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Comment: Re:Counterclockwise? (Score 4, Interesting) 141

by Convector (#48988531) Attached to: The Strangest Moon In the Solar System

According to the IAU definition, the north pole for a major planet (or one of its satellites) is the pole on the same side of the ecliptic as the Earth's north pole, the North Celestial Hemisphere. By this definition, Venus and Uranus are retrograde rotators -- they rotate clockwise about their north poles.

For comets and minor planets (including Dwarf planets), the north pole is the pole about which the body rotates counterclockwise. So the north pole of a retrograde-rotating asteroid points into the South Celestial Hemisphere.

This brings us (as do all topics that mention the IAU) to Pluto. Pluto rotates retrograde. It was once considered a major planet, so it's north pole would have been on the same side of the ecliptic as ours. But as a dwarf planet, the opposite definition applies. Even before the 2006 decision, the convention was inconsistently applied. Papers have been published using each definition of the north pole, and they're not always good about stating which convention they used. With New Horizons on the doorstep, we're going to need consistency for mapping and navigation. So I believe the mission has decided to use the current IAU definition consistently to avoid any confusion. There was a huge fight over the coordinate system of Vesta on the Dawn mission, and we don't want that.

Comment: Re:Antipodal eruptions (Score 2) 78

by Convector (#48641865) Attached to: Massive Volcanic Eruptions Accompanied Dinosaur Extinction

I've never found the antipodal argument convincing. Seismic waves converge at the antipode of an impact only if the target is spherically symmetric and isotropic. In the actual Earth, you have reflections off all kinds of laterally varying boundaries. Also, the sound speed differs substantially between continental and oceanic crust, so the path matters quite a bit.

The Chixulub impact is also not that big (as planetary-scale impacts go). The projectile was what, 10 km? Shock heating is only significant within a few times the projectile diameter.

Comment: Phase I study (Score 1) 79

It's not clear from the summary (or the linked article), but this isn't a mission at this point. This is a concept selected for Phase I study.

From the NASA Innovative Advanced Concepts (NIAC) news release:
"NIAC Phase I awards are approximately $100,000, providing awardees the funding needed to conduct a nine-month initial definition and analysis study of their concepts. If the basic feasibility studies are successful, proposers can apply for Phase II awards, which provide up to $500,000 for two more years of concept development."

This effort is independent of the ongoing Europa mission studies (e.g. the Clipper concept.) The Draper concept may end up getting a mission if the results prove promising. Personally, I have doubts that this will prove credible, but that's the whole point of the NIAC studies.

Somebody's terminal is dropping bits. I found a pile of them over in the corner.

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