We study the effect of a close encounter of a passing star on the shape of the inner Oort Cloud , using the impulse approximation .
The deviation of the perturbed Oort Cloud from sphericity adds angular fluctuations to the brightness of the Cosmic Microwave Background ( CMB ) due to thermal emission by the comets .
An encounter with a solar-mass star at an impact parameter of 2100 { AU } , as expected based on the abundance and velocity dispersion of stars in the solar neighborhood , leads to a quadrupole moment C _ { 2 } = 4.5 ( 3.5 ) \times 10 ^ { -15 } , 6.7 ( 1.1 ) \times 10 ^ { -12 } , 1.1 ( 0.11 ) \times 10 ^ { -9 } at \nu = 30 , 353 , 545 { GHz } , respectively in intensity and ( temperature ) fluctuations .
We also quantify the quadrupole spectral distortions produced by the Scattered Disc , which will exist irregardless of any perturbation and the subsequent shape of the Oort Cloud .
For comparison , the temperature fluctuation quadrupole moment predicted by the current cosmological model is C _ { 2 } = 1.76 \times 10 ^ { -10 } , which corresponds to fluctuation in the CMB intensity of C _ { 2 } = 2.9 \times 10 ^ { -10 } , 6.8 \times 10 ^ { -9 } , 1.6 \times 10 ^ { -8 } at \nu = 30 , 353 , 545 { GHz } .
Finally , we discuss how a measurement of the anisotropic spectral distortions could be used to constrain the trajectory of the closest stellar fly-by .