Orbiter missions with no (or little) fuel usage for deceleration, Target planet capturing the spacecraft w/o extensive fuel usage |
Orbiter missions with no (or little) fuel usage for deceleration, Target planet capturing the spacecraft w/o extensive fuel usage |
![]()
Post
#1
|
|
![]() Newbie ![]() Group: Members Posts: 11 Joined: 6-July 15 From: Russia, Saint Petersburg Member No.: 7559 ![]() |
I was involved in a casual conversation recently about the exploration of bodies in Solar System, in particular about fly-by approach vs. orbiter approach. In particular, I was saying that the main challenge with orbiter missions is that either you have to decelerate once you reach your target (requires tons of fuel on board), or you have to travel along the trajectory that would take an unreasonable amount of time to reach the target. As an example of the latter, I said that you could launch a spacecraft into a trajectory that would be a part of an elliptical orbit around the Sun with the perihelion around Earth and the aphelion around the destination. That way, when the spacecraft reaches its target, its speed relative to the planet will be slow enough for it to be "picked up" by the planet's gravity, and it will start orbiting the planet.
The other person in the conversation pointed out that it's been shown that when two bodies pass each other and influence each other gravitationally, it's not possible for them to start orbiting each other (or, in the case of one object being much more massive than the other (planet vs. spacecraft), simply "one orbiting another"). He said that either the more massive object will simply alter the trajectory of the passing smaller object, but not capture it, or the smaller object will crash into the bigger one. And this is something that, to me, "intuitively" shouldn't be right, but I don't have enough expertise to prove that it's wrong. My counter-arguments are: - Some of the natural satellites in the Solar System are believed to be captured by the planet as they were passing by (true, these are mainly hypotheses, but people wouldn't make such hypotheses if this wouldn't be possible?). - If the object is passing by the planet at the speed less than what is required to enter the orbit, then it will crash down onto the planet. If the object is passing at the speed greater than the escape velocity, then it will continue flying without being captured by the planet. Surely if the object's speed is between these two values, it has to start orbiting the planet? (Not necessarily in a perfect circular orbit of course). As further proof for the second point, I calculated the elliptical orbit with the perihelion at Earth (1 AU) and aphelion at Uranus (19.2 AU). Sure, it would take 16 years to get to Uranus, but the required takeoff speed would be 41.1 km/s (relative to Earth, that would be 11.3 km/s, so just barely above Earth's escape velocity, so we're good here), and the spacecraft's speed when it arrives to Uranus would be 2.1 km/s relative to Sun, and -4.6 km/s relative to Uranus. This 4.6 km/s speed happens to be the speed of a circular orbit at 266000 km from the center of Uranus. So, in my understanding, we can launch the spacecraft from Earth at 11.3 km/s into the elliptical orbit, and then some 16 years later it will pass by Uranus and be captured by it. If we make some small course corrections along the way so that it passes 266000 km from Uranus, it will even be a circular orbit. ... but, like I said, maybe I'm missing something that won't allow the spacecraft to be captured by the planet's gravity? My whole point was that in this scenario you don't have to use fuel for anything else other than takeoff. Of course, some course corrections would be inevitable, but at least you won't have to try to decelerate from New Horizons-like speeds. |
|
|
![]() |
![]()
Post
#2
|
|
Member ![]() ![]() ![]() Group: Members Posts: 915 Joined: 4-September 06 From: Boston Member No.: 1102 ![]() |
I have not done orbital calculations, but was a physics major for the first two years of college--so have a feel but not a solid answer. The problem I see with your thought experiment is you can drop a spacecraft into the proper position and velocity for a circular orbit of a 3rd body, but when you start working backwards towards the joint orbit from earth you will find you would have had to fire rockets to slow the probe to get there. If you think of an elliptical orbit of earth pointing directly away from the sun out 2 AU, and look at the second focus spot and calculate how massive an object you would need to capture a spacecraft at the spacecraft's velocity at it furthest point out, then drop the 3rd body into place when our spacecraft is at the point of its orbit closest to earth (and also sun), and then follow what happens. As it goes in its orbit, it will speed up as it approaches the 3rd object and not go into orbit because it is going too fast. As you play with increasing the mass of the 3rd body to try and capture it, you get closer and closer to orbital capture....but just as you think you are about to succeed...SPLAT... litho-braking... Regardless of where you drop the 3rd object or how massive it is, you will always be (sped up to be) going too fast to orbit, but crash landing can be arranged.
-------------------- |
|
|
![]() ![]() |
![]() |
Lo-Fi Version | Time is now: 23rd June 2024 - 03:58 AM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |
![]() |