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Raumfahrt - Why Is It So Damn Hard to Land on the Moon? After India failed to safely land on the lunar surface at the last minute—the latest mission to just miss the mark—space travel remains as t

11.09.2019


Millions watched Friday as India attempted to set its Vikram lander down near the moon’s south pole. After separating from its orbiter earlier in the week, Vikram sped toward the lunar surface at a top velocity of roughly 5,381 feet per second. In order to land safely, it would have to come to a near full stop in a matter of minutes.

And then, suddenly, just a mile above its destination, silence.

Vikram stopped communicating with mission control. Across multiple screens in the mission control room, the small icon plotting the lander’s trajectory froze. Indian Space Research Organization (ISRO) engineers worked quickly to regain contact with the lander, but were unable to. Had Vikram landed safely, India would have been the fourth country to land on the lunar surface, after Russia, the U.S., and China.

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This morning, ISRO officially announced that it had located Vikram⁠—toppled over on its side, but likely still intact. The fate of the mission is unknown. It’s unlikely that the Pragyan rover, which was housed inside the lander, will be able to complete its planned 1,640-foot trek.

Now, the agency is working to connect with the lander to find answers to the singular question on everyone’s mind: What went wrong?

What Does This Mean for Vikram?

“The story's still ongoing,” Ryan Kobrick, an aerospace engineer at Embry-Riddle Aeronautical University, tells Popular Mechanics. “There's a possibility they might be able to communicate with [the lander] and there's a possibility they may be able to get some science out of it.”

He suspects Vikram might have suffered from a "communications data overload." As the lander speeds toward the moon, its computer is rapidly processing large streams of data. As with computers here on Earth, that rapid influx of data can really slow things down.

The Chandrayaan-2 mission certainly hasn’t been a failure. The orbiter is still zipping around the moon, snapping pictures at an incredibly high resolution. We’ll still gain insight about the moon thanks to India’s mission. "The most important part of all of this is the data, Kobrick says. As long as you're getting high resolution data, that's the most important thing."

Ultimately, space travel is just difficult⁠—especially if the mission has components that an agency or private entity hasn’t tried before.

“You try to anticipate every single thing that could happen,” Paulo Lozano, a propulsion engineer at Massachusetts Institute of Technology, tells Popular Mechanics. “But at the end of the day, you cannot.”

So ... What's Happening on the Lander?

Everything a lander does has to be autonomous. "You cannot interact with the lander in real time,” says Lozano. Communication is often delayed or impossible during the landing. All the multiple systems aboard the lander must work together seamlessly in order for the launch to be successful.

Inertial sensors aboard the lander pick up information about the acceleration, orientation, and trajectory of the lander as it barrels toward the surface. All this information⁠—plus visual and radar information⁠—is then fed into the lander’s computer, which then processes the data and makes commands in a matter of seconds.

“The computers are an essential part of the process,” Lozano says. “Even if all the hardware is working perfectly, there's the software” that can malfunction.

If the lander is traveling too fast, these sensors can inform the propulsion system, which will slow the spacecraft from breakneck speeds as it approaches the surface. If these sensors malfunctioned, landing would be impossible.

“It would be like you're approaching a stoplight and you're applying the brakes, and then all of the sudden your brakes just cut out,” says Kobrick.

If these systems sense that the lander becomes perilously tilted upon arrival, they can communicate with the propulsion system to stabilize the spacecraft, spurring engines on either side to fire and right the craft. “The goal is to be as upright as possible, that's what mapping the ground below is good for. Nothing is perfectly flat,” says Kobrick. “There's no flat moon.”

Why Is It So Hard to Land on the Moon?

Spacecraft descending toward the lunar surface face a number of challenges. There are high levels of radiation to contend with, and as a spacecraft begins its descent, solar reflections can saturate the instruments and backscattered communication signals may interfere with the primary communication system, says Kobrick.

But the last 10 meters of the descent can be especially challenging. As the spacecraft gets closer to the surface, it kicks up dust, which can jam the lander’s many navigational sensors. Lidar instruments, for example, which use lasers to map the moon’s surface in order to help the craft find the ideal landing spot, may struggle to pinpoint important surface features.

Vikram endeavored to be the first lander to reach the moon’s resource-rich south pole. When ISRO’s Chandrayaan-1 discovered water on the moon, most of the chemical signatures for water ice deposits were found in craters along the moon’s poles. Ice found inside these craters and within the lunar soil could provide a vital source of water as humans venture back to the lunar surface and eventually to Mars. “That’s why the poles are such a big target,” Kobrick says.

Previous missions have opted for landing sites closer to the equator because it takes less energy and less propulsion to get there, Kobrick says. The trajectory to get to the moon’s equator is much simpler, and requires fewer complicated maneuvers. And because the poles are tucked away in the shadows, communication is tougher to establish.

Kobrick argues that, no matter what, the Chandrayaan-2 mission increased ISRO's mission. “Around the world, there were people watching who were not thinking about going to the moon,” says Kobrick. “The more we see international investment into humanity's future in the cosmos, the higher probability we'll actually get people interested in making it happen.”

Quelle: PM

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