Null result from Spitzer suggests Oumuamua was small

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The uncertainty of science: The inability of the infrared Spitzer Space Telescope to detect the interstellar object Oumuamua as it exited the solar system suggests the object is small.

The fact that ‘Oumuamua was too faint for Spitzer to detect sets a limit on the object’s total surface area. However, since the non-detection can’t be used to infer shape, the size limits are presented as what ‘Oumuamua’s diameter would be if it were spherical. Using three separate models that make slightly different assumptions about the object’s composition, Spitzer’s non-detection limited ‘Oumuamua’s “spherical diameter” to 1,440 feet (440 meters), 460 feet (140 meters) or perhaps as little as 320 feet (100 meters). The wide range of results stems from the assumptions about ‘Oumuamua’s composition, which influences how visible (or faint) it would appear to Spitzer were it a particular size.

The new study also suggests that ‘Oumuamua may be up to 10 times more reflective than the comets that reside in our solar system – a surprising result, according to the paper’s authors.

These results fit the models that explain Oumuamua’s fluctuations in speed as caused by the out gassing of material, like a comet. They also do not contradict the recent hypothesis that the object might have been an alien-built light sail.

The simple fact is that we do not have enough data to confirm any of these theories.



  • Steve Burrows

    This whole event is utterly fascinating! It’s almost worth considering the construction of an Orion drive craft to go see it. Even if it is only a rock, could be the last one seen in a lifetime, the knowledge gained would be spectacular.

  • Max

    a light sail has very little thust and would need to be extended once it passes the sun. Otherwise it would have slowed down as it approached the Sun in the same manner as a comets tail always points away from the sun.
    A black sail, that absorbs light and radiates heat, has more thrust. As a radiometer demonstrates. It would be hard to see in an optical telescope but would be bright in a thermal telescope.

    I love the idea that it could’ve been a spent alien fuel tank. A static charge on a metal surface will collect dust to make it appear like a rock but once it passes the Sun, heat could have ruptured the reserve fuel causing the acceleration.

  • Max

    A different article with the news post by NASA that shows Solar system illustration of the change in trajectory. Short and interesting.

    Again, Spitzer telescope would have seen a solar sail in the infrared. It did not detect the object at all. A comet has outgassing that makes it very visible, A coma of dust/gas and a tail. None was visible, making out gassing to speed up the object unlikely. I love a good mystery.

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