Voyager and the Limits of Exploration

Monday, June 28, 2010, marked the 12,000th day of the Voyager 2 mission. Since 1977, Voyager 2 (along with its companion Voyager 1) has been exploring the solar system and pushing the envelope of the extent of our exploration of the universe. How far these probes have gone, and how far humanity has ventured away from Earth (albeit indirectly), are intertwined mesofacts. And the current status is that Voyager 2 is 14 billion kilometers from the sun, and Voyager 1 is more than 17 billion kilometers from the sun. Of course, these are both less than two-tenths of a percent of a single light-year, meaning that we have our exploration cut out for us. In addition, the Voyager probes are not actually the farthest probes; Pioneers 10 and 11 hold that distinction. However, they are no longer operational, and so while they continue to move through space, they no longer actively explore.

An interesting side-note: I learned of this milestone courtesy of Voyager 2’s Twitter feed, @Voyager2, which was described as follows:

12,000 days since launch, & still going strong. Thank you, to all who designed me, put me together, talk to me, & keep me going to this day.”

I can only imagine that this is a subtle allusion to the ancient Jewish prayer known as the Shehecheyanu, which is traditionally said to celebrate special occasions. It is translated as follows:

“Blessed are You, Lord our God, Ruler of the universe, who has granted us life, sustained us, and enabled us to reach this occasion.”

For further reading about the Voyager probes, I recommend checking out Todd Sieling’s wonderful paean to the Voyager mission, showing its effect on a single person.

(reposted from

3 Responses to “Voyager and the Limits of Exploration”

  1. Indy July 20, 2010 at 1:22 pm Permalink

    Voyager 1 is indeed the farthest probe. The NASA page you link says, “On 17 February 1998, Voyager 1’s heliocentric radial distance equaled Pioneer 10 at 69.4 AU and thereafter exceeded Pioneer 10 at the rate of 1.02 AU per year.” Voyager is now at 113.2 AU, and Pioneer-10 at slightly more than 100.

    Despite being 29 years older than New Horizons, Voyager I is still the probe with the fastest solar-system escape speed, at 17,000 meters per second (vs NH at 13 km/sec), because it had the advantage of a rare and fortuitous alignment of the planets. Since then, there has been zero progress in this regard, and thus, in three decades, we’ve gotten no closer to accelerating a probe into interstellar space anywhere near as fast as Voyager, let alone faster.

    Now, the nearest sun-like star, Alpha-Centauri, is 275,000 AU away. Voyager would take 76,400 years to get there if it were heading that way. So, even if by some miracle we could make a craft go 8 times faster (and we have failed any improvement in a full generation), it would still take almost a millennium to get to the our nearest neighbor. No RTG power source, even in dormancy, could last that long.

    If we made 10% improvements every year, it would take a century to have something that could get to a-centauri in a decade. We had to endow Voyager-I with 200 Gigajoules of kinetic energy to get it up to its slow speed. Something at only 10% the speed of light would require three million times as much.

    Actually, it would require much much more, because you have to accelerate the fuel that is going to be used for later thrust. Solve the rocket equation for a speed of 0.1 c, and you’ll see that the order of magnitude of the energy requirements is far beyond prohibitive.

    And for what? Only for pictures that would look remarkably like those already taken of our own sun by SOHO and other solar probes. No real major scientific merit or public excitement of a great human achievement value. And what of farther objects?

    In other words – we are trapped by natural law to the domain of our solar system and true meaningful interstellar travel (even by robotic probe) is a practical impossibility. We will only ever be able to look at the cosmos from out distant vantage point. One must engage in a kind of magical thinking about violating the current understanding of Physics to maintain hopefulness that the problem will somehow ever be overcome.

  2. Samuel Arbesman July 20, 2010 at 1:33 pm Permalink

    Thanks for the correction! Someone over at noted that Voyager 1 is indeed the farthest probe. Thanks for this.

  3. Indy July 20, 2010 at 7:58 pm Permalink

    I showed my comment to a friend and they wanted to know what the “rocket equation” bit meant. I hope you don’t mind if I add just a little bit here to explain.

    To a first order approximation, you can solve the equation of the final velocity of a probe Vp, by knowing 1. The velocity of the exhaust gas Vx, and 2. The ratio of rocket fuel weight to probe weight: r. The equation is logarithmic as follows:

    Vp = Vx * ln (1+r)

    For liquid bi-propellants, Vx is about 4500 m/s. Voyager I is going about 17000 m/s so, using this equation (not exactly accurate, but close enough), r would be about 43. So, Voyager weighs 700 kilograms, and it would have take 43 times more rocket fuel – about 30 tonnes, to get it up to speed (ignoring gravity).

    So, the logarithm kills us here – look at this table of multiples of Voyager’s speed (MV) vs. multiples of Voyager’s fuel-to-probe ratio (MR).

    MV MR
    1 1
    2 44
    4 85,000
    8 310 Billion
    16 4 Trillion Trillion

    And that 16 times Voyager’s velocity? The one that needs 200 times the mass of the earth in chemical rocket fuel? That’s still only a single *thousandth* the speed of light.

    So that’s clearly out as an option.