In this same vein, I've always thought that Titan would be much easier to colonize...if it weren't so darn far away. Maybe if there's a next leap in propulsion or something.

New Horizon spacecraft launch from Earth with sun escape velocity:

"New Horizons is a space probe launched by NASA on 19 January 2006 to study the dwarf planet Pluto ....

it flew by the orbit of Mars on 7 April 2006, Jupiter on 28 February 2007, the orbit of Saturn on 8 June 2008; and the orbit of Uranus on 18 March 2011"

http://en.wikipedia.org/wiki/New_HorizonsSo from Earth it took it 2 1/2 year to get to Saturn orbit.

If same New Horizon launch vehicle had launched instead from planet Mercury instead of Earth, it could got to Saturn in less than 1/2 the time.

Due to number of reasons.

One reason is from planet Mercury, there is lower sun escape velocity needed than compared to Earth.

But this is not only reason. But I will go over this.

So let look three sun escape of Earth, Mercury, and Mars. Wiki:

Earth: 42.1 km/sec

Mars: 34.1 km/sec

Mercury: 67.7 km/sec

Earth has orbital velocity of:

"Mean orbital velocity (km/s) 29.78

Max. orbital velocity (km/s) 30.29

Min. orbital velocity (km/s) 29.29"

So to reach Earth escape, one adds earth's orbital vehicle to delta-v of rocket. 42.1 minus 30 is 12.1.

So one needs 12.1 km/sec of delta-v added to Earth's orbital velocity.

Mars has orbital velocity of:

Mean orbital velocity (km/s): 24.13

Max. orbital velocity (km/s): 26.50

Min. orbital velocity (km/s) : 21.97

So Mars solar escape is 34.1 km/sec, minus 26 or 8.1 km/sec of delta-v needed.

Mercury has orbital velocity of:

Mean orbital velocity (km/s): 47.87

Max. orbital velocity (km/s): 58.98

Min. orbital velocity (km/s) : 38.86

So Mercury solar escape is 67.7 km/sec, minus 58 or 9.7 km/sec of delta-v needed.

In order, delta-v needed is 12.1 km/sec {Earth}, 8.1 km/sec {Mars}, and 9.7 km/sec {Mercury}.

So Mars has lowest velocity needed to do sun escape, and it's nearer in distance to Saturn [or

Jupiter], but with sun escape velocity, one get to Saturn [or Jupiter] faster from Mercury as compared

to Mars or Earth.

New Horizon left Earth on 19 January 2006 and "flew by the orbit of Mars on 7 April 2006". So in 78 days

it travel from Earth distance to Mars distance.

This rock:

http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2007%20PR25;orb=1 Went from Mercury distance to Earth distance, starting March 15 2013 at Mercury and going to Earth distance on April 25. So 41 days. Then traveled to Mars distance on June 2 2013. So another 38 days.

In total it took 79 days from Mercury to Mars. But this rock has velocity of less that sun escape.

So with sun escape from Mercury not only would get Jupiter quicker than from Earth escape, but cross Mars

orbit, quicker than from Earth sun escape.

Next we look difference: Earth needs 12.1 km/sec delta-v added. Mercury needs 9.7 km/sec.

If use same rocket at Mercury, than 12.1 minus 9.7 is 2.4 km/sec "extra". Or can add 2.4 to the escape velocity, thereby going faster the Mercury sun escape.

What does this mean?

Well if look at the space rock as goes out from Mercury it's slowing down as reaches Earth and Mars distance. With Mercury Sun escape velocity, it slows down less than what this rock does. And with velocity

over sun escape, it slows down less than the sun escape.

And added velocity has greater effect the further you get from the sun.

Just guessing, but compared rock getting to Earth distance from Mercury, if going sun escape, instead of 41 days, it could be say, +30 days [or save 10 days] but difference time to Mars would greater, instead 38 days, could closer to 2 weeks [save 14 days]. But such shorten of time is more dramatic at Saturn or Jupiter distance, when go slightly above Mercury escape velocity.

So we difference of 2.4 km/sec, or 2.4 km above Mercury sun escape.

Another factor is leaving earth one gets gravity losses. So Mercury has lower gravity and no atmosphere.

Or with Earth one lose about 2 km/sec leaving Earth. Leaving Mercury with such rocket would have number

near zero. In addition the rocket engine performs better in vacuum. Or rocket that gets 10 km/sec leaving Earth get more if rocket engines are functioning in vacuum.

We are not even including the changes one do to the rocket's design so performs better. In other words

we using rocket designed for launching from Earth, and same rocket on Mercury. That rocket will simply perform a lot better launched from Mercury.

Let's go into the weeds a bit:

Altas V First Stage:

Thrust at Sea Level 3,827kN

Thrust (Vacuum) 4,152kN

http://www.spaceflight101.com/atlas-v-401.htmlSo 10% more thrust in vacuum.

Or in terms of ISP:

"Thrust at Sea Level, 3,827kN. Isp SL, 311s. Thrust (Vacuum), 4,152kN (933,369 lbf). Isp Vac, 338s"

And a launch of rocket would "look" very different. It's starting thrust instead being say 150% of the weight of rocket. So on earth 1000 ton rocket has 1500 tons of thrust. On Mercury the it's 333 tons and more 1500 tons of thrust. So it jumps off the launch pad. It would be very dissimilar looking than the Earth

rocket launch.

Plus there are "details" such as from Oberth effect.

But one is added about 6 km/sec above Mercury sun escape by rocket not designed to function from

Mercury.

Edit: So said Mercury sun escape get to Saturn less than 1/2 time of Earth sun escape. I thought

provide rough answer of how much it is less than 1/2 of the time. And I use the Cassini spacecraft to illustrate it. It should noted that Cassini would be slower than Mercury sun escape trajectory. But gets to

~~Saturn~~ edit: Jupiter [and Mars orbital distance] faster than New Horizon earth escape.

So we start from point Cassini crosses Earth orbital distance [it's on way to Saturn, after spend time in inner system getting gravity assist, and gets last gravity assist off Earth, on August 18th 1999, and then 39 days later cross Mars orbit [September 26 1999]. Mercury sun escape would do this in about 1/2 the time, but it's about twice as fast as New Horizon.

Then going past Mars, Cassini goes to Jupiter [where does another gravity assist], and crosses Jupiter

on 2001 Jan 5. So about 6 months from Earth to Jupiter. And Mercury sun escape would same distant is

less than 3 months. Or from beginning at Mercury distance [rather crossing Earth] less than 4 months

to Jupiter distance.

Screen shot of simulator of Cassini, 2001 Jan 5:

http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=-82&vbody=1001&month=1&day=5&year=2001&hour=00&minute=00&rfov=60&fovmul=-1&bfov=20&porbs=1&showsc=1&showac=1 Or go here put in what you want:

http://space.jpl.nasa.gov/And take until 2004 June 26 to cross Saturn's outer moon, Iapetus.

And 4 days later to cross Titan's orbital distance. And a day later to cross closest to Saturn.

Or 2004 July 1 to cross Saturn's orbit.

So Cassini gets to Jupiter pretty quick, but enter Saturn orbit it requires a lot more time, and not

just because Saturn's distance.

Cassini gets to Juptier twice as fast at New Horizon "New Horizons received a Jupiter gravity assist with a closest approach at 05:43:40 UTC on 28 February 2007 when it was 2.3 million kilometres (1.4 million miles) from the planet." -wiki. Or slightly over year.

But flew by Saturn's orbital distance on June 10 2008. From Jupiter arrived much faster to Saturn distance.

Or from Mercury sun escape [or faster] you get to Jupiter in less than 4 months, and flyby Saturn in less than 4 month, but if land on Titan, the leg from Jupiter should be slower [and/or less straight] . Which can do with gravity assist using Jupiter. And/or braking at Saturn, atmospheric braking of Saturn and/or Titan moon.

And same applies if want go to say Jupiter's moon Europa. Or getting to Europa from Mercury in less than 4 month would be somewhat hard, and involve gravity assist with say Earth, and/or aerobraking with Jupiter. Of course Jupiter is wonderful for gravity assists.

Or from Mercury you don't need gravity assists to get out to outer planets, but to land or go into orbit they are more or less required.