steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
I keep seeing articles about how Voyager 1 is leaving the solar system, or about to leave the solar system, or may just have left the solar system. An example is a science blog post titled More evidence that Voyager has exited the solar system.

These articles are talking about a real and interesting phenomenon, but I find the description of it as "leaving the solar system" misleading.

What they are primarily talking about is Voyager leaving the sun's heliosphere, the region of space affected by the charged particles of thesolar wind. Outside the heliosphere is the interstellar medium. Passing the heliopause between the heliosphere and the interstellar medium is crossing a significant boundary between a kind of space that is heavily influenced by the sun and one that isn't. So it is a significant event that Voyager 1 is passing this boundary, and in the process giving us evidence about the location of the boundary and what conditions are like at it and on either side of it.

Voyager 1 is currently about 122 astronomical units (au) from the sun, or 122 times earth's distance from the sun. That's a long ways away, but there are still many objects in the solar system whose orbits take them out that far; not only many comets, but also Sedna, which goes as far as 940 au from the sun at its most distant. And the Oort cloud, a hypothesized reservoir of comets that have never yet come close to the sun, is hypothesized to be between around 5000 and 50,000 au from the sun. All these orbits are still orbiting the sun, and thus still part of the solar system. It seems misleading to me to say that Voyager 1 is leaving the solar system when it's still well within the orbits of many things orbiting the sun.

But crossing the heliopause is still neat and noteworthy.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Tiny, Gentle Pluto May In Fact Be A Killer

A bit of an overdramatic headline, but the article's got interesting stuff in it. The core idea is that the New Horizons spacecraft is on the way to the Pluto system, scheduled to arrive in 2015; when the project was first designed, Pluto was only known to have one moon, the rather large Charon, but since then, 3 smaller moons have been discovered - two that were first suspected in 2005 and confirmed in 2006, and one that was discovered in 2011. And there are suspicions that there may be more small moons; partly this is a matter of "well, if there were three hiding until recently, maybe there are more that are still hiding", but there's a more concrete aspect: there were actually two specific satellite candidates that were found, but more observation is needed in order to figure out if they're actually satellites or not. In addition, there's a possibility that impacts onto these moons may create rings around Pluto. The more moons and rings there are around Pluto, the more risk there is that New Horizons will smash into something and be damaged or destroyed when it gets to the Pluto system.

I'd heard before about the concerns for New Horizons about Pluto having more moons and rings. What was new to me in this article, and particularly interesting, was that there are specific identified candidates for further moons, not just a general sense that there could be more moons.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Extreme Ballooning -- on Titan

On efforts to develop a hot air balloon for a mission to Titan; this has an advantage over a lander that a balloon can move around and give you a view of a variety of different places. A major challenge for developing a balloon for Titan is that it's so very cold.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Three press releases today:
Number one: ESA finds that Venus has an ozone layer too. It was already known that Mars as well as Earth has an ozone layer. They found Venus's by analyzing the spectra of stars seen through the very outer edge of Venus's atmosphere.

Number two: The Secrets of Asteroid Minerva and its Two Moons. Moons let scientists figure out Minerva's mass. They figured out its diameter from a combination of watching it occult a star, and infrared measurements, both of which pointed to a diameter of about 156 km. This lets them calculate its density - about 1.9 grams per cubic centimeter; based on the density of what they think it's made of, that indicates that it must have about 30% empty space in its interior.

Number three: Series of bumps sent Uranus into its sideways spin. This is about a new hypothesis, not a new discovery. The previous standard account for Uranus being tilted on its side was that a single large impact had changed its orientation. The problem with that explanation has always been that it didn't explain why its moons orbit it in similarly tilted orbits. New computer simulation results show that this can be explained if Uranus was hit before the moons formed, while it was still surrounded by a protoplanetary disc out of which the moons later formed, and if it was tilted by two or more impacts, rather than by a single one.

And what I got from Twitter. As before, don't put too much confidence in it; I could very well have misinterpreted things or expanded them wrongly.

I'm not sure how much of this is new, but: Venus has lightning, and like on Earth, some days have more lightning than others, depending on the weather. The lightning rate on Venus is probably comparable to that on earth, about 100 flashes a second worldwide, but that's hard to prove.

Venus's cloud tops are generally 72 km above the surface, except at the poles where they're only 65 km up.

Global climate models for Earth aren't working for trying to understand Venus, which indicates that they might not be working right for Earth either. To understand Earth's climate, we have to understand Venus too.

Saturn's moons
One of the puzzles about Saturn's moon Iapetus is why it has a ridge running around its equator. One hypothesis has been that it used to be spinning faster, which made it more oblate (i.e., bigger around at the equator than at the poles) due to centrifugal effects, but its rotation slowed, so gravity made it more spherical than oblate, but the rearrangement process from more oblate to less oblate created an equatorial ridge. The problem with that is how exactly the spin change could have worked to create that result. The proposal now presented is that if Iapetus once had a satellite of its own, that might make it possible to reduce Iapetus's spin enough to create the ridge. (I don't know how a satellite would help that, though.)

It was discovered over a year ago that Mimas has a weird Pac-Man-shaped temperature pattern; there's a relatively warm area that looks like a Pac-Man shape on images, and relatively cold area on the leading hemisphere that looks like the inside of Pac-Man's mouth. News: The cold part of the leading hemisphere - the area inside Pac-Man's mouth - is correlated with an area in the leading hemisphere that is dark in UV light. Also, as of September 2011, a similar anomaly seems to have been observed on Tethys. (I think this means both the temperature pattern and the correlation with a UV-dark leading hemisphere area have been observed on Tethys, but I'm not sure about the second half.) There's a hypothesis, which still sounds quite tentative, that it might be caused by being bombarded by electrons, which somehow (not clear from Twitter) increase the thermal inertia of the moon's material. (Also not entirely clear to me whether the electrons are bombarding the warmer area or the colder area.)

The outer irregular satellites of Saturn that Cassini observes are so faint that if you were sitting on Cassini, you couldn't see them with the naked eye.

Other stuff
Almost all near-earth asteroids smaller than 60 metres diameter rotate really fast - in less than 2 hours.

Dust from collisions between irregular satellites of Jupiter might have left 100 metres of dust on Callisto and 20-30 metres of dust on Ganymede.

It sounds as if there was some interesting stuff about Europa, but not that I could contextualize well enough to pick out relevant tweets and expand on their significance as needed.

Good news: Jim Green, the Director of Planetary Science for NASA, says that production of Plutonium-238 will be restarted, so there will be a reliable power source for outer solar system missions.

Twitter sources:
[ profile] DrFunkySpoon
[ profile] kat_volk
[ profile] gsinfinite
[ profile] elakdawalla
[ profile] jeanlucmargot
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Vesta - an asteroid full of surprises

Talks a bit about what's been seen and what's being looked for at Vesta.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
NASA's Journey Above Vesta

A narrated video tour of Vesta, about 2 minutes long. It's worthwhile to follow the link to the High Definition on the right side of the linked page.
steorra: Part of Saturn in the shade of its rings (Default)
[personal profile] steorra
The $15 Million Budget Battle That May End Outer Solar System Exploration

Solar panels don't work very well in the outer solar system, because there's less sunlight out there. The most successful power source for spacecraft in the outer solar system uses Plutonium 238. Due to budget issues, Plutonium 238 isn't getting made; without it, making new outer solar system spacecraft will be practically impossible.

(Unless someone comes up with a new viable power source...)
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
The MESSENGER mission to Mercury has just released a neat map-exploring site. It includes two main kinds of maps: images of Mercury, and indications of which parts of Mercury its various instruments have observed and will observe. To view images of Mercury, the layers you probably want to look at are:

Basemaps > MESSENGER & Mariner 10 Mosaic (Covers most of the surface, uses images from Mariner 10 and the MESSENGER flybys of Mercury before it got into orbit.)

Global Mosaic Campaigns > MDIS 750 nm mosaic (PDS) (Covers probably more than half but less than three quarters of the surface at this point; images taken by MESSENGER in orbit around Mercury.)

You can also turn on and off a latitude/longitude grid, found under Location Overlays.
steorra: Part of Saturn in the shade of its rings (Default)
[personal profile] steorra
(Cross-posted to my own journal)

Today's Astronomy Picture of the Day confirms something I'd noticed from looking at some of the early reasonably detailed pictures of Vesta. As I observed here, Vesta's southern region is not very heavily cratered. There are some craters on it, but they're not all-over-the-place-everywhere. At that time, I hadn't seen any good picture of the northern hemisphere to know whether it was similarly lightly cratered, but I suspected it was more heavily cratered, and as the APOD commentary says, it is indeed more heavily cratered.

Here's the APOD's commentary on the cratering:
Why is the northern half of asteroid Vesta more heavily cratered than the south? No one is yet sure. This unexpected mystery has come to light only in the past few weeks since the robotic Dawn mission became the first spacecraft to orbit the second largest object in the asteroid belt between Mars and Jupiter. The northern half of Vesta, seen on the upper left of the above image, appears to show some of the densest cratering in the Solar System, while the southern half is unexpectedly smooth.

Now, in general, the more heavily cratered a chunk of terrain is, the older it is. When new terrain is formed, it doesn't have any craters yet, and as time goes on, stuff smashes into it and it gets craters. So the older it is, the more time it's had for more stuff to smash into it and make more craters. It's a bit more complicated than that - in the early history of the solar system, there was more stuff around in space to smash into things, so the rate of cratering decreases as we get farther away from the early solar system. But still, it works out to the fewer craters a chunk of surface has, the newer it is. (I think crater size plays into the formula too, but I don't remember much about that.)

So the relative lack of craters in Vesta's southern half suggests that it's relatively recent terrain.

Even before Dawn got to Vesta, it had been proposed that Vesta's south half is shaped as it is because a large impact had basically knocked off a huge chunk of the southern hemisphere. It seems like a fairly obvious guess to make that the age of the southern terrain is an indicator of the age of that impact.

The APOD also mentions the grooves that encircle Vesta in the southern part of the equatorial region.

Pictures of Vesta from Dawn are available here and here.

There's also a NASA news conference video about Vesta from yesterday, but I haven't watched it yet.
steorra: Part of Saturn in the shade of its rings (Default)
[personal profile] steorra
So, a few days ago, Dawn entered orbit around Vesta, and is now slowly spiralling in towards its intended science orbit. Already it has sent back some pictures that show interesting amounts of detail: this one from July 17 (press release for the image) and this one from July 18 (press release for the image).

Both pictures largely show the south polar terrain, where a large impact is believed to have taken quite a chunk off of Vesta; in the middle of the south polar terrain is a large peak.

What catches my attention is the texture of the south polar terrain. There are a lot of grooves and ridges that I suppose were probably formed in some way by the impact. There are craters on it, but it's not thoroughly cratered, which ought to indicate that it's not very very old. (It would be nice to see how cratered the rest of Vesta appears, for comparison, but the pictures we have so far don't show that very well; you can see a bit of the non-south-polar terrain in the July 17 picture.)

Pluto's moons
On Wednesday, the discovery of a fourth moon orbiting Pluto was announced! It was discovered with the Hubble Space Telescope.
jeweledeyes: Sailor Venus thinks you're a loser (Kowalski science)
[personal profile] jeweledeyes
More about possible fuel methods for Project Icarus: tapping into the "gas mines of Uranus"!
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