Tag: gravity

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NASA Tries Crash Landing on Mars

Like a car’s crumple zone, the experimental SHIELD lander is designed to absorb a hard impact.

NASA has successfully touched down on Mars nine times, relying on cutting-edge parachutes, massive airbags, and jetpacks to set spacecraft safely on the surface. Now engineers are testing whether or not the easiest way to get to the Martian surface is to crash.


Rather than slow a spacecraft’s high-speed descent, an experimental lander design called SHIELD (Simplified High Impact Energy Landing Device) would use an accordion-like, collapsible base that acts like the crumple zone of a car and absorbs the energy of a hard impact.


The new design could drastically reduce the cost of landing on Mars by simplifying the harrowing entry, descent, and landing process and expanding options for possible landing sites.


“We think we could go to more treacherous areas, where we wouldn’t want to risk trying to place a billion-dollar rover with our current landing systems,” said SHIELD’s project manager, Lou Giersch of NASA’s Jet Propulsion Laboratory in Southern California. “Maybe we could even land several of these at different difficult-to-access locations to build a network.”


Much of SHIELD’s design borrows from work done for NASA’s Mars Sample Return campaign. The first step in that campaign involves the Perseverance rover collecting rock samples in airtight metal tubes; a future spacecraft will carry those samples back to Earth in a small capsule and safely crash land in a deserted location.
Studying approaches for that process led engineers to wonder if the general idea was reversible, said Velibor Ćormarković, SHIELD team member at JPL.


“If you want to land something hard on Earth, why can’t you do it the other way around for Mars?” he said. “And if we can do a hard landing on Mars, we know SHIELD could work on planets or moons with denser atmospheres.”
To test the theory, engineers needed to prove SHIELD can protect sensitive electronics during landing. The team used a drop tower at JPL to test how Perseverance’s sample tubes would hold up in a hard Earth landing. Standing nearly 90 feet (27 meters), it features a giant sling – called a bow launch system – that can hurl an object into the surface at the same speeds reached during a Mars landing.


On Aug. 12, the team gathered at the drop tower with a full-size prototype of SHIELD’s collapsible attenuator – an inverted pyramid of metal rings that absorb impact. They hung the attenuator on a grapple and inserted a smart phone, a radio, and an accelerometer to simulate the electronics a spacecraft would carry.


Sweating in the summer heat, they watched SHIELD slowly rise to the top of the tower.


“Hearing the countdown gave me goose bumps,” said Nathan Barba, another SHIELD project member at JPL. “The whole team was excited to see if the objects inside the prototype would survive the impact.”


In just two seconds, the wait was over: The bow launcher slammed SHIELD into the ground at roughly 110 miles per hour (177 kilometers per hour). That’s the speed a Mars lander reaches near the surface after being slowed by atmospheric drag from its initial speed of 14,500 miles per hour (23,335 kilometers per hour) when it enters the Mars atmosphere.


Previous SHIELD tests used a dirt “landing zone,” but for this test, the team laid a steel plate 2 inches (5 centimeters) thick on the ground to create a landing harder than a spacecraft would experience on Mars. The onboard accelerometer later revealed SHIELD impacted with a force of about 1 million newtons – comparable to 112 tons smashing against it.


High-speed camera footage of the test shows that SHIELD impacted at a slight angle, then bounced about 3.5 feet (1 meter) into the air before flipping over. The team suspects the steel plate caused the bounce, since no bounce occurred in the earlier tests.


Upon opening the prototype and retrieving the simulated electronic payload, the team found the onboard devices – even the smart phone – survived.


“The only hardware that was damaged were some plastic components we weren’t worried about,” Giersch said. “Overall, this test was a success!”


The next step? Designing the rest of a lander in 2023 and seeing just how far their concept can go.


https://www.jpl.nasa.gov/news/why-nasa-is-trying-to-crash-land-on-mars

AR Issue #79

The Roswell Miracle Metal

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Telling up from Down for Insects and Drones

While drones typically use accelerometers to estimate the direction of gravity, the way flying insects achieve this has been shrouded in mystery until now, as they have no specific sense of acceleration. In a new study, European scientists1 have shown that drones can assess gravity using visual motion detection and motion modeling together.

To develop this new principle, scientists have investigated optical flow, that is, how an individual perceives movement relative to their environment. It is the visual movement that sweeps across our retina when we move. For example, when we are on a train, trees next to the tracks pass by faster than distant mountains. The optical flow alone is not enough for an insect to be able to know the direction of gravity.


However, the research team discovered that it was possible for them to find this direction by combining this optical flow with a modeling of their movement, i.e. a prediction of how they will move. The conclusions of the article show that with this principle it was possible to find the direction of gravity in almost all situations, except in a few rare and specific cases such as when the subject was completely immobile.


During such perfect stationary flights, the impossibility of finding the direction of gravity will destabilize the drone for a moment and therefore put it in motion. This means the drone will regain the direction of gravity at the next instant. So these movements generate slight oscillations, reminiscent of insect flight.


Using this new principle in robotics could meet a major challenge that nature has also faced: How to obtain a fully autonomous system while limiting payload. Future drone prototypes would be lightened by not needing accelerometers, which is very promising for the smallest models of the size of an insect. (https://www.nature.com/articles/s41586-022-05182-2)

AR Issue #122

Flying Machines: the Downside

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Measuring Gravity

The gravitational constant G determines the strength of gravity—the force that makes apples fall to the ground or pulls the Earth in its orbit around the sun. It is part of Isaac Newton’s law of universal gravitation, which he first formulated more than 300 years ago. The constant cannot be derived mathematically; it has to be determined through experiment.

Over the centuries, scientists have conducted numerous experiments to determine the value of G, but the scientific community isn’t satisfied with the current figure. It is still less precise than the values of all the other fundamental natural constants—for example, the speed of light in a vacuum.


One reason gravity is extremely difficult to quantify is that it is a very weak force and cannot be isolated: when you measure the gravity between two bodies, you also measure the effect of all other bodies in the world.
“The only option for resolving this situation is to measure the gravitational constant with as many different methods as possible,” explains Jürg Dual, a professor in the Department of Mechanical and Process Engineering at ETH Zurich. He and his colleagues conducted a new experiment to redetermine the gravitational constant and have now presented their work in the scientific journal Nature Physics.


To rule out sources of interference as far as possible, Dual’s team set up their measuring equipment in what used to be the Furggels fortress, located near Pfäfers above Bad Ragaz, Switzerland. The experimental setup consists of two beams suspended in vacuum chambers. After the researchers set one vibrating, gravitational coupling caused the second beam to also exhibit minimal movement (in the picometre range – i.e., one trillionth of a metre). Using laser devices, the team measured the motion of the two beams, and the measurement of this dynamic effect allowed them to infer the magnitude of the gravitational constant.


The value the researchers arrived at using this method is 2.2 percent higher than the current official value given by the Committee on Data for Science and Technology. However, Dual acknowledges that the new value is subject to a great deal of uncertainty: “To obtain a reliable value, we still need to reduce this uncertainty by a considerable amount. We’re already in the process of taking measurements with a slightly modified experimental setup so that we can determine the constant G with even greater precision.” Initial results are available but haven’t yet been published. Still, Dual confirms that “we’re on the right track.” (https://www.nature.com/articles/s41567-022-01642-8)

AR Issue #73

Defying Gravity

by Len Kasten