Colossal asteroid impact forever changed the balance of the moon

An ancient collision is to blame for all the "holes" on the dark side of the moon.

One side of the moon is littered with far more craters than the other, and researchers finally know why: A massive asteroid that slammed into the moon around 4.3 billion years ago wreaked havoc in the moon's mantle, according to a new study.

More than 9,000 visible craters pockmark the moon, thanks to barrage of impacts from meteors, asteroids and comets over billions of years, according to the International Astronomical Union. However, these craters are not evenly distributed across the lunar surface. The far side of the moon, which people never see from Earth because the moon is tidally locked (meaning that it takes the same amount of time for the moon to rotate and orbit Earth), has a considerably higher concentration of craters than the visible nearside.

The nearside of the moon has fewer pits because the surface is covered in lunar maria — vast stretches of solid lava that we can see with the naked eye on Earth as dark patches on the moon. These lava fields likely covered up the craters that would otherwise have marked the moon's nearside. The far side of the moon has almost no lunar maria, which is why its craters are still visible.

Scientists have long suspected that lunar maria formed in the wake of a massive collision around 4.3 billion years ago. This collision created the South Pole–Aitken basin (SPA), a huge crater with a maximum width of around 1,600 miles (2,574 kilometres) and a maximum depth of 5.1 miles (8.2 km), which is the largest pit on the moon and the second largest confirmed impact crater in the solar system. However, until now researchers were unable to explain why only the nearside of the moon has lava fields.

Related: How many space rocks hit the moon every year?

The new study finds that the SPA impact created a unique phenomenon inside the moon's mantle, the layer of magma below the crust, that affected only the nearside.

"We know that big impacts like the one that formed SPA would create a lot of heat," lead author Matt Jones, a doctoral student of planetary science at Brown University, said in a statement. "The question is how that heat affects the moon's interior dynamics."

Researchers already knew the near side’s lava fields originated within the moon's mantle, because lunar samples brought back by the Apollo missions contained radioactive, heat-generating elements such as potassium, phosphorus and thorium that are all suspected to be found in abundance within the lunar mantle, according to the statement.

In the new study, computer simulations revealed that the SPA impact would have created a heat plume within the mantle that pushed the radioactive elements toward the crust. The researchers repeated the simulation for a number of possible scenarios of the SPA impact, including direct hits and glancing blows, and found that regardless of how the asteroid hit, the mantle impacts would have only affected the nearside of the moon.

Put another way, when a space rock collided with the moon, it caused lava from the mantle to pour out on the nearside, burying many of its older impact craters.

"What we show is that under any plausible conditions at the time that SPA formed, it ends up concentrating these heat-producing elements on the nearside," Jones said. "We expect that this contributed to the mantle melting that produced the lava flows we see on the surface."

The researchers are pleased to have solved what they described as "one of the most significant questions in lunar science," according to the statement.

"The SPA impact is one of the most significant events in lunar history," Jones said. Being able to better understand how it shaped the two sides of the moon we see today is "really exciting," he added.

James Webb’s 30 Days of Terror

It’s been a long and winding road getting the James Webb Space Telescope from concept to reality. And finally, after decades of planning, work, delays, and cost overruns, the next generation of space telescopes is finally ready to launch. But even now, as the telescope might be secretly traveling by cargo ship to the European Space Agency (ESA) launch site in French Guiana, everyone involved with the JWST project knows a successful launch isn’t the final victory.

In reality, post launch is when the real nail-biting begins. While the Mars rover teams undergo “Seven Minutes of Terror” to land their spacecraft on the Red Planet, the JWST teams will have more than 30 days of excruciating, slow-motion terror as the telescope embarks on its month-long-day, 1.5-million-kilometer (million-mile) journey out to the second Lagrange point (L2).

And all the while, JWST will be unfolding to its desired configuration, with more than 40 major deployments of various systems, needing hundreds of actuators to fire and hold mechanisms to release, along with cables to unspool, joints to work and electrical systems to activate.

Everything has to work perfectly during the 30 straight days of make-or-break for the mission, all taking place in the unyielding environment of space, with the telescope on its own. Not only are there 30 days of terror, there could also be 30 sleepless nights for everyone involved.

Artist conception of the James Webb Space Telescope. Credit: NASA

Of course, it all starts with the launch, a terror in itself.

“We’re putting this incredibly precious resource on top of a controlled explosion,” said Heidi Hammel, an interdisciplinary scientist and vice president of the Association of Universities for Research in Astronomy. “It’s frightening, but rocket science is what it is. It will be a huge sigh of relief to have a successful launch.”

If all goes well, the deployment excitement/terror starts about 30 minutes into the flight. The Ariane 5 rocket will provide thrust for roughly 26 minutes, sending JWST about 10,400 kilometers on its trip. After second stage cutoff, Webb will separate detach from the Ariane 5’s second stage.

“This will trigger the solar arrays to deploy 30 minutes after launch,” said Massimo Stiavelli, head of Webb’s mission office at the Space Telescope Science Institute (STScI). “This is crucially important because we need power. But this is only the first of a number of important deployments on the way out to L2.”

The next event is what keeps Stiavelli up at night. While the Ariane will put JWST on a direct route to L2, without first orbiting Earth, an important thruster firing will ensure the telescope is headed exactly in the right direction.

“We have to turn on the observatory’s rocket engine to put us out towards the desired orbit of L2,” he said. “The Mid Course Correction 1 (MCC1) could take place about 12. 5 hours after launch. This is the most important burn of the mission.”

“JWST has to go into the orbit at L2, that’s how the mission is designed,” said Hammel. “If the thrusters don’t fire to get us there, there goes the mission.

Following the thruster firing is another important moment, the release and deployment of the high gain antenna to for communication to the telescope, and for the all-important science data to be sent back to Earth.

Within Webb’s first week in space will be a second trajectory correction maneuver, and then comes a sequence of major deployments with nearly 200 actuators needed to work, just to prepare for JWST’s sunshield to unfold. This includes booms extending and radiators releasing and deploying.

During a test, engineers and technicians fully deployed all five layers of the James Webb Space Telescopes sun-shield. Image Credit: NASA/Chris Gunn

Here’s where the real nail-biting starts. The tennis-court-sized sunshield itself requires over 150 release mechanisms to fire correctly over the course of three days.

“The number of unexploded actuators can give one a little bit of a headache, as all of them have to work,” said Helmut Jenkner, a longtime scientist at STScI. “At even over 99.9% reliability, if you multiply that by the number of actuators, you get to a fairly hair-raising number.”

The complicated sunshield deployment involves around 7,000 parts, including 400 pulleys, numerous cables and eight motors. But the sunshield’s deployment is crucial to shading the telescope from any heat or light from the Sun, Earth and Moon, to keep the telescope’s infrared components as cold as possible. This will allow JWST to detect the faint signatures of distant object in the universe. The telescope and scientific instruments will start to cool rapidly in the shade of the sunshield, but it will take several weeks for them to cool all the way down and reach stable temperatures.During the second week after launch, the telescope will begin to take shape, first with the secondary mirror deployment. Then comes the big moment, when JWST’s 6.5 primary mirror begins to unfold. The 18 gold-plated beryllium segments will unfurl, beginning with the side wings. Then 132 small actuators will push or pull each of the mirror segments into a micron-precise alignment, putting the primary mirror into focus. Again, everything must work perfectly. While the first month is the tensest part of deployment, it will take six months for all the instruments to be turned on, calibrated and commissioned. Only then will scientists see “first light” from the telescope. “There are myriad ways that things could go wrong,” Hammel admitted. “But over the past 20 years and especially over the past 5 years, we have tested this telescope and all the systems in every way imaginable: shaking it, thermal cycling it, putting it into zero pressure. We’ve really exercised it in order to find out all the little things that might go wrong, making sure we’ve done everything we can to ensure a successful mission.”

Learn more about JWST at this NASA website, or at the STScI website.

A new measurement puts the Sun 2,000 light-years closer to the center of the Milky Way

Since 1985, astronomers have calculated that we're 27,700 light-years away from the center of the Milky Way, but it's always been hard to estimate because of the gas and dust that block our view of the galactic center. A team of astronomers has made meticulous distance measurements to other stars orbiting within the galaxy and used their movements to estimate our location. They've calculated that we're actually 25,800 light-years from the center.

Knowing Betelgeuse’s size is crucial to understanding its recent bizarre behavior — and predicting when it will go supernova. But it’s harder to figure out than you might think.

Earlier this year, furor surrounded Betelgeuse — the star had “fainted,” dimming more than expected in its usual cycle of brightness changes, leading some so far as to suggest the star might go supernova sometime soon. But even as astronomers begin to get a handle on the red giant’s unexpected behavior, they’re still struggling to understand its basic properties — namely its size and distance.

Meridith Joyce (Australian National University) and colleagues reported in the Astrophysical Journal (arXiv preprint available here) that Betelgeuse is actually smaller, and therefore closer, than previously thought. Not everyone agrees with the results. Nevertheless, the study represents a new — and needed — approach toward understanding this enigmatic giant.