September 2005 Archives

The Pioneer Anomaly

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This is one of the most interesting questions raised by the fleet of unmanned spacecrafts floating around (and outside) the Solar System. In short, the Pioneer probes are not where they should be, and nobody knows why.

The two Pioneers (10 and 11) were launched in late 1972 and early 1973 and are currently two of the most distant man-made objects in existence. Neither of them is active anymore; the last signal from Pioneer 11 arrived in 1995, and from Pioneer 10 in 2003. They are still moving away from us in more or less opposite directions: 10 is heading to Aldebaran, which it should reach in about 2 million years, and 11 will fly by the constellation of Aquila in 4 million years.

Being this far away from us and having travelled for such a long time, they are in an excellent position to show us unexpected news about the way the universe works, and they may have done just that. Their actual position over the last few years was consistently "wrong" when compared to their expected position; the error indicated that they were being subjected to a small force towards the Sun that exceeds what should be expected from gravity and other known effects.

Nobody knows what that force is, of course. It could be something as simple as a gas leak, but it would be definitely strange that this would cause exactly the same amount of force applied in the same direction in both probes (then, again, they are identical; if it's a design problem or side-effect, it's not so far fetched as it sounds). It could also be an observational error; old data is being revisited to try to rule this out.

We do have different probes that are as far away as the Pioneers: Voyagers 1 and 2 are still operating and have already left the outer-most planets behind; Voyager 1 is, in fact, farther away from the Sun than any other probe (Pioneer 10 trails it by almost 1 billion kilometres, and Voyager 2 by 3 billion). However, the Voyagers are too different from the Pioneers: their stabilization system is different, and it causes thrusters to fire quite often to keep them pointed in the right direction; these thrusters interfere with the trajectory of the ships and, thus, with the very small effect of this unknown force. Still, they seem to be affected in the same way, although data is not quite conclusive. The probes Galileo and Ulysses, which are stabilised in the same way as the Pioneers, never ventured so far away from the Sun that the effect on them could be conclusively measured.

It is expected that New Horizons, the new mission to Pluto, planned to be launched early next year and to reach Pluto in 2015, could bring some light to this issue; as the Pioneers, it is spin-stabilised and should be affected by the same forces. Whatever the reason for the anomaly turns out to be, and even if it is indeed something quite prosaic, we will certainly learn something new: either about some aspect of space flight that was previously overlooked, or about some new facet of the inner workings of the universe.

Nine, ten, eleven... how many planets?

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Funny how things come and go. Last year, when Sedna was discovered, it was announced as a possible 10th planet; however, there was also talk of not only not considering Sedna as a planet but also demoting Pluto to a more general "Kuiper belt object", leaving us with 8 planets again.

Then, earlier this year, an as-yet-unnamed object (2003 UB313, unofficially referred to as Xena or Lila) was discovered that is larger than Pluto, and considerably farther away (Sedna is smaller than Pluto and, therefore, more easily brushed aside). This is one of possibly thousands of similar objects orbiting the Sun from very, very far away. Are those objects planets?

It turns out that it is very difficult to come up with a scientific definition of planet that will include the current set of nine and nothing else. It would be easy if Pluto were demoted; then you would be able to define based on size (it's very unlikely, although not impossible, that there's any undiscovered object larger than Mercury orbiting the sun) or even on distance from the Sun (which is somewhat arbitrary, though), or you could pick the definition currently chosen by many scientists: a large body that dominates a particular orbit. Pluto does not fit (Neptune would be the dominant body in that orbit), and neither do the inner Solar System asteroids or any of the Kuiper Belt objects (no single body dominates those orbits). However, any definition that tries to include Pluto will, certainly, include many other bodies.

The current set of nine planets is a historical accident: when Pluto was discovered, it was thought to be an one-of-a-kind object, which would make it an odd planet but not much of a problem. The subsequent discovery of the Kuiper belt opened a can of worms, of which at least two jumped out so far: Sedna and "Lila". Nowadays, the working definition of "planet" is "whatever the IAU says is a planet", and the IAU is working (without much success) to come up with a ruling on the new bodies and, presumably, any new ones that pop up.

Personally, I'd like to see "Lila" accepted as a planet, but I recognize that this would bring a different problem: we could end up with hundreds of planets over the next few decades, and that would trivialize the word "planet". Nine is ok, ten is fine, eleven is all right... 137 may be a little too much. I think the easiest solution is to keep the name for the current nine planets for historical reasons (or add "Lila" as the 10th planet to recognize its discoverers), retire the word "planet" as a scientifically significant word (therefore freezing the set of planets in the actual configuration) and adopt an official naming convention based on the characteristics of the objects being named. This would probably make everyone happy without being too disruptive.

Space exploration

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There seems to be a renewed interest in space exploration, especially in big projects, lately. Or is that just my impression?

We had NASA saying that we'll have people on the moon again by 2018, which seems to fit with what Pres. Bush said a few months ago. Very interesting project, and I hope it does happen (still, 13 years is a long time to sustain funding...).

Then the August edition of IEEE Spectrum has a cover story on space elevators, making the point that now would be a good time to start building one. A space elevator could reduce the cost of putting materials in orbit from US$20,000 a kilogram to around $200. According to a presentation I found at, the budget for a single elevator is around $10 billion US dollars, already including legal costs (I can imagine the cost of liability insurance...). Meanwhile, an American private company (Liftport Group) got FAA clearance to perform tests related to building an elevator; their homepage even counts down to their expected lift-off date (12 April 2018; what is it with 2018?).

Interestingly, the NASA plans for space exploration in the late 1960s included permanent moon bases by the early 1980s, and manned missions to Mars by 1985. Even the Shuttles should have been in space by 1973, if I'm not mistaken (the first launch was in 1981). We all know what happened, of course. Even the Apollo program only really succeeded because of the Russian competition, which prompted Kennedy's challenge (put a man on the moon and bring him back by 1969); we don't have anything like that now. Let's wait and see.

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