StarDate

The Moon is lumpy. In fact, its gravity is the lumpiest of any body in the solar system. That makes it tricky for spacecraft to maintain the right speed and altitude as they orbit the Moon.

The lumps are known as mass concentrations – “mascons.” They were discovered by Lunar Orbiter 1, which arrived at the Moon 60 years ago. You can’t see them – only feel their gravitational pull.

Lunar Orbiter was designed to survey possible landing sites for Apollo astronauts. But as it circled the Moon, its orbit changed in unexpected ways. As it flew over some parts of the Moon, it sped up a little and dropped closer to the surface. The same effect was seen in the orbits of later missions as well.

The effect could be dramatic. Apollo 16 left a small satellite in orbit in April of 1972. It was supposed to stay in orbit for several months. Instead, it crashed after just one month.

Many of the mascons are associated with giant impact basins. They formed when asteroids slammed into the Moon long ago. Dense molten rock bubbled up below the scars, then cooled and hardened. This rock is denser than the surrounding rock. That makes its gravitational pull a little stronger.

Today, we have good maps of the mascons – allowing satellites to escape their fatal attraction.

The crescent Moon is quite low in the sky as twilight fades the next couple of nights. Venus, the “evening star,” stands close by.

Script by Damond Benningfield

The first of a series of lunar “spy” satellites entered orbit around the Moon 60 years ago today. There wasn’t any secret about it – it was conducted in full public view. But its camera system was adapted from one built for an Air Force spy satellite.

Lunar Orbiter 1 was the first American spacecraft to orbit the Moon – a half-dozen earlier attempts had all failed. Its main goal was to snap high-resolution pictures of possible landing sites for Apollo astronauts. But it would also map a good portion of the lunar surface, and take a close-up look at a few spots on the far side. And it would measure the Moon’s gravitational and magnetic fields.

Its camera system used two lenses – one for close-ups, the other for wider views. The images were recorded on 65-millimeter film, then processed in an on-board lab. Finally, they were scanned and transmitted to Earth.

Eastman Kodak had created the camera system for a spy satellite called SAMOS. But for that craft, the film was dropped back to Earth, where it was grabbed by an airplane as it parachuted toward the surface – something they couldn’t do from a quarter of a million miles away.

Lunar Orbiter 1 took more than 200 pictures. They were combined with the images from four successor missions to produce the most comprehensive atlas of the lunar surface to that time. And scientists continue to study the images today.

Script by Damond Benningfield

A stellar vanishing act led to a revelation about an iceball far from the Sun: It has a thin atmosphere. It’s the smallest object with a known atmosphere in the entire solar system.

The object is 2002 XV93. It’s in the Kuiper Belt – a wide “doughnut” beyond the orbit of Neptune, the Sun’s outermost major planet. The belt contains millions of icy bodies, including Pluto.

XV93 is about 300 miles in diameter – just one-fifth as big as Pluto.

Astronomers in Japan watched the little iceball in January of 2024. They’d calculated that XV93 would pass in front of a star, hiding the star from view. As they watched the star disappear, it didn’t blink out instantly. Instead, it dimmed a bit before it passed behind XV93. And it took a moment to reach full brightness when it returned to view. That meant that something was partially obscuring the star: an atmosphere. It’s only about one percent as thick as Pluto’s atmosphere, which is a bare wisp.

XV93’s gravity is extremely weak, so it can’t hang on to any atmosphere for long. So the gases might vent into space from its interior, which would keep the atmosphere going. Or they might have surrounded the object after it was hit by a small comet. In that case, the atmosphere would start to vanish quickly – eventually leaving XV93 airless.

Script by Damond Benningfield

After teaming up with the Sun to produce a solar eclipse today, the Moon will make another skywatching spectacle possible tonight – the Perseid meteor shower. It’s expected to be at its peak tonight. And since the Moon is new, it won’t be around to interfere with the sparklers.

The Perseids occur every August, as Earth flies through the orbital path of Comet Swift-Tuttle. The comet is a ball of frozen gases mixed with bits of rock and metal. As it approaches the Sun, some of its gas vaporizes. That releases some of the heavier material, known as comet dust.

As Earth zips through this material, some of the dust grains slam into our atmosphere at more than a hundred thousand miles per hour. They vaporize instantly, creating meteors – streaks of light across the sky.

Swift-Tuttle returned to the inner solar system a third of a century ago. That was its first appearance since the Civil War, so the Perseids were especially good for a while. With the comet retreating into the outer solar system, though, there’s less comet dust to feed the shower.

The meteors enter the atmosphere from the direction of Perseus – hence the name. But they can streak across any part of the sky, so you don’t have to look to a specific region to see them.

To view the Perseids, find a safe, dark observing site away from the pesky glare of city lights. Then watch the sky for cosmic sparklers.

Script by Damond Benningfield

The top of the world will see a total solar eclipse tomorrow. A partial eclipse will encompass a wider slice of the globe, including Alaska and parts of the northeastern United States.

A solar eclipse occurs when the Moon passes directly between Earth and the Sun, blocking the Sun from view. The Sun’s hot but faint outer atmosphere, the corona, looks like silvery tendrils radiating away from the Moon.

This eclipse begins when the lunar shadow first touches Earth, over northeastern Russia. The eclipse path then passes near the north pole, slides across parts of Greenland and Iceland, and finally over northern Spain before vanishing over the Mediterranean Sea. At its peak, off the coast of Iceland, totality will last for 2 minutes, 18 seconds.

A partial eclipse, where the Moon covers only a portion of the solar disk, will flank that path. In addition to parts of North America, it’ll encompass much of western Europe and western Africa.

From far-northern Alaska, the Moon will cover more than half of the Sun’s disk, at about 8:30 a.m. From Anchorage, about a quarter of the Sun will be covered. And from the northeast, the greatest eclipse takes place in northern Maine, shortly before 2 p.m.

If you’re in one of those areas, remember to protect your eyes. Watch the eclipse only through special glasses or dark welder’s glass, and never look at the unfiltered Sun.

Script by Damond Benningfield