StarDate

Our solar system is an oddball. The way it’s laid out is different from every other planetary system yet discovered. In part, that’s probably because solar systems like ours are harder to find. But it also means that there’s no template for how a planetary system should look.

The solar system consists of eight planets. The closest to the Sun is Mercury, at roughly a third of the distance between the Sun and Earth. The most distant planet is Neptune, 30 times farther than Earth is. The four inner planets are small and rocky, while the outer planets are big and either gassy or icy.

Most of the other known systems consist of only one or two planets. Most of them are quite close to their parent stars — often much closer than Mercury is to the Sun. Some of these planets are much heavier than Jupiter, the giant of the solar system. But they’re so close in that they’re blazing hot.

Several systems contain at least six planets, with some topping out at eight. In most of these, the planets are all packed tight. In some cases, all the planets are closer in than Earth is to the Sun.

And many planets in other systems are unlike anything in our own. They’re between the mass of Earth — the biggest of the little worlds — and Neptune — the smallest of the big worlds. The fact that the solar system has none helps solidify its standing as an oddball — a system that’s different from all the rest.

Script by Damond Benningfield

Earlier this year, a Chinese lander photographed something never before seen on the Moon: growing plants. Cotton seeds in a small chamber on the lander had sprouted, producing green leaves. Alas, the plants were doomed. They withered when night fell — and outside temperatures dropped to a couple of hundred degrees below zero.

The Moon experiences some of the wildest temperature swings in the solar system — all because the Moon doesn’t have an atmosphere.

On Earth, the atmosphere traps heat from the Sun and distributes it around the planet. So the difference between the hottest and coldest spots on Earth typically varies by a couple of hundred degrees Fahrenheit.

On the Moon, though, there’s no atmosphere to keep things in check. So at the equator, daytime temperatures peak at about 250 degrees. At night, all that heat radiates into space. As a result, the temperature around the equator can drop to almost 300 below.

The polar regions are colder. In fact, the bottoms of some craters at the poles never see sunlight. Temperatures in the craters can reach more than 400 below — as cold as Pluto. There are deposits of ice in some of those craters — possible supplies of water, rocket fuel, and more for future lunar colonies.

The Moon has a bright companion tonight. Regulus is the brightest star of Leo, the lion. It’s below the Moon at nightfall, and even closer to the Moon as they set in the wee hours of the morning.

Script by Damond Benningfield

Ten years from today, the asteroid Apophis will pass just 20,000 miles from Earth — close enough to look like a faint star streaking across the night sky.

When Apophis was discovered, though, it looked like the light show might be much more ominous. Astronomers initially gave Apophis a 2.7 percent chance of hitting Earth in 2029. Since the asteroid is about a quarter of a mile wide, an impact would devastate thousands of square miles.

That scary thought inspired a scary name. Apophis is the Greek name for Apep, the Egyptian god of chaos. He was depicted as a snake-like creature who lived in the dark underworld. Each night, as the Sun passed through that realm, Apep tried to eat the Sun god, Re, and plunge the world into darkness and chaos.

As astronomers got a better track of Apophis’s orbit, though, they ruled out a collision in 2029. But there was still a possibility that it could hit us in 2036. It would require Apophis to fly through a point in space only a few hundred yards wide during the 2029 encounter. More tracking has ruled out that possibility as well. But we’re still not completely safe. There’s a one-in-150,000 chance it’ll hit us in 2068.

Earth will have a big effect on Apophis during the 2029 encounter. It’ll dramatically change its orbit, moving its average distance from the Sun outward by more than 15 million miles.

Script by Damond Benningfield

OSIRIS-Rex is starting a new job about now. The spacecraft has spent the last couple of months mapping the surface of a small asteroid known as Bennu. Now, it’s beginning to zero in on the equator. It’ll identify as many as a dozen areas that might be good spots for picking up a couple of ounces of dirt. It’ll swoop down on the winning location next summer, then bring its bounty to Earth in 2023.

OSIRIS-Rex entered orbit around Bennu in December. The asteroid is only about a third of a mile in diameter, and it looks a bit like a spinning top. The craft’s instruments have found minerals on Bennu that formed in the presence of water.

Bennu’s orbit periodically brings it close to Earth. It’ll come within half a million miles in 2060, for example. Predicting its orbit beyond that is tricky. But astronomers give Bennu about a one-in-2700 chance of hitting Earth in the late 22nd century.

OSIRIS-Rex will help narrow down those odds. Precise tracking of the spacecraft will reveal how Bennu’s orbit changes as a result of the Yarkovsky effect. It produces a slight “thrust” as areas warmed by sunlight cool off on the asteroid’s nightside.

Ground-based tracking has shown a “push” of about a hundred miles in 12 years — enough to alter calculations of Bennu’s position. The time OSIRIS-Rex spends with Bennu will refine estimates of the Yarkovsky effect, helping astronomers predict the motions of this possible threat to Earth.

Script by Damond Benningfield

Asteroids are easy to push around. The gravity of a planet can shove one into a new orbit. A nudge by another space rock can also give it a big push. And even sunlight can alter its path. That makes it tough to predict an asteroid’s orbit very far into the future.

The change caused by sunlight is known as the Yarkovsky effect. It’s named for an engineer who first published the idea more than a century ago.

As an asteroid orbits the Sun, it spins on its axis, just as Earth does. The side that faces the Sun gets warmer, while the side away from the Sun gets cooler. It cools off by emitting infrared radiation, which gives the asteroid a tiny “push.” Over time, the push builds up, altering the orbit.

It’s especially important to understand this process for asteroids that come close to Earth. A tiny change in orbit can change the odds that an asteroid will hit our planet.

The Yarkovsky effect is hard to predict, though. Scientists have to know the asteroid’s mass, size, and shape. They also need to know the patterns of light and dark on its surface, because lighter colors reflect more sunlight, while darker colors absorb more.

The best measurement of the Yarkovsky effect so far is on an asteroid named Bennu. The effect has altered Bennu’s position by about a hundred miles in just a dozen years. A spacecraft that arrived at Bennu last year is providing an even better measurement. More about that tomorrow.

Script by Damond Benningfield

When engineers first pondered how to send astronauts to the Moon, they thought big. They planned on sending a single rocket — like the rockets in 1950s science fiction. It would fly directly to the Moon, land, then take off and head straight for home.

Such a rocket would have been expensive and taken many years to develop. Astronauts wouldn’t have landed until well after the end-of-the-’60s deadline imposed by President John F. Kennedy.

But an engineer who was born 100 years ago today had a better idea — one that met Kennedy’s goal.

John Houbolt was born in Iowa and grew up in Illinois. He earned degrees in engineering and, in 1942, he went to work for NASA’s predecesor.

When NASA began planning the lunar landing, Houbolt advocated a technique called lunar orbit rendezvous. It would use two spacecraft. Astronauts would leave the mothership and climb into a separate lander. After they’d finished their work on the Moon, they’d rendezvous with the mothership in lunar orbit. Houbolt said the technique would be cheaper, faster, and safer than a one-rocket show.

NASA brass did not agree, though. Houbolt finally pitched the idea in a letter to the associate administrator. Houbolt recalled what happened next:

The scheme worked — sending astronauts to the Moon and safely back to Earth.

Script by Damond Benningfield

One of the brightest stars in northern skies travels through the galaxy with a few friends. But astronomers have had a hard time figuring out if they’re locals, or if they came from beyond the Milky Way.

Arcturus is low in the east as darkness falls, and climbs high across the sky later on. It shines yellow-orange — the result of its relatively cool surface. The star is probably at least seven billion years old. It’s leaving the prime of life, so it’s puffing up — getting bigger, brighter, and redder.

Arcturus travels through the galaxy with more than 50 known companions. Together, they form the Arcturus stream. The stars are all about the same age, and they move in the same speed and the same direction.

But that direction is different from most of the stars in the Milky Way. While most stars orbit in the plane of the galaxy’s flat disk, those of the Arcturus stream orbit perpendicular to the disk. And they move much faster than most of the other stars around us — about 275,000 miles per hour.

That suggested that the stars were born in a smaller galaxy that once orbited the Milky Way. The Milky Way captured the galaxy and stripped away its stars.

Research in the last few years, though, shows that the composition of the stars in the stream matches that of stars born in the Milky Way. So the stars of the Arcturus stream may be home-grown, and pushed into an odd path by the Milky Way itself.

Script by Damond Benningfield

The crescent Moon has two blushing companions the next couple of nights — the planet Mars and the star Aldebaran, the “eye” of the bull. The two orange orbs are above the Moon this evening, with Aldebaran on the left and fainter Mars on the right. They’ll be below the Moon tomorrow night.

Mars and Aldebaran are rare spots of color in a basically monochrome sky. Most objects are so far and faint that we don’t see any color from them at all.

Mars and Aldebaran produce their color in completely different ways. Mars’s tint comes from dust. The dust contains a lot of iron oxide — the compound that gives rust and blood their reddish color. In fact, Mars got its name from that color. It reminded ancient skywatchers of blood, so they named it for the god of war: Ares in Greece, and Mars in Rome.

Martian winds blow the powdery dust around much of the planet, giving it an overall rusty patina — something not seen on any other world.

Aldebaran’s color, on the other hand, is quite common — it’s shared by many other stars.

Aldebaran is nearing the end of its life, so it’s puffed up to giant proportions. As its outer layers of gas expanded, they got cooler. At their current temperature, Aldebaran shines brilliant orange. Many stars have a similar temperature, so orange stars are quite common. Unfortunately, though, most of them are so faint that the eye doesn’t pick up that color, so the stars look white.

Script by Damond Benningfield

The brightest star of the constellation Puppis, which represents the stern of the ship Argo, is one in a million — or one in several million, to be more precise. It’s a member of the hottest and heaviest class of stars, known as Class O. This class is also the rarest of all — there are no more than a few tens of thousands of members in the entire galaxy.

Zeta Puppis is low in the south as night falls this evening. It’s far to the lower left of Sirius, the brightest star in the night sky.

Zeta Pup looks only a fraction as bright as Sirius, but that’s only because of their distances. Sirius is less than nine light-years away, but Zeta Pup is more than a hundred times farther. So if you put the two stars side by side, Zeta Puppis would shine thousands of times brighter.

Class O stars are the most massive “main-sequence” stars — stars that are in the prime of life. They’re many times heavier than the Sun. Such stars are squeezed more tightly by gravity, heating their cores to billions of degrees. That also heats their surfaces, making them much hotter and bluer than the Sun.

The heat in an O star’s core cranks up the rate of nuclear reactions. So even though Zeta Puppis is more massive than the Sun, it’ll live a much shorter life — only about a million years, versus 10 billion years for the Sun. And at the end, it’s likely to blow itself to bits as a supernova — a brilliant demise for a brilliant star.

Script by Damond Benningfield

The constellation Puppis rows low across the south on early spring evenings. It represents the stern of the Argo, the boat that carried Jason and the Argonauts through many adventures.

Most of the stars of Puppis are faint. In fact, its leading light barely ranks among the 70 brightest stars of the night sky.

In most constellations — especially those from the ancient world — the brightest star has a name beginning with “alpha,” the first letter of the Greek alphabet. So in this case, the star would be known as Alpha Puppis. Instead, though, the star is called Zeta Puppis, using the sixth letter of the alphabet.

That naming system was devised by Johann Bayer more than four centuries ago. He generally gave the “alpha” designation to a constellation’s brightest star. The rest of the stars were labeled based on their brightness or their position in the sky.

At the time Bayer came up with his system, though, Puppis didn’t exist. Instead, it was part of a much larger constellation — Argo Navis — the ship Argo.

Well after Bayer’s time, another astronomer, Nicolas de Lacaille, split Argo Navis into three constellations. Puppis represents the ship’s stern, Carina is its keel, and Vela is its sail. But Lacaille kept the Greek letter designations from Argo Navis. So Carina has the first three letters, Vela has the next two, and Puppis starts with number six — Zeta.

We’ll have more about Zeta Puppis tomorrow.

Script by Damond Benningfield