>To get a sense of how cold this is, Pluto's average surface temperature is seven degrees warmer.

That doesn't give me a good sense because I have no idea how Pluto's average surface temperature compares to typical on-Earth temperatures that I have experienced. This gives me a better sense:

26 K = -247.2 C

IMHO you're better off thinking in Kelvins. Room temperature is 295 Kelvins, the cosmic background radiation (the leftover glow of the Big Bang) is 2.7 Kelvins, and between the stars, far from any particular star, an object naturally cools to the universe's background temperature of 2.7 Kelvins.

Is that with the windchill?

No atmosphere, therefore no wind, thus no windchill.

You must be fun at parties.

Or -413 F for us silly Americans.

Oh, so something like Canadian winter. ;)

Fun bit of trivia: If a moon rover could maintain speed of about 11 miles per hour (17 Kilometers per hour) forever, it could circle the moon in 28 days, and would be moving fast enough to stay on the light side of the moon. That doesn't seem very fast, but with the moons low gravity and all.... You'd have to be doing science while catching big air.

That said, temperature works differently in hard vacuum - with no air to conduct/convect the heat away, isolated objects don't actually get "cold" the way they would in a pool of liquid nitrogen or something. Notice you don't hear about the ISS freezing at night.

The ISS is only in shade for 45 minutes at a time, which is orders of magnitude less than a lunar night.

But speaking of that: how long is too long to spend at 26K in a vacuum? Are there any good articles/videos that talk about what happens to various materials at such extreme temperatures in a vacuum?

I think the point of the parent comment was that simply being in a cold vacuum doesn't make you instantly cold. That still requires a process of heat transfer, which occurs in a fluid via conduction, convection, and radiation.

When immersed in a vacuum the conduction and convection heat transfer paths are sharply reduced, so the question then becomes how fast do you cool off via radiation alone?

I'm not a physicist so I don't know, but it's certainly nowhere near as fast as conduction/convection (in fact I believe heat dissipation was actually problematic for many spacecraft designs).

As it turns out, even on earth's surface, radiation is a more efficient heat transfer method than conduction and convection through the air. Objects under a clear sky quickly drop below air temperature in many circumstances.

Thank you! I did an hour jog tonight, and most of that time was spent trying to figure out why there was frost on the ground with an air temperature above freezing and a warm breeze.

I wonder if heat might also be conducting downwards to the frozen ground below.

What he's trying to say is that if you toss a rock into liquid helium or a 4 Kelvin zero g vacuum it's going to get colder faster via direct contact with the helium than via radiation in the vacuum.

Wow, that seems amazing but now that you mention it I think I've heard of that happening even in deserts.

No, actually, a vacuum is an ideal environment for radiating heat away -- better than an atmosphere, certainly. Surfaces in lunar shadow really do drop to an extremely low temperature over a period of days.

You changed the entire context by sticking the word 'radiating' in there. Yes vacuum is fine for radiation, but radiation is much slower than physical contact with standard pressure air.

> Yes vacuum is fine for radiation, but radiation is much slower than physical contact with standard pressure air.

Actually, it's the other way around. Under a clear sky at the surface of the earth, radiation is much more efficient than convection and conduction to air. This is why objects at the surface readily cool below air temperature at night.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/coobod.htm...

The linked example is a person losing heat energy under normal circumstances (23 C ambient, body temperature of 34 C). The example shows that a body loses 17 watts to perspiration, 11 watts to conduction (to the air and environment), and 133 watts to radiation.

Contrary to what many people think, a planet without an atmosphere loses heat to space very efficiently, solely by radiation -- indeed, that mechanism is more efficient in the absence of an atmosphere. But even with an atmosphere, radiation is the major heat loss mechanism, most more efficient than convection.

A great deal of effort has gone into keeping the ISS at civilized temperatures:

http://science.nasa.gov/science-news/science-at-nasa/2001/as...

It's got 100,000 watts of solar power, so that combined with good insulation means it won't freeze as long as the panels keep working...

> Pluto's average surface temperature is seven degrees warmer.

What about the temperature in Pluto's shaded regions (if there are any)?

Maybe Pluto's atmosphere keeps the temperature warmer? I assume it's too far from the sun to have any unshaded versus shaded regions.

The average "temperature" of empty space is 3K, due to heating by background radiation. That's a big enough difference to have a warmer sun-facing side and a cold, shaded side.

Suppose China were to send another Rover up in the next 5 years, and it found this one. In theory, could this one be repaired? Or will the "electronics self-destruct" be thorough by then?

Some assumptions I have to make are:

• The RTG in Yutu (the rover already up) will last that long

• Yutu does not move, get struck by a meteor, or other normal events in space that will damage it further

• The team who understands Yutu is still around in 5 years

Then China could, in theory:

• Assemble a rover with the repair parts on board

• Launch it to the moon

• Land on the moon intact

• Locate Yutu

• Do a remotely-operated repair

There's a lot of risk in each of those steps. It might be an interesting mission to push the boundaries of tele-operated robotics etc. Realistically it seems less expensive to launch a second rover that has been improved based on the lessons learned from Yutu. Hopefully China can think of lots of things to improve the second time.

Interesting thought experiment, though.

Does China have the capabilities and reserves of radioactive material to make probes with RTGs?

Online accounts say there's a radiation source for heating during the lunar night, but it seems one of the solar panels used for normal operations didn't fold up properly (by folding up these panels are closer to the heat source) before the sun went down, and that exposed component froze and was ruined, which sealed the spacecraft's fate.

Source: http://www.dvice.com/2014-2-12/chinas-jade-rabbit-lunar-rove...

Looks like it: http://en.wikipedia.org/wiki/Timeline_of_the_Republic_of_Chi...

They had capability to produce required material decades ago.

They also have sufficient nuclear arsenal to warrant maintaining stockpiles of the stuff: http://en.wikipedia.org/wiki/China_and_weapons_of_mass_destr...

You are confusing the Republic of China, commonly called Taiwan (which is the subject of your first link) with the People's Republic of China, which sent this robot to the moon.

Both of them have nuclear power though (but only PRC has nuclear weapons) and radioactive material in sufficient quantities.

It has a radioisotope heating unit https://en.wikipedia.org/wiki/Yutu_%28rover%29#Specification...