New Horizons probe successfully arrives at Pluto

[SpottedKitty]

Damn, are those two objects actually that close to each other?

That might be a line-of-sight effect, they're really a bit further apart than that. There's an animation over a few days that was taken last week, showing the rotation of the Pluto-Charon binary — and yes, it is generally accepted now to be a binary system — and they're just big enough you can see the two always showing the same face to each other. It's actually quite a bit more lopsided than the Earth-Moon system; the centre of rotation of that is deep under the Earth, but the centre of Pluto-Charon is well above Pluto's surface. The other moons, and there might still be one or two more hiding in the data waiting to be downloaded, are just tiny little rocks barely big enough for Hubble to see them, pulled along for the ride.

[jwl]

It turns out the radius of pluto is slightly bigger than we thought, apparently. It confirms its place as the biggest known dwarf planet in radius.

[Executor32]

And here's the last image taken before flyby:

[Borgholio]

That, my friend, is wallpaper material.

[Eternal_Freedom]

There are already people pointing out that the large white area in the southern hemisphere looks sorta like Pluto the cartoon dog's head, which is fitting.

[Borgholio]

Now that you mention it, I can never look at Pluto the same way again.

[SpottedKitty]

That, my friend, is wallpaper material.

And it does indeed look very nice on my monitor.

BTW, that pic linked on the last page of Pluto and Charon together is a composite, according to the Astronomy Picture of the Day site.

[Borgholio]

Ah ok. On the NASA site it said it was a composite of black and white + color data, not that the image was a composite of the two bodies put together. Thanks for the clarification.

[Irbis]

It turns out the radius of pluto is slightly bigger than we thought, apparently. It confirms its place as the biggest known dwarf planet in radius.

Only assuming the exact same error wasn't made while measuring all the other dwarf planets, no?

[Borgholio]

At that distance, some error is to be expected. Once the probe explores deeper into the Kuiper belt, we will probably get better at estimating the sizes of these things.

[ndryden]

It turns out the radius of pluto is slightly bigger than we thought, apparently. It confirms its place as the biggest known dwarf planet in radius.

Only assuming the exact same error wasn't made while measuring all the other dwarf planets, no?

Previous estimates of Pluto's radius were that it was in the 1150-1200 km range. These were based upon observations of it occulting stars, but the uncertainty is due to Pluto's (relatively speaking) dense atmosphere causing interference. The new estimate is 1185 +/- 10 km (towards the upper end of the old range). This was made based upon comparison of images of Pluto with Charon, the radius of which is well-established due to the lack of atmospheric interference (it is 603.5 +/- 1.5 km). Pluto is now known to be the largest (by radius) object in the Kuiper belt; however, Eris is more massive than Pluto.

For the other dwarf planets, off the top of my head, we have pretty good error bars for Eris and Ceres, but Makemake and Haumea are still rather poor. Though these are far enough from spheres that "radius" is maybe not a very accurate term...

[Borgholio]

Well Ceres is much closer and has no air so that one is pretty easy. But Eris and Sedna are going to need a visit to get accurate measurements.

[ndryden]

Well Ceres is much closer and has no air so that one is pretty easy. But Eris and Sedna are going to need a visit to get accurate measurements.

Definitely they need to be visited, though the next viable low-energy launch window looks to be a Jupiter gravity assist in late 2028 (unless we could somehow launch a mission late this or next year). Sedna in particular is fascinating due to its peculiar orbit, and Eris's large mass is another oddity.

That said, Eris's radius (and mass, and a few other things) has pretty low (see here for a summary from the EPSC) error bars: 1163 +/- 6 km, due to a lucky occultation event and an atmosphere about 10000 times thinner than Pluto's.

[SpottedKitty]

Well, the engineering data download seems to indicate everything worked. There should be a few nice close-up pictures released later tonight.

[Borgholio]

Closeup of some mountain ranges that are over 2 miles in height...and they're fairly young too. Pluto may still be somewhat geologically active.

[Broomstick]

Between those mountains and the relative lack of craters it seems fairly certain that Pluto is still geologically active, at least if you're talking about geological time spans. Likewise Charon.

Very interesting....

[SpottedKitty]

<squee!!!>

<ahem>

Yes, that does seem to be a very young surface; one guesstimate I read earlier put it at probably under a hundred million years old. I wonder what could be driving it — Pluto's much too old for the usual culprit in radioactive heating, Aluminium-26, which decays in just a few million years, and Charon's orbit is settled and virtually circular, which rules out tidal heating. It's a head-scratcher, all right.

And an interesting feature on Charon, too — a big chasm, hundreds of miles long and about five miles deep. Wonder how old that is?

One picture so far of one of the smaller moons, unfortunately very low-res, as it's such a titchy little thing. You thought the asteroid Eros looked like a banana? This looks like a hockey stick...

[Simon_Jester]

Between those mountains and the relative lack of craters it seems fairly certain that Pluto is still geologically active, at least if you're talking about geological time spans. Likewise Charon.

Very interesting....

The lack of craters may also be because Pluto's out in a part of the solar system that contains very few large objects to smack into and a lot of space for them to buzz around in.

Or it could be because small amounts of various substances routinely boil off and redeposit randomly across the surface during Pluto's 'seasonal' cycle, until craters are filled in with a layer of nitrogen hoarfrost or whatever. Mountains would still be exposed under such conditions, although their contours might be blurred and obscured under ice.

<squee!!!>

<ahem>

Yes, that does seem to be a very young surface; one guesstimate I read earlier put it at probably under a hundred million years old. I wonder what could be driving it — Pluto's much too old for the usual culprit in radioactive heating, Aluminium-26, which decays in just a few million years, and Charon's orbit is settled and virtually circular, which rules out tidal heating. It's a head-scratcher, all right

You wouldn't need very intense heat to drive cryovulcanism on Pluto. I don't know if cryovulcanism would result in geological activity on Pluto's surface, though, as opposed to just random plumes and sprays.

[SpottedKitty]

You wouldn't need very intense heat to drive cryovulcanism on Pluto.

A good point, I didn't think of that. I'm not sure if we've got a complete list yet of all the different ices that might be below the surface; enough chunks with a different specific heat capacity jumbled together could easily be sensitive in weird and unexpected ways to temperature changes.

[Simon_Jester]

Yeah. Among other things, some of them undergo phase transitions at the kind of temperatures Pluto experiences, which could result in considerable amounts of pressure and unusual absorption and release of energy.

[Borgholio]

Here's a pic of Charon's surface. Definitely something out there that's causing the surface to smooth itself out after a period of time. I wonder if due to how close Pluto and Charon are to each other, they can create their own gravitational heating even when there isn't a larger planet around?

[Simon_Jester]

Charon looks a bit more moonlike- the dark patch in the northern hemisphere looks a LOT like a lunar mare (the singular of 'maria,' you know, the lava plains created by major impact basins). I can't remember what the speculation before the flyby was about whether Charon had a rocky surface or an icy one; almost all the talk I remember at the presentation and elsewhere indicated that people expected an icy surface on Pluto.

[LaCroix]

All these pictures and the new data resulting from it are absolutely awesome. The only way they could have made this mission even cooler was if they named the probe Event Horizon...

[Borgholio]

almost all the talk I remember at the presentation and elsewhere indicated that people expected an icy surface on Pluto.

I think it IS icy...just not frozen methane like they thought. It's mostly water-ice with a thin layer of nitrogen and methane as a crust on top. Those mountains the size of the Rockies are all water ice.

[SpottedKitty]

I wonder if due to how close Pluto and Charon are to each other, they can create their own gravitational heating even when there isn't a larger planet around?

How close together they are doesn't really have much effect on heating. What does matter is changes — one of the best examples being Jupiter's moon Io. It's in a constant tug-of-war between Jupiter and the other three Galilean moons, and it's in orbital resonance with two of them, preventing its orbit from becoming more circular and stable. These constant changes in the pull of gravity are what causes tidal heating, so we end up with a moon that has the most volcanoes anywhere in the solar system.

The Pluto/Charon system has none of that — their orbits seems to be pretty much stable, they're locked into mutual synchronous rotation, and the other moons are tiny in comparison. There might well be tidal bulges because they are so close together, but unlike other systems e.g. the Earth and the Moon, these bulges can't move. If there's no friction from the tides, there's no heating. Whatever's going on here, it isn't that obvious.