From answers to this question i've learned that the Lagrangian L$_1$ point of Venus is not stable, despite the almost circular orbit of the planet and the fact that it has no moon.

Nevertheless would it be possible that a flat structure, composed of panels made of Smart glass, perpendicular placed on the Sun-Venus axis right behind the L$_1$ point, seen from Venus, could stay there ?

A voltage applied to an electrochromic device would change the opacity of the glass material and thus would change the radiation pressure exerted by the sunlight.
And a voltage applied to micro-blinds would block light by stretching out very small, thin metal blinds on glass.

Because of its place right behind the L$_1$ point this "solar sail" would have a slight pull from the gravity of the Sun and the radiation pressure could be changed sufficiently to counteract upon it.

The square structure would be surrounded at the edges by 45$⁰$ tilted panels to regulate that the structure would not deviate from the Sun-Venus axis.

Would visual location of Venus be sensitive enough to allow the smart window structure to act upon variation in distance and position to the planet in an adequate way ?

  • $\begingroup$ This is a really interesting question! Station keeping in a halo orbit around L1 using purely radial delta-v is definitely a thing. I know that SOHO uses this, and there may be others. $\endgroup$
    – uhoh
    Commented Nov 3, 2019 at 23:39
  • 4
    $\begingroup$ Since fully reflective solar sails able to twist their shape can already control their attitude on 3 axis (and therefore are able to keep station), using smart glass instead of a very thin layer of material becomes a heavy solution. $\endgroup$
    – user19132
    Commented Nov 4, 2019 at 17:27

1 Answer 1


It couldn't stay at L1 for longer than a few months.

The electrochromic material's degradation due to ultraviolet radiation would limit how long the panels would operate. On Earth, outdoors, at sea level, they would last at most "a few years." Earth's atmosphere blocks 77% of UV, so the UV just outside Earth's atmosphere is 4.3 times stronger. Venus is about 0.7 AU from the sun, so the UV is twice as strong there. So the panels would last "an eighth of a few years," just a few months. Mechanical devices would last much longer.

  • 5
    $\begingroup$ 1. Even after the smart glass' ability to station keep was lost, a carefully placed object would remain near L1 for longer than a few months. 2. A thin UV filter applied to the front surface might be all that's needed to solve that problem. 3. An accelerometer would only detect propulsive or drag effects. It would not have any sensitivity to orbital perturbations. The test mass inside the accelerometer is in the same orbit as the rest of the spacecraft, so "The accelerometers would be sensitive enough." is wrong on a fundamental level. $\endgroup$
    – uhoh
    Commented Nov 3, 2019 at 23:46
  • $\begingroup$ Sorry if i didn't inform you well enough, but there are also so-called micro-blinds. en.wikipedia.org/wiki/Smart_glass#Micro-Blinds $\endgroup$
    – Cornelis
    Commented Nov 4, 2019 at 9:31
  • $\begingroup$ @uhoh Thanks for your comment, i've learned from it. So changed the last sentence. $\endgroup$
    – Cornelis
    Commented Nov 4, 2019 at 15:50
  • $\begingroup$ Just slap on a transparent material that totally blocks/reflects the UV frequencies. Something like maybe this?: prnewswire.com/news-releases/… $\endgroup$ Commented Jul 28, 2021 at 7:42
  • $\begingroup$ Something that's thin, UV-opaque, and otherwise sunlight-transparent might itself degrade from the UV quickly enough to shorten the mission. Usable lifetime is conspicuously missing from that news release, for instance. $\endgroup$ Commented Jul 28, 2021 at 18:36

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