Telescopes could not get an image of the interstellar asteroid Oumuamua. Recent research shows that the asteroid had an acceleration. It deviated from a Keplerian orbit. It increased speed and changed direction. The deviation was with super-high statistical significance (30 standard deviations). Harvard researchers calculate that one explanation is that the asteroid was an alien solar sail.
Harvard researchers Bialy and Loeb offer a counter-explanation to the comet theory. If ‘Oumuamua were in fact a comet, why then did it not experience outgassing when it was closest to our Sun? In addition, they cite other research that showed that if outgassing were responsible for the acceleration, it would have also caused a rapid evolution in ‘Oumuamua’s spin (which was not observed).
They calculate that the movement is explained if was a light sail with sub-millimeter thickness.
If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin.
Oumuamua entered the Solar System from north of the plane of the ecliptic. It had a speed of 26.35 km/s initially. It reached its maximum speed of 87.71 km/s (315,800 km/h) as it passed south of the ecliptic on 6 September and made a sharp turn upward at its closest approach to the Sun (perihelion) on 9 September at a distance of 0.255 AU (38,100,000 km; 23,700,000 mi) from the Sun. It was 17% closer than Mercury’s closest approach to the Sun. It headed away from the Sun towards Pegasus at an angle of 66° from the direction of its approach.
Accounting for Vega’s proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega. But as a nearby star, Vega was not in the same part of the sky at that time.
Considering an artificial origin, one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment. Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative.
This would account for the various anomalies of ‘Oumuamua, such as the unusual geometry inferred from its lightcurve, its low thermal emission, suggesting high reflectivity, and its deviation from a Keplerian orbit without any sign of a cometary tail or spin-up torques. Although ‘Oumuamua has a red surface color, similar to organic-rich surfaces of Solar-System comets and D-type asteroids, this does not contradict the artificial scenario, since irrespective of the object’s composition, as it travels through the ISM its surface will be covered by a layer of interstellar dust, which is itself composed of organic-rich materials.
Alternatively, a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization. Based on the PAN-STARRS survey characteristics, and assuming natural origins following random trajectories, Do et al. (2018) derived that the interstellar number density of ‘Oumuamua-like objects should be extremely high, ∼ 2×10^15 per cubic parsec, equivalent to ∼ 10 15 ejected planetisimals per star, and a factor of 100 to 100 million larger than predicted by theoretical models. This discrepancy is readily solved if ‘Oumuamua does not follow a random trajectory but is rather a targeted probe. Interestingly, ‘Oumuamua’s entry velocity is found to be extremely close to the velocity of the Local Standard of Rest, in a kinematic region that is occupied by less than 1 to 500 stars.
The observations of the interstellar asteroid were not sufficiently sensitive to provide a resolved image of ‘Oumuamua, and one can only speculate on its possible geometry and nature. Although periodic variations in the apparent magnitude are observed, there are still too many degrees of freedom (e.g., observing angle, non-uniform reflectively, etc.) to definitely constrain the geometry. The geometry should not necessarily be that of a planar sheet, but may acquire other shapes, e.g., involving a curved sheet, a hollow cone or ellipsoidal, etc. Depending on the geometry our estimated value for the mass-to-area ratio will change, but the correction is typically of order unity.
30 standard deviations away from normal orbit – certainty of anomalous acceleration
‘Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, Micheli et al. (2018) reported that ‘Oumuamua showed deviations from a Keplerian orbit at a high statistical significance.
Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state. Micheli reported the detection, at 30σ significance, of non-gravitational acceleration in the motion of ‘Oumuamua.
A 20 sigma-event corresponds to an expected occurrence period measured in years that is 10 times larger than the higher of the estimates of the number of particles in the Universe. A 25-sigma event corresponds to an expected occurrence period that is equal to the higher of these estimates but with the decimal point moved 52 places to the left.
Solar sail as the reason
The observed trajectory is best explained by an excess radial acceleration ∆a ∝ r^−2, where r is the distance of ‘Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that ‘Oumuamua is not an active comet. they explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is (m/A) ≈ 0.1 g cm−2. For a thin sheet this requires a width of ≈ 0.3−0.9 mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of ∼ 5 kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. We discuss the possible origins of such an object including the possibility that it might be a lightsail of artificial origin. Our general results apply to any light probes designed for interstellar travel