A comet targeted for a flyby with NASA's Deep Impact spacecraft cannot be found, forcing mission planners to send the probe to a different comet. The comet may have evaded telescopes simply because its predicted orbit was incorrect, or, more intriguingly, it might have disintegrated completely.
The Deep Impact spacecraft completed its main mission in 2005, when it slammed a metal impactor into comet Tempel 1 and watched the debris fly. After the successful encounter with Tempel 1, the mission team had hoped to carry out a second rendezvous, this time with a comet called 85P/Boethin, in late 2008.
But the team now says comet Boethin is nowhere to be found, forcing them to target a different comet called Hartley 2 instead.
Comet Boethin has been spotted only twice, first when it was discovered during a close approach to the Sun in 1975, and again during a second close passage in 1986.
The comet was not seen when it was expected to approach the Sun most recently in 1997. But that is not surprising since it was behind the glare of the Sun as seen from Earth that time, says Michael A'Hearn of the University of Maryland in Baltimore, US, chief scientist for the extended mission.
In October, some of the world's most powerful telescopes, including the Very Large Telescope (VLT) array in Paranal, Chile, and the Subaru observatory in Hawaii combed the skies for the comet, but failed to see it.
It is possible that the comet was destroyed during the 1997 Sun encounter, disintegrating from the Sun's heat, A'Hearn says. But comet Boethin never comes closer to the Sun than just beyond Earth's orbit, making it unlikely to have disintegrated, he says.
"Disappearing in the sense of breaking up and dissipating is actually very rare" for such a comet, he told New Scientist. "If it disappeared, then that is fascinating in itself – only one other comet has done that in recent memory." Comet Linear-S4 disintegrated and disappeared in 2000. A comet called 73P/Schwassmann-Wachmann 3 was seen fragmenting in 2006, but it did not crumble away to invisibility.
A somewhat more likely possibility is that the comet broke into a few large chunks that are still intact but have drifted too far from the original comet's orbit to have been spotted in searches to date, A'Hearn says.
However, the most likely explanation of all is that telescopes have simply been searching in the wrong place, he says. Because the comet has been briefly spotted only twice, scientists have not been able to compute its path around the Sun very precisely.
Despite the uncertainty in comet Boethin's orbit, the Deep Impact team had selected it as a target because it would have been relatively quick and easy to get to, reducing the cost of the extended mission.
Now, NASA has decided to divert the spacecraft to Hartley 2 instead. Hartley 2 is about 1.6 kilometres across, about the same size as comet Boethin. Although it will take longer to get there – the encounter will not occur until 2010 – Hartley 2 is more active than Boethin was, which will give the spacecraft more to look at.
The spacecraft turned on its rocket engine for three minutes on 1 November, setting up for a flyby of Earth on 31 December, the first of three Earth encounters that will use our planet's gravity to adjust the spacecraft's trajectory for its new mission.
The spacecraft will pass within about 1000 kilometres of the comet. Trying to get much closer than that would risk the spacecraft getting pummelled by dust particles, which can do a lot of damage due to their high speeds, A'Hearn says.
Only a small number of comets have been seen up close. Adding another to the mix will help scientists understand better which features tend to be the same and which vary from comet to comet, he adds.
The Deep Impact spacecraft's extended mission has been named EPOXI. The spacecraft will also use its camera to watch stars with so-called transiting planets – planets known to pass in front of their parent star as seen from Earth, periodically blocking some of the starlight.
The way the starlight varies as the planet moves in front of the star could lead to the discovery of rings or moons around the known planets, and possibly result in the discovery of additional planets with masses as small as three times that of Earth.
Comets and Asteroids - Learn more in our special report.
By Mats Andersson
Mon Dec 17 14:46:28 GMT 2007
So, maybe comet Boethin was actually an alien spacecraft, which changed its trajectory to go to the equivalent of a sunny beach somewhere in the asteriods?By Lloyd
Mon Dec 17 20:57:56 GMT 2007
Doesn't it strike you as strange that NASA would choose a body to explore, one that they've only seen twice and couldn't have calculated its trajectory properly? Has planning gone out of the window here???!By Anonymous
Fri Dec 21 18:17:30 GMT 2007
You labor under a misunderstanding. They couldn't find NOT because they didn't know its orbit well (they would not have selected it as a target if they didn't). They couldn't find it because they could not even see it - it vanished. Comet nuclei - especially smaller ones - are fragile things and have a tendency to fall to bits. There are many on record that have vanished that way.All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
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