"Future destinations in our solar system neighborhood include potential probe missions to a few moons of Jupiter, Saturn, and Neptune -- mainly by virtue of them being possible candidates for life, with their large oceans buried beneath icy crusts, plus intense volcanic activity. But getting humans to explore these possibly habitable worlds is a big issue in space travel. The record for the fastest-ever human spaceflight was set by the Apollo 10 crew as they gravita­tionally slingshotted around the Moon on their way back to Earth in May 1969. They hit a top speed of 39,897 kilo­meters per hour (24,791 miles per hour); at that speed you could make it from New York to Sydney and back in under one hour. Although that sounds fast, we've since recorded un-crewed space probes reaching much higher speeds, with the crown currently held by NASA's Juno probe, which, when it entered orbit around Jupiter, was traveling at 266,000 kilometers per hour (165,000 miles per hour). To put this into perspective, it took the Apollo 10 mission four days to reach the Moon; Opportunity took eight months to get to Mars; and Juno took five years to reach Jupiter. The distances in our solar system with our current spaceflight technology make planning for long-term crewed explora­tion missions extremely difficult.

"So, will we ever explore beyond the edge of the solar system itself? The NASA Voyager 1 and 2 spacecraft were launched back in 1977 with extended flyby missions to the outer gas giant planets of Jupiter and Saturn. Voyager 2 even had flyby encounters with Uranus and Neptune -- it's the only probe ever to have visited these two planets.

"The detailed images you see of Uranus and Neptune were all taken by Voyager 2. Its final flyby of Neptune was in October 1989, and since then, it has been traveling ever farther from the Sun, to the far reaches of the solar sys­tem, communicating the properties of the space around it with Earth the entire time. In February 2019, Voyager 2 reported a massive drop off in the number of solar wind particles it was detecting and a huge jump in cosmic ray particles from outer space. At that point, it had finally left the solar system, forty-one years and five months after being launched from Earth.

"Voyager 1 was the first craft to leave the solar system in August 2012, and it is now the most distant synthetic object from Earth at roughly 21.5 billion kilometers (13.5 billion miles) away. Voyager 2 is ever so slightly closer to us at 18 billion kilometers (11 billion miles) away. Although we may ultimately lose contact with the Voyager probes, they will continue to move ever farther away from the Sun with nothing to slow them down or impede them. For this reason, both Voyager crafts carry a recording of sounds from Earth, including greetings in fifty-five differ­ent languages, music styles from around the world, and sounds from nature -- just in case intelligent life forms happen upon the probes in the far distant future when the future of humanity is unknown. "The next nearest star to the Sun is Alpha Centauri at an epic 39,923,400,000,000 kilometers (that's almost 25 trillion miles) away.

"It takes light a little over four years to travel that dis­tance at 299,792 kilometers per second (186,282 miles per second). At the more realistic speed of Voyager 1, it would take the probe around seventy-four thousand years to reach Alpha Centauri, except that it's not heading anywhere near there. Instead, it's heading in the direction of the constellation of Ophiuchus, so that in around forty thou­sand years, Voyager 1 will come within 16 billion kilome­ters (10 billion miles) of a star in the constellation of Ursa Minor, and its closest star will no longer be the Sun, which gave life to all the recordings it carries.

"Unless we manage to come up with another, more efficient, and faster way of powering spacecraft, it's not going to be an easy task to get a human being to abandon their friends and family by signing up for a one-way trip to the far reaches of the solar system and beyond. If we ever do want to explore beyond the safety of our Sun's gravi­tational sway with interstellar travel, we're going to need better technology and some extremely intrepid explorers. I wonder, reader, if you were given the chance to go today, would you?"