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While Bill Nye has recently gotten quite upset at us for some things (strong language for Bill Nye warning), in the video below he expresses heartfelt thanks to all members of the Planetary Society and for contributors to the Kickstarter program(s) associated with the Lightsail project.

In the video he announces "Mission Success!" for using solar photons to increase the orbital energy of the cubesat.

Questions:

  1. What is the total delta-v they expect to produce with the sail by the planned end of mission?
  2. Why stop using the sail so quickly and then end the mission burning up in the atmosphere rather than seeing just how high it can sail over the following years? Source
    • Orbit raising: 23 July through 23 August, 2019 (estimated)
    • Deorbit: August 2020 (estimated)

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The image below shows the answer. Lightsail only gets thrust while moving away from the Sun (on the lower half of the orbit as drawn here). Thrusting on one side of an orbit raises (or lowers) the other side but has no effect on the side where you do the thrusting. Meanwhile drag is slowly lowering the whole orbit, so the overall effect is that the orbit gets higher on one side of the Earth and lower on the other. Once it gets a bit lower than its starting perigee, there will be enough drag at perigee to exceed the thrust. At that stage it will stop raising the apogee, and there is no way to get more thrust than drag ever again.

enter image description here

Image from http://www.planetary.org/blogs/jason-davis/ls2-deploys-sail.html

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    $\begingroup$ Lightsail-2 (2019-036AC, 44420) is in LEO with an inclination of 24 degrees, so apsidal precession due to Earth's J2 will naturally put it in position to start raising its periapsis instead, within a few months, won't it? (also here) And even if the apses didn't precess, in six months the Sun would be on the other side and it could then be raising the periapsis, so the fixed GIF doesn't explain by itself why they can't walk their way up in a nearly circular orbit over years. $\endgroup$ – uhoh Aug 1 at 12:10
  • $\begingroup$ I've editing in a little more explanayion $\endgroup$ – Steve Linton Aug 1 at 12:38
  • $\begingroup$ Interesting answer. But still, I find @uhoh's argument about the sun being on the opposite side after 6 months rather convincing. $\endgroup$ – Everyday Astronaut Aug 1 at 13:19
  • $\begingroup$ @SteveLinton I see, this seems to be a key part of your answer; "Once it gets a bit lower than its starting perigee, there will be enough drag at perigee to exceed the thrust." Can that be supported using a calculation or citing an authoritative source? Periapsis is over 700 km, so it's not necessarily a self-evident truth that drag is greater than thrust there. $\endgroup$ – uhoh Aug 1 at 14:45
  • $\begingroup$ just fyi there's some really interesting information in Ars Technica's So, how much did the Planetary Society’s light sail really sail? which links to Jonathan McDowell's tweet. $\endgroup$ – uhoh Aug 4 at 14:08

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