发信人: ZZGR (闲逛), 信区: Aviation [删除]
标 题: [NS]Space probe targets third comet after second vanishes
发信站: BBS 未名空间站 (Tue Dec 18 10:52:26 2007)
* 10:23 17 December 2007
* NewScientist.com news service
* David Shiga
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
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
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.
Pummelled by dust
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'
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.
※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 155.198.]