#Sentinel-1 reached orbit!

April 4, 2014

So why is it that +European Space Agency, ESA made #Sentinel-1 head north at launch instead of the regular east during launch? Usually you’d start east to take maximum advantage from earth’s rotation (which turns counter-clockwise if north is up) to save fuel. However, Sentinel-1 ist on a solar synchronous orbit [1], which is slightly retrograde (if I looked it up correctly [3], it should have an inclination of about 98° at a orbital periode of 96 minutes and a height of 574 km). If you’d start straight ahead to the east you’d end in a very low-inclined orbit which is directly related to Kourou’s space port lattitude (~5° North). When you’re already in orbit, changing the inclination is hard. You need a lot of delta-v – changes in velocity – needing a lot of fuel – which you need to carry with you – increasing the mass – needing even more fuel [2]! And considering that changing velocity is dependent on the current mass and velocity – kinetic energy – it’s best to change the inclination as soon as possible, while you’re not at already at thousands of meters per seconds. It still needs way more fuel to get an object into an polar or (worse) retrograde orbit, but you better do it early and better do it fast.

Edit: Fun fact: Sentinel-1’s velocity is supposed to be 0.89 times the speed of General Zod’s spaceship in the movie Man Of Steel. Thank you, +Wolfram|Alpha :-) http://www.wolframalpha.com/input/?i=%282*%CF%80*%28polar+radius+earth+%2B+%28%28578+km%29%2F2%29%29%29%2F96+minutes

1. See https://en.wikipedia.org/wiki/Sun-synchronous_orbit
2. See rocket formula, an ordinary differential equation describing how much fuel is needed to reach a certain velocity, taking into account that you’re losing mass when burning fuel: https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
3. See https://en.wikipedia.org/wiki/Sun-synchronous_orbit#Technical_details – Table 1