When a space probe passes a planet, it is said to either speed up or slow down. Explain why the difference.?

Question

If you follow space news, you sometimes hear where an object (space probe or maybe an asteroid) passes a planet. Sometimes it is said that the passing object speeds up and is flung out. Other times, it is slowed down by the encounter and may even end up captured by it. In laymans terms, when could an object be expected to speed up and when could it be expected to slow down?

Answer 1

As an object approaches a planet it is pulled toward the planet at an ever-increasing speed by the planets gravity. As the object passes the planet and moves away from the planet, the gravity of the planet is still pulling on the object, ergo it begins to lose some of it's speed but still departs the area of the planet (called "escape velocity"). Sometimes the object may not gain enough speed, as it is approaching the planet, to escape the pull of the planets gravity and will either be pulled into orbit around the planet or actually crash into the planet.

Answer 2

The most common example is a space probe in orbit around the Sun and it has an elliptical orbit that just reaches Jupiter when the probe is farthest from the Sun. Jupiter, being in a near circular orbit around the Sun, will be moving around the Sun faster than the probe and so will catch up to the probe. The probe will execute a near-parabolic orbit around Jupiter and will leave Jupiter in the direction opposite to what it had when Jupiter approached it. Hence, relative to the Sun, the probe will now be moving in the same direction as Jupiter, but *faster* than Jupiter. The new orbit will now extend much farther out from the Sun than Jupiter is, and can perhaps escape the Sun's gravity altogether!

Similarly, if the probe has an orbit around the Sun that just reaches Venus when the probe is closest to the sun; the probe will be traveling faster than Venus and, and, after Venus flings it around, it will be traveling slower than Venus and can then perhaps fall into an orbit that can intercept Mercury's orbit, just as the Messenger Mercury probe did last year.

In a classic two-body problem, a planet cannot capture a probe that does a flyby since there is no way that the planet can extract energy from the probe's orbit. However, if an extended object such as an asteroid or comet passes very close to a planet such as Jupiter, then the asteroid or comet will be distorted by tidal forces from the planet; this will create internal heat in the asteroid or comet that *does* extract energy from the orbit, so it is possible for such an extended object to be captured. This apparently happened to comet Shoemaker-Levy when it passed close to Jupiter in 1992; it was captured into a close orbit to Jupiter and impacted Jupiter in 1994.

Answer 3

Gravity can be used to effect the speed and path of a spacecraft as it nears, and passes a planet. It all depends on the trajectory of the object as it encounters the gravitational pull of a planet. Get it just right,and the effect is similar to the game of "crack the whip". The object is held by gravity ,making it curve around the planet. As it does that,it's speed increases. As it leaves the pull of gravity,the object is hurled off into space at a greater speed,and in a new direction. That is one way scientists can send a spacecraft so far into the solar system. If the object approaches the planet at any other trajectory,then the effect of gravity will slow it down. At that point the object can either crash to the surface,or remain in orbit.

Answer 4

The only way to give a truly satisfactory answer to your question is to go through the mathematical details of the orbital mechanics of a specific encounter. I remember a classical mechanics book by Barger and Olsson that had a good example in it.

I know what you are talking about, because the Voyager spacecraft got gravitational slingshot boosts to get to the outer planets, while probes to the inner planets need energy removed from them so they can fall into an orbit closer to the sun. I forget the name of the one that needed multiple encounters with Venus and Earth to do this -- was it Magellan?

Answer 5

Go in your backyard with a friend. Ask the friend to run in circles around you - then grab hands briefly and rotate with him/her for a few revolutions - then let go - he/she will fly off.

This is centripedal acceleration.

When an object having sufficient velocity not to get captured by a planets gravity comes in proximity with that planet, the gravity briefly 'grabs' hold and swings the object around --- speeding it up ---- but is incapable of holding and 'flings' it away on a new vector at an accelerated velocity.

Other times, a space probe travelling at a lower velocity, or vectored to 'aerobrake' by friction with the upper atmosphere of a planetary body (such as Mars) will be slowed down and captured.

Thankfully, physicists and rocket scientists have figured out how to do this - for these reasons we have been able to explore the outer solar system to some extent. Sadly, our simple chemical rocket technology is insufficient without these assists to reach much farther than beyond the orbit of Mars around the sun. We need to play 'planetary' billiards (banking the shot for Jupiter off from interaction with Venus) to explore the further reaches of our solar system.

Hopefully in a better future, America (or more probably China, India, Japan ) will embrace space exploration and new methods of propulsion such as ion power, solar sails, ram gun acceleration, and maybe even antimatter propulsion to continue the manifest destiny of our species.

Great question and...

KEEP STUDYING SCIENCE!!!!! Science is one of the greatests hopes for the future of America and humanity. Sadly, ignorant religious zealots who have seized the government and culture of the United States have facilitated our demise as a world power since we no longer have the talent of homegrown engineers such as those that made the Manhattan project, Apollo program and semiconductor revolution possible. In part, the election of ignorant anti-science politicians, such as George W. Bush, is responsible for our downfall. He is the worst President in modern American history.