How Does the International Space Station Maintain Its Orientation?

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iStock

How does the ISS keep its orientation?

Robert Frost:

Nominally, attitude control is provided by four control moment gyroscopes (CMGs). Each CMG contains a wheel that is 220 pounds (100 kg). That wheel spins at 6600 rpm, resulting in an angular momentum of 3500 ft-lb-s (4742.5 N-m-s). The basic idea is that if a torque induces a rotation on the ISS, those wheels can rotate about their gimbals to change the angular momentum of the ISS, creating a counter torque. Using CMGs is much more subtle than using thrusters, so microgravity experiments are not impacted. CMGs do have limits, though, so thrusters can assist, if needed. That assistance is needed whenever the torques are large.

To minimize thruster assists, during quiescent operations, we do a type of attitude control called momentum management (MM). This is done by maneuvering the ISS to a torque equilibrium attitude (TEA) that was analyzed by the ground a year or more in advance. This TEA is an attitude that, with meanderings of up to 15 degrees, will result in the gravity torques and atmospheric torques adding up, over an orbit, to close to zero. The CMGs then take up the slack to make that zero.

We often can't be in a TEA during critical operations. For those we need to be in an attitude hold (AH). An example of this is a docking or berthing. Attitude holds are challenging because they require a lot more work, often too much for the CMGs to handle alone, and yet firing thrusters during critical operations can be problematic.

For these operations we design a matrix for the flight rules to ensure safety. For example, we do not allow thrusters to fire whenever the end of the robotic arm is within 2 feet (0.6 m) of the vehicle. The last thing we need is for a thruster firing to shake the arm and cause it to hit the side of a module, puncturing the module. If the timeline indicates the arm will be that close, ADCO (the attitude control flight controller) will inhibit thruster assist.

Dockings and berthings can produce sudden changes in momentum. During these activities we inhibit the entire attitude control system to ensure we do not introduce forces that could damage a docking or berthing mechanism. You might notice, on NASA TV, that the vehicle can get considerably out of attitude at these times.

The attitude control computer (GNC MDM) contains the software that does all of the necessary calculations for attitude control. It takes in the actual attitude and subtracts the commanded attitude to determine the error it needs to correct. It knows the rates of the ISS. That is very sensitive, so sensitive that we can tell when the crew wake up by watching the behavior of the CMGs as the crew start to move around the vehicle. The software also needs a set of user provided parameters such as the vehicle mass properties and inertia tensors. These are located in data slots called CCDBs (controller configuration databases). We have a stockpile of these CCDBs for different vehicle configurations. For example, if a Progress cargo vehicle arrives and docks to the Russian Segment, we will have a CCDB slot designed for that configuration. When it leaves, we will swap to another one.

This post originally appeared on Quora. Click here to view.

Why Are Marathons 26.2 Miles Long?

iStock/ZamoraA
iStock/ZamoraA

What's the reason behind the cursed distance of a marathon? The mythical explanation is that, around 490 BCE, the courier Pheidippides ran from Marathon to Athens to delivers news that the Greeks had trounced the Persians at the Battle of Marathon. The trouble with that explanation, however, is that Pheidippides would have only covered a distance of approximately 25 miles. So what accounts for the extra 1.2 miles?

When the modern marathon appeared in the late 19th century, the race distance was inconsistent. During the first Olympic games in 1896, runners jogged along Pheidippides’s old route for a distance of 40,000 meters—or 24.85 miles. (That race, by the way, was won by a Greek postal worker.) The next Olympic games saw the distance bumped to a pinch over 25 miles. And while subsequent marathons floated around the 25 mile mark, no standard distance was ever codified.

Then the Olympics came to London. In 1908, the marathon, which stretched between Windsor Castle and White City Stadium in London, lasted 26.2 miles—all for the benefit of England's royal family.

It wasn't supposed to be that way. Like previous races, the original event was supposed to cover a ballpark of 25 miles. The royal family, however, had other plans: They wanted the event to start directly in front of Windsor Castle—as the story goes, the royal children wanted to see the start of the race from the castle nursery. Officials duly agreed and moved the starting line, tacking on an extra mile to the race.

As for the pesky final 0.2? That was the royal family’s fault, too. The finish line was extended an extra 385 yards so the race would end in front of the royal family’s viewing box.

Those extra 1.2 miles proved to be a curse. The race’s leader, an Italian pastry chef named Dorando Pietri, collapsed multiple times while running toward the finish line and had to be helped to his feet. One of the people who came to his aid was a journalist named Arthur Conan Doyle. Afterward, Conan Doyle wrote about Pietri's late-race struggles for the Daily Mail, saying, "Through the doorway crawled a little, exhausted man ... He trotted for a few exhausted yards like a man galvanized into life; then the trot expired into a slow crawl, so slow that the officials could scarcely walk slow enough to keep beside him."

After the London Olympics, the distance of most marathons continued to hover between 24 and 26 miles, but it seems that Conan Doyle's writing may have brought special attention to the distance of 26.2, endowing it with a legendary "breaker-of-men" reputation. Indeed, when the International Amateur Athletic Federation convened to standardize the marathon, they chose the old London distance of 26 miles and 385 yards—or 26.219 miles.

Writing for Reuters, Steven Downes concluded that, "the marathon race may have been as much a Conan Doyle creation as Sherlock Holmes."

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What Are the Santa Ana Winds?

Satellite image of Santa Ana winds in Southern California.
Satellite image of Santa Ana winds in Southern California.
NASA/JPL-Caltech, Wikimedia Commons // Public Domain

Two massive wildfires burning in California have now become the state's deadliest and most destructive. In Northern California, the Camp Fire near Chico decimated the town of Paradise and killed 29 people as of November 12, 2018. In Southern California, the Woolsey Fire started near Simi Valley northwest of Los Angeles, and has torched hundreds of homes in Malibu and other communities.

The National Weather Service says that a combination of high temperatures, low humidity, and gusty Santa Ana winds have created perfect conditions for cataclysmic fires.

What are these Santa Ana winds and why do they help create fire conditions?

Santa Anas are dry, warm (often hot) winds that blow westward through Southern California toward the coast. They're usually seasonal, and typically occur between October and March and peak in December. They originate when high pressure systems form over the high-elevation deserts of the Great Basin between the Sierra Nevadas and the Rocky Mountains. Air from the system flows clockwise, so winds on the southern side of the system push west towards the Pacific Ocean.

The winds pass over the mountains between coastal California and the inland deserts. As they flow downslope, the air gets compressed and rises in temperature at a rate of almost 29 degrees per mile of descent. While air's temperature rises, its relative humidity drops, commonly to less than 20 percent and sometimes to even less than 10 percent. The winds also increase dramatically in speed when they're forced through narrow mountain passes and canyons.

By the time the winds hit the coastal areas, they're very dry, warm, and moving fast. This is what makes them problematic. They dry out vegetation, making it better fuel for a fire—and once a fire starts, the winds fan the flames and help spread them.

WHAT'S IN A NAME?

So, why are the winds called "Santa Ana winds"?

"While the origin and cause of the Santa Ana winds are not in dispute," writes Robert Fovell, currently a professor of atmospheric and environmental sciences at SUNY Albany, "the origin of the name is."

One fairly popular explanation is that the name comes from a Native American word, santana, which means "devil wind" and was corrupted into Santa Ana. But according to Fovell, the Los Angeles Times, and other sources, no one has found any words similar to santana with that definition in any of the native languages of the area.

Another explanation is that the winds were named for Mexican politician and general Antonio López de Santa Anna, possibly in reference to dust storms kicked up by the cavalry he commanded. Santa Anna never operated in southern California, though, and spelled his name with two n's. The Oxford English Dictionary dismisses this etymology as having no foundation.

In the early 1930s, an article in the United States Naval Institute Proceedings suggested that the name might have originated with early Spanish explorers, who had a "custom of naming places and events for the saint's day on which they happened or were discovered." In this case, they might have noted the winds on St. Anne's day and named them for her. This also seems unlikely to historians, though, because a few Santa Ana winds, experienced for the first time, probably wouldn't have warranted naming—and the winds aren't recorded with any name until much later, anyway. St. Anne's feast day is also July 26, when a Santa Ana wind is unlikely.

The most common and accepted etymology, says Fovell, is that the winds' name simply derives from the Santa Ana canyon in Orange County.

This article was originally published in 2014 and has been updated.

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