Image: Voyager 1’s cameras were turned back on in early 1990 to take pictures of our Solar System. The spacecraft took some 60 pictures of the Sun and 6 of the planets; the 60 frames were combined to make the mosaic seen above. The six individual shots were taken when Voyager 1 was more than 4 billion miles from Earth. Earth appears framed in brightness due to the amount of light scattered while taking the picture with Earth so close to the Sun. Credit: NASA/Caltech.

Looking Back from Deep Space


It’s reasonable to call the two Voyager spacecraft our first interstellar probes, in the sense that they are approaching the heliopause and are still transmitting data. Long before controllers shut them down — which should occur somewhere in the 2020s — Voyager 1 will have left the Solar System and we’ll have data on what happens when the solar wind gives way to the stellar winds from beyond. A case could be made for the Pioneer craft as interstellar probes as well, but while Pioneer 10 has reached a distance of 107 AU, the Pioneers are no longer transmitting data. Voyager 1 is now 123.45 AU out, for a round-trip light time of 34 hours, 15 minutes.

But does leaving the Solar System mean we’ve truly entered interstellar space? An entertaining piece called Postcards from the edge, published in early February by The Economist, notes that much depends on how we define ‘interstellar.’ Gravity, says its author, defines the universe at the largest scales, and if we’re talking about gravity, Voyager is still deeply in the grip of the Sun. In fact, Voyager 1 would have to travel another 14,000 years to reach the roughly 50,000 AU distance where the Sun’s gravity would cease to be a factor.

Read more: Looking Back from Deep Space — Centauri Dreams.

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