The Ulysses Spacecraft was sent to Jupiter to change it's orbital inclination via a gravity assist, and enter a nearly polar heliocentric orbit with an inclination of 79° in respect to the solar plane.

enter image description here

Entering an equatorial LEO in the direction of Earth's rotation requires about 9.4 km/s of $\Delta V$. The amount of $\Delta V$ to enter a Jupiter-Earth transfer orbit requires about 6 km/s from LEO. So I don't see a major problem from a Delta-V budget standpoint to enter a polar LEO, and then escape Earth's gravitational pull to enter a heliocentric orbit.

Question: Why didn't Ulysses, or any future spacecraft intended to study the poles of the Sun, enter a polar orbit around Earth, then fire it's rocket engine to obtain an inclined heliocentric orbit?

Note: Ulysses was launched aboard Space Shuttle Discovery, and the Space Shuttle couldn't enter a polar orbit, but let's ignore this issue as this question isn't specifically about Ulysses but about all polar heliocentric orbit satellites.

  • $\begingroup$ I think this question is one of the ones that particularly benefits from trying it in Kerbal Space Program. $\endgroup$ – ikrase May 24 '20 at 18:03
  • $\begingroup$ Actually, the Space Shuttle could enter a polar orbit, but never actually did. A launch from Vandenberg (STS-62-A) was scheduled for July 1986 to put a satellite in polar orbit, but this was cancelled after the Challenger disaster. $\endgroup$ – Astrid_Redfern Jan 1 at 15:09

A polar LEO orbit is still a equatorial heliocentric orbit (since that describes the Earth's orbit around the sun).

To change from a equatorial heliocentric to polar heliocentric requires a lot of Delta v.

The final orbit wasn't circular, but we can get an order-of-magnitude figure for the energy required by using the circular plane change approximation.

$$\Delta v_i = 2v \sin \left( \frac{\Delta i}{2}\right)$$ $$\Delta v_i = 60 \text{km/s} \sin(0.69 \text{rad}) = 38\text{km/s}$$

You can get some of that back by not having a final circular orbit. But the change necessary to do the maneuver immediately at the earth's orbit is still quite large.

  • $\begingroup$ So what would happen if a satellite in a polar earth-orbit burns its engines pro-grade at the night-side of Earth? It would still be in a relatively equatorial heliocentric orbit? $\endgroup$ – Star Man May 23 '20 at 0:16
  • $\begingroup$ Depends on how much of a burn. It's starting in a 30km/s heliocentric orbit around the sun. A "northward" burn of single-digit km/s can get it to escape the earth, but maybe not change the (solar) orbit too significantly. Total energy increases, so orbit would be larger as well. $\endgroup$ – BowlOfRed May 23 '20 at 3:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.