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.