And the thing is, it didn't detect anything. That non-discovery is what's causing physicists to rethink how they think about dark matter.

Holding up galaxies

Dark matter is an answer to a nearly century-old problem. In the early 1930s, shortly after astronomers realized that the universe consisted of countless galaxies, scientists turned their attention to understanding the dynamics of how stars orbited within the galaxies — essentially, how galaxies rotate. Dutch astronomer Jan Oort applied Newton's laws of motion and gravity to the observed matter in our own Milky Way and found that our galaxy rotated faster than he had calculated. It appeared that the Milky Way had twice the mass that astronomers had estimated. Of course, this was in an era in which precise galactic astronomy was coming into existence and a disagreement between calculation and measurement of only a factor of two was considered excellent agreement.

However, shortly after Oort's measurement, Bulgarian-Swiss astronomer Fritz Zwicky was studying the Coma Cluster, a large group of more than a thousand galaxies that were bound together by their mutual gravity in an enormous structure. When he measured the velocity of the galaxies, he found that they, too, were moving far too fast for gravity's gentle tug to hold them together. By all rights, the cluster should have torn itself apart. But it didn't. He calculated that the cluster held 400 times more matter than could be seen by ordinary telescopes. Modern measurements have reduced that number, but the discrepancy still is thought to be a factor of 100. Zwicky proposed that there was a type of unseen matter holding together the cluster that he called "dunkle Materie," or dark matter. [TED-Ex Talk: How Does Dark Matter Explain a Star's Speed?]

In the 1970s, astronomer Vera Rubin was trying to find a noncontroversial topic to study when she turned her attention to the rotation curves of galaxies. This is a measure of the orbital speed of stars in galaxies as a function of their distance from the center. She found that the measurements agreed with predictions very well at the center of the galaxy and even approaching the periphery. But, in the very outskirts of the galaxy, stars were orbiting far faster than could be accommodated by the known laws of physics and the observed matter. It looked like Zwicky's dark matter might be showing up in the interiors of galaxies as well. (Clearly, Rubin failed epically in her attempt to find a noncontroversial topic.)

Over the decades, several ideas have been proposed to explain an extensive array of astronomical mysteries, from the possibility that Newton's laws of motion might not apply when accelerations become small, to the idea that both Newton and Einstein were wrong aboutgravity. These hypotheses have not survived rigorous testing. Another idea was that perhaps there exist types of matter in the universe that do not emit electromagnetic energy … this was Zwicky's dark matter.

But, even here, there were many possibilities. The most plausible option was that the universe was home to a menagerie of black holes, brown dwarfs, rogue planets and other dark objects that consisted of the same kind of ordinary matter that makes up the visible component of the universe. These objects are massive and compact and cold enough that they don’t emit light like stars.  Some objects like these were found, but not enough to solve the mystery.  And so, astronomical surveys in the 1990s completely ruled out this idea, too. Taking a page from Sherlock Holmes in “The Sign of Four,” in which he said, “When you have ruled out the impossible, whatever remains, however improbable, must be the truth,” scientists have been forced to conclude that a new form of invisible dark matter permeates the universe. Perhaps even more surprising, there appear to be five times more dark matter than ordinary matter.

The properties of dark matter

We have never directly observed dark matter, but we know a great deal about what it must be: It must be massive (because it affects the rotation of galaxies); it must be electrically neutral (because we can't see it); it must be different from ordinary matter (because we see no evidence for it interacting with matter in the usual ways); and it must be stable (because it has existed since the dawn of the universe). These properties are unequivocal.

However, we don't know exactly what it is. In the most popular generic theory, the dark-matter particle is called a WIMP, for weakly interacting massive particle. WIMPs are kind of like heavy neutrons (but definitely not neutrons), with a mass of 10 to 100 times heavier than a proton. They were created in great quantities during the Big Bang and a small relic remainder persists to this day.

When cosmologists add the WIMP idea to their ideas of the Big Bang, they can calculate how it would interact. They find that in the earliest stages of the universe, WIMPs were a dominant form of matter, but, as the universe expands and cools and the interaction rate drops to zero, the relic amount of WIMPs is about five times as massive as ordinary matter. Combined with the fact that the WIMP idea can also explain a pressing theoretical mystery of why the Higgs boson particle has such a small mass, scientists call this the "WIMP miracle," in view of the fact that the WIMP idea seems to answer so many questions.

A dark non-discovery

So this brings us back to the LUX experiment. It is simply the latest and most powerful experiment designed to detect WIMPs. The idea is that the detector will sit underground for a long time, say a year or more, and a rare WIMP will bump into a xenon atom and be detected. (The detector is buried underground to protect it from cosmic rays, which would bump into xenon atoms and fake a dark matter detection. Being half a mile or a mile underground stops nearly all cosmic rays.)

On Aug. 27, the LUX researchers announced their results. They saw no evidence for dark-matter WIMPs.

So that's not actually that surprising. There have been dozens of experiments that have looked for dark matter and found nothing. In this sense, LUX has joined a venerable group. But LUX is also far more capable. It was expected to have a better shot at finding WIMPs than any other apparatus.

But it didn't.

So does that mean that the WIMP idea is dead? No, not really. Dark matter detectors are optimized to be most sensitive to certain masses, kind of like how a particular microphone might pick up tenor voices better than they hear soprano or bass ones.  LUX was optimized to find a WIMP with a mass of a few tens of times heavier than a proton. In fact, it ruled out a WIMP having a mass 50 times of a proton with impressive rejection. However, LUX doesn't do so well at detecting WIMPs with a mass below 10 times that of a proton.  And, if WIMPs exist and have a mass of 1,000 times heavier than a proton, this is also not LUX's optimum range for investigation. So there remains a range of masses where a WIMP might exist.

Axions, dark life and dark charge

Even so, WIMPs were simply the most popular idea for dark matter. There are other hypotheses. One hypothesis points to a particle called a sterile neutrino, which is a cousin of the more familiar neutrino generated in nuclear reactions. In fact, neutrinos from the biggest nearby nuclear reactor (the sun) are constantly barraging the Earth.  Unlike regular neutrinos, which are ultralight and interact via the weak nuclear force, sterile neutrinos are heavy and don't experience the weak force. It is the heavy mass and non-interacting qualities that make the sterile neutrino an ideal dark-matter candidate.

Another possible dark-matter particle, the axion, was proposed in 1977 as a way to ensure that the strong nuclear force treated matter and antimatter on equal footing (so as agree with observations). The axion is a very light, but still massive, hypothetical particle. The LUX detector is not designed to study axions.

Then, of course, there is the even more creative hypothesis, which suggests that dark matter isn't a single, neutral, non-interacting particle. After all, ordinary matter is pretty complicated. At the quantum scale, we have quarks and leptons and four forces. At the macro scale, we have you and me and sugar and stars and volcanoes and all the various ways they interact. Ordinary matter has all sorts of interactions and constituents.  Why not dark matter? [7 Strange Facts About Quarks]

Under this hypothesis, dark matter might have a "dark charge," or dark matter's form of electrical charge. In the same way that dark matter doesn't experience dark charge, maybe ordinary matter doesn't experience dark charge. This dark charge would interact with dark photons, or the dark twins of ordinary light particles.

And, even more daring, perhaps dark matter experiences many forces and there is an entire complex dark sector, with dark atoms and dark molecules and perhaps even dark life. We are able to set constraints on these possible dark interactions; for instance, we know enough to rule out dark stars and planets, but dark asteroids are possible. Again, LUX is silent on these new ideas.

It's not fair to say that the LUX measurement leads to a crisis in particle physics and cosmology. But it certainly gives scientists pause and suggests that maybe we should take another look at this WIMP thing. Maybe other ideas need to be revisited.  On the other hand, scientists who want to continue to pursue the WIMP idea still have something to look forward to as technology advances. LUX uses a third of a ton of liquid xenon. In 10 or 15 years, scientists are planning to build detectors that might contain 100 tons, providing even more chances of capturing that rare WIMP interaction. These are heady times to be a dark-matter scientist.

But, in the end, we still don't know. We just know that the capabilities of LUX are good enough that maybe it's time to broaden our thinking. In the words of the rock band Buffalo Springfield, "There's something happening here. What it is ain't exactly clear..."

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.

Defending Earth

After the sun formed, leftover dust and rock circled the newborn star in a disk. The material smashed together, building planetary embryos. Some bits of debris hurled down on the young planets, while some material was cast out of the solar system forever. Over time, the fiery rain slowed as the population of dust and rock dwindled.

As life evolved on Earth, bits of remaining debris occasionally crashed down — at times, wiping out plants and animal species. While most of the largest rocks have already collided with planets or have left the solar system, enough leftover material remains to pepper our planet. 

"Over a hundred tons of material fall to Earth every day," Johnson said. 

Most of that is dust, but occasionally, a solid chunk of rock makes it to the ground (a meteorite). Those meteorites can be chunks that have broken off of asteroids, but they also may be debris that was once part of a planet or the moon and was blown off in an early collision. For example, Earth is covered in meteorites that were once part of the moon and Mars — places humans have been able to study up close with various probes, rovers and (in the case of the moon) human missions. 

But asteroids are more challenging. The sample of Bennu that scientists are aiming to return in the OSIRIS-REx mission will help scientists better understand the composition of other asteroids and space debris if they were to prove threatening.

During the OSIRIS-REx mission, scientists also will study how the sun affects the orbital paths of small rocky bodies. As sunlight strikes a rotating asteroid, the radiating heat provides a tiny thrust that can change the orbit of the rock — a process known as the Yarkovsky effect. 

"We can't just determine where an asteroid is going to go just by orbital dynamics," said Alex Young, a heliophysics scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We have to account for this Yarkovsky effect."

Observing how the sun ever so slightly redirects Bennu will help scientists to improve their models of other asteroids and their potential to impact Earth.

On top of all that, guiding OSIRIS-REx into orbit around Bennu will help NASA learn more about manipulating a spacecraft around small bodies.

"NASA's gotten very skilled at going to Mars, Saturn, through multibody systems," said NASA Chief Scientist Ellen Stofan. But smaller objects are different, and maneuvering OSIRIS-REx is a step toward landing humans on asteroids and the moons of Mars, she said.

Shifting orbits

Debris from the early solar system still floats between the planets. While most of the largest objects have been identified, NASA estimates that only half of the rocks the size of Bennu have been spotted.

"We've got to find them first before we can do anything about them," Johnson said.

For that, planetary defense researchers turn to NASA's NEOWISE, a repurposed telescope mission to identify near-Earth objects. The instrument has helped scientists to build a catalog of rocks near the planet, some passing closer than satellites.

Observation isn't the only tool for planetary defense. In partnership with the European Space Agency, NASA is working on the Double Asteroid Redirection Test (DART). The two agencies plan to send a spacecraft to rendezvous with an asteroid and its moon. By deliberately crashing into the tiny moonlet, DART will shift its course by "a fraction of a percent"according to NASA. The mission will test the ability of the space agencies to deflect an asteroid years before it collides with Earth.

Johnson described DART as "a kinetic impactor that would impact the moon of Didymos, which we affectionately call Didymoon."

Another upcoming project is planning to bring a boulder-size asteroid sample to the moon, where astronauts can remove samples to carry back to Earth. After traveling to an asteroid and selecting a hunk to take home, the Asteroid Redirect Mission (ARM) will attempt to change the path of the asteroid by using its mass as what Johnson called "a natural tug rope." 

"Gravity will pull the asteroid off its natural orbit," he said.

Asteroid Basics: 

Asteroids are fascinating for lots of reasons. They contain a variety of valuable resources and slam into our planet on a regular basis, occasionally snuffing out most of Earth's lifeforms. How much do you know about space rocks?

Of course, NASA doesn't have a mission ready to deflect incoming danger. Johnson told Space.com that it will take somewhere from five to 10 years to launch a mission that will effectively deflect a specific threat.

Earth-threatening asteroids aren't prolific in the solar system. Johnson said that serious impacts that might be worth deflecting happen only about once every couple of centuries.

"It's not something I worry about a lot on a regular basis," he said.

But that doesn't mean we should become complacent. 

"Since we don't know everything that's out there, it's something that could happen almost any day," he said. 

He suggested that the best way to avert disaster is to improve the ability to identify potentially hazardous objects. The sooner large, dangerous objects are identified, the more time humans on Earth have to take the necessary steps.

"We need to improve our capabilities to detect and find them further out in space so we have time to prepare," he said.

Follow Nola Taylor Redd on Twitter @NolaTRedd and Google+. Follow us at@Spacedotcom, Facebook or Google+. Originally published on Space.com.

In the new paper, Gros argues that it would be possible to terraform planets(effectively make them more like Earth), using robotic probes that would carry terrestrial organisms and deposit them on the surface of the alien planet. However, the technology for such probes is not even in the design phase, making it uncertain when such a project could come to light.

"The Genesis project is about giving life [the chance] to evolve all by itself on other planets," said Gros in an e-mail to Space.com. "This process is robotic only in the sense that the initial seeds will be transported by robotic interstellar probes."

Gradual evolution

Upon deployment, a probe would send down "nano-size re-entry capsules" that would carry an array of terrestrial organisms. Ahead of deploying it, scientists would decide which terrestrial organisms might be best suited to the chosen planet's particular environment. Over time, the scientists would watch how the planet reacts to the arrival of the new organisms. 

If scientists find a planet that is generally hospitable to life and seed it with the right kind of organisms, Gros says there could very likely be "global-scale ecological disasters" in the first while, such as "uncontrolled blossoming of unicellular algae." But, he said, the planet would be expected to stabilize in a few thousand years. Whether or not the newly deposited life-forms survive, or even thrive, will depend on several factors, such as how much carbon dioxide is in the planet's atmosphere, the average temperature of the planet, and how much sunlight reaches the surface (or if the life-forms must rely on another energy source, likehydrothermal activity).

Of course, once a real-world planet is "seeded" with life, things won't play out the way they do in the movie "The Search for Spock" (1984): evolution will proceed at a normal pace. And unlike the film, the planet wouldn't age rapidly, either.

Scientists have strong evidence that life appeared on Earth about 1.2 billion years after the planet's formation, but there are also hints that organisms may have appeared only half a billion years after the planet's birth. Multicellular animals and planets only appeared with the Cambrian explosion, close to 550 million years ago. While something like the Genesis project could jump-start this process on another planet, Gros said, it would depend on the specifics of the planet; for example, it would still take at least 10 million to 100 million years for the terrestrialorganisms to produce oxygen and have it accumulate in the planet's atmosphere, making way for more advanced life-forms. "Enough time to watch more than a few 'Star Trek' movies," he joked.

Gros added that he thinks the technology needed to send these terraforming probes to other planets should be available in 50 to 100 years. But he said his outlook will be influenced by the success of a project called Breakthrough Starshot, which aims to send postage-stamp-size probes to the star system nearest to Earth, using a massive laser to kick-start the probe on its journey. The Starshot project was just announced in April; its leadership includes famed physicist Stephen Hawking and billionaire investor Yuri Milner.

"The Genesis project is quite unique in the sense that there will be no tangible benefit for humanity," he added. "There is, however, no doubt that the project will be realizable both scientifically as well as economically in the not too far future. Personally I find the thought of life flowering elsewhere in the universe very appealing."

As described in the paper, two of the reasons that Gros feels the device could be feasible at some point are the success of exoplanet searches in finding small, rocky planets that could have some characteristics that make them potentially hospitable to life, and early-stage studies on robotic interstellar travel (including the Breakthrough Starshot proposal). However, it is unclear exactly when a device like the one described by Gros could be ready, as much of the work in these fields is in the early stages.

Ethical questions

Should Genesis be deployed, Gros acknowledges an ethical dilemma with the situation. If some form of life already exists on the planet (something the scientists who launched Genesis didn't notice), it's possible the newly introduced life-forms would wipe out the native life-forms. 

Gros writes that there are two general points of view for planetary protection, which contrast the scientific benefit for humanity with the idea that independently evolved life has "value per se." Gros advocates balancing these options in each individual situation to figure out what the best solution is.

Gros' research has been accepted in the journal Astrophysics and Space Science, and is available now on the pre-publishing site Arxiv.

Follow Elizabeth Howell @howellspace, or Space.com @Spacedotcom. We're also on Facebook and Google+. Original article on Space.com.

The main goal of the $800 million OSIRIS-REx mission (whose name is short for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) involves learning more about the role asteroids may have played in seeding Earth (and perhaps other worlds) with the building blocks of life.

If all goes according to plan, OSIRIS-REx will arrive at Bennu in August 2018, grab at least 2 ounces (60 grams) of asteroid material two years later and return this sample to Earth in September 2023.

Snagging samples from an asteroid in deep space is a tricky operation under any circumstances, but it's especially difficult when the space rock is less than 660 feet (200 meters) or so wide, Richey said. That's because small asteroids rotate fast — sometimes fast enough to fling material off the surface, leaving little stuff for a visiting probe to grab.

"Furthermore, it's difficult [for a spacecraft] to match the rotation speed of a rapidly rotating object," Richey said.

But Bennu is nearly 1,640 feet (500 m) in diameter, and it rotates just once every 4.3 hours — slowly enough that OSIRIS-REx should be able acquire samples via a "safe, smooth, slow high-five" of the surface, she added.

The nearly spherical asteroid is also quite accessible, thanks to a relatively Earth-like orbit. Bennu completes one lap around the sun every 1.2 years and comes into Earth's neighborhood every six years, Richey said. In addition, the asteroid's orbit is inclined by just 6 degrees compared to that of Earth.

"So it really is optimal for accessibility, for the spacecraft to go to and then return from" in a relatively short amount of time, Richey said.

Then there's the composition.

Bennu "has a very dark, black surface that we think indicates high concentrations of carbon, and hopefully in the form of organic material," OSIRIS-REx principal investigator Dante Lauretta, of the University of Arizona, told Space.com last month.

"We're really interested in bringing back a pristine sample of the early solar system," he added. "Did these kinds of bodies [like Bennu] deliver organic material and water, in the form of hydrated minerals like clay, to the surface of our planet that created the habitability and the environments that may have led to the origin of life?"

But Bennu wasn't the only asteroid that met all of these necessary criteria. In fact, the OSIRIS-REx team eventually identified five suitable finalists, Lauretta said.

"We selected asteroid Bennu as the final target of OSIRIS-REx because it was so well characterized, with the radar data and telescopic data, that we felt that really reduced risk to the mission profile," he said during Tuesday's news conference.

Indeed, Bennu, which was discovered in 1999, is the best-understood near-Earth asteroid that has never been visited by a spacecraft, Richey said.

Bennu offers some bonus perks as well. It's classified as a potentially hazardous asteroid, and there's aslight chance it could slam into Earth in the late 22nd century, scientists have said. OSIRIS-REx's observations will allow researchers to fine-tune their understanding of Bennu's orbit, and thus the threat it poses to Earth, Lauretta said. The mission should also shed light on the forces that can nudge an asteroid this way and that along its trajectory, Lauretta said.

One such force comes from the solar energy that asteroids radiate away as heat, in a phenomenon known as the Yarkovsky effect.

"When that happens, it acts like a thruster and changes the trajectory of the asteroid," Lauretta told Space.com in August. "If you want to be able to predict where an object like Bennu is going to be in the future, you have to account for this phenomenon, and we're going to provide the best-ever scientific investigation of this fascinating concept."

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us@Spacedotcom, Facebook or Google+. Originally published on Space.com.

"Humans have a lot of salt in our waste," Crusan told Space.com. So, in late June, NASA awarded Paragon Space Development Corp. a $5.1 million contract to create a Brine Processor Assembly for flight in 2018. This assembly is expected to remove brine and recover up to 94 percent of the water from urine, NASA officials said in a statement.

Ongoing technology developments also allow astronauts to manufacture their own tools using 3D printing and to use atmospheric monitors to check the air in the cabin environment for contaminants. Those monitors shrink huge gas chromatography mass spectrometry units, which identify different substances in test samples, to about the size of a toaster.

All of these are important considerations in sending a future crew to Mars in an Orion spacecraft, along with one to three other habitat modules attached to provide extra room, Crusan said. This "Orion plus" spacecraft would likely have solar electric propulsion capability — engines that ionize noble gases to give a small amount of thrust and run for long periods of time, Crusan said. 

"Humans have a lot of salt in our waste," Crusan told Space.com. So, in late June, NASA awarded Paragon Space Development Corp. a $5.1 million contract to create a Brine Processor Assembly for flight in 2018. This assembly is expected to remove brine and recover up to 94 percent of the water from urine, NASA officials said in a statement.

Ongoing technology developments also allow astronauts to manufacture their own tools using 3D printing and to use atmospheric monitors to check the air in the cabin environment for contaminants. Those monitors shrink huge gas chromatography mass spectrometry units, which identify different substances in test samples, to about the size of a toaster.

All of these are important considerations in sending a future crew to Mars in an Orion spacecraft, along with one to three other habitat modules attached to provide extra room, Crusan said. This "Orion plus" spacecraft would likely have solar electric propulsion capability — engines that ionize noble gases to give a small amount of thrust and run for long periods of time, Crusan said.