We have sent a probe deep into space. In 1977, two unmanned space probes, Voyager 1 and Voyager 2 were sent deep into space. Although their original mission was to study only the planetary systems of Jupiter and Saturn, Voyager 2 continued on to Uranus and Neptune. The Voyagers now explore the outer boundary of the heliosphere in interstellar space; their mission has been extended three times and they continue to transmit useful scientific data. Neither Uranus nor Neptune has been visited by a probe other than Voyager 2.
On 25 August 2012, data from Voyager 1 indicated that it had become the first man-made object to enter interstellar space, traveling “further than anyone, or anything, in history”.
As of 2013, Voyager 1 was moving with a velocity of 17 kilometers per second (11 mi/s) relative to the Sun.
On 5 November 2018, data from Voyager 2 indicated that it also had entered interstellar space.
On 4 November 2019, scientists reported that, on 5 November 2018, the Voyager 2 probe had officially reached the interstellar medium (ISM), a region of outer space beyond the influence of the Solar System, and has now joined the Voyager 1 probe which had reached the ISM earlier in 2012.
Their speeds right now, are approx. 17261 m/s. which is almost the square root of the speed of light. I hope you get how fast the speed of light actually is. Also, by travelling at the speed of light, You would travel time (theoretically).
Coming at your question, Voyager spacecrafts are actually decelerating at a rate of 0.000002592832 m/s^2. This is because after entering interstellar space, Very less forces are acting upon the probes. Also in vacuum, unless an external force is acted upon, their acceleration must be 0, but it could be that they have encountered a repelling force that has reduced their acceleration or the plutonium-238 fuel that resides in the probe is radiating in direction opposite to motion, hence decelerating the probe with respect to the sun.
If you are thinking the probes must accelerate forever, that can’t and won’t happen. This is due to 2 things, the main source of natural thrust(not engines or any sort of thrusters) in our solar system, is the solar wind. The solar wind is a stream of energized, charged particles, primarily electrons and protons, flowing outward from the Sun, through the solar system at speeds as high as 900 km/s and at a temperature of 1 million degrees (Celsius). It is made of plasma. But its affect is negligible, after we cross the Heliopause, This is due the fact that these particles lose their acceleration when they reach the edge of the sun’s gravitational field, i.e Heliopause or edge of the solar system.
I wanted to include a lot more points to this, much has not been said. Interstellar space has been made possible, but it is still unmanned. Travelling a whole light year would take the Voyager spacecraft nearly 50.000 years and it will lose contact with earth in 2025.
I hope we travel at the speed of light very soon, although we would not be around to witness it. 🙂
Infinite Physics 