Voyager 1 Enters Interstellar Space

I was originally planning to write about how the Syrian crisis shows up serious flaws in the international system, or else about the recent political crisis in Taiwan. But while I may write about these things later, this time I decided to write about a completely different and rather more positive news item.

One of the cooler bits of news lately was the announcement by NASA that Voyager 1 has entered interstellar space. For those who aren't familiar with it, the robotic spacecraft Voyager 1 was launched in 1977 and visited Jupiter in 1979 and Saturn in 1980, making major discoveries at those two planets (its twin, Voyager 2, also became the first and so far only spacecraft to visit Uranus and Neptune). Since then it has been heading out of the Solar System at a velocity of about 17 kilometers per second (11 miles per second), or about 3.6 AU per year (an AU or astronomical unit is the mean distance from the Earth to the Sun, or roughly 150 million kilometers). Over the past couple of years there has been debate over the indications that Voyager 1, which is moving slightly faster than Voyager 2 or the earlier but slower Pioneer 10 and 11 (though Pioneer 10 is currently still more distant than Voyager 2), had left the heliosphere, the bubble in space created by the solar wind, but in the latest announcement NASA stated that they finally have definitive evidence that Voyager 1 is now beyond the heliosphere and the boundary area known as the heliopause and has entered interstellar space.

This is a pretty amazing achievement. Voyager 1 is currently almost 19 billion kilometers from the Sun, about 125 times the distance from the Earth to the Sun and over three times the average distance of Pluto, and it is still functioning after 36 years in space. At the speed of light, it now takes signals about 17 hours to travel from Voyager 1 to the Earth, and we are still receiving them. As one article pointed out, Voyager 1 has accomplished this with far less computing power than a so-called smartphone. It is incredible to contemplate that a human-made object has traveled so far into space.

On the other hand, the Voyagers and the Pioneers (and the other, more recent spacecraft on a recent trajectory toward interstellar space, New Horizons) also serve to remind us of how huge the distances in space are. While many headlines have reported that Voyager 1 has left the solar system, this is not really accurate. As others have pointed out, while it is in interstellar space in the sense that it is beyond the solar wind, it is still well within the region gravitationally dominated by the Sun. For example, the most distant substantial object that we know of in the solar system is Sedna, a trans-Neptunian object that probably qualifies as a dwarf planet under the current classification system. Sedna is currently "only" 80-some AU away, but it is now nearing its perihelion, or closest approach to the Sun. At the most distant point in its orbit, Sedna is 937 AU or 140 billion kilometers from the Sun, something like seven times the current distance of Voyager 1. The comets in the Oort Cloud at the edge of the Sun's gravitational grip are many times further than that. So in this sense, it will be thousands of years before Voyager 1 truly leaves the solar system (a point mentioned even in one or two mainstream news articles).

Another way of looking at it is to remember that while Voyager 1 is an impressive 17 light hours away, meaning it takes light 17 hours to reach us from it at a velocity of 300,000 kilometers per second, it won't reach the distance of 1 light day (about 172 AU) for well over a decade, around the time it is expected to cease functioning completely, even if it suffers no glitches in the meantime. The nearest star other than the Sun is over 4 light years away. Even if Voyager 1 were heading directly toward it (which it isn't), it would take it tens of thousands of years to reach it. So while this milestone could be said to mark humanity's first step toward the stars, it is a very tiny step. If we really want to explore the stars, it will take a major leap forward in propulsion technology. This isn't to say we shouldn't be impressed at Voyager 1's achievement, but rather than just patting ourselves on the back, we should be inspired to go even further and faster in the future. If we make the effort, maybe by the end of this century we'll have one or more spacecraft passing the Voyagers on the way to the stars.