JWST M51 image credit: ESA/Webb, NASA & CSA, A. Adamo (Stockholm University) and the FEAST JWST team

Tonight, around 10:00 p.m. Eastern time, the Moon will be about a quarter degree from Saturn in the sky! That’s fairly close, about half the Moon’s diameter, so it should make for a pretty sight. You don’t need any equipment, though binoculars will help; the Moon will be just a day past full so it’s a lot brighter than Saturn.

The exact time of closest approach depends on your latitude and longitude, so I suggest going out as soon as it’s dark out, face the southeast, and take a look. They’ll be close together to anyone observing from Earth, so it’ll be pretty no matter what!

Even better, for some people the Moon will occult (pass directly in front of) Saturn, but how close they get depends on where you are. A map at the IOTA site shows where you need to be to see the occultation. Timings for some major cities are listed too, to help you know when to look.

If you don’t live in the right place, or it’s cloudy, or whatever else might prevent you from seeing it, Gianluca Masi of the Virtual Telescope Project will hold a livestream with a view through a telescope. That starts on August 21 at 03:30 UTC (Aug. 20 at 11:30 p.m.). Take a look! Saturn occultations are extremely cool.

Speaking of Gianluca Masi…

In BAN Issue 761 I wrote about the ESA Jupiter mission JUICE, and how it swings by Earth over the next couple of days to do a gravity assist and drop toward Venus. It will pass our planet by a little under 7,000 km, less than Earth’s diameter.

On August 10/11, Gianluca Masi with the Virtual telescope Project used a 43-centimeter telescope to take images of the spacecraft on its way in. He combined them to produce this cool shot:

He stacked up many images keeping JUICE centered, so the stars smear out into lines. The spacecraft is the dot in the center (the odd fuzzy thing to the upper right is a small edge-on spiral galaxy). He also created an animated GIF which was too big for me to embed here, but he has it on his website.

JUICE will be taking images of Earth and the Moon as it passes, and I’m excited to see those! I’ll post them when they go up.

We usually think of the Moon as being airless, but it does have an extremely tenuous layer of atoms above the surface called an exosphere. It’s usually called the Moon’s “atmosphere” but it hardly qualifies: the pressure at the surface is only about a quadrillionth of the atmospheric pressure at Earth’s sea level. The total mass is probably only a couple of dozen tons. Mind you, that’s spread over the entire surface of the Moon, about 38 million square kilometers! So it’s thin. Earth’s air has a mass of about 5 quadrillion tons, and is about 100 trillion times denser at the surface.

The Moon’s exosphere so thin the atoms don’t even bump into each like they do in a dense gas, so they aren’t suspended by pressure like on Earth; instead they either travel upward in ballistic arcs that bring them back to the surface or they’re lost to space if they get enough energy to escape (a velocity of roughly 2.4 km/s). The source of this material has long been assumed to be the regolith, the pulverized rocks on the lunar surface. Something is causing them to eject atoms that then fly up.

What’s causing the atoms to get flung away? Scientists have assumed the rocks are zapped by some combination of the solar wind and barraged by micrometeorite impacts. The solar wind is made up of subatomic particles moving at extremely high speed (over a million km/hr) and when these hit rocks they can dislodge atoms that then fly away. Teeny tiny rocks (smaller than a millimeter, typically) orbiting around the solar system can also impact the surface and dislodge atoms, again causing them to be ejected and feed the exosphere.

But which is it?

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