Engineers hope Juno’s Earth flyby yesterday will help solve a mystery seen in previous flybys by unmanned probes.

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The uncertainty of science: Engineers hope Juno’s Earth flyby yesterday will help solve a mystery seen in previous flybys by unmanned probes.

Since 1990, mission controllers at ESA and NASA have noticed that their spacecraft sometimes experience a strange variation in the amount of orbital energy they pick up from Earth during flybys, a technique routinely used to fling satellites deep into our Solar System. The unexplained variation is noticed as a tiny difference in the expected speed gained (or lost) during the passage.

The variations are extremely small: NASA’s Jupiter probe ended up just 3.9 mm/s faster than expected when it swung past Earth in December 1990. The largest variation– a boost of 13.0 mm/s – was seen with NASA’s NEAR asteroid craft in January 1998. Conversely, the differences during swingbys of NASA’s Cassini in 1999 and Messenger in 2005 were so small that they could not be confirmed.

The experts are stumped.

It is likely that these small variations are related in some way with simple engineering and not some unknown feature of gravity. Nonetheless, it remains a mystery.



  • Well, I actually solved the problem. Here is my press release concerning the flyby ->

  • Pzatchok

    And here I was just thinking it could be the variations in the Earths surface.

    As a craft moves past the Earth if it passes over the Himalayas the surface would be closer to the craft than if it passed by over the open flat ocean.

    More mass miles closer to the craft causes an increase in the working gravity.

  • Edward


    The gravitational variations in the density and elevations of the Earth are well mapped and included in orbit equations. These variations are even used to routinely change orbital planes ( e.g. so that a spacecraft can be constantly in sunshine — called a sun-synchronous orbit).

    My first thought was that the lunar influences were miscalculated (a three-body equation has been elusive), but that seemed rather unlikely, given the large number of accurate calculations given by the many orbital calculators.

    Although I do not understand how spacecraft spin affects orbital mechanics, Kimmo Rouvari’s hypothesis/theory seems more plausible than anything I can think of (such as influence by “space aliens” who are just messing with our minds).

    Unfairly, I am making these comments without having read Rouvari’s “Theory of Everything by illusion,” perhaps because it just seems illusory to me (pun intended — in fact, the whole reason for this sentence was to make that pun — I hope you take this with amusement, Kimmo).

  • No problem Edward! I have a great sense of humour :-) Besides, you have plenty of time to read my paper after the flyby results are public. My kind advice… sit while you read ;-)

  • Units should be mm/s, my bad!

  • Here’s latest regarding Juno Earth flyby

    1.111 mm/s at perigee! Champagne is ready to go ;-)

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