General Discussion
In reply to the discussion: On this day in 1781, William Herschel discovered the planet [View all]royable
(1,263 posts)The two STEREO spacecraft, labeled "STEREO A" and "STEREO B," are each in their own orbit around the sun. They are quite free of Earth's gravity. After their launch, NASA used the Moon for a gravitational slingshot to move them into orbits where one is slightly closer to the sun than the Earth, the other slightly more distant. As a result, the closer-to-the-sun one orbits faster than the Earth and has moved "ahead" of us (STEREO A), and the further-from-the-sun one orbits more slowly than Earth and has moved "behind" us (STEREO B). They moved ahead and behind, and then further ahead and further behind, until first the A , then the B, passed around the far side of the sun several years ago. They are now continuing in those orbits and are halfway back to Earth. See
https://gong2.nso.edu/products/scaleView/view.php?configFile=configs/pfssModels.cfg&productIndex=6
for a map, generated hourly, showing where they are right now in a view looking down at the Sun's and Earth's north poles.
While the spacecraft were behind and almost behind the sun, indeed, we would not have been able to receive their signals, and so they were put into a standby mode for a few weeks.
If there were an Earth-sized body on the opposite side of the sun, the orbits of these two spacecraft would have been altered by it and those who aim the antennas to pick up their signals would have not found the space craft to be where they were expected to be. There is no hidden planet opposite the Sun from us.
See https://www.nasa.gov/mission_pages/stereo/main/index.html for lots of info on the STEREO mission and findings.