how does space vehicles travels outside solar system?

Law of physics:

"An object in motion tends to stay in motion".

In space, where there is no air and nothing to cause friction, a space ship will STAY MOVING until it hits something. But without fuel, it can not change direction, adn it cannot accelerate (or decelerate)! it will continue to travel in a straight line.

Solar power provides ONLY electricity to power computers and such. Solar power DOES NOT provide any FUEL needed for changing course or changing speed.

You can use the "slingshot effect" to let gravity help speed up a space ship, but once the maneuver is done, there is no more effect from gravity -- until you come near to ANOTHER sun in another solar system!

So, without fuel, solar power, or gravity, there is NO CHANCE of accelerating the space craft any greater.

Right now, there are FIVE space craft that are on path to leaving our solar system -- Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizon. Pioneer 10 and 11 are DEAD -- they no longer function. Voyager 1 and 2, and New Horizon, are still functioning -- a little bit.

New Horizons has not yet left the solar system, but is on path to do so.

You can find more information here:

heavens-above.com/solar-escape.asp

If we were to use gravity or solar energy as propulsion for an interstellar spaceship, then no it would not be able to 'accelerate' whilst in the void between stars (although I'm unsure at what distances either means would become insignificant, especially as inertia needs to be factored). But it will already have built up a high speed and will maintain that speed as there will be nothing to slow it down...I think.

As for gas fuel, or similar methods of propulsion, then yes it would still accelerate. But given the distance to the edge of the solar system to do so would require unrealistic amounts of fuel, unless of course you use the other ways whilst still in the solar system.

The "solar sail" idea has been on the table for decades, but hasn't ever been put to use for a practical spacecraft means of propulsion.

All spacecraft that have ever been launched use chemical rockets. They carry fuel with them as payload, and react the propellants to get a gaseous exhaust, and use conservation of momentum to have the remaining payload alter the course as needed.

The solar panels on spacecraft exist do obtain electrical energy to operate the onboard equipment, not to do any propulsion. And for the record, photovoltaics on spacecraft barely can operate well enough on Mars. Most spacecraft sent to the outer solar system carry with them the means of supplying on-board equipment.

When sending probes to other planets of the solar system, the methods used as energy saving tricks are Hohmann transfer orbit and gravitational slingshot.

Once outside the solar system, with no opposing force to the spacecraft drifting away...the spacecraft just keeps drifting as per the property of inertia. An object in motion remains in its state of motion.

There are only four spacecraft outside the solar system, and they are Voyager 1 & 2, and Pioneer 1&2. They have pretty much completed their goals of photographing the objects on the journey...now they are just coasting outward, symbolically representing the farthest pieces of manmade equipment ever to exist. There really isn't a reason to propel these probes anywhere...just let them coast.

The satellites that have gone out of our solar system, only had enough fuel to use the guiding thrusters.

As for the solar wind, the craft dod not have any type of solar sail, and if they had one it would have had to been larger than a foot ball field, and any thing using a solar sail can only travel in a straight line out from the sun.

The way NASA increases the speed of the craft was by using the sling shot effect, the craft was blasted out into space away from from the earth, then the craft was guided back toward the earth so the earths gravity would increase the crafts speed, it came back past the earth I believe it was three times before flew out toward jupiter, and as jupiter's gravity pulled the craft toward it the craft gained more speed, then it was sling shot out toward saturn, saturn gravity was used to in cease the crafts speed even faster, then the craft was sling shot out away from saturn at even a greater speed, from there on to the outer reaches beyond the solar system the craft was moving fast enough to travel to who knows where.

Since there is nothing to stop it or slow it down in the vacuum of space, the spacecraft would keep on going. Fuel is only needed to alter its speed or direction.

Without the Sun to rely on for energy, a spacecraft can use thermoelectric generators that use heat from radioactive material.

"An object in motion will continue in motion unless acted on by an external force" (Newton's First Law)

Once it has the escape velocity to leave the solar system, it will keep on coasting out of the solar system. It doesn't require fuel to do so. Yes, it would require fuel to accelerate, but it doesn't have any need to accelerate, just as asteroids and planets don't require fuel to coast along their paths.

Solar power is used by spacecraft to power instruments. Since they're ours, we like them to talk to us, not just drift. So we want them to continue collecting data, running their onboard computers, and sending us radio messages. That takes power. But maneuvering takes fuel, and satellites have very little extra carrying capacity. So they don't do very much maneuvering far away from earth.

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With no friction in a vacuum a vehicle can travel in theory forever with one push. Minute gravity pulls may slow or divert it.

It needs no energy whatsoever to keep right on going