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The Sun, Couch potatoes, and Planetary Orbits:


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NASA team studies middle-aged Sun by tracking motion of Mercury

January 18, 2018

by Elizabeth Zubritsky,

 NASA's Goddard Space Flight Center

Like the waistband of a couch potato in midlife, the orbits of planets in our solar system are expanding. It happens because the Sun's gravitational grip gradually weakens as our star ages and loses mass. Now, a team of NASA and MIT scientists has indirectly measured this mass loss and other solar parameters by looking at changes in Mercury's orbit.

The new values improve upon earlier predictions by reducing the amount of uncertainty. That's especially important for the rate of solar mass loss, because it's related to the stability of G, the gravitational constant. Although G is considered a fixed number, whether it's really constant is still a fundamental question in physics.

"Mercury is the perfect test object for these experiments because it is so sensitive to the gravitational effect and activity of the Sun," said Antonio Genova, the lead author of the study published in Nature Communications and a Massachusetts Institute of Technology researcher working at NASA's Goddard Space Flight Center in Greenbelt, Maryland.



Read more at: https://phys.org/news/2018-01-nasa-team-middle-aged-sun-tracking.html

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the paper:: 

https://www.nature.com/articles/s41467-017-02558-1

Solar system expansion and strong equivalence principle as seen by the NASA MESSENGER mission: 

 

Abstract: 

The NASA MESSENGER mission explored the innermost planet of the solar system and obtained a rich data set of range measurements for the determination of Mercury’s ephemeris. Here we use these precise data collected over 7 years to estimate parameters related to general relativity and the evolution of the Sun. These results confirm the validity of the strong equivalence principle with a significantly refined uncertainty of the Nordtvedt parameter η = (−6.6 ± 7.2) × 10−5. By assuming a metric theory of gravitation, we retrieved the post-Newtonian parameter β = 1 + (−1.6 ± 1.8) × 10−5 and the Sun’s gravitational oblateness, J2⊙J2⊙ = (2.246 ± 0.022) × 10−7. Finally, we obtain an estimate of the time variation of the Sun gravitational parameter, GM⊙°∕GM⊙GM⊙°∕GM⊙ = (−6.13 ± 1.47) × 10−14, which is consistent with the expected solar mass loss due to the solar wind and interior processes. This measurement allows us to constrain ∣∣G°∣∣∕GG°∕G to be <4 × 10−14 per year.

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