Close encounters of the Lunar Kind: The Korean lunar orbiter, called Danuri, passed just a few kilometers below NASA’s Lunar Reconnaissance Orbiter (or LRO) on three separate occasions over the evening of March 5/6, 2024. They move in opposite directions as they circle the Moon, so imaging opportunities were very brief as they passed at a relative speed of over 3 kilometers per second, but the folks who operate LRO are good.

Really, really good: 

That image was taken when Danuri was only 4 km from LRO. The exposure time was 0.000338 seconds — 0.3 milliseconds! — but the huge speeds blurred out the Korean spacecraft into a streak. LRO had to tip way over by 25° to get this shot; usually it points straight down to get the highest resolution images it can of the Moon’s surface.

Danuri is shoebox-shaped, about 1.8 x 2.1 x 2.3 meters in size. Seeing it at all at those speeds is pretty remarkable. But orbital mechanics can be a very precise science, so they knew exactly where to point LRO and when to capture this shot.

The LRO page has a much wider-field image you can scan and pan, zooming in to see just how small the Korean mission is compared to the vast lunar landscape. It’ll really give you a sense of just how amazing this sort of shot is.

Astronomers have determined that, apparently, 8% of all Sun-like stars eat at least some of their planets.

Right away I will gently remind you of the Greek myth of Cronus, the king of the gods who, um, ate his children.

This statistic is actually pretty hard to determine. We can determine the chemical composition of a star by taking very high-resolution spectra, breaking its light up into thousands of individual colors. Different elements and molecules absorb different but generally specific colors, so we see less light in some places than others, allowing us to determine what the star is composed of.

But a lot of stars have relative abundances of elements that are also common in planets (silicon, for example, or iron and magnesium). How can you tell if the star ate a planet or not? 

This bit is clever: They looked at pairs of stars that are known to have formed together. Sometimes these stars are binary, orbiting each other, and sometimes they happen to be moving through space together, a sure sign they were born at the same time in the same place. If so, you’d expect their chemical compositions to be very similar.

The astronomers looked at 91 pairs of stars very similar in mass to the Sun [link to journal paper]. They took spectra of both stars in each pair, and found clear evidence of chemical differences between the two stars in seven of those pairs. In those seven pairs, one star showed a higher abundance of elements of planet-like elements (sometimes called refractory elements, ones hard to melt) than the other, which the astronomers take to mean those stars ate some of their planets.

Or, it’s also possible that those materials fell onto the stars as planets were forming from a disk surrounding the star, called the protoplanetary disk. They can’t distinguish between those two scenarios, but they find the planet eating more likely.

Such a thing isn’t so ridiculous. At first, a lot of planets from that disk, and as they encounter each other they can interact gravitationally. Some planets will collide and merge, some will collide and vaporize each other, and some will get ejected from the system entirely, slingshot into interstellar space. But some will have their orbits dropped down close to the star, where they can then fall into the star itself. If so, they can vaporize and become a part of the star, their materials mixing in with the star’s own composition.

This may very well have happened with our own solar system. There’s indirect evidence there used to be a fifth gas giant that yanked and tugged on Jupiter, Saturn, Uranus, and Neptune, and was flung away after a close planetary encounter, lost to the galaxy. Another hypothesis is that there used to be much large rocky planets in the inner solar system that collided and scattered debris all over the places, much of it dropping into the Sun. The smaller remainders formed the inner, smaller rocky planets Mercury, Venus, Mars, and our own Earth.

We see lots of evidence stars swallow planets after the star dies and becomes a white dwarf, but it’s only recently become clear this is happening in stars like the Sun, too. Interesting! It gives one pause to think that a long time ago, if some ancient and now long-gone planet had zigged instead of zagged, we might not be here at all, but instead just be various elements scattered around inside the Sun.

Be thankful for the little things, some people say, but yeah: Be thankful for the big ones, too.

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