The Solar System boundary and the week in review (September 8-14)

Cthulhu at NASA Wallops, for the LADEE launch last weekend. (I didn't wear the hat the whole time. I'm not that weird.)

Cthulhu at NASA Wallops, for the LADEE launch last weekend. (I didn’t wear the hat the whole time. I’m not that weird.)

‘Twas a busy week!

  • High-resolution observations show how black hole jets churn galactic gas (Ars Technica): One portion of my PhD thesis involved galactic feedback. That’s the process by which jets from black holes at the center of galaxies push material away, potentially affecting star formation and other activity. This article addressed the observation of galactic feedback, showing exactly where the hot jet of plasma from the black hole meets the colder atoms in galactic clouds. Very awesome stuff!
  • Parallel Earth and the Evil Matthew Hypothesis (Double X Science): I don’t know if Star Trek was the original source of the “evil twin from a parallel world” trope, but it’s the most famous. The idea is that there’s a mirror universe to ours, in which things are almost the same, but not quite. I discussed that trope in light of the multiverse, the concept that during rapid expansion right after the Big Bang, the Universe split into a number of disconnected regions that might obey different laws than our own.
  • Do-it-yourself science at GeekGirlCon (Galileo’s Pendulum): We’re still raising money to send a group of us to GeekGirlCon in Seattle next month! We’re willing to embarrass ourselves in public to accomplish this! However, the real purpose is to have hands-on science activities at the con.
  • Status of the book-in-progress (Galileo’s Pendulum): On a more somber note, I have suspended work on my book indefinitely and released my agent. I haven’t completely given up on either the book or getting it published, but the frustrations around the whole process have exhausted me, so it’s time for a break.
  • Cosmic coincidence and a potato eclipse (Double X Science): The Moon is nearly the same size as the Sun in our sky, which has led to all sorts of mystical musings and apocalyptic fears, especially during eclipses. However, that appearance is a coincidence, which we can understand using simple geometry. What’s even more fun to contrast our Moon to Phobos, the larger of Mars’ two moons, which is much smaller than our own but manages to create its own eclipses.
  • Voyager 1 really has left the Solar System…probably (Ars Technica): Sometime last year, the venerable spacecraft Voyager 1 crossed into interstellar space. While there have been a lot of announcements along these lines (I compared the number with Spinal Tap drummers), this time the probe seems to have actually done it. The necessary measurement is the plasma density, which is much higher in interstellar space, but Voyager’s plasma instrument had been knocked out by a solar flare. Researchers pieced together the appropriate data from other instruments. There’s still an anomalous measurement that needs to be accounted for — the magnetic field doesn’t behave as predicted — but I think it’s pretty safe to say that’s an issue for theorists, not ambiguity about Voyager’s position. (See below for a discussion of whether Voyager has actually “left the Solar System” or not.)
  • Mapping the dark matter in the tiniest of galaxies (Galileo’s Pendulum): Dwarf spheroidal galaxies don’t look like galaxies at all. They have so few stars and so little gas or dust, they’re nearly see-through, yet they have as much as 1000 times more dark matter than ordinary matter. (In regular galaxies, dark matter is more like 10 times the amount.) Two astronomers analyzed the motion of stars within dwarf spheroidals to see if they could map the distribution of dark matter, and they found something similar to what is seen in larger galaxies.
  • Finally, I participated in the Weekly Space Hangout, sponsored by Universe Today and CosmoQuest. I joined hosts Fraser Cain and Nicole Gugliucci, along with Amy Shira Teitel, David Dickinson, and Nancy Atkinson to talk about the space and astronomy news from the last week. The whole thing is archived at Google+, or you can watch the video on YouTube.

Where’s the edge of the Solar System?

Returning to Voyager 1, I think stories about its passage into interstellar space fell into two major categories: those saying “Voyager 1 has left the Solar System!” and variations on “Stop saying Voyager 1 has left the Solar System!” Despite what the headline on my story said, the second group of people (which includes writers I respect like Phil Plait and Amy Shira Teitel) is correct: the Solar System includes the Oort Cloud, a diffuse region of icy bodies loosely bound to the Sun by gravity.

A radio image of Voyager 1, as seen by the Very Long Baseline Array (VLBA) and the Green Bank Telescope. Click for a larger image and more information. [Credit: Alexandra Angelich, NRAO/AUI/NSF]

A radio image of Voyager 1, as seen by the Very Long Baseline Array (VLBA) and the Green Bank Telescope. Click for a larger image and more information. [Credit: Alexandra Angelich, NRAO/AUI/NSF]

However, if you want to say Voyager has left the Solar System, I’ll back you up: the boundary between the Oort Cloud and the “rest of the galaxy” isn’t very well defined. Gravity technically extends forever, though it weakens substantially with larger distances. As a result, the Oort Cloud is a fuzzy edge, and one we can’t measure. Is the end of the Solar System the point where the last Oort Cloud body resides?

Now, I agree with the pedants that the Oort Cloud truly does define the end of the Sun’s influence, and therefore is the edge of the Solar System. But the magnetic boundary of the Solar System, which is arguably the more important one from the point of view of astronomy, is defined by the edge of the heliopause, where the solar wind hits interstellar gas. That boundary, while it fluctuates with solar weather, is a much clearer division, and one we could conceivably measure near other stars.

So, I’m a both/and kind of guy in this case. Since there’s no single, sharp boundary between the Solar System and “everything else”, let’s just say there are two edges: one for the Sun’s electromagnetic influence (the heliopause), and one for its gravitational influence (the Oort Cloud). Voyager crossed the first one, but won’t reach the second one for 300 years. Now, can we get back to talking about how awesome Voyager is?

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The week in review (September 1-7)

A full-size wooden mock-up of the Lunar Atmosphere and Dust Environment Explorer (LADEE). [Credit: moi]

A full-size wooden mock-up of the Lunar Atmosphere and Dust Environment Explorer (LADEE). [Credit: moi]

The last week was especially busy because I attended the launch of the Lunar Atmosphere and Dust Environment Explorer (LADEE) at NASA’s Wallops Flight Facility. I will have a lot to say about that launch, LADEE, and related topics later on, but suffice to say it was a great experience — increased because it was my first successful rocket launch viewing. (I attempted to watch the Antares rocket test in April, but that was scrubbed at the last minute and I couldn’t attend the rescheduled launch.) So, here’s my very small list of articles published this week.

  • Turbulence ahead: Interstellar wind changes direction, blows faster (Ars Technica): The Solar System orbits the center of the Milky way, and as it does, it’s passing through a diffuse nebula known as the Local Interstellar Cloud (LIC). Various satellites and probes have measured the passage of atoms through the Solar System since the 1970s; analysis of that data shows that the direction of this wind has changed and its rate has picked up. That reveals some interesting new detail about the environment surrounding the Solar System.
  • Of maps and math and Buckminster Fuller (Galileo’s Pendulum): Mapmaking will never be perfect because there’s no way to create a flat representation of the curved surface of Earth without some distortions. This post goes a little into the math of projection, the art of converting positions on a globe onto a flat map, and how the eccentric utopian architect Buckminster Fuller tried to solve the problem. The result was his wonderful Dymaxion map, which as a physicist I’m very fond of.

I also wrote a brief viewing guide for the LADEE launch, which is now necessarily obsolete. However, you can find a lot of photos and video from the launch at NASA’s LADEE site.