Dynamical Constraints On A Population of Massive Interstellar Objects
Dynamical Constraints On A Population of Massive Interstellar Objects
1
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
e-mail: oem.trivedi@vanderbilt.edu
2
Astronomy Department, Harvard University, 60 Garden St., Cambridge, MA 02138, USA
e-mail: aloeb@cfa.harvard.edu
arXiv:2509.06300v1 [astro-ph.EP] 8 Sep 2025
September 9, 2025
ABSTRACT
Context. The detection of kilometer-scale interstellar objects (ISOs) places strong dynamical constraints on their underlying popula-
tion and distribution.
Aims. We aim to assess whether the observed encounter rates of large ISOs can be explained by their expected velocity distribution,
or whether anomalous velocity anisotropies are required.
Methods. We first derive the cumulative encounter rate scaling with size from brightness-limited detection models. We then employ
the Eddington inversion method to link power-law density profiles ρ(r) ∝ r−k with phase-space distributions, illustrating how steep
slopes imply strong velocity-space focusing. Finally, we develop a Liouville mapping formalism based on energy and angular momen-
tum conservation, propagating the interstellar distribution inward to the detection region while explicitly incorporating gravitational
focusing and anisotropy.
Results. We show that encounter rates of large ISOs require flux enhancements far beyond expectations, that steep density profiles
produce the necessary inward velocity bias, and that Liouville mapping provides a physically self-consistent way to reproduce the ob-
served size dependent detection rates. The main results are framed in the context of the parameters for 3I/ATLAS, but the implications
are general and go on to sharpen the distinction between natural dynamical mechanisms and potential artificial origins for ISOs.
Key words. Interstellar Objects – 3I/ATLAS – Non-Natural Trajectories
radius interpretations of 3I/ATLAS, we demonstrate how the Assuming spatial uniformity, the number of objects per unit vol-
rarity of its trajectory places stringent constraints on the abun- ume in a size range [R, R + dR] is
dance of massive interstellar objects. This analysis provides a
dN
new perspective on the emerging census of ISOs and directly n(R)dR = , (8)
addresses the question of whether the detection of 3I/ATLAS dV
is dynamically compatible with a natural origin, or whether and so
alternative explanations such as artificial or directional delivery
need to be considered. n(R)dR ∝ R−q dR, (9)
which implies the differential number density scales as
2. Encounter rate analysis
n(R) ∝ R−q . (10)
Since direct detections of ISOs remain rare, the statistical rate of
encounters provides a powerful indirect diagnostic of the under- Now, rather than compute the differential detection rate, we
lying ISO population. In particular, encounter rate formulations derive the cumulative detection rate which is the rate of en-
allow us to quantify the scaling of detection probabilities with countering all ISOs with radius greater than or equal to R. To
object size, velocity distribution, and survey sensitivity. This is do so, we integrate the differential rate from R to ∞. Since
especially interesting in the case of an object like 3I/ATLAS, as Γ1 (R′ ) ∝ n(R′ ) · σ(R′ ), and we have just shown that n(R′ ) ∝ R′−q
it could highlight the enhancements in flux required to recon- and σ(R′ ) ∝ R′ , the integrand becomes R′−(q−1) . The cumulative
cile the observed ISO detections with natural population models. encounter rate is
Z ∞
We begin with the standard expression for the detection Γ1 (R) ∝ R′−(q−1) dR′ . (11)
rate of ISOs, written as an encounter rate R
where n(R) is the number density of objects within a narrow bin Γ1 (R) ∝ R−(q−2) . (12)
dR of radius R, ⟨v⟩ is the average encounter speed of interstel- Finally, including the average encounter speed ⟨v⟩, which is
lar objects relative to the Sun, and σ(R) is the effective detection computed from the shifted Maxwellian distribution as
cross section for an object of radius R. Rather than modeling the
cross section geometrically, we instead adopt a brightness-based 2
!
v2⊙
!
detection model, following a similar formulation as in Seligman ⟨v⟩ = √ σ s 1 + , (13)
et al. (2025). In this approach, the detection cross section is set π 6σ2s
by the maximum distance at which an object of size R is de- with σ = 25 km/s v⊙ = 17.4 km/s. We have taken the velocity
tectable, given a fixed limiting apparent magnitude mlim . Using dispersion σ s to be of this value, given recent results showing
standard photometric relations for reflected sunlight, the appar- that the dispersion in the range 20 − 30 km/s regime is reason-
ent magnitude of an object at heliocentric distance r ≫ 1 au, able. For example, Kohandel et al. (2020) showed a velocity dis-
geocentric distance ∆, and radius R, for a fixed geometric albedo persion in the region 23 − 28km/s region, Eubanks et al. (2021)
p, is given approximately by discussed a value of 26.14 for local stellar population as well,
while other recent works have also also considered this regime
m ≈ H + 5 log10 (r∆), (2)
Hopkins et al. (2025b,a). The final expression for the cumulative
where the absolute magnitude H itself depends on the object ra- detection rate becomes
dius as
p Γ1 (R) = k · R−(q−2) · ⟨v⟩. (14)
H = 17.1 − 2.5 log10 − 5 log10 (R). (3) To fix the normalization, we impose the condition for 3I/ATLAS,
0.04
Assuming constant albedo and phase behavior, we can express Γ1 (0.6) = 0.2 yr−1 (15)
the apparent magnitude as
which gives
m ∝ −5 log10 (R) + 5 log10 (r∆). (4)
0.2
Fixing the apparent magnitude to the detection threshold mlim , k= . (16)
(0.6)q−2 · ⟨v⟩
we can then invert the above expression to solve for the maxi-
mum distance at which an object of size R can be detected. This Thus, the full expression becomes
leads to R −(q−2)
0.2
rmax (R) ∝ R 1/2
, (5) Γ1 (R) = · (0.6)q−2 · R−(q−2) · ⟨v⟩ = 0.2 · . (17)
⟨v⟩ 0.6
and hence, since the detection cross section scales with the ob- This allows us to define the flux enhancement factor as
servable area (a circle of radius rmax ), we have
Γ2 R q−2
σ(R) ∝ rmax
2
∝ R. (6) ≈ , (18)
Γ1 0.6
To determine the number density n(R), we assume that the ISOs which gives the required enhancement in flux beyond the natural
follow a differential size distribution of the form population in order to explain the detection rate of objects larger
dN than a given radius R. This formulation is based on cumulative
∝ R−q . (7) detection rate considerations and incorporates both the size
dR
Article number, page 2 of 6
Oem Trivedil and Abraham Loeb: Dynamical Constraints on a Population of Massive Interstellar Objects
with radial velocity that it is possible to match the observational size distribution
r slope by tuning the anisotropy in J, there is still no known
J2 astrophysical mechanism that would actually generate such a
vr (r, E, J) = 2 E + GM⊙ /r − 2 . (35) distribution of low angular momentum interstellar objects in
r
the first place. This gap leaves room for speculation that the
To extract the effective slope k(R) that measures the relative required anisotropy could be the result of artificial intervention,
enhancement of encounter rates with object size, we compute like from a directed release of objects with engineered orbits.
ρ(r; R, a) in a narrow logarithmic window around the detection
radius rmax = 4 AU and fit a local power law
5. Conclusions
ρ(r; R, a) ∝ r−k(R) . (36) In this work we have developed a dynamical framework to con-
This procedure gives an estimate of k(R) by comparing ρ at radii strain the population of large ISOs using three complementary
slightly above and below rmax and since only relative values approaches. Encounter rate analysis demonstrated that detection
matter, we normalize such that k(R0 ) = 0 at R0 = 0.6 km, in of ISO with radii of tens of kilometers cannot be reconciled
accordance to Loeb (2025). with a purely Maxwellian background, implying the need for
significant flux enhancements. By applying the Eddington
We then adjust the anisotropy amplitude a to be in agree- inversion method, we established how steep radial density
ment with the required enhancement implied by the cumulative profiles naturally correspond to phase-space distributions biased
detection rates and once a is fixed, the function k(R) follows toward radial orbits, providing a clear dynamical mechanism for
uniquely from the dynamics. The resulting plot in Figure 3 of gravitational focusing near the Sun. Extending this discussion,
we then employed a Liouville mapping in (E, J) space to prop-
agate the interstellar velocity distribution inward under explicit
conservation laws, showing that the required size dependent
enhancement of encounter rates can arise from anisotropies that
favor low angular momentum trajectories.
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