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J002E3

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> One explanation could have been that it was a 30-meter-wide piece of rock, but University of Arizona astronomers found that the object's electromagnetic spectrum was consistent with white titanium dioxide paint, the same paint used by NASA for the Saturn V rockets.

I know this is a really old thing but the idea that we can determine composition of the surface of an object from so far away simply by looking at the light it reflects is absolutely incredible to me. Very science fiction. :)

It's not completely accurate, however; this one was also thought to be a rocket from its spectrum: http://en.wikipedia.org/wiki/2006_RH120

Nice catch. Here's a link to some positional data collected from that one: http://www.skyandtelescope.com/astronomy-news/earths-other-m...

This is a pretty fundamental part of astronomy. Just look at what you can do with Fraunhofer lines (http://en.wikipedia.org/wiki/Fraunhofer_lines). They are an almost steganographic signature written into the elements.

And more generally spectroscopy: http://en.wikipedia.org/wiki/Spectroscopy

Just to reinforce jgc's point: this object was not "far away" by spectroscopy standards. You can use the same technique for looking at the most distant stars and galaxies in the sky.

When people talk about Hubble (the guy, not the telescope) detecting the expansion of the universe by looking at redshift from distant galaxies, what he was actually looking at were spectral signatures. He saw all the patterns he expected to see from burning stars, but the lines were smeared out. Because he knew the patterns always form a consistent picture of the atomic structure, and he understood the doppler effect, he was able to estimate the distances of those objects quite accurately. Amazing stuff.

Not quite. From the doppler-shifted spectrosocopy lines, we can figure out what speed everything is moving relative to us, but that by itself doesn't tell us anything other than their speed.

Noting that all the doppler shifts of other galaxies are red-shifts tells us that all the galaxies are moving away from us, but it still doesn't tell us whether speed correlates to distance in any way.

Figuring out the distance of galaxies is a different matter entirely, and is done with standard candles[0].

It's from the standard candles that we can measure distance, and only once we know the distance that way can we make a correlation between speed and distance, from which we see that the further away a galaxy is, the faster it is receding.

Only once we have found that there is a speed/distance curve can we estimate the distance to an object based on its doppler shift. We can't get it from the doppler shift alone.

[0] http://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard...

Might be worth mentioning the use of doppler shift of the known patterns of spectral lines associated with elements to get information about radial velocity of e.g. a star being observed.

https://www.e-education.psu.edu/astro801/content/l4_p7.html

Might be worth mentioning the use of Doppler shift of the known patterns of spectral lines associated with elements to get information about radial velocity of e.g. a star being observed.

https://www.e-education.psu.edu/astro801/content/l4_p7.html

Hyperfine splitting of spectral lines in hydrogen gas caused by the interaction between electron and neutron spin allows you to map the hydrogen in our local Galaxy.

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/h21.html

As parent post says this is pretty standard astrophysics, just find any reasonable text in your local library. Or just Google 'astronomy .pdf' for a huge range of handbooks and presentations.

Spectroscopy is pretty much the reason we have been able to infer as much as we do about stars, their lifecycles, and what happens inside them.

Oh, for fuck's sake, mods. There was a useful, informative, uncontroversial title here, and you've turned it into something meaningless. You've actually made it more clickbaity. Stop doing this, for the love of all that is good.

Original title:

    J002E3 - piece of Apollo 12 returned
             to Earth orbit after 31 years

That was much, much more useful and informative.

For those interested, NASA's Jet Propulsion Lab runs a telnet server that provides information about objects like this via its HORIZONS system.

To avoid an onslaught of HN traffic to the telnet server, I will just say to go to ssd.jpl.nasa.gov, and click through to find it. Once you're telnetted in, enter J002E3.

My Kerbals will be honored to know that even NASA loses track of used stages. :)

Apollo 12 was struck by lightening shortly after launch. I wondered initially if this was the reason for the "extra long burn of the ullage motors".

But according to the Apollo 12 wikipedia page: "a small error in the state vector in the Saturn's guidance system caused the S-IVB to fly past the Moon at too high an altitude to achieve Earth escape velocity".

http://en.wikipedia.org/wiki/Apollo_12

> ...the object's electromagnetic spectrum was consistent with white titanium dioxide paint...

Isn't this just a fancy way of saying that it was white?

if 'fancy way of saying' means it has a specific chemical composition then sure, yes it's a way of saying it was white.

Not exactly, some compounds absorb and reflect different parts of the electromagnetic spectrum. In paticular, titanium dioxide:

"Some compounds, like titanium oxide, only appear in the spectra of very cool stars." [1]

Obviously this wasn't a star, so it must be a manmade object.

[1] From http://skyserver.sdss.org/dr1/en/proj/advanced/spectraltypes...

No. It means it, spectroscopically, looked specifically like titanium dioxide paint, and not just any arbitrary white material. Its spectral lines were consistent with titanium dioxide.

http://en.wikipedia.org/wiki/Spectral_line

The see-also links below this article are pretty interesting too.

Now this is a much clearer title!

Sarcasm?

Godspeed, J002E3. Next time you come back around, we'll bring you home.

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