Thursday, December 6, 2012

The Long Journey of Voyager 1

Voyager 1 (artist depiction)
Mankind is about the leave the solar system. Well, sort of anyway. Voyager 1, the space probe launched by NASA over 35 years ago, has reached a point in space about 18.5 billion miles from the sun, give or take. NASA, which still monitors and communicates with the probe, announced earlier this week that Voyager 1 has entered a region of space called the “magnetic highway” a boundary area where highly charged particles from deep space interact with solar particles. This region is very close to what’s been termed the heliopause, the very outside edge of the heliosphere, which is (the heliosphere) the bubble of space where the Sun’s solar wind dominates the background particles that permeate space. The heliosphere is used by cosmologists to demarcate our solar system from interstellar space.1 One way to conceptualize the heliosphere is to think of it like the solar system’s version of Earth’s atmosphere, which encompasses us and separates us from space. The further from the Earth’s surface you get the thinner the atmosphere becomes until eventually it stops and space dominates. Same concept with the heliosphere2, the further from the Sun you get the less its radiation dominates space until eventually its influence ceases altogether.

It may actually take Voyager 1 another year or two before it technically reaches interstellar space, such is the vastness of space, but still this is a good time to reflect on the spacecraft and just how far it’s travelled.

The Flights of Voyager 1 and 2
Jupiter with moon Io and Europa as photographed by Voyager 1
By a quirk of planetary orbital dynamics, in the late 1970s and 1980s the outer planets were in a favorable alignment for a space probe to observe each one at close range (they were all on the same side of the Sun). The relative position of the planets would allow for each planet’s gravity to be used to assist in redirecting the probe onto the next planet. This alignment was realized in the late 1960s and astronomers knew that this favorable positioning wouldn’t occur again for 175 years, so time was of the essence. Fortunately NASA, in the wake of the concluded Apollo lunar missions, took advantage and developed two probes, Voyager 1 and its sister craft Voyager 2, which would be sent on close-up flybys of each planet. Each probe weighed 1,500 pounds and was instrumented to observe the planets in just about any way NASA engineers could want. NASA launched both probes in late summer 1977.3 Initially, owing to post-Apollo budget cuts the two spacecraft were only going to observe Jupiter and Saturn, and indeed that’s all Voyager 1 did. It reached the Asteroid belt three months after launch, and approached Jupiter in early 1979. At its closest approach it came nearer the Jovian “surface” than the Moon is to Earth. Among other things, the Voyager probes discovered that Jupiter had rings and that its moon Io was volcanically active. Voyager 1 then headed on to Saturn. It flew by the planet in November 1980, just 77,000 miles above Saturn’s outer atmosphere. Voyager 1 not only observed Saturn, but its moon Titan and the combined gravities of these two bodies hurled the spacecraft (as planned) toward deep space. Its primary mission was over.

I was born just before Voyager 1 reached Saturn; for all intents and purposes, the probe has been racing out of the solar system for my entire life.4 More on this below.

Neptune as seen by Voyager 2
After the success of Voyager 1, NASA decided to direct Voyager 2 to Uranus and Neptune. Voyager 2 traveled slower than Voyager 1 (it reached Jupiter shortly after Voyager 1 and Saturn about eight months after) reaching Uranus in late 1985 and finally Neptune in mid-1989. To call the missions a success would be an understatement. Along with valuable scientific data about all of the gas giants, they provided gorgeous photographs. These are just the type of results that both advance science and fire our imaginations, exciting us to further explore and learn about space.

Both probes have enough power to operate until at least 2025. After that, barring a collision with some interstellar object, they will continue on into oblivion. Both probes include a Golden Disk that presents information about Earth and mankind (including audio recordings). The chances may be infinitesimal, but maybe sometime, millions and millions of years from now and many many light-years away, some other intelligent species will find these markers of man.

The Lessons of Voyager 1 for Deep Space Travel
I've always been interested in the stark contrast between the realities of space and the fantastic ways that space travel is portrayed in science fiction. The journey of Voyager 1 illustrates this discrepancy. Voyager 1 is one of the fastest moving manmade objects. It’s currently travelling away from the sun at more than 38,000 miles per hour, that’s over 10.7 miles every second. Even at that speed it still took it 32 years to travel from Saturn to the edge of the solar system5, a distance of roughly 17.6 billion miles. The nearest star to Earth is Proxima Centauri, 4.24 light-years distant. A light-year is equivalent to about 5.87 trillion miles (light travels at about 186,000 mi/s). 4.24 light-years is a bit less than 25 trillion miles. Don't bother trying to conceptualize this distance, it's far greater than anything we humans can relate to. At the current speed of Voyager 1, it would take the probe more than 75,000 years to reach that star (and to be clear, it’s not headed towards Proxima Centauri). That’s more than 1,000 lifetimes.6

I highlight these huge numbers to show you just how inconceivable it is for man to travel to another star system. The Apollo missions used the Saturn V rocket to accelerate the lunar spacecraft to about 25,000 miles per hour (Earth’s escape velocity). This is as fast as man has ever travelled, and had the astronauts been headed to Proxima Centauri instead of the Moon, it would have taken 114,000 years. In fact had Apollo 11 been on a mission to the stars when launched in July 1969, it would be about 9.5 billion miles from Earth by now, barely half way out of the solar system. Double, triple, multiply by tenfold the speed of human spacecraft and the time to approach the nearest stars don’t get any more reasonable.

I’ve written before about the questionable purpose of human spaceflight beyond low Earth orbit. But while I think this debate is largely academic (at least in the present fiscal climate), destinations like the Moon, maybe Mars, and perhaps thinking more fancifully, some distant moon of Jupiter or Saturn are at least thinkable. The simple reality of human existence and mortality demonstrate that no one will ever leave our solar system.

The overwhelming odds are that for thousands or even millions of years (or much longer) the Voyager spacecraft (along with the Pioneer and other distant probes) will transit through interstellar space, a virtual emptiness, passing nothing of note and experiencing nothing worth remembering. That’s no trip for humans to take and no place for humans to be.



1. It worth a quick discussion of what exactly comprises the solar system: There’s the Sun at the center with all of the planets, moons, dwarf planets, asteroids, comets, and miscellaneous other space objects that orbit the Sun. Less familiar, and well beyond the orbit of Neptune is the Kuiper belt, which is like a much larger version of the Asteroid belt. Beyond that is a less cohesive collection of objects called the Scattered disc, which is where most periodic comets are believed to originate. Beyond that are the limits of the heliosphere, including the termination shock, heliosheath, heliopause, “magnetic highway” and other boundaries that mark the progressive decrease in the dominance of the Sun over surrounding space.

Beyond these traditional (and very distant) limits of the solar system there other highly scattered objects like Sedna (observed) and the Oort Cloud (hypothesized) that do/may orbit the sun over very long orbital periods.

2. This is a much simplified analogy. In reality the heliosphere is more like a combination of our atmosphere and Earth’s magnetic field, which is critical in deflecting solar radiation and is a crucial boundary separating the Earth below from space beyond.

3. Voyager 2 was actually launched two weeks before Voyager 1.

4. In 1990, Voyager 1 did take a long range picture of all the planets together (excepting Mercury and Pluto, which was still a planet then).

5. Voyager 1 picked up speed after it passed Saturn (it stole some of Saturn gravitational energy), so it left Earth slower than it’s travelling today.

6. Using the biblical three score and ten years definition of a lifetime.


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