet.

Keywords: ʻOumuamua (1I/2017 U1); extraterrestrial intelligenceISM: individual objects (1I&2017 U1)minor planets, asteroids: generalminor planets, asteroids: individual (1I&2017 U1).


ʻOumuamua (1I/2017 U1) is an object from outside of the solar system by first observed by Panoramic Survey Telescope and Rapid Response System(Pan-STARRS) located at Haleakala Observatory, Hawaii, USA (“IAU MPC”), from the IAU website, we can find that ʻOumuamua is also the first object human ever observed from outside the solar system. We are trying to figure out where exactly it came from. The thing is, there are several things shows it is not an ordinary object and can possibility to be a spacecraft may be made by aliens. However, I do not agree with this statement; I tend to describe it is an unusual comet by against three main theories exist which say ʻOumuamua is not a comet.

The ratio and shape of ʻOumuamua

Since we cannot use an optical telescope to truly see how ʻOumuamua looks like, the only way we can use it make the scientific hypothesis of the shape of it.   For a rigid body, it will rotate around the maximum inertia axis(MIA), which is the axis with the most considerable moment of inertia. Dynamically, it can be proved that the rotation of the rigid body around other inertia spindles is unstable if we consider the slight deformation damping of the object.

Figure 1 Explorer_1

Figure 1 Explorer_1

Just like the graph of Explorer_1, which is the first artificial satellite launched by America, it has a long and thin shape. The blue line points the long spindle of Explorer_1, and that is the minimum inertia spindle of this structure, the red line shows the maximum inertia spindle. People at that time hope the satellite could rotate around the long axis, but the result of that be the Explorer_1 cannot stabilize its posture at all, even they did successfully launch the satellite, but Explorer_1 cannot do many planned tasks.

In this way, we can assume that ʻOumuamua is rotating around its MIS. And the diagram below helps us to figure what could be the possible shape of ʻOumuamua.

Light curve of ʻOumuamua.** All of the magnitudes have been scaled using the measured colors; the error bars indicate 1_σ photometric errors. (Meet, et al. 2)

Light curve of ʻOumuamua.** All of the magnitudes have been scaled using the measured colors; the error bars indicate 1_σ photometric errors. (Meet, et al. 2)

Light curve of ʻOumuamua. All of the magnitudes have been scaled using the measured colors; the error bars indicate 1_σ_ photometric errors. (Meet, et al. 2)

According to the Light curve of ʻOumuamua, the red dotted line corresponds 10:1:1 triaxial ellipsoid with a 20% variation in albedo across the surface (Meech et al. 2), and the dotted curve made by observation data from different telescopes of ʻOumuamua shows a nearly perfect fitness to the light curve model of a 10:1:1 triaxial ellipsoid. By analyzing the interval between two relative curve minima points, we could approximate the rotating period as 7.34 h. Also, by the hydrostatic balance theory, the size of ʻOumuamua cannot construct itself to be a sphere.

For a rigid body, it is tough to rotate in this way, it will quickly be rapture, and it from outer space and we even cannot figure out it from which galaxy (Ye, et al. 3), so it must have traveled in space for a long time, and that means ʻOumuamua should keep rotating like this for a long time, the possibility of that should be very small, because if P(x)= . P(d) should be minimal (d refers to the distance ʻOumuamua travel, x= ʻOumuamua keep this long and thin shape), and because this shape is too unusual for human, so many people believe ʻOumuamua is an artificial object or spacecraft.

However, the evidence of this hypothesis has some vulnerabilities.

First, the model of the shape of ʻOumuamua have some problem, since we do not have the graphic evidence of the shape, if only according to the light curve, there are much more shapes could fit this model, or there is a light absorbing material covered on, and the following statement about “rigid body” cannot works because other shapes could prevent them from rapture.

Second, we do need to assume that ʻOumuamua is a rigid body, because for a rigid body, they will rupture because of their inside force, so if ʻOumuamua is not a rigid body but made by relatively elasticity materials, in this way, ʻOumuamua could keep a long and thin shape but not broke.

Third, I think even ʻOumuamua truly looks long and thin, that is not a strange thing to human, because if one thing could exist in this world by way we know about physics, then it is properly, “Practice is the sole criterion for testing knowledge”, we should believe in data, whatever how impossible it looks like.

In this way, I think the shape of ʻOumuamua has nothing to doubt about.

The Orbit of ʻOumuamua

Figure 2 Meech, et al. 3

Figure 2 Meech, et al. 3

The path of ‘Oumuamua (1I/2017 U1) through the Solar System.

“The orbit of a typical Halley-type comet is shown for comparisonas the solid grey line. The inset shows the inner Solar System; the solid linesegment along the trajectory taken by ‘Oumuamua (dashed)indicates the short window of two weeks during which it was bright” (Meech, et al. 1), As we can see that its track eccentricity is only 1.1956 ± 0.0006, it looks got into the solar system vertically to the zodiac and soon go back the same way, it just like aim to the sun. So, it could easily be seems as a spacecraft want to use the gravity of the sun to redirect its orbit because it is the best way we can think to save the energy for a shuttle.

Orbit offset

Marco Micheli’s team analyzed the full observational dataset, which includes 177 ground-based and 30 HST based astrometric positions (for a total of 414 scalar measurements), applying the procedures and assumptions discussed in Methods. Their analysis shows that the observed orbital arc cannot be fitted in its entirety by a trajectory governed solely by gravitational forces due to the Sun, the eight planets, the Moon, Pluto, the 16 most massive bodies in the asteroid main belt and relativistic effects14. As shown in diagram D1-a, the residuals in right ascension and declination of the best-fitting gravity-only trajectory are incompatible with the formal uncertainties: ten data points deviate by more than 5σ in at least one coordinate, and 25 are discrepant by more than 3σ. Furthermore, the offsets (as large as 22″ for the 2017 October 14 Catalina observation) are not distributed randomly but show clear trends along the trajectory. (Micheli 1)

Diagram 1(D2):

The most reasonable reason for the offset of the orbit is the force of comet dust which will give a power to the object when it spread out of the object, however, by the data from Micheli’s team (Micheli 3), they built a model of comet could have this kind of behavior, they found even 2kg of the dust from the object is enough for telescope  to observe, the thing is, from the actual diagram, we cannot see any dust from ʻOumuamua.

Figure 3Micheli 3

Figure 3Micheli 3

Is that means ʻOumuamua is not a comet but could change its orbit by itself and become a spacecraft? It could be a reasonable guess, but I think there is a better explanation that the object has released only a small quantity of dust, probably in the form of larger grains which are hard to see. 

Also, by the theory of Fitzsimmons’s team, we have another way to explain that there is a cover on ʻOumuamua’s surface which prevents the ice inside of ʻOumuamua spread out from itself, and the cover is so thick that even the sun cannot put effects on ʻOumuamua in that short time, that they reported the spectroscopic characterization of ‘Oumuamua,  and finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. The observable ISO population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity implies a lack of surface ice. (Fitzsimmons, Alan, et al 4). This is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. An internal icy composition cannot, therefore, be ruled out by the lack of activity, even though ‘Oumuamua passed within 0.25 AU of the Sun.

So ʻOumuamua is more likely to be a comet in this way.

The theory of it being a LightSail

Because of the usual orbit of ‘Oumuamua, it could consider an artificial origin; one possibility is that Oumuamua is a LightSail, floating in interstellar space as debris

from advanced technological equipment (Bialy 1), As a solar sail, its propulsion is dependent on solar radiation alone. Solar photons exert radiation pressure on the sail, producing a small degree of acceleration. Thus, the solar sail will be propelled by force from sunlight itself, and not by the charged particles of the solar wind.

Even Mr. Bialy mentioned that ‘Oumuamua could be alien craft since its orbit is so unusual: “Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.” (Bialy 4)

It is just an educated guess of the orbit of ‘Oumuamua, but I think this hypothesis did not consider more, because we already prove that ‘Oumuamua is rotating, but if LightSail works, it must let one side of itself toward the sun, so the theory of a LightSail could not work.

The reflectivity of the surface

Since all hypothesis of saying is not a comet be proved as wrong, here I can give relevant evidence that ‘Oumuamua is a comet.

Figure 4 Meech 3

Figure 4 Meech 3

The reflectivity of the surface of ‘Oumuamua.

The surface reflectivity of ‘Oumuamua is consistent with D-type asteroids and comets. Data are normalized to 1 at 0.65 μ m, and the error bars reflect the 1_σ_ standard deviation. (Meech 3)

From the data given by Meech’s team,‘Oumuamua’s reflectivity is pretty close to D-type asteroids and comets, compare to Comet 67P, Comet 9P, and Comet 103P, it does not have much differentness.

One possible way to check what ‘Oumuamua truly is

The challenge of reaching the object within a reasonable timeframe is formidable due to its high heliocentric hyperbolic excess velocity of about 26 km/s (Hein 1); Much faster than any vehicle yet launched, Launching a spacecraft to ’Oumuamua in a reasonable timeframe of 5-10 years requires a hyperbolic solar system excess velocity between 33 to 76 km/s for mission durations between 30 to 5 years. (Hein 4) It is demonstrated that based on existing technologies now human have such as from the Parker Solar Probe, launchers such as the Falcon Heavy and Space Launch System could send spacecraft with masses ranging from dozens to hundreds of kilograms to 1I/’Oumuamua, if launched in 2021. A further increase in spacecraft mass can be achieved with an additional Saturn flyby post solar Oberth maneuver. The potential of more advanced technologies such as laser electric propulsion, solar and laser sails would also allow for chasing ’Oumuamua, although their development will likely push launch dates farther into the future and might be more attractive for reaching future ‘Oumuamua-like objects. And we can chase it in 2029.


Even the shape of ’Oumuamua is strange. Its orbit changes but we cannot observe any dust, and the model is suitable to be a LightSail, but all we can find a reasonable explanation. So `Oumuamua is a comet –a tiny one, with perhaps different chemistry and an unusual shape, but colored like Solar System comets, and outgassing like our comets or covered with an organic layer. So, we were indeed lucky to find it, but we did not need to be too fortunate to find an interstellar asteroid.  And we still have the chance to take a clear picture of this comet to see how exactly it looks like if we want to.


Bialy, Shmuel, and Abraham Loeb. “Could Solar Radiation Pressure Explain ‘ʻOumuamua’s Peculiar Acceleration?” The Astrophysical Journal Letters 868.1 (2018): L1. Accessed on 19 December 2018.

Fitzsimmons, Alan, et al. “Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘ʻOumuamua.” Nature Astronomy 2.2 (2018): 133. Accessed on 19 December 2018.

Hein, Andreas M., et al. “Project Lyra: Sending a Spacecraft to 1I/’ʻOumuamua (former A/2017 U1), the Interstellar Asteroid.” arXiv preprint arXiv:1711.03155 (2017). Accessed on 19 December 2018.

Meech, Karen J., et al. “A brief visit from a red and extremely elongated interstellar asteroid.” Nature 552.7685 (2017): 378. Accessed on 19 December 2018.

Micheli, Marco, et al. “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘ʻOumuamua).” Nature 559.7713 (2018): 223.

<>. Accessed on 19 December 2018.

“The International Astronomical Union Minor Planet Center.” IAU Minor Planet Center, Accessed on 8 January 2018

Ye, Quan-Zhi, et al. “1I/2017 U1 (‘ʻOumuamua) is hot: imaging, spectroscopy, and search of meteor activity.” The Astrophysical Journal Letters 851.1 (2017): L5. Accessed on 19 December 2018.