# Tag Info

## Hot answers tagged gravity-assist

70

In order to land on the Moon, you must, at some point, be moving towards the Moon (decreasing your distance from it, to be more precise, you may also be moving sideways) and close enough that the Moon's gravity dominates that of the Earth and the Sun. From that point on, your kinetic energy (relative to the Moon's centre of mass) can only increase as you get ...

65

It's the gravitational attraction of the Sun. Voyager is moving away from the Sun and is pulled back by its gravity. Since Voyager is not moving directly away from the Sun, it's trajectory also curves.

57

@SteveLinton's answer is right, no matter how gently you try, by the time you get to the surface the Moon's gravity will have accelerated you to something like 2,400 m/s. There are ways to use the gravity of the Earth and Sun to make a tiny reduction in this, but it's a very small effect. The simplest way to argue this is that rocks on the Moon don't ...

49

Yes, Trajectory Correction Maneuvers (TCMs) are always performed during cruise phases, whether before or after gravity assist flybys. This NASA tutorial serves as a good general reference. One source of error resulting in an imperfect trajectory (one that would miss its aimpoint at the next destination, whether the ultimate destination or an intermediate ...

46

why weren't they completely attracted by their gravitational field? How much a trajectory is changed, depends on 3 factors: the mass of the planet, the speed of the spacecraft, the distance between spacecraft and planet Voyager's speed and distance were chosen to make sure Voyager wouldn't enter orbit around the planet. Voyager's speed before approaching ...

43

The mission was to fly by the outer planets. Once it got to Neptune that mission was complete. From Wikipedia: Because this was the last planet of the Solar System that Voyager 2 could visit, the Chief Project Scientist, his staff members, and the flight controllers decided to also perform a close fly-by of Triton, the larger of Neptune's two originally ...

38

Understanding the Principle Let's start by understanding the mechanism of a gravity assist. As a spacecraft approaches a planetary body, it gets affected by the planets gravitational pull. Getting nearer, the pull increases, and eventually when the spacecraft passes the planet, the pull decreases. If you think about a stationary planet as an absolute ...

36

They wanted a close flyby of Triton. Triton's orbit is at a large angle to the ecliptic plane, and Triton was below Neptune at the time of Voyager's flyby, so they needed a course change that pointed "down" from Neptune. From the Voyager Neptune travel guide (large PDF), page 118-119 of the PDF (page number 107-108 indicated on the page): However, ...

34

Additionally, the probe has just passed a large planetary body with its own gravity well. The probe has to climb up out of that planet's gravity well which costs momentum. There is a net gain in momentum overall, the probe is leaving with more momentum than it entered the slingshot manoever, but the highest velocity is about where the probe is moving ...

29

This sort of spacecraft is known as a "cycler". You hit on the problem with it: you have to match its trajectory/velocity exactly in order to dock with it, so if you can reach the cycler, you could already reach the cycler's destination. There's no slowing down of the cycler for the same reason. In principle, if you connected to the cycler with a very ...

26

First order analysis Given that we have practical ion thrusters, it's time to look at them. Deep Space 1 The DS1 probe massed 387kg, had 83kg of fuel, operated for 162 days, and generated 92mN. So, it generated about 0.2mm/s^2. The craft is not tanks-dry, either. It has approximately 6 months (180 days) of fuel per design. That's a roughly 20% fuel ...

23

It was said in here that the time to reach Pluto was shortened by 3 years. It's also said that after the Jupiter flyby the probe gained ~ 4 km/s accelerating to the speed of 23 km/s relative to the Sun. We can use simple Keplerian estimate (ignoring all complexities of the actual orbital mechanics) to obtain the speed when the probe approaches Pluto, \begin{...

21

If you only count planets, then I believe it's MESSENGER at six. Though your question was explicitly: "Which probe that we have launched has received the most gravitational assists?" The winner there is Cassini, hands down. It is on Titan flyby number 93 125, so far. And it's flown by other moons of Saturn. Plus the four planetary flybys on the way to ...

21

The speed of the probe doesn't change with regard to the assisting body. It is direction that is changed. If a hyperbolic orbit about the sun comes in with a Vinfinity of 5 km/s, it will exit with a Vinfinity of 5 km/s. Unless it happens to swing by a planet. From the planet's point of view incoming and outgoing speed are also the same. Again, it's ...

21

To add to the answers @Hobbes & @Steve Linton posted, the mission designers indeed knew Jupiter's gravity field quite well from the orbits of Jupiter's moons. But before the Voyagers arrived they got additional measurements from the close flybys of two other spacecraft, Pioneers 10 and 11. @Steve Linton correctly describes the effect of the "sideways" ...

21

How small do you want to get? $F=G{Mm \over r^2}$ applies regardless of size. If you remove enough disturbances from other bodies you can get two neutrons to orbit a common barycenter on gravity alone - or send them against each other on a near-miss trajectory and they'll pass influencing each other gravitationally in essence performing a slingshot against ...

20

Whilst the main factors involved in transferring energy to space craft during a flyby are well known, I believe this is a reference to several observed anomalies that have occurred to various space probes, covering everything from the early Pioneer probes to the much more recent Rosetta probe. The researchers looked at five deep-space probes — Galileo to ...

18

The STEREO satellites used multiple gravitational assists from the Moon to significantly decrease the amount of fuel needed to put those two satellites into heliocentric orbits. The first flyby resulted in STEREO ahead (STEREO-A) being ejected from the Earth-Moon system with a semi-major axis slightly less than that of the Earth-Moon system. STEREO-A has a ...

18

Considering that Voyager 1 is already 126 AU from the Sun 36 years since launch, there should be no reason that it would not be possible energetically using a normal launch, small maneuvers, and planetary flybys. Just a Jupiter flyby should be sufficient. Jupiter will also provide the necessary change in inclination. Designing a probe that is assured to ...

18

It is absolutely possible, just not advised. New Horizons was launched at Solar System Escape Velocity, meaning it could have visited anywhere beyond Earth without stopping. It did visit Jupiter, however, that was to allow it to leave even faster, the Jupiter stop was purely optional. As for the issue of human capable spacecraft, again, it could be done, if ...

17

No. What a gravity assist does is change the velocity with respect to other objects, but not the one you were approaching. Nasa provided a nice diagram to assist with understanding this. In fact, in more ordinary terms, it could be though of as the below diagram shows. The baseball is thrown at the train at 30 miles per hour. From the view point of the ...

16

Yes, those are the three factors. Your third factor shows up as the $v_\infty$ of the spacecraft relative to the object. The first two are the GM of the object, $\mu$ and the closest approach distance $r$. The $\Delta V$ you can get is: $$2\,v_\infty\over 1+{r\,v_\infty^2\over\mu}$$ As you surmised, lower $v_\infty$ is good since you spend more time ...

15

In general, Gravity Assists do not reduce the amount of time, unless you are going really far out there. For instance, Galileo took 6 years to make it to Jupiter after making 3 flybys, one of Venus, two of Earth. Comparatively, New Horizons made it there in just over a year, Voyager 18 months, and Pioneer about 2 years. However, there is considerable fuel ...

15

A gravity assist (or a slingshot) is one of the many compromises in mission design. Instead of going somewhere directly you go somewhere else first and use the momentum of a planetary body to speed up your own movement thus fitting into a Delta-V budget. So the crux of the problem is that gravity assists take time. For unmanned missions, this is acceptable ...

15

Let's try and understand how gravity works in space. This is kind of key idea to understanding lots of issues in space travel and astronomy. So imagine a space probe, or rock, which is heading in from deep space, aimed almost, but not quite towards a planet. We can break that motion into two parts -- the part towards the planet and the "sideways" part. The ...

14

Hefty gravity wells can give a healthy Oberth benefit. Doing a burn deep in Neptune's well makes sense. Suggesting an Oberth maneuver near a Pluto sized object is pretty silly. Don't know if Randall Munroe knows this. Maybe that's part of his joke. Heading all the way back from the Kuiper belt to the inner solar system take 30 years. Then back out another ...

14

I've been trying to figure out where I read about this, and then realized that I didn't read it. I saw it in a video. The NASA New Horizons channel on Youtube made some short videos called "Pluto in a Minute". In the video, How Did Pluto Accelerate New Horizons?, the speaker explains how a planetary body's sphere of influence works, and then mentions that ...

14

Arthur Dent caught the basic problem with gravity assists - you can't decide where and when you can use them - you must plan your mission around opportunities for gravity assists, and that means a very complex and often very long trajectory. It takes a lot of computational power to plan. It sets a rigid launch window; the planets won't wait for new budget ...

14

Accumulating 0.1c (30000 km/s) with gravity turns alone within the bounds of Solar system isn't possible. Reason is: system escape velocity (sometimes referred to as third cosmic velocity) is about 42km/s (at Earth orbit, and the farther the lower). Once craft reaches this speed, it still can accumulate a bit more with right escape path, but generally not ...

14

The mass of the Voyager is $\approx$ 825 kg. The mass of the Jupiter is $\approx 1.9 \cdot 10^{27}$ kg. According this question, the Voyager accelerated from $\approx 10.2 \frac{km}{s}$ to $\approx 27.8 \frac{km}{s}$ by its Jupiter flyby at 1979.7.29: Thus, the Voyager got $\approx 17.6 \frac{km}{s}$ velocity from the Jupiter by its gravitational slingshot ...

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