"The space probe Voyager 1 left the planet in 1977. Any month, day, minute, second now it will enter interstellar space and become the furthest-reaching man-made object, and the first to leave the heliosphere. This will be one of the biggest moments in scientific history and we will never know exactly when it happened. Three things would signify that Voyager 1 had crossed the border of the heliopause: an increase in galactic cosmic rays, reversal of the direction of the magnetic field, and a decrease in the temperature of charged particles. Voyager 1 reports show a 25 percent increase per month of cosmic rays. But its signals take 17 hours to travel back to Earth at the speed of light."