Asteroid vs. Comet

Mars and Jupiter are siblings – planets born at the same time, from the same disk of debris around the infant Sun. And they’re fairly close together – the fourth and fifth planets out. But other than that, they don’t seem to have much in common. They don’t look alike, their composition is different, and they’re wildly different sizes. The two planets will huddle close together in the morning sky the next few days. They climb into good view by about one o’clock, and are high in the sky at dawn. Jupiter looks like a brilliant star. Mars is just above it as they rise tomorrow, but will slide past Jupiter over the following few days. At their closest, they’ll be separated by about one degree – the width of a pencil held at arm’s length. Jupiter is the largest and heaviest planet in the solar system – about 11 times the diameter of Earth. It’s a big ball of gas with a small, dense core. It formed far enough from the Sun that it was surrounded by lots of ice and gas, which it pulled in to grow to its enormous proportions. Mars is only a little more than half of Earth’s diameter – roughly five percent the diameter of Jupiter. It’s a dense ball of rock and metal. It formed much closer to the Sun, where there was much less ice and gas. And Jupiter’s gravity might have stirred up things where Mars was born, limiting its growth. So Mars remained small – a mere fraction the size of its giant sibling.   Script by Damond Benningfield

Our solar system is filled with millions of leftovers – the chunks of rock and ice that weren’t incorporated into the planets and moons. The icy ones are known as comets, while the rocky ones are asteroids. But there’s not that much difference between them. They have similar ingredients, but in different amounts. How we classify them has more to do with where they were born and how close they get to the Sun. The planets formed from a disk of material that encircled the infant Sun. Small grains of ice and rock stuck together to make larger bodies. These larger objects then stuck together to make the planets and some of their moons. In the outer solar system, it was cold enough for these building blocks to contain a lot of ice. But closer to the Sun it was hotter, so the building blocks consisted mainly of rock and metal. The outer planets kicked many of the leftovers deeper into space. One of them occasionally falls toward the Sun. Solar energy vaporizes some of the ice, releasing some of the solid particles as well. The Sun pushes some of this gas and dust away, forming a long, bright tail – a comet. Many asteroids have a fair amount of ice as well, just not enough to make a tail. Most asteroids are concentrated in a broad belt between Mars and Jupiter. About one-and-a-half million have been cataloged, with millions more awaiting discovery – leftovers from the birth of the solar system. Script by Damond Benningfield

If anyone ever makes it to Pluto, they might want to stay away from the bottoms of steep craters. Occasional jiggling and shaking on the surface might trigger big, fast landslides – some of them big enough to bury a small city. Planetary scientists recently found evidence of several massive pileups in images snapped by New Horizons. The spacecraft flew past Pluto in 2015, giving us our only close look at everyone’s favorite dwarf planet. The aftermath of six landslides was detected on the inner slopes of three craters. All of the craters are near the edge of Sputnik Planitia, a heart-shaped ice plain that’s Pluto’s most prominent feature. The images revealed either fields of large boulders or indentations in the crater walls where material had slumped away. Because of Pluto’s weak gravity and icy surface, the slides traveled a long way. Material slid from about six to nine miles before coming to rest, and dropped up to a mile and a half from a crater’s rim to its floor. The largest slide covered an area large enough to bury Boston or Fargo. One of the landslides could have been triggered by an impact that gouged a small crater near the edge of the crater that contained the debris. The others could have been powered by temperature changes. Some of the ices at the surface vaporized, weakening the crater walls. So whatever the cause, it might be wise to stay away from the slopes of Pluto’s craters. Script by Damond Benningfield

Two “super-puff” planets appear to orbit a bright star about 1100 light-years from Earth. They’re as big as Jupiter, the giant of our own solar system. But they’re only a few percent the mass of Jupiter. That makes them extremely puffy – the least-dense worlds yet seen – less dense than cotton candy. Such planets are called super-puffs. The planets orbit the star TOI 791. It’s a little bigger and heavier than the Sun, and a few times brighter. But it’s too far south to see from all but a sliver of the United States. The planets were discovered by TESS, a planet-hunting space telescope. But it took years for astronomers to winnow out their details. They used almost a dozen ground-based telescopes, including one near the south pole. The planets periodically pass in front of their star, dimming the star’s light. But the gravity of each planet tugs at the other, changing the timing of these transits. The scope of the change revealed the masses of the planets. One planet orbits at a little less than the distance from Earth to the Sun, while the other is a bit closer. They might have been born farther out, then migrated to their current spots – a scenario that could explain their puffiness. Astronomers plan to study the system with James Webb Space Telescope. It can reveal the composition of the planets’ atmospheres, telling us more about where the planets formed – and how they became super-puffs.   Script by Damond Benningfield

You can forget about sending a welcome basket to any neighbors on the planets orbiting Barnard’s Star, the second-closest star system to Earth. Conditions on all four of its planets are likely to be hostile to life. Barnard’s Star is just six light-years away – only the triple-star system Alpha Centauri is closer. The star is much smaller and lighter than the Sun. And at visible wavelengths, it’s less than one-thousandth as bright as the Sun. In 2024, four planets were discovered orbiting the star. None of them is more than about four million miles out – about four percent the distance from Earth to the Sun. All of them probably are locked so that one hemisphere always faces the star, just as one hemisphere of the Moon always faces Earth. And they orbit in such a way that the system has remained stable over the star’s 10-billion-year lifetime. A recent study provided new details on all four planets. All of them are less massive than Earth and Venus, but heavier than Mars. The combination of their masses and the composition of Barnard’s Star itself suggests they’re dense and rocky, like Earth. They’re probably quite dry, and none of them is likely to have an atmosphere. That makes them unlikely places to find neighbors. Barnard’s Star is well up in the west-southwest at nightfall. It’s not far from the brightest star of Ophiuchus. But it’s so faint that you need a telescope to pick it out.   Script by Damond Benningfield

The planet Venus is climbing higher into the morning sky day by day. It’ll reign as the “morning star” until summer, when it passes behind the Sun. It’ll return to view as the evening star in the fall. Just about every major culture figured out that the morning star and evening star are really a single object. The people of Sumeria probably knew it as far back as 5,000 years ago. But that didn’t stop people from giving the two apparitions different names. In ancient Greece, morning Venus was known as Phosphorous – “bringer of light.” Evening Venus was Hesperos. They originally were thought of as separate bodies, but the names were kept even after Venus’s true nature was discovered. In Rome, morning and evening were Lucifer and Vesper. And in China, the morning star was sometimes called The Great White, while the evening star was The Excellent Western One. Venus commutes between morning and evening because of the relative orbits of Venus and Earth. Venus spends eight or nine months in the morning sky, then disappears behind the Sun for about three months. Then it’s eight or nine months in the evening sky. It then disappears for a few days as it crosses between Earth and the Sun – starting the cycle all over again. Watch for Venus as it climbs higher into the sky over the next couple of months. Right now, it’s quite close to the star Spica. But it’ll pull away from the star over the coming days.   Script by Damond Benningfield

Two planetary siblings are headed for a family reunion. They’re moving toward one another, and will be at their closest in a little more than a week. And they’re both bright, so you can follow their progress day by day. Jupiter and Mars climb into good view in the east by about 1:30 or 2. They’re high in the sky at dawn. Jupiter is the brightest object in the night sky after the Moon and Venus, so it’s an especially easy target. Mars rises close above it. It’s only a few percent as bright as Jupiter, but it’s brighter than most of the true stars. And it has a definite orange tint, helping it stand out. The planets are appearing close together because of their orbits around the Sun. Mars is only about a third as far as Jupiter, so its orbital speed is much higher. And it’s closer to Earth, which makes it appear to move even faster. So as it slides eastward against the background of stars, Mars will overtake its brighter and bigger sibling. The two planets will be especially close on the mornings of the 15th through the 18th. They’ll be separated by about one degree – the width of a pencil held at arm’s length. As a bonus, the star Regulus – the heart of the lion – tags along behind them. Mars will pass by it later in the month. So keep an eye on Mars as it stages two beautiful encounters in the early morning sky. Script by Damond Benningfield

Early risers are in for a treat tomorrow – a tight triangle formed by the Moon, the planet Venus, and the star Spica. It’s in the east-southeast as dawn twilight paints the sky. But the group is quite low, so there’s not much time to look for it. And you’ll need a clear eastern horizon. The Moon is about as close to the Sun as it can get and still be visible. The Sun will light up only about three percent of the lunar hemisphere that faces our way. But you can see the rest of the lunar disk thanks to earthshine – sunlight reflected off of Earth. You should be able to make out many of its features with your eyes alone. But binoculars will really enhance the view. By Sunday morning, the Moon will be lost in the Sun’s glare as it heads toward “new” later in the day – the start of a new cycle of phases. It’ll return to view in the evening sky on Wednesday, also a thin crescent. Venus is the brilliant “morning star,” so it’s an easy target. It’s climbing away from the Sun and into better view day by day. It’ll be in good view well before twilight in a few days. Spica, the leading light of Virgo, is quite close to the left of Venus. It’s the faintest member of the dawn trio – about half a percent as bright as Venus. But its proximity to the morning star will help you pick it out – completing this beautiful treat in the early morning sky. We’ll talk about two other morning planets tomorrow.   Script by Damond Benningfield

Two crescents hang out in the twilight the next couple of mornings: the Moon and the planet Venus. They’re quite low in the east at dawn. Venus is the “morning star.” It’s below the Moon tomorrow, but will be closer above the Moon on Saturday. Venus and the Moon appear as crescents for the same reason – both of them are passing between Earth and the Sun. That means their daylit sides are facing the Sun, while we see mainly their nightsides – the Sun is lighting only a sliver of the hemisphere that’s facing Earth. It’s obvious that the Moon is a crescent. But you need strong binoculars or a telescope to see Venus as one. If you take a close look, you’ll notice that Venus and the Moon are almost equally lit up – less than 10 percent of the visible surface is in sunlight tomorrow, with the rest in the dark. Venus and the Moon are headed in opposite directions. The Moon is dropping toward the Sun in our sky. It will cross the line between Sun and Earth on Monday. It’ll be “new” then, beginning a new cycle of phases. Venus crossed that line two weeks ago, so it’s pulling higher into the morning sky. So the crescent will grow a little fatter day by day. Even though we’re seeing mostly its nightside, Venus is quite bright. That’s because it’s closest to Earth about now, so it’s a big target. And it also reflects a lot of the sunlight that strikes it in our direction – adding to the luster of this beautiful crescent.   Script by Damond Benningfield

Two Mars-bound spacecraft have been just loitering about. They were launched a year ago. But the timing wasn’t right for them to reach Mars. So they were sent to a point in space where the gravity of Earth and the Sun are balanced – a place where they could “loiter.” But now, their time has come. They’ll leave that spot in a few days, beginning their trip to the Red Planet. Escapade consists of two small spacecraft, called Blue and Gold. After they enter orbit around Mars, they’ll measure its magnetic field and its interactions with the solar wind – a flow of particles from the Sun. That will help scientists understand how Mars lost much of its atmosphere. Escapade was supposed to launch several years ago. But there were problems with its potential rides to space, so it kept getting delayed. And if you miss a launch window, it’s a long time until the next one opens. You can’t send a craft to Mars anytime you want. You have to wait for the proper alignment – when Earth and Mars are going to be close together. Otherwise, the distance between them is so great that you just can’t get there. That alignment comes about every 26 months. NASA decided to go ahead and launch Escapade a year early and have the two craft wait in space. They’ll soon fire their engines to nudge them toward Earth. They’ll use Earth’s gravity to kick them toward Mars, with arrival at the Red Planet in September. Script by Damond Benningfield