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Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?

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‘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. 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. We 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 thickness 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.
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ACC EP TE D FO R PU BL IC ATIO N IN T HE AS TRO PH YS IC AL JO UR NAL LE TT ER S
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COULD SOLAR RADIATION PRESSURE EXPLAIN ‘OUMUAMUA’S PECULIAR ACCELERATION?
SHMUEL BIA LY?AND ABRAHAM LOE B
Harvard Smithsonian Center for Astrophysics, 60 Garden st., Cambridge, MA, 02138
Accepted for publication in the Astrophysical Journal Letters
ABSTRACT
‘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. The observed trajectory is best explained by an excess radial acceleration ar2, where ris
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. We explore the possibility that the excess acceleration results from Solar radiation pressure. The
required mass-to-area ratio is (m/A)0.1 g cm2. For a thin sheet this requires a thickness of 0.30.9 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.
Subject headings: ISM: individual objects (1I/2017 U1) – minor planets, asteroids: individual (1I/2017 U1) –
General: extraterrestrial intelligence – Minor planets, asteroids: general
1. INTRODUCTION
On October 19, 2017, the first interstellar object in the So-
lar System, ‘Oumuamua (1I/2017 U1) was discovered by the
PAN-STARRS1 survey. It has a highly hyperbolic trajectory
(with eccentricity e=1.1956 ±0.0006) and pre-entry veloc-
ity of v26 km s1(Meech et al. 2017). Based on the
survey properties and the single detection, Do et al. (2018) es-
timated the interstellar density of objects like ‘Oumuamua or
larger to be n2×1015 pc3, 2-8 orders of magnitude larger
than expected by previous theoretical models (Moro-Martin
et al. 2009). The large variations in its apparent magnitude
and the non-trivial periodicity of the lightcurve, suggest that
‘Oumuamua is rotating in an excited spin state (tumbling mo-
tion), and has an extreme aspect ratio of at least 5 : 1 (Fraser
et al. 2018;Drahus et al. 2018), an unprecedented value for
previously known asteroids and comets in the Solar System.
Belton et al. (2018) have shown that if ‘Oumuamua rotates
in its highest rotational energy state, it should be extremely
oblate (pancake-like).
Recently, Micheli et al. (2018) reported the detection of
non-gravitational acceleration in the motion of ‘Oumuamua,
at a statistical significance of 30σ. Their best-fit to the data
is obtained for a model with a non-constant excess accelera-
tion which scales with distance from the Sun, r, as ar2,
but other power-law index values are also possible. They con-
cluded that the observed acceleration is most likely the result
of a cometary activity. Yet, despite its close Solar approach of
r=0.25 AU, ‘Oumuamua shows no signs of a any cometary
activity, no cometary tail, nor gas emission/absorption lines
were observed (Meech et al. 2017;Knight et al. 2017;Jewitt
et al. 2017;Ye et al. 2017;Fitzsimmons et al. 2017). From
a theoretical point of view, Rafikov (2018) has shown that
if outgassing was responsible for the acceleration (as origi-
nally proposed by Micheli et al. 2018), then the associated
outgassing torques would have driven a rapid evolution in
‘Oumuamua’s spin, incompatible with observations.
?SBialy@cfa.harvard.edu
If not cometary activity, what can drive the non-
gravitational acceleration observed? In this Letter we explore
the possibility of ‘Oumuamua being a thin object accelerated
by Solar radiation pressure, which would naturally result in
an excess acceleration ar2.1However, for radiation
pressure to be effective, the mass-to-area ratio must be very
small. In §2we derive the required mass-to-area ratio and find
(m/A)0.1 g cm2, corresponding to an effective thin sheet
of thickness w0.30.9 mm. We explore the ability of such
an unusually thin object to survive interstellar travel, consid-
ering collisions with interstellar dust and gas (§3), as well as
to withstand the tensile stresses caused by rotation and tidal
forces (§4). Finally, in §5we discuss the possible implications
of the unusual requirements on the shape of ‘Oumuamua.
2. ACCELERATION BY RADIATION PRESSURE
Micheli et al. (2018) had shown that Oumuamua’s experi-
ences an excess radial acceleration, with their best fit model
a=a0r
AUn
(1)
with n=2,
a0= (4.92 ±0.16)×104cm s2.(2)
The value for a0is averaged over timescales much longer than
‘Oumuamua’s rotation period.
An acceleration of this form is naturally produced by radi-
ation pressure,
P=CR
L
4πr2c,(3)
where Lis the Solar luminosity, cis the speed of light, and
CRis a coefficient of order unity which depends on the ob-
ject’s composition and geometry. For a sheet perpendicular
to the Sun-object vector CR=1+εwhere εis the reflectivity.
For a perfect reflector ε=1, and CR=2, whereas for a perfect
absorber ε=0 and CR=1.
1Interestingly, a similar approach was adopted historically regarding the
anomalous orbit of Phobos (Shklovskii 1962).
2
FIG . 1.— The maximum allowed travel distance through the interstellar
medium (ISM), as a function of (m/A). The blue and red lines are limita-
tions obtained by slow-down due to gas accumulation, and vaporization by
dust-collisions, respectively. The plotted results are for a mean ISM proton
density of hni ∼ 1 cm3. All lines scale as 1/hni. The dashed magenta line
is our constraint on ‘Oumuamua based on its excess acceleration. The Solar
Galactrocentric distance is also indicated.
For an object of mass mand area Athe acceleration would
be
a=PA
m=L
4πr2cA
mCR(4)
=4.6×105r
AU2m/A
g cm21
CRcm s2.
Comparing Eq. (1) and (4) we find that the requirement on the
mass-to-area ratio is
m
A= (9.3±0.3)×102CRg cm2.(5)
For a planer body with mass density ρ, this translates into a
requirement on the body’s thickness
w=m
Aρ= (9.3±0.3)×102ρ1
0CRcm ,(6)
where ρ0=ρ/(100g cm3). Typically, ρ013, giving
a thin sheet of 0.3–0.9 mm thickness. Other geometries are
also possible, and are discussed in §5. The force exerted by
the Solar wind on a solid surface is negligible compared to
that of the Solar radiation field and is neglected hereafter.
The observed magnitude of ‘Oumuamua constrains its area
to be A8×106α1cm2, where αis the albedo (Jewitt
et al. 2017). This corresponds to an effective radius, Reff
A/π=16α1/2meters. For our estimation of the mass-to-
area ratio, this area translates into a mass of m740(CR/α)
kg.
3. MAXIMUM DISTANCE FOR INTERSTELLAR TRAVEL
Next we explore the implications of impacts with interstel-
lar dust-grains and gas particles, in terms of momentum and
energy transfer. We obtain general requirements for the ob-
ject’s mass-to-area ratio, or alternatively, for the maximum
interstellar distance that can be traveled before it encounters
appreciable slow-down or evaporation.
3.1. Momentum Transfer - Slow Down
An object with a cross sectional area Atraveling a distance
Lthrough the ISM would accumulate an ISM gas mass of,
Mgas =AΣgas =1.4mphniLA ,(7)
where Σgas is the accumulated mass column density of inter-
stellar gas, mpis the proton mass, hniis the mean proton
number density averaged along the object’s trajectory, and
the factor 1.4 accounts for the contribution of helium to the
mass density of the ISM. For trajectories that span Galactic
distances the contribution of the Solar System to the accumu-
lated column is negligible. The contribution of accumulated
dust to the momentum transfer is also negligible, because the
typical dust-to-gas mass ratio in the Galaxy is 1/100.
Once MISM approaches the object’s mass, m, the momen-
tum of the traveling object will decrease by a significant
amount. The requirement MISM /m1 translates into a max-
imum allowed value on the object’s mass-to-area ratio, giving
m
Amin,p=Σgas =1.4mphniL(8)
=7.2×103hni0L0g cm2.
In the last equality we normalized to the typical values hni0=
hni/(100cm3)and L0=L/(100kpc).
Given a mass-to-area ratio, the maximum travel distance is
Lmax,p=1
1.4mphnim
A(9)
=14hni1
0m
A1kpc .
In the second equation we denoted (m/A)1
(m/A)/(101g cm2). Figure 1shows the results from
Eq. (9) as a function of (m/A). The dashed vertical line
indicates our constraint on (m/A) for ‘Oumuamua (Eq. 5).
Evidently, ‘Oumuamua can travel Galactic distances before
encountering appreciable slow-down.
3.2. Energy Transfer - Collisions with Dust-Grains
Collisions with dust grains at high velocities will induce
crater formation by melting and evaporation of the target ma-
terial. Since the typical time between dust collisions is long
compared to the solidification time, any molten material will
solidify before the next collision occurs, and thus will only
cause a deformation of the object’s surface material, not re-
duction in mass. On the other hand, atoms vaporized through
collisions can escape and thus cause a mass ablation.
We would like to estimate the minimum mass-to-area ratio
required for the object to not lose significant fraction of its
mass upon dust-grain collisions. Let mdbe the colliding dust-
grain mass, and φthe fraction of the kinetic energy that is
converted into vaporization of the object’s body. The total
number of vaporized atoms per collision is then,
Nv=mv
¯m=φmdv2
2Uv
,(10)
where mvis the mass of vaporized material in the object, ¯mis
the mean atomic mass of the object, and Uvis the vaporiza-
tion energy. Although highly simplistic, this analysis captures
the results of the detailed theoretical model of Tielens et al.
(1994) and the empirical data from Okeefe & Ahrens (1977).
A good match to the numerical results is obtained for φ=0.2.
3
Over a distance Lthere will be many collisions. Adopting
the conservative assumption that for each collision all the va-
porized material escapes to the ISM, we can account for all the
collisions along a path-length, L, by replacing mdin Eq. (10)
with the total accumulated dust mass,
Md=ΣdustA=Σgasϕd gA=1.4mphniLϕdg A,(11)
where Σdust is the dust column density and φdg is the dust-to-
gas mass ratio. This gives,
mv
¯m=φ1.4mphniLϕdg Av2
2Uv
.(12)
Requiring that the total vaporized mass not exceed half of the
object’s mass, we obtain a constraint on the minimum mass-
to-area ratio of the object,
m
Amin,m=1.4mphniLφϕdg ¯mv2
Uv(13)
=3.1×104hni0L0ϕ2¯m12v2
26
U4g cm2.
Here we defined the normalized parameters, ϕ2=ϕdg /102,
¯m12 =¯m/(12mp), as appropriate for carbon based materials
(e.g., graphite or diamond); U4=U/(4 eV), as appropriate for
typical vaporization energies (e.g., for graphite, Uv=4.2 eV);
and v26 =v/(26 km s1), the velocity at infinity of ‘Oumua-
mua. For a given mass-to-area ratio, the maximum allowed
distance before significant evaporation is
Lmax,d=1
1.4mphniUm
φϕdg ¯mv2m
A(14)
=330hni1
0U4
ϕ2¯m12v2
26 m
A1kpc .
For our constrained value for the mass-to-area ratio, ‘Oumua-
mua can travel through the entire galaxy before a significant
fraction of its mass is evaporated. Evaporation becomes im-
portant at higher speeds. Comparing Eqs. (8) and (13) we find
that only for speeds above
vcrit =s2Um
¯mϕφ (15)
=130U4
ϕ2¯m12 1/2
km s1,
vaporization dominates over slow-down.
3.3. Energy Transfer - Collisions with Gas Particles
When an object travels at a high speed, collisions with
atoms in the ISM can potentially transfer sufficient energy to
produce sputtering. This process was studied in the context of
dust grains in hot shocks (Tielens et al. 1994). For an object
traveling at a velocity v, over a distance Lthrough the ISM,
the total number of sputtered particles is,
Ns=ms
¯m=2ALhniYtot ,(16)
where Ytot =iYixiis the total sputtering yield (which de-
pends on the kinetic energy), summed over collisions with
different species (i.e., H, He, and metals), and xiis the abun-
dance of the colliding species relative to hydrogen.
TABLE 1
TEN SI LE ST RE NG TH S
material tensile strength (dyne cm2)
67P/Churyumov-Gerasimenko110-50
Meteorites2(15)×107
Iron 3 ×107
Diamond 2 ×1010
Silicon (monocrystalline) 7 ×1010
1Attree et al. (2018)
2Petrovic (2001)
The minimal mass-to-area ratio below which half of the ob-
ject’s mass will be sputtered is,
m
Amin,s=¯mhniLYtot (17)
=6.2×105¯m12hni0L0Y3g cm2.
In the second equality we normalized to Ytot =103, corre-
sponding to kinetic energies E30 100 eV, corresponding
to v40 70 km s1. For lower speeds, as that of ‘Oumua-
mua, the yield is even lower, further decreasing the value of
(m/A)min,s. At higher speeds, the yield increases but typically
remains below 0.01 (Tielens et al. 1994), thus at any velocity,
vaporization and slow-down remain the dominating processes
limiting the allowed distance an object can travel through the
ISM.
Cosmic-rays are expected to cause even less damage. Al-
though their energy density is comparable to that of the ISM
gas, they deposit only a very small fraction of their energy as
they penetrate through the thin object.
4. TENSILE STRESSES
A thin object can be torn apart by centrifugal forces or tidal
forces if its tensile strength is not sufficiently strong. Typical
values for the tensile strengths of various materials are shown
in Table 1. Next, we calculate whether centrifugal or tidal
forces can destroy ‘Oumuamua.
4.1. Rotation
Oumuamua’s lightcurve shows periodic modulations on an
order of 6-8 hours. Ignoring the tumbling motion, let us esti-
mate the tensile stress originating from the centrifugal force.
The largest stress is produced if the object is elongated such
that the longest dimension is perpendicular to the rotation
axis. We denote this dimension as d. Considering the object
as made of two halves, each located with a center of mass at a
distance d/4 from the rotation axis, and ignoring self gravity,
a radial force of magnitude,
F=1
2m2d
4,(18)
will be exerted on each half. The associated tensile stress is,
P
rot =1
4ρd22(19)
=0.25 ρ0d2
42
4dyne cm2,
where d4d/(104cm),4/(104s1). This is much
smaller than typical tensile strengths of normal materials, and
even of that of the comet 67P/Churyumov-Gerasimenko (see
Table 1). Thus, even when self-gravity is ignored, ‘Oumua-
mua can easily withstand its centrifugal force.
4
4.2. Tidal Forces
The tidal force will be maximal if the long dimension of the
object is parallel to the Sun-object vector. Again, modeling
the object as consisting of two halves as in §4.1, the difference
in the gravitational force experienced by the far and near ends
of the object is,
δF1
4dGMm
r3,(20)
where ris the distance of the center of mass from the Sun.
The associated tensile stress,
P
tid 1
4ρd2GM
r3(21)
=9.9×107ρ0d2
4r
AU3
dyne cm2.
Even at perihelion (r=0.25 AU), the tensile stress is negligi-
ble.
The critical distance below which tidal forces dominate
over centrifugal is
Rtid =GM
21/3
=3.42/3
4M
M1/3
R.(22)
Thus, unless ‘Oumuamua encountered an extremely close ap-
proach to a star in its past, it is unlikely that tidal forces played
any significant role.
5. SUMMARY AND DISCUSSION
We have shown that the observed non-gravitational accel-
eration of ‘Oumuamua, may be explained by Solar radia-
tion pressure. This requires a small mass-to-area ratio for
‘Oumuamua of (m/A)0.1 g cm3. For a planar geometry
and typical mass densities of 1–3 g cm3this gives an effec-
tive thickness of only 0.9–0.3 mm, respectively. For a mate-
rial with lower mass density, the inferred effective thickness
is proportionally larger. We find that although very thin, such
an object can travel over galactic distances, maintaining its
momentum and withstanding collisional destruction by dust-
grains and gas, as well as centrifugal and tidal forces. For
‘Oumuamua, the limiting factor is the slow-down by accu-
mulated ISM mass, which limits its maximal travel distance
to 10 kpc (for a mean ISM particle density of 1 cm3).
Our inferred thin geometry is consistent with studies of its
tumbling motion. In particular, Belton et al. (2018) inferred
that ‘Oumuamua is likely to be an extremely oblate spheroid
(pancake) assuming that it is excited by external torques to its
highest energy state.
While our scenario may naturally explains the peculiar ac-
celeration of ‘Oumuamua, it opens up the question what kind
of object might have such a small mass-to-area ratio? The ob-
servations are not sufficiently sensitive to provide a resolved
image of ‘Oumuamua, and one can only speculate on its pos-
sible geometry and nature. Although periodic variations in the
apparent magnitude are observed, there are still too many de-
grees of freedom (e.g., observing angle, non-uniform reflec-
tively, etc.) to definitely constrain the geometry. The geom-
etry 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 es-
timated value for the mass-to-area ratio will change (through
CRin Eq. 5), but the correction is typically of order unity.
Known Solar System objects, like asteroids and comets
have mass-to-area ratios orders of magnitude larger than our
estimate for ‘Oumuamua. If radiation pressure is the acceler-
ating 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.
Considering an artificial origin, one possibility is that
‘Oumuamua is a lightsail, floating in interstellar space as a de-
bris from an advanced technological equipment (Loeb 2018).
Lightsails with similar dimensions have been designed and
constructed by our own civilization, including the IKAROS
project and the Starshot Initiative2. The lightsail technology
might be abundantly used for transportation of cargoes be-
tween planets (Guillochon & Loeb 2015) or between stars
(Lingam & Loeb 2017). In the former case, dynamical ejec-
tion from a planetary System could result in space debris of
equipment that is not operational any more 3(Loeb 2018),
and is floating at the characteristic speed of stars relative
to each other in the Solar neighborhood. This would ac-
count for the various anomalies of ‘Oumuamua, such as the
unusual geometry inferred from its lightcurve (Meech et al.
2017;Fraser et al. 2018;Drahus et al. 2018;Belton et al.
2018), its low thermal emission, suggesting high reflectivity
(Trilling et al. 2018), and its deviation from a Keplerian or-
bit (Micheli et al. 2018) without any sign of a cometary tail
(Meech et al. 2017;Knight et al. 2017;Jewitt et al. 2017;
Ye et al. 2017;Fitzsimmons et al. 2017) or spin-up torques
(Rafikov 2018). Although ‘Oumuamua has a red surface
color, similar to organic-rich surfaces of Solar-System comets
and D-type asteroids (Meech et al. 2017), this does not con-
tradict 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 com-
posed of organic-rich materials (Draine 2003).
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-STARRS1
survey characteristics, and assuming natural origins following
random trajectories, Do et al. (2018) derived that the inter-
stellar number density of ‘Oumuamua-like objects should be
extremely high, 2×1015 pc3, equivalent to 1015 ejected
planetisimals per star, and a factor of 100 to 108larger than
predicted by theoretical models (Moro-Martin et al. 2009).
This discrepancy is readily solved if ‘Oumuamua does not
follow a random trajectory but is rather a targeted probe. In-
terestingly, ‘Oumuamua’s entry velocity is found to be ex-
tremely close to the velocity of the Local Standard of Rest, in
a kinematic region that is occupied by less than 1 to 500 stars
(Mamajek 2017).
Since it is too late to image ‘Oumuamua with existing tele-
scopes or chase it with chemical propulsion rockets (Selig-
man & Laughlin 2018, but see Hein et al. 2017), its likely
origin and mechanical properties could only be deciphered by
searching for other objects of its type in the future. In addi-
tion to the vast unbound population, thousands of interstellar
‘Oumuamua-like space-debris are expected to be trapped at
any given time in the Solar System through gravitational in-
teraction with Jupiter and the Sun (Lingam & Loeb 2018).
2A list of books and papers on lightsails is provided in
http://breakthroughinitiatives.org/research/3. The IKAROS project is
discussed in http://global.jaxa.jp/projects/sat/ikaros/.
3Note that ‘Oumuamua was found not to show any radio emission down
to a fraction of the power of a cell phone transmission (Enriquez et al. 2018;
Tingay et al. 2018;Harp et al. 2018).
5
Deep wide-area surveys of the type expected with the Large
Synoptic Survey Telescope (LSST)4will be particularly pow-
erful in searching for additional members of ‘Oumuamua’s
population of objects.
A survey for lightsails as technosignatures in the Solar Sys-
tem is warranted, irrespective of whether ‘Oumuamua is one
of them.
We thank Manasvi Lingam, Paul Duffell, Quanzhi Ye, and
an anonymous referee for helpful comments. This work was
supported in part by a grant from the Breakthrough Prize
Foundation.
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4See https://www.lsst.org/
... More specifically, LH can be made precise in two different ways. One possibility (Bialy and Loeb 2018) is that 'Oumuamua is a non-operational light sail, floating in interstellar space, the debris of advanced technological equipment which is now derelict. As mentioned above, projects such as IKAROS and the Starshot Initiative have used light sails technology that exhibits similar shape features, and same dimensions as those of 'Oumuamua. ...
... Along the same lines, the other possibility (Bialy and Loeb 2018) is that 'Oumuamua is an operational, targeted probe intentionally sent to our galaxy by an alien civilization. This exotic hypothesis is supported by the fact that if 'Oumuamua's trajectory were really random, the interstellar number density of 'Oumuamua-like objects should be extremely high, which would contradict the results of our theoretical models (Bialy and Loeb 2018). ...
... Along the same lines, the other possibility (Bialy and Loeb 2018) is that 'Oumuamua is an operational, targeted probe intentionally sent to our galaxy by an alien civilization. This exotic hypothesis is supported by the fact that if 'Oumuamua's trajectory were really random, the interstellar number density of 'Oumuamua-like objects should be extremely high, which would contradict the results of our theoretical models (Bialy and Loeb 2018). In this vein, much work has been done to consider spacecraft designs that use hydrogen fuel and that may have either a thermal propulsion architecture or a solar electric propulsion architecture (Sheerin & Loeb 2020). ...
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Astrophysicist Abraham Loeb suggests that the interstellar interloper 1I/2017 ‘Oumuamua, detected in our solar system in 2017, is alien space debris or even an alien operational probe (Bialy and Loeb 2018; Sheerin & Loeb 2020). Does this conjecture have significant epistemic support, such that it can be justified as a viable hypothesis? In this paper, I propose that the meta-empirical confirmation approach, developed and defended by philosopher and physicist Dawid (2006, 2013, 2018, 2019), provides an appropriate framework to answer this question. I defend this proposal by elucidating how meta-empirical confirmation applies to the ‘Oumuamua case and what specific meta-empirical arguments could support Loeb’s hypothesis. Even though Loeb would not himself endorse meta-empirical confirmation, because it is not traditional empiricism, this case shows that meta-empirical confirmation is not in fact a threat to empiricism.
... The celestial body designated 1I/'Oumuamua is the first object to be discovered and identified as an interstellar object, stimulating much interest, debate and speculation from the scientific community (Bannister et al. 2019). Theories to explain the nature of 1I/'Oumuamua have included a fractal dust aggregate (Flekkøy et al. 2019), a hydrogen iceberg (Seligman & Laughlin 2020), a nitrogen iceberg , an alien solar sail (Bialy & Loeb 2018), fragments of a tidally disrupted planet (Raymond et al. 2018) and so on. All explanations have one feature in common -they are extraordinary. ...
... An alternative strategy would be to send a spacecraft to 1I. Despite initial statements against the feasibility of this option (Bialy & Loeb 2018), a series of publications has argued that such a mission would be feasible with near-term technologies (Hein et al. 2019;Seligman & Laughlin 2018;Hibberd & Hein 2021a,b;Hibberd et al. 2020). ...
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To settle the question of the nature of the interstellar object 1I/’Oumuamua requires in-situ observations via a spacecraft, as the object is already out of range of existing telescopes. Most previous proposals for reaching 1I/’Oumuamua using near-term technologies are based on the Solar Oberth Manoeuvre (SOM), as trajectories without the SOM are generally significantly inferior in terms of lower mission duration and higher total velocity requirement. While the SOM allows huge velocity gains, it is also technically challenging and thereby increases programmatic and mission-related risks. In this paper, we identify an alternative route to the interstellar object 1I/’Oumuamua, based on a launch in 2028, which does not require a SOM but has a similar performance as missions with a SOM. It instead employs a Jupiter Oberth Manoeuvre (JOM) with a total time of flight of around 26 years or so. The efficacy of this trajectory is a result of it significantly reducing the ΔV to Jupiter by exploiting the VEEGA sequence. The total ΔV of the trajectory is 15.8 kms−1 and the corresponding payload mass is 115 kg for a SLS Block 1B or 241 kg for a Block 2. A further advantage of the JOM is that the arrival speed relative to 1I/’Oumuamua is approximately 18 kms−1, much lower than the equivalent for the SOM of around 30 kms−1.
... Thin films of the required surface-area per unit mass could be produced naturally by coagulation of dust particles in the midplane of protoplanetary disks (Moro-Martín 2019; Luu et al. 2020) or artificially by technological civilizations like ours (Bialy & Loeb 2018;Loeb 2021). ...
... Interstellar films withw ≫ w c (Bialy & Loeb 2018), would enter the solar system and could be identified by the upcoming Legacy Survey of Space and Time (LSST) on the Vera C. Rubin Observatory (Bianco et al. 2022), as they would reflect sunlight during their passage close to Earth, similarly to the first reported interstellar object 1I/2017 U1/'Oumuamua (Meech et al. 2017). ...
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I show that interstellar films of material thinner than a micron, drift away from the Galactic plane as a result of stellar radiation pressure. Such films, whether produced naturally by dust coagulation in proto-planetary disks or artificially by technological civilizations, would accumulate over the age of the Milky-Way and hover above the Galactic disk at a scale-height set gravitationally by the dark matter halo. Limits on scattered starlight imply that this population carries a fraction below 2x10^{-3} of the interstellar medium mass.
... It has a hyperbolic excess velocity of 26.33 km/s (94,800 km/h), the speed relative to the Sun when traveling in interstellar space. In addition, there has been interesting discussion about its origin [136]. ...
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Recent years have seen unprecedentedly fast-growing prosperity in the commercial space industry. Several privately funded aerospace manufacturers, such as Space Exploration Technologies Corporation (SpaceX) and Blue Origin have transformed what we used to know about this capital-intense industry and gradually reshaped the future of human civilization. As private spaceflight and multi-planetary immigration gradually become realities from science fiction (sci-fi) and theory, both opportunities and challenges will be presented. In this article, we first review the progress in space exploration and the underlying space technologies. Next, we revisit the K-Pg extinction event and the Chelyabinsk event and predict extra-terrestrialization, terraformation, and planetary defense, including the emerging near-Earth object (NEO) observation and NEO impact avoidance technologies and strategies. Furthermore, a framework for the Solar Communication and Defense Networks (SCADN) with advanced algorithms and high efficacy is proposed to enable an Internet of distributed deep-space sensing, communications, and defense to cope with disastrous incidents such as asteroid/comet impacts. Furthermore, perspectives on the legislation, management, and supervision of founding the proposed SCADN are also discussed in depth.
... 'Oumuamua was the first interstellar object detected in the Solar System by Pan-STARRS (Meech et al. 2017;Micheli et al. 2018). Several follow-up studies of 'Oumuamua were conducted to better understand its origin and composition (Bannister et al. 2017;Gaidos et al. 2017;Jewitt et al. 2017;Mamajek 2017;Ye et al. 2017;Bolin et al. 2018;Fitzsimmons et al. 2018;Trilling et al. 2018;Bialy & Loeb 2018;Hoang et al. 2018;Siraj & Loeb 2019a,b;Seligman et al. 2019). Its size was estimated to be 20m -200m, based on Spitzer Space Telescope constraints on its infrared emission given its temperature (Trilling et al. 2018). ...
Preprint
The earliest confirmed interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the abundance of interstellar objects of its size $\sim 100\;$ m. This was followed by the discovery of Borisov, which allowed for a similar calibration of its size $\sim 0.4 - 1 \mathrm{\; km}$. One would expect a much higher abundance of significantly smaller interstellar objects, with some of them colliding with Earth frequently enough to be noticeable. Based on the CNEOS catalog of bolide events, we identified in 2019 the meteor detected at 2014-01-08 17:05:34 UTC as originating from an unbound hyperbolic orbit with 99.999\% confidence. In 2022, the U.S. Department of Defense has since verified that "the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory," making the object the first detected interstellar object and the first detected interstellar meteor. Here, we discuss the dynamical and compositional properties of CNEOS 2014-01-08, and describe our plan for an expedition to retrieve meteoritic fragments from the ocean floor.
... Since 1I/'Oumuamua [1], was discovered on an unquestionably hyperbolic orbit, i.e. with e = 1.2 ( > 1), the possibility of flyby or rendezvous missions have been the subject of speculation and debate. With their significant heliocentric hyperbolic excess speeds (for 'Oumuamua V∞ > 26.3kms -1 ), the problem of the feasibility of spacecraft missions to ISOs after they have passed beyond their perihelia and are leaving the solar system, has been debated by scientists [2,3]. The resolution of this problem is no longer a matter for conjecture, it can be quantified by the application of the necessary software tool, for example Optimum Interplanetary Trajectory Software (OITS) [4]. ...
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This paper explicates the concept of an Intermediate Point (IP), its incorporation as a node along an interplanetary trajectory, and how it permits the determination and optimization of trajectories to interstellar objects (ISOs). IPs can be used to model Solar Oberth Manoeuvres (SOM) as well as Vinfnity Leveraging Manoeuvres (VLM). The SOM has been established theoretically as an important mechanism by which rapid exit from the solar system and intercept of ISOs can both be achieved; the VLM has been demonstrated practically as a method of reducing overall mission Delta-V as well as the Characteristic Energy, C3, at launch. Thus via these two applications, the feasibility of missions to interstellar objects (ISOs) such as 1I/Oumuamua can be analysed. The interplanetary trajectory optimizer tool exploited for this analysis, OITS, permits IP selection as an encounter option along the interplanetary trajectory in question. OITS adopts the assumption of impulsive thrust at discrete points along the trajectory, an assumption which is generally valid for high thrust propulsion scenarios, like chemical or nuclear thermal for instance.
Article
Avi Loeb has defended the hypothesis that the interstellar object, ‘Oumuamua, detected in 2017, is in fact an extraterrestrial artefact. His hypothesis has been widely rejected by the scientific community. On examination however it is not clear why. The puzzle is at the level of argument structure. The scientific community's responses to Loeb's hypothesis appear to point to explanations of ‘Oumuamua's properties that are mere possibilities. Yet this is something that Loeb does not contest. I appeal to broadly philosophical considerations to understand and bolster the response to Loeb. These considerations concern the structure of his argument, the role of prior confidences within it, and the presence of ‘unconceived alternative’ explanations. I then generalise. ‘Oumuamua will surely not be the last object that does not admit of straightforward natural explanation and that is claimed to be evidence of an extraterrestrial artefact. I use the preceding discussion of Loeb's argument and the scientific community's response to make some general remarks for future debate about similar cases.
Article
Based on the occurrence rates implied by the discoveries of 1I/‘Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect ≥one interstellar object every year. We advocate for future measurements of the production rates of H 2 O, CO 2 , and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. By quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO 2 relative to H 2 O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q (CO)/ Q (H 2 O) < 0.2 and Q (CO)/ Q (H 2 O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.
Article
I show that interstellar films of material thinner than a micron, drift away from the Galactic plane as a result of stellar radiation pressure. Such films, whether produced naturally by dust coagulation in proto-planetary disks or artificially by technological civilizations, would accumulate over the age of the Milky Way and hover above the Galactic disk at a scale-height set gravitationally by the dark matter halo. Limits on scattered starlight imply that this population carries a fraction below 2 × 10 ⁻³ of the interstellar medium mass.
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'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 1 . Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state1-4 and that the physical properties of its surface resemble those of cometary nuclei5,6, even though it showed no evidence of cometary activity1,5,7. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing 8 . Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population 9 . Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of 'Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r-2 or r-1 (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from 'Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that 'Oumuamua has thermal properties similar to comets.
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Models of the Solar System's evolution show that almost all the primitive material leftover from the formation of the planets was ejected to the interstellar space as a result of dynamical instabilities ¹ . Accordingly, minor bodies should also be ejected from other planetary systems and should be abundant in the interstellar space ², giving hope for their direct detection and detailed characterization as they penetrate through the Solar System 3,4 . These expectations materialized on 19 October 2017 ut with the Panoramic Survey Telescope and Rapid Response System's discovery of 1I/'Oumuamua ⁵ . Here, we report homogeneous photometric observations of this body from Gemini North, which densely cover a total of 8.06 h over two nights. A combined ultra-deep image of 1I/'Oumuamua shows no signs of cometary activity, confirming the results from other, less sensitive searches ⁶⁻⁹ . Our data also show an enormous range of rotational brightness variations of 2.6 ± 0.2 mag, larger than ever observed in the population of small Solar System objects, suggesting a very elongated shape of the body. Most significantly, the light curve does not repeat exactly from one rotation cycle to another and its double-peaked periodicity of 7.56 ± 0.01 h from our data is inconsistent with earlier determinations 6,7,10-12 . These are clear signs of a tumbling motion, a remarkable characteristic of 1I/'Oumuamua's rotation that is consistent with a collision in the distant past. Bearing marks of a violent history, this first-known interstellar visitor tells us that collisional evolution of minor body populations in other planetary systems might be common.
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The discovery1 of 1I/2017 U1 (1I/‘Oumuamua) has provided the first glimpse of a planetesimal born in another planetary system. This interloper exhibits a variable colour within a range that is broadly consistent with local small bodies, such as the P- and D-type asteroids, Jupiter Trojans and dynamically excited Kuiper belt objects2–7. 1I/‘Oumuamua appears unusually elongated in shape, with an axial ratio exceeding 5:1 (refs 1,4,5,8). Rotation period estimates are inconsistent and varied, with reported values between 6.9 and 8.3 h (refs 4–6,9). Here, we analyse all the available optical photometry data reported to date. No single rotation period can explain the exhibited brightness variations. Rather, 1I/‘Oumuamua appears to be in an excited rotational state undergoing non-principal axis rotation, or tumbling. A satisfactory solution has apparent lightcurve frequencies of 0.135 and 0.126 h−1 and implies a longest-to-shortest axis ratio of ≳5:1, although the available data are insufficient to uniquely constrain the true frequencies and shape. Assuming a body that responds to non-principal axis rotation in a similar manner to Solar System asteroids and comets, the timescale to damp 1I/‘Oumuamua’s tumbling is at least one billion years. 1I/‘Oumuamua was probably set tumbling within its parent planetary system and will remain tumbling well after it has left ours. The brightness variations of the interstellar object 1I/’Oumuamua observed during six nights are incompatible with a unique rotation rate, indicating that the body is tumbling. Colour measurements suggest a heterogeneous surface, with a large red region.
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We directly measure twenty overhanging cliffs on the surface of comet 67P/Churyumov-Gerasimenko extracted from the latest shape model and estimate the minimum tensile strengths needed to support them against collapse under the comet's gravity. We find extremely low strengths of around one Pa or less (one to five Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features and implied previous collapse of another, suggests that they are prone to failure and that true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of 67P's nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties with size, over the $\sim10-100$ m range studied here, or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small (tens of km) body.
Article
We examine data from the Murchison Widefield Array (MWA) in the frequency range 72-102 MHz for a field of view that serendipitously contained the interstellar object 'Oumuamua on 2017 November 28. Observations took place with a time resolution of 0.5 s and a frequency resolution of 10 kHz. Based on the interesting but highly unlikely suggestion that 'Oumuamua is an interstellar spacecraft, due to some unusual orbital and morphological characteristics, we examine our data for signals that might indicate the presence of intelligent life associated with 'Oumuamua. We searched our radio data for (1) impulsive narrowband signals, (2) persistent narrowband signals, and (3) impulsive broadband signals. We found no such signals with nonterrestrial origins and make estimates of the upper limits on equivalent isotropic radiated power (EIRP) for these three cases of approximately 7 kW, 840 W, and 100 kW, respectively. These transmitter powers are well within the capabilities of human technologies, and are therefore plausible for alien civilizations. While the chances of positive detection in any given search for extraterrestrial intelligence (SETI) experiment are vanishingly small, the characteristics of new generation telescopes such as the MWA (and, in the future, the Square Kilometre Array) make certain classes of SETI experiments easy, or even a trivial by-product of astrophysical observations. This means that the future costs of SETI experiments are very low, allowing large target lists to partially balance the low probability of a positive detection. © 2018. The American Astronomical Society. All rights reserved.
Article
We show that `Oumuamua's excited spin could be in a high energy LAM state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze `Oumuamua's lightcurve using 818 observations over 29.3~days. Two fundamental periodicities are found at frequencies (2.77$\pm$0.11) and (6.42$\pm$0.18)~cycles/day, corresponding to (8.67$\pm$0.34)~h and (3.74$\pm$0.11)~h, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77~cycles/day and a single minimum at 6.42~cycles/day. This is characteristic of an excited spin state. `Oumuamua could be spinning in either the long (LAM) or short (SAM) axis mode. For both, the long axis precesses around the total angular momentum vector with an average period of (8.67$\pm$0.34)~h. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48~h, with 54.48~h being the most likely. `Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where `Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48~h, the latter as the most likely. In this case any nutation will occur with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the TAMV, and the average total spin period may be possible with a direct model fit to the lightcurve. We find that `Oumuamua is "cigar-shaped"', if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state for its total angular momentum.
Article
A rapid accumulation of observations and interpretation have followed in the wake of 1I `Oumuamua's passage through the inner Solar System. We briefly outline the consequences that this first detection of an interstellar asteroid implies for the planet-forming process, and we assess the near-term prospects for detecting and observing (both remotely and in situ) future Solar System visitors of this type. Drawing on detailed heat-transfer calculations that take both `Oumuamua's unusual shape and its chaotic tumbling into account, we affirm that the lack of a detectable coma in deep images of the object very likely arises from the presence of a radiation-modified coating of high molecular weight material (rather than a refractory bulk composition). Assuming that `Oumuamua is a typical representative of a larger population with a kinematic distribution similar to Population I stars in the local galactic neighborhood, we calculate expected arrival rates, impact parameters and velocities of similar objects and assess their prospects for detection using operational and forthcoming facilities. Using `Oumuamua as a proof-of-concept, we assess the prospects for missions that intercept interstellar objects (ISOs) using conventional chemical propulsion. Using a "launch on detection" paradigm, we estimate wait times of order 10 years between favorable mission opportunities with the detection capabilities of the Large-Scale Synoptic Survey Telescope (LSST), a figure that will be refined as the population of interstellar asteroids becomes observationally better constrained.
Article
We estimate the capture rate of interstellar objects by means of three-body gravitational interactions. We apply this model to the Sun-Jupiter system and the Alpha Centauri A\&B binary system, and find that the radius of the largest captured object is a few tens of km and Earth-sized respectively. We explore the implications of our model for the transfer of life by means of rocky material. The interstellar comets captured by the "fishing net" of the Solar system can be potentially distinguished by their differing ratios of oxygen isotopes through high-resolution spectroscopy of water vapor in their tails.
Article
We provide a calculation of Pan-STARRS' ability to detect objects similar to the interstellar object 1I/2017 U1 (hereafter 'Oumuamua), including the most detectable approach vectors and the effect of object size on detection efficiency. Using our updated detection cross-section, we infer an interstellar number density of such objects ($n_{IS} \approx 0.2~\rm{au}^{-3}$). This translates to a mass density of $\rho_{IS} \approx 4M_\oplus~\rm{pc}^{-3}$ which cannot be populated unless every star is contributing. We find that given current models, such a number density cannot arise from the ejection of inner solar system material during planet formation. We note that a stellar system's Oort cloud will be released after a star's main sequence life time and may provide enough material to obtain the observed density. The challenge is that Oort cloud bodies are icy and \OBJECT was observed to be dry which necessitates a crust generation mechanism.