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I say wait 20 years until we have the propulsion technology to go into orbit around Pluto and its moon, Charon.

I don't understand. We can put spacecraft into orbit around other planets. Why is Pluto different? Why can't present technology put something into orbit around, say Mars or Venus, but not Pluto? I understand Pluto is a lot further away, but I don't understand why the greater distance is relevent.

I have a guess as to why Pluto is harder to get an orbit for. It is a LOT further away than Jupiter and Saturn. It also has a much smaller mass (which naturally means its gravitational pull is very small also). It may be that since it is so very far away, the spacecraft must travel much faster to get there in a "reasonable amount of time". It will therefore need tremendous effort to get it to slow down enough to attain orbit, rather than fly by. Pluto's gravitational weakness will hamper the use of its gravity alone for this purpose. If I am not mistaken (and I may well be, I am out of the loop on scientific advances here in South America) we have never even orbited a craft around Uranus or Neptune, but did get some nifty shots of them with a Voyager fly by.

I think that it might still be worth doing even if we can only manage a fly-by type expedition--look at what the Voyagers let us see. Alternately, I doubt that it will really have to be 20 years before an orbiter is possible. If the US puts its money on a project and is allowed by its space budget to dedicate enough research to it, it could be much earlier. It took less than ten years to go from having every other rocket blow up on the launch pad to landing actual people on the moon.

The trajectory of a craft on its way towards a planet is drastically different to the trajectory of the planet itself. To go from the one to the other takes a great deal of effort.

When we're sending things to Jupiter and other massive bodies, we can take advantage of their own gravitational forces to alter the energy state of the spacecraft. With Pluto being so tiny: it's really little more than a big asteroid; its puny gravitational strength is not conducive to a good transition. It's not impossible, but it is very impractical, to orbit Pluto.

Personally, I'm not convinced by the desire to send a craft to Pluto. Very little is to be gained by investigating the outer planets. I'd much prefer to see missions much closer to home (lunar missions and Mars).

Andrew

To put something in orbit around Pluto, you have to slow it down to orbital velocity. This means we would have to send the spacecraft to Pluto at a slow speed, and it would take 20 years to get there.

To get a spacecraft there in under 10 years, it has to go very fast. Therefore, it would take a large amount of fuel to slow it to orbital velocity. The weight of the fuel would cause the whole mission to cost more.

Twenty years from now Pluto will have moved far enough away from the Sun that its tenuous atmosphere will have frozen out and snowed onto the surface, there to remain for another 120 years. Time is running out to observe Pluto's unique atmosphere, one of only five solid bodies in the known universe to have one.

The reason why it's so much more difficult to orbit Pluto is that you have to take along all the fuel you need to slow down enough to get into orbit, and in order to take that fuel, you have to push it up to tens of thousands of kilometers per hour, which takes more fuel at the outset. And the faster you go, the more fuel you need to slow down.

It's taking the Cassini probe, which is to go into orbit around Saturn in 2004 and which is travelling at 30,000 kilometers per hour with respect to the Sun, 7 years to get to Saturn even after it was launched on the most powerful booster available, the Titan IV.

Pluto is 4.47 billion kilometers away, versus Saturn's 1.47 billion kilometers, three times as far. Keeping the Galileo probe functional since its October 1989 launch and its 1995 arrival at Jupiter has been an ongoing struggle, one that is not likely to succeed much longer, and that's over the course of a mere 13 years.

It took from 1977 to 1989 for Voyager 2 to reach Neptune to fly past it at tens of thousands of kilometers per hour using a gravity assist opportunity that comes along only once in 176 years.

The upshot is that with current propulsion technology, going into orbit around Pluto would take decades, far beyond the expected lifetime of space-worthy systems, and thus miss the atmosphere. Here's the web page of the Pluto/Kupier Express Mission.

Pluto has the most eccentric (football-shaped) and inclined orbit of all of the planets. At the moment, it is actually CLOSER to the sun than Neptune, the next most distant planet in terms of average distance. This condition exists for about 40 years, or about 10% of its complete trip around the sun.

In addition, its current proximity to the orbital plane potentially allows a gravity-assisted path that could considerably shorten the travel time. This method has been used for other interplanetary missions several times in the past, with great success. The spacecraft is routed near one of the other planets, close enough that the planet attracts - and accelerates - it, cutting significant time off of the total time. Less time also means less exposure to failure.

These are not definitive arguments, but they are real.