August 2006 Archives

European probe to crash on the Moon

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And on purpose. SMART-1 will impact on the Lunar surface at some point on the 2nd or 3rd of September, ending its mission in a spectacular way. The impact will likely be visible from the Earth even to amateur astronomers, and the ESA is calling for people to help with observations.

The expected impact is a 05:41 UTC on 03/09; that is 15:41 in Eastern Australia, which means that the impact will not be visible here (the moon will be visible, but the impact will be lost in the daylight). It is, however, possible that the impact will happen in the previous orbit, at 00:36 UTC, or even the one before, at 19:33 UTC on 02/09 (or, conceivably, the one after, at 10:43 UTC, which is 20:43 AEST). This page has visibility charts for the possible impact times; the impact will happen on the "night" side of the Moon, which will make it much easier to spot from the Earth. It will not be visible with the naked eyes from anywhere on Earth, though.

Atlantis not rolled back after all

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NASA decided not to roll Atlantis back to the VAB after all, and it will remain on the launch pad during tropical storm Ernesto. The decision was taken because Ernesto proved to be not as strong as expected, and NASA stopped the rollback midway through and sent Atlantis back to the original position.

NASA is currenly expecting to launch Atlantis between Sept. 6th and 8th, depending on the effects of the storm.

Supernova caught in the act

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Astronomers reported in Nature that a supernova was caught on camera as it happened for the first time. This was possible because the blast was preceded by a somewhat long gamma ray burst that acted as an "early warning system" of sorts.

Supernova 2006aj (in the constellation Aries) also gives us the first confirmation that supernova blasts do have early signals that can be detected so that the explosion can be predicted and its progress followed; previously, supernovas were only detected visually, and that only became possible several days after the explosion happened.

STS-115 delayed

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The untimely arrival of tropical storm Ernesto forced NASA to delay the launch of Space Shuttle Atlantis. There is no new date set for the launch, as the shuttle is actually being rolled back into the Vehicle Assembly Building to ensure it is safe from the weather.

The current launch window lasts until 13 September, but NASA wants to send the shuttle up before the 7th because a Russian supply rocket is also scheduled to go up to the ISS. Since the rollback causes a very long delay (at least eight days after it is moved back to the launch pad), this may not be possible, and the next lauch window is only in October (but, then again, the Russian Soyuz might be delayed to make way for the shuttle).


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The whole Pluto story has been all over the blogosphere and the regular media, so I didn't talk about it much. In the end, I liked that the IAU reached a decision (disputed as it may be), even though I actually preferred the option that was announced the week before (12 planets, not 8).

The Australian "Cosmos" magazine has a good editorial online about why Pluto had to go; and, to be honest, I do hope this subject dies quickly and that people get it over with.

No, it's eight

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Straight from the "mouth" of the IAU:


The IAU therefore resolves that "planets" and other bodies in our Solar
System be defined into three distinct categories in the following way:

(1) A "planet"1 is a celestial body that (a) is in
orbit around the Sun, (b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape, and (c) has cleared the neighbourhood around its

(2) A "dwarf planet" is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its self-gravity to overcome rigid
body forces so that it assumes a hydrostatic equilibrium (nearly round)
shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Dark matter directly "observed"

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NASA has announced yesterday that it has found direct evidence of dark matter in the collision of two large galaxy clusters. This was possible because dark matter does not interact with "normal" matter or with itself in any way other than by gravity, while normal matter is also affected by friction; because of that, "normal" and dark matter were separated during the collision and the gravity-lensing effects of both could be independently measured.

This gives a serious push to the theory that most of the matter of the universe is actually dark matter; the theory explains many observations that would not be compatible with the way we understand gravity to work if only "normal" matter were involved. Other competing theories exist, but none of them explains the observed effects from the galactic collision observed by the Chandra X-ray Observatory. That's not to say that none will show up in the future, but this does add one more hurdle to competing theories.

STS-115 launch date set

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NASA has decided to launch the Space Shuttle Atlantis on 27 August at 4:30pm (Eastern US time; that's 28 August 8:30am Melbourne time), on mission STS-115. The main purpose of this mission is to restart the construction of the ISS by installing a new truss that will include new solar arrays, batteries and associated electronics, over the course of three space walks. The new structure will add some 16 metric tons to the mass of the station.

Ok, so it's 12 now?

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The 12 planetsAs reported here and in many other places, we will possibly end August with 12 planets in our Solar System: the current set of nine (including Pluto), plus Ceres, 2003UB313 and Charon. The unofficial definition, to be voted next Thursday, is that a planet is any body that orbits a star and is large enough that gravity makes it spherical (or almost spherical).

Ceres is one of the bodies in the asteroid belt, between the orbits of Mars and Jupiter, and it joins the group because it is, indeed, round. It was briefly considered a planet shortly after it was discovered, so it actually regains the status.

2003UB313 is the large trans-Neptunian body discovered in 2003 and recognised last year as being larger than Pluto. Despite a very weird orbit, it actually makes sense to consider it as a planet is Pluto is one.

Now, Charon is a weird one. It is, until now, considered one of the moons of Pluto (and, until recently, the only one); so, it might seem that it does not orbit the Sun, but Pluto. However, Charon is so large compared to Pluto that the center of gravity of the combined system is actually outside Pluto; that is, it's not so much Charon orbiting Pluto, but both orbiting each other. By the "new rules", this makes the system a binary planet, and therefore both count as planets.

Interestingly enough, our own Moon is also quite large, in comparison with the moons of other planets. It's not that large, though, and the centre of gravity of the system is very much inside Earth. But the Moon is getting away from Earth: the average distance increases by some 4cm a year. This means that, in a few million years, the centre of gravity will be outside Earth, and the Moon will be a planet. I really don't think the IAU rules will still exist (or be relevant) by then, though.

The order of planets our kids will have to learn will, then, be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003UB313 (which will by then have a new name). Now, I don't know why Sedna and Quaoar were not invited to the party. Maybe that will come later.

Still about Pluto...

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One other proposal that has surfaced for the definition of a planet is to accept anything that has an intrinsic magnitude (brightness) that is equal than or larger than Pluto's. This would keep Pluto as a planet, and would also include 2003UB313, a.k.a. Xena, which would then (I suspect) be named in accordance with the naming rules for planets (names of Greek gods). Nothing else would make the cut, though — Sedna, Ceres etc. would not be considered planets.

The IAU conference happens over the next two weeks, and a blog with information on everything that happens (not only about Pluto, of course) is at

Pluto still a planet, probably

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Pluto and Charon - photo NASANPR reports that a IAU panel has unanimously recommended that Pluto (in the photo with its larger moon, Charon) keep its status as a planet, and it is possible that we may end up with several objects being recognised as new "dwarf planets".

I mentioned a while ago that the IAU was going to decide on an unambiguous definition for the word "planet", and that it was likely that this would change the number of planets in our solar system. The decision has not yet been reached; this should happen later this month. However, several members of the aforementioned panel suggest that Pluto will join a new group called "dwarf planets" (we already have "terrestrial planets" and "giant gas planets"), which might include any body that orbits the Sun and that is large enough that its own gravity makes it spherical (or nearly so); this would include anything larger than some 700km across.

This definition would include not only Pluto and Xena (a.k.a. 2006UB313), but also other smaller bodies such as Ceres (an asteroid between Mars and Jupiter). This would take the number of known planets to something like 13 or 14, but it is unsure whether this will be approved.

The IAU general assembly opens tomorrow, 12 August, in Prague.

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This page is an archive of entries from August 2006 listed from newest to oldest.

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