A couple of titbits I got while perusing the New Scientist website today:
THIS is a tale of two spacecraft. Pioneer 10 was launched in 1972; Pioneer 11 a year later. By now both craft should be drifting off into deep space with no one watching. However, their trajectories have proved far too fascinating to ignore.
That's because something has been pulling - or pushing - on them, causing them to speed up. The resulting acceleration is tiny, less than a nanometre per second per second. That's equivalent to just one ten-billionth of the gravity at Earth's surface, but it is enough to have shifted Pioneer 10 some 400,000 kilometres off track. NASA lost touch with Pioneer 11 in 1995, but up to that point it was experiencing exactly the same deviation as its sister probe. So what is causing it?
Nobody knows. Some possible explanations have already been ruled out, including software errors, the solar wind or a fuel leak. If the cause is some gravitational effect, it is not one we know anything about. In fact, physicists are so completely at a loss that some have resorted to linking this mystery with other inexplicable phenomena.
Bruce Bassett of the University of Portsmouth, UK, has suggested that the Pioneer conundrum might have something to do with variations in alpha, the fine structure constant. Others have talked about it as arising from dark matter - but since we don't know what dark matter is, that doesn't help much either. "This is all so maddeningly intriguing," says Michael Martin Nieto of the Los Alamos National Laboratory. "We only have proposals, none of which has been demonstrated."
Nieto has called for a new analysis of the early trajectory data from the craft, which he says might yield fresh clues. But to get to the bottom of the problem what scientists really need is a mission designed specifically to test unusual gravitational effects in the outer reaches of the solar system. Such a probe would cost between $300 million and $500 million and could piggyback on a future mission to the outer reaches of the solar system
"An explanation will be found eventually," Nieto says. "Of course I hope it is due to new physics - how stupendous that would be. But once a physicist starts working on the basis of hope he is heading for a fall." Disappointing as it may seem, Nieto thinks the explanation for the Pioneer anomaly will eventually be found in some mundane effect, such as an unnoticed source of heat on board the craft.
And...
IF YOU travel out to the far edge of the solar system, into the frigid wastes beyond Pluto, you'll see something strange. Suddenly, after passing through the Kuiper belt, a region of space teeming with icy rocks, there's nothing.
Astronomers call this boundary the Kuiper cliff, because the density of space rocks drops off so steeply. What caused it? The only answer seems to be a 10th planet. We're not talking about Quaoar or Sedna: this is a massive object, as big as Earth or Mars, that has swept the area clean of debris.
The evidence for the existence of "Planet X" is compelling, says Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado. But although calculations show that such a body could account for the Kuiper cliff (Icarus, vol 160, p 32), no one has ever seen this fabled 10th planet.
There's a good reason for that. The Kuiper belt is just too far away for us to get a decent view. We need to get out there and have a look before we can say anything about the region. And that won't be possible for another decade, at least. NASA's New Horizons probe, which will head out to Pluto and the Kuiper belt, is scheduled for launch in January 2006. It won't reach Pluto until 2015, so if you are looking for an explanation of the vast, empty gulf of the Kuiper cliff, watch this space.
