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The Physics of NASA’s New Mars Helicopter

5 min read

Sending a rover to Mars is cool—but sending one to Mars along with a helicopter is even better. Yes, that is the plan for the next NASA Mars rover, scheduled for 2020. The idea is to have a driving rover that brings along a small coaxial helicopter. The helicopter will be self-powered and fly for a few minutes a day. The main advantage of the helicopter is that it can scout ahead of the rover and take pictures and stuff—maybe some epic rover selfies. But really, it should be a huge advantage over previous rovers.

Besides being a cool Mars mission, this Mars helicopter is also perfect for some physics questions. Here are some questions and answers regarding this flying robot.

Why doesn’t it have a tail rotor?

The Mars helicopter is not your traditional helicopter. For a helicopter with a single rotor (like the ones you usually see on Earth), the tail rotor is required to counteract the change in angular momentum (and the frictional torque from the air) due to the main rotor spinning. Without the tail rotor, the helicopter would spin out of control and crash—or at least get the passengers very dizzy. A coaxial helicopter has two counter rotating blades. Since the two rotors are spinning in opposite directions, the total angular momentum is zero and there is no need for torque from an extra tail rotor.

Getting rid of the tail rotor also does something else—it saves space. You can make a smaller helicopter by using a coaxial rotor. Smaller is better—especially with the limited space on board the rover. Getting a bigger helicopter on the rover would be like trying to fit an overstuffed rolling suitcase in the overhead bin on your last cross-country flight. Also, I should point out that the Mars helicopter doesn’t go back to the rover, but it does start there.

Will a helicopter work on Mars?

With regards to helicopter flight, there are two big differences between Earth and Mars. First, the density of the atmosphere on Mars is significantly lower than on Earth (only about 1 percent of our atmospheric density). Second, the gravitational field is also lower on Mars (just 38 percent of the gravity on the surface of the Earth). The lower atmospheric density makes it more difficult to fly a helicopter, but the lower gravity makes it easier.

The real question—how do helicopters fly? In a very basic model of helicopter thrust, the rotors take air above the helicopter and throw this air down. Since the “thrown” air has an increase in momentum, this requires a force—which is the lift force. Also, we can imagine this mass of air is in the shape of a cylinder with the radius of this air-cylinder the same as the area of the helicopter rotors.

The momentum of the downward thrown “air” (I’m going to use air for the stuff on Mars and Earth) depends on the speed that it’s moving down and the mass. But what about this mass of air? Since the thrust force depends on the rate of change of momentum, I don’t actually need to know the height of this cylinder of air.However, the rate of change of air-mass does indeed depend on the size of the rotors and the density of air. From that, I can get the following rough approximation for the thrust force from a helicopter.

You can probably guess that the A is the rotor area and v is the speed the air is pushed down. The density of air is represented by the Greek letter ρ. But what about gravity? Well, if the helicopter is to hover then the thrust force must be equal to the weight. We can calculate the weight as the product of mass (m) and gravitational field (g).

Since the air density on Mars is just 0.01 that of on Earth but the gravitational field is 0.38 as much, the lower gravity benefit does not match the loss in air density. It’s more difficult to hover on Mars than on Earth. Oh, please notice that this is just a rough model for the thrust force from a helicopter. Please do not use this calculation to attempt to build your own human-carrying helicopter. However, you could use it to calculate the correct size of the S.H.I.E.L.D. helicarrier rotors.

Actually, here is a quick homework question for you. Based on the known mass and size of the Mars helicopter, what is the air speed from the rotors in order to make this thing hover?

What about the flight time?

NASA says that the Mars helicopter will be able to fly for about two to three minutes per day. The rest of the time, it will use its solar panel on the top of the vehicle to recharge the battery. So, how big of a battery does this thing need? There are actually two ways to estimate the battery size (other than asking NASA—that’s the boring way).

The first method considers the charging time. Basically, this thing will recharge for a whole day. On Mars, each day is almost exactly the same as an Earth day (but 37 minutes longer). But the total day doesn’t matter—what matters is the length of sunlight time. Just like on Earth, the length of daylight on Mars changes with the seasons. So, let’s just estimate that the sun is visible of 10 hours a day (I just picked that for fun).

The next thing to determine is to think about the solar irradiance. This is the power per unit area that the is in the light reaching the surface of the planet. For Earth, this has a maximum of about 1,000 Watts per square meter. On Mars, it’s only about 2590 Watts/m2—but remember that is the maximum. Over the course of a day, the sun appears to change position. If you have a static facing solar panel, the average irradiance will be smaller. I’m just going to go with 295 Watt/m2. Since the size of the solar panel appears to be approximately the size of the cube part of the robot, I will estimate its radius (it looks circular) at 7 cm. With this size, irradiance, length of a day, and an efficiency of 25 percent, that would give a total energy of about 40,000 Joules. That’s very close to the energy stored in the battery of the iPhone X—note, there are some tricks going from units of mAh to Joules.

Now for the second method. How much power would it take for the Mars helicopter to fly for three minutes? How do you calculate flying power? Well, think of it this way. Power is the rate at which you use energy. What is the helicopter doing with its energy? It is taking air above it and increasing its speed (an increase in kinetic energy). I’m skipping some steps, but from that I can get an expression for the power needed to hover—you can find all the details in my calculation of the power needed for a human propelled helicopter (yes, that’s a real thing).

To calculate the flying power, I need to know the rotor size (14 cm diameter) and the helicopter mass (about 1 kg). From this, I can calculate the thrust air speed needed to hover. And with the speed, I can calculate the rate of change of air kinetic energy—which is the power. With these values (along with the density of air and the gravitational field on Mars), I get a flying power of 374 Watts (note: here are my calculations in python).

But wait. NASA has already released the power requirement for the helicopter at 220 Watts. Personally, I consider this to be close enough for a win-win. Win for me for doing this and win for you because you read it.

If I go with the NASA power of 220 Watts I can calculate the energy required to run for three minutes since power = energy/time (don’t forget to convert the time to seconds). This gives a power usage of 3.96 x 104 Joules. Boom. Right around the energy value for the other calculation.

I probably should have left this whole thing as a homework problem—but I couldn’t help myself. I wanted to see if the numbers worked out.

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Few Rules Govern Police Use of Facial-Recognition Technology

4 min read

They call Amazon the everything store—and Tuesday, the world learned about one of its lesser-known but provocative products. Police departments pay the company to use facial-recognition technology Amazon says can “identify persons of interest against a collection of millions of faces in real-time.”

More than two dozen nonprofits wrote to Amazon CEO Jeff Bezos to ask that he stop selling the technology to police, after the ACLU of Northern California revealed documents to shine light on the sales. The letter argues that the technology will inevitably be misused, accusing the company of providing “a powerful surveillance system readily available to violate rights and target communities of color.”

The revelation highlights a key question: What laws or regulations govern police use of the facial-recognition technology? The answer: more or less none.

State and federal laws generally leave police departments free to do things like search video or images collected from public cameras for particular faces, for example. Cities and local departments can set their own policies and guidelines, but even some early adopters of the technology haven’t done so.

Documents released by the ACLU show that the city of Orlando, Florida worked with Amazon to build a system that detects “persons of interest” in real-time using eight public-security cameras. “Since this is a pilot program, a policy has not been written,” a city spokesperson said, when asked whether there are formal guidelines around the system’s use.

“This is a perfect example of technology outpacing the law,” says Jennifer Lynch, senior staff attorney at the Electronic Frontier Foundation. “There are no rules.”

Amazon is not the only company operating in this wide open space. Massachusetts based MorphoTrust provides facial-recognition technology to the FBI, and also markets it to police departments. Detroit police bought similar technology from South Carolina’s Data Works Plus, for a project that looks for violent offenders in footage from gas stations.

The documents released Tuesday provide details about how Orlando, and the sheriff’s department of Oregon’s Washington County use Amazon’s facial recognition technology. Both had previously provided testimonials about the technology for the company’s cloud division.

Orlando got free consulting from Amazon to build out its project, the documents show. In a prior testimonial, Orlando’s chief of police John Mina said that the system could improve public safety and “offer operational efficiency opportunities.” However a city spokesperson told WIRED that “this is very early on and we don’t have data to support that it does or does not work.” The system hasn’t yet been used in investigations, or on imagery of members of the public.

Washington County uses Amazon’s technology to help officers search a database of 300,000 mugshots, using either a desktop computer or a specially built mobile application. Documents obtained by the ACLU also show county employees raising concerns about the security of placing mugshots into Amazon’s cloud storage, and the project being perceived as “the government getting in bed with big data.”

There’s no mention of big data in the US Constitution. It doesn’t provide much protection against facial recognition either, says Jane Bambauer, a law professor at the University of Arizona. Surveillance technology like wiretaps are covered by the Fourth Amendment protections against search and seizure, but most police interest in facial recognition is in applying it to imagery gathered lawfully in public, or to mugshots.

State laws don’t generally have much to say about police use of facial recognition, either. Illinois and Texas are unusual in having biometric privacy laws that can require companies to obtain permission before collecting and sharing data such as fingerprints and facial data, but make exceptions for law enforcement. Lynch of EFF says hearings by the House Oversight Committee last year showed some bipartisan interest in setting limits on law enforcement use of the technology, but the energy dissipated after committee chair Jason Chaffetz resigned last May.

Nicole Ozer, technology and civil liberties director at the ACLU of Northern California, says the best hope for regulating facial recognition for now is pressuring companies like Amazon, police departments, and local communities to set their own limits on use of the technology. “The law moves slowly, but a lot needs to happen here now that this dangerous surveillance is being rolled out,” she says. She says Amazon should stop providing the technology to law enforcement altogether. Police departments should set firm rules in consultation with their communities, she says. In a statement, Amazon said all its customers are bound by terms requiring they comply with the law and “be responsible.” The company does not have a specific terms of service for law enforcement customers.

Some cities have moved to limit use of surveillance. Berkeley, California, recently approved an ordinance requiring certain transparency and consultation steps when procuring or using surveillance technology, including facial recognition. The neighboring city of Oakland recently passed its own law to place oversight on local use of surveillance technology.

Washington County has drawn up guidelines for its use of facial recognition, which the department provided to WIRED. They include a requirement that officers obtain a person’s permission before taking a photo to check their identity, and that officers receive training on appropriate use of the technology before getting access to it. The guidelines also state that facial recognition may be used as investigative tool on “suspects caught on camera.” Jeff Talbot, the deputy spokesperson for the Washington County Sheriff’s Office, said the department is not using the system for “public surveillance, mass surveillance, or for real-time surveillance.”

Ozer and others would like to see more detailed rules and disclosures. They’re worried about evidence that facial recognition and analysis algorithms have been found to be less accurate for non-white faces, and not accurate at all in law enforcement situations. The FBI disclosed in 2017 that its chosen facial-recognition system only had an 85 percent chance of identifying a person within its 50 best guesses from a larger database. A system tested by South Wales Police in the UK during a soccer match last year was only 8 percent accurate.

Lynch of EFF says she believes police departments should disclose accuracy figures for their facial recognition systems, including how they perform on different ethnic groups. She also says that despite the technology’s largely unexamined adoption by local police forces, there’s reason to believe today’s free for all won’t last.

Consider the Stingray devices that many police departments began to quietly use to collect data from cellphones. Amid pressure from citizens, civic society groups, and judges, the Department of Justice and many local departments changed their policies. Some states, such as California, passed laws to protect location information. Lynch believes there could soon be a similar pushback on facial recognition. “I think there is hope,” she says.

Louise Matsakis contributed to this article.


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Luminaries from across Israel’s tech ecosystem are joining us onstage in Tel Aviv – TechCrunch

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Tickets are going fast for our inaugural Tel Aviv event and no one should miss out on the opportunity to see some the nation’s rising stars discuss the challenges and opportunities ahead for mobility technologies.

Hear from some of the architects and creators of Israel’s latest technology marvels like Orit Nissan Messing, the co-founder and Chief Architect of Iguazio. And government officials like Anat Lea Bonshtien, the chairman and director of the Fuel Choices and Smart Mobility Initiative in the Prime Minister’s Office, who are driving mobility technologies forward.

Fiona Darmon, the Chief Operating Officer of one of Israel’s pre-eminent venture funds, JVP, will join us alongside Natalie Refuah, a partner with the growth capital investment firm Viola Growth, to discuss how businesses can scale and make the right moves as they navigate their inevitable international expansion.

They’re all part of a stellar line up that we’ve put together to take the pulse of one of the hottest trends in tech and one that’s increasingly reliant on Israeli technology companies to fulfill the promise of its potential.

These phenomenal speakers will be sharing insights that no one would want to miss, and they’ll be exclusively available to our audience in Tel Aviv.

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Sleep Tight pits kids against monsters from under the bed on July 26

1 min read

When the creepy-crawlies come out from under the bed, some kids might pull the covers over their heads. But not the ones in Sleep Tight. In this twin-stick shooter, you fight back against the bogeyman by building forts and arming yourself with an arsenal of toy weapons. It’s developer We Are Fuzzy’s debut game, and it launches July 26 on PC and Nintendo Switch.

To fight off waves of nighttime monsters, players can choose from a roster of 12 kids, each of whom has their own special perk. For instance, Lynn sports a star-studded astronaut outfit. She just came back from space camp, and she researches new skills faster than others. Wyatt, on the other hand, takes his inspiration from cowboys on the range, and he starts with a buckshot to fend off enemies. Players who prefer to beef up defense can select Rosie the engineer, whose perk enables her to build tougher pillow fort walls.

The new indie studio has triple-A roots, and its team has individually worked on games like Far Cry 5 and Tom Clancy’s Rainbow Six Siege. It also recruited Disney illustrator Dylan Ekren, who contributed to Wreck-It Ralph and Zootopia. The veteran talent shows in Sleep Tight’s new trailer, which is brimming with character and polish.

Sleep Tight seems to feature a lot of strategic gameplay and fast-paced shooting, and it’s neat that it has gone for a colorful, Pixar-like aesthetic rather than something gritty or blood-splattered.



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