In 2013 the spacecraft Voyager 1 left the solar system, moving at around 62,000 km/h. In the last few weeks, Voyager 2 followed. They bear messages to any alien civilizations that might find them, including recordings of sounds of Earth. However, space is so huge it is unlikely anyone will come across them.
Launched in 1977, the two spacecraft were intended to give us close views of the outer planets. Back then, we did not have the technology to slow the spacecraft down to go into orbit around those planets. Instead they just shot past at high speed, taking pictures and making other observations as they went. As seems usual these days, the results that came back from those robot explorers have forced us to discard a lot of our favourite theories about the workings of the outer solar system.
When we fly outward from Earth, we eventually reach the magnetopause, where our planet’s magnetic field ends and we enter the realm of the solar wind. There is a definite point where we leave “Near-Earth space” and enter interplanetary space. The solar wind flows outward past all the planets, with most of them having their own backyards, enclosed by their magnetic barriers. Eventually, the solar wind meets the magnetic field of our galaxy, the Milky Way. The meeting point is called the heliopause. Inside the heliopause, we are in the solar system, beyond that we are in interstellar space. The solar system is our cosmic backyard, and we are now venturing over the back fence into interstellar space.
This brings us face-to-face with unimaginably huge distances and spans of time. Light takes about 100,000 years to travel from one side of our galaxy to the other, so we say it has a diameter of 100,000 light years. Light takes about eight minutes to travel from the Sun to us. The solar system is less than a light day in diameter, and it took the Voyager 1 spacecraft since 1977 to reach the heliopause. The nearest star after the Sun lies 4.3 light years away. If Voyager 1 were heading in the right direction, it would take around 80,000 years to reach it. Images from the Hubble Space Telescope show millions of galaxies, extending out billions of light years from us. To explore even our own galaxy would require spacecraft able to travel at huge speeds, far faster than anything we can achieve now. This brings us to Mother Nature’s speed limit. Albert Einstein showed, and experiments have proved him right, that any material object cannot travel faster than the speed of light. At that speed it would still take 4.3 years to get to the nearest star. However, as we get closer and closer to the speed of light, funny things happen to time. We could return from our trip to the nearest star, after a two-way journey time of 9-10 years, depending on how long we spent exploring, and find that centuries or millennia had passed on Earth. To all intents, we would have to regard the trip as one-way, because when we would get back, everything we were familiar with would be long gone. In science fiction, this problem has been “solved” with ideas like “warp drive”, “jumps through hyperspace” and so on. The latest work in physics suggests such things might well be possible. However, at the moment, we have no ideas as to how we can make them happen.
One stunning thing we have found from our space probes out there exploring the solar system is how long they have continued to work. Probes have been sending us back data after decades or more. One explanation for this might be that those robots are well beyond the reach of engineers and scientists who have bright ideas as to how to tweak those devices to make them “work better”.
Since this is my last article for 2018, I am taking this opportunity to wish you a wonderful Christmas and a Happy New Year.
Mars lies in the south after dark. Venus shines low in the dawn glow, with Mercury and Jupiter below it. The Moon will reach Last Quarter on the 29th
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton. E-mail: firstname.lastname@example.org