Wednesday, June 28, 2017

Success of Gravity-Wave Satellite Paves Way for 3-Craft Mission

Europe’s gravitational-wave hunters are celebrating. On July 1, a satellite will wrap up its mission to test technology for the pioneering quest to measure gravitational ripples in the stillness of space. Over the past year, the craft has performed much better than many had hoped. That success has convinced the European Space Agency (ESA) to give the go-ahead to a full-scale version able to sense cataclysmic events that can’t be felt on Earth. The LISA Pathfinder mission, launched in late 2015, beat its precision target by a factor of 1,000 and quieted critics who have doubted its potential, says project scientist Paul McNamara, an astrophysicist at ESA in Noordwijk, the Netherlands. “This is not the impossible task that some people believed it was.” Currently set to fly in 2034, the full-scale Laser Interferometer Space Antenna (LISA) will be the space analogue of the Laser Interfero-meter Gravitational-Wave Observatory (LIGO), two machines in the United States—each with a pair of 4-kilometre-long arms—that first detected the ripples by ‘hearing’ the merger of two black holes.

LISA Pathfinder—shown before being encapsulated into a rocket for launch—allowed scientists to test technology for detecting gravitational waves. 

 LISA’s three probes will fly in a triangle, millions of kilometres apart, making the mission sensitive to much longer gravitational waves, such as the ripples produced by the collisions of even larger black holes.

The mission will bounce laser beams between the three LISA craft—or, more precisely, between test masses suspended in a vacuum inside each satellite. Taking advantage of the vibration-free conditions of space, it will measure tiny variations in the distances between the test masses that reveal the passage of space-warping gravitational waves.

LISA Pathfinder’s goal was to show that such variations could be measured in zero gravity and with a precision of one pico-metre, or one-billionth of a millimetre. High-precision thrusters adjusted Pathfinder’s route so that it would closely follow the gravitational free fall of two test masses inside the craft and not interfere with their orbit. At the same time, the probe bounced a laser beam between the two masses—a pair of 2-kilogram cubes made of a gold and platinum alloy—and measured fluctuations in their separation.

The €400-million (US$447-million) probe was declared a success in February 2016, two weeks after LIGO announced its first detection. Pathfinder did not detect gravitational waves—which would not have appreciable effects over the short distance inside the probe—but it showed that it could detect motions 100 times smaller than the pico-metre requirement. Since then, the experiment’s performance has improved by another order of magnitude (M. Armano et al. Phys. Rev. Lett. 118,171101; 2017).

By early June this year, LISA Pathfinder had almost run out of thruster fuel, and mission control used what was left to nudge the spacecraft out of its operating orbit and into its final orbit around the Sun. On 1 July, Pathfinder will stop collecting data, and the spacecraft will be put to sleep for good on 18 July.

Pathfinder was “a triumph”, says William Klipstein, a physicist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who works on LISA development but was not involved in ESA’s Pathfinder mission. Its performance “removes the last major technical barrier for proceeding with a long-planned ESA-led gravitational-wave mission”, he says.

In a unanimous decision on June 20, ESA’s Science Programme Committee officially selected LISA as the third of the agency’s large, or €1-billion-class, mission in its current science programme. The approval was long-awaited but had been in little doubt after Pathfinder’s success and LIGO’s gravitational-wave discoveries, says Karsten Danzmann, a director of the Max Planck Institute for Gravitational Physics in Hanover, Germany, and Pathfinder’s co-principal investigator.

The decision is not final, but it means that industrial partners will now be involved in detailed design and cost projections. Once those are finished, ESA will decide whether to ‘adopt’ the mission and commit the funding to build it. The United States—which was an equal partner in the mission until 2011, when it reduced its participation to save costs—is expected to provide important components.

ESA has chosen two other large missions to go ahead before LISA—one to the moons of Jupiter, slated to launch in 2022, and an X-ray observatory for 2028. This puts LISA on schedule to be launched in 2034. But Pathfinder principal investigator Stefano Vitale, a physicist at the University of Trento in Italy, and others hope that its schedule can be accelerated. ESA’s call for proposals to lead the gravitational-wave observatory—won by Vitale’s team—was put out in late 2016, instead of late 2019 as the agency had planned. Vitale and other gravitational-wave researchers hope the agency will push the launch date forward so that LISA can start sending back data before too many of the current key researchers have retired.

Sunday, June 25, 2017

Hubble Captures Massive Dead Disk Galaxy that Challenges Theories of Galaxy Evolution

By combining the power of a "natural lens" in space with the capability of NASA's Hubble Space Telescope, astronomers made a surprising discovery—the first example of a compact yet massive, fast-spinning, disk-shaped galaxy that stopped making stars only a few billion years after the big bang. Finding such a galaxy early in the history of the universe challenges the current understanding of how massive galaxies form and evolve, say researchers. When Hubble photographed the galaxy, astronomers expected to see a chaotic ball of stars formed through galaxies merging together. Instead, they saw evidence that the stars were born in a pancake-shaped disk.

Acting as a “natural telescope” in space, the gravity of the extremely massive foreground galaxy cluster MACS J2129-0741 magnifies, brightens, and distorts the far-distant background galaxy MACS2129-1, shown in the top box. The middle box is a blown-up view of the gravitationally lensed galaxy. In the bottom box is a reconstructed image, based on modeling that shows what the galaxy would look like if the galaxy cluster were not present. The galaxy appears red because it is so distant that its light is shifted into the red part of the spectrum.
Credits: NASA, ESA, S. Toft (University of Copenhagen), M. Postman (STScI), and the CLASH team

This is the first direct observational evidence that at least some of the earliest so-called "dead" galaxies — where star formation stopped — somehow evolve from a Milky Way-shaped disk into the giant elliptical galaxies we see today.

This is a surprise because elliptical galaxies contain older stars, while spiral galaxies typically contain younger blue stars. At least some of these early "dead" disk galaxies must have gone through major makeovers. They not only changed their structure, but also the motions of their stars to make a shape of an elliptical galaxy.

"This new insight may force us to rethink the whole cosmological context of how galaxies burn out early on and evolve into local elliptical-shaped galaxies," said study leader Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute, University of Copenhagen, Denmark. "Perhaps we have been blind to the fact that early "dead" galaxies could in fact be disks, simply because we haven't been able to resolve them."

This artist's concept shows what the young, dead, disk galaxy MACS2129-1, right, would look like when compared with the Milky Way galaxy, left. Although three times as massive as the Milky Way, it is only half the size. MACS2129-1 is also spinning more than twice as fast as the Milky Way. Note that regions of Milky Way are blue from bursts of star formation, while the young, dead galaxy is yellow, signifying an older star population and no new star birth.
Credits: NASA, ESA, and Z. Levy (STScI)

Previous studies of distant dead galaxies have assumed that their structure is similar to the local elliptical galaxies they will evolve into. Confirming this assumption in principle requires more powerful space telescopes than are currently available.

However, through the phenomenon known as "gravitational lensing," a massive, foreground cluster of galaxies acts as a natural "zoom lens" in space by magnifying and stretching images of far more distant background galaxies. By joining this natural lens with the resolving power of Hubble, scientists were able to see into the center of the dead galaxy.

The remote galaxy is three times as massive as the Milky Way but only half the size. Rotational velocity measurements made with the European Southern Observatory's Very Large Telescope (VLT) showed that the disk galaxy is spinning more than twice as fast as the Milky Way.

Using archival data from the Cluster Lensing And Supernova survey with Hubble (CLASH), Toft and his team were able to determine the stellar mass, star-formation rate, and the ages of the stars.

Why this galaxy stopped forming stars is still unknown. It may be the result of an active galactic nucleus, where energy is gushing from a supermassive black hole. This energy inhibits star formation by heating the gas or expelling it from the galaxy. Or it may be the result of the cold gas streaming onto the galaxy being rapidly compressed and heated up, preventing it from cooling down into star-forming clouds in the galaxy's center.

But how do these young, massive, compact disks evolve into the elliptical galaxies we see in the present-day universe? "Probably through mergers," Toft said. "If these galaxies grow through merging with minor companions, and these minor companions come in large numbers and from all sorts of different angles onto the galaxy, this would eventually randomize the orbits of stars in the galaxies. You could also imagine major mergers. This would definitely also destroy the ordered motion of the stars."

The findings are published in the June 22 issue of the journal Nature. Toft and his team hope to use NASA's upcoming James Webb Space Telescope to look for a larger sample of such galaxies.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

The Very Large Telescope is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of Northern Chile.

Tuesday, May 30, 2017

VLT snaps an exotic exoplanet

Astronomers hunt for planets orbiting other stars (exoplanets) using a variety of methods. One successful method isdirect imaging; this is particularly effective for planets on wide orbits around young stars, because the light from the planet is not overwhelmed by light from the host star and is thus easier to spot. This image demonstrates this technique. It shows a T-Tauri star named CVSO 30, located approximately 1200 light-years away from Earth in the 25 Orionis group (slightly northwest of Orion’s famous Belt). In 2012, astronomers found that CVSO 30 hosted one exoplanet (CVSO 30b) using a detection method known as transit photometry, where the light from a star observably dips as a planet travels in front of it. Now, astronomers have gone back to look at the system using a number of telescopes. The study combines observations obtained with the ESO’s Very Large Telescope (VLT) in Chile, the W. M. Keck Observatory in Hawaii, and the Calar Alto Observatory facilities in Spain.

Using the data astronomers have imaged what is likely to be a second planet! To produce the image, astronomers exploited the astrometry provided by VLT’s NACO and SINFONI instruments.

This new exoplanet, named CVSO 30c, is the small dot to the upper left of the frame (the large blob is the star itself). While the previously-detected planet, CVSO 30b, orbits very close to the star, whirling around CVSO 30 in just under 11 hours at an orbital distance of 0.008 au, CVSO 30c orbits significantly further out, at a distance of 660 au, taking a staggering 27 000 years to complete a single orbit. (For reference, the planet Mercury orbits the Sun at an average distance of 0.39 au, while Neptune sits at just over 30 au.)

If it is confirmed that CVSO 30c orbits CVSO 30, this would be the first star system to host both a close-in exoplanet detected by the transit method and a far-out exoplanet detected by direct imaging. Astronomers are still exploring how such an exotic system came to form in such a short timeframe, as the star is only 2.5 million years old; it is possible that the two planets interacted at some point in the past, scattering off one another and settling in their current extreme orbits.

Wednesday, July 8, 2015

New Horizons Back on Track for Pluto Flyby

WASHINGTON — NASA’s New Horizons spacecraft has exited a protective safe mode that project officials said July 6 was triggered when the spacecraft’s primary computer became overloaded.
New Horizons, which entered safe mode July 4, briefly cutting off communications with the Earth, will resume normal science observations on July 7, and the project’s leadership said they remain confident the spacecraft will operate normally through its July 14 flyby of Pluto.
“The spacecraft is in excellent health and is back in operation,” said Jim Green, director of NASA’s planetary science division, during a conference call with reporters July 6.
The problem took place, said Glen Fountain, New Horizons project manager at Johns Hopkins University Applied Physics Laboratory, because the spacecraft’s primary computer was doing too much at one time. Ground controllers were transmitting a set of commands to the spacecraft, called a “command load,” that it will carry out during the flyby. At the same time, the computer was compressing data stored from previous observations that controllers did not plan to immediately transmit back to Earth, freeing up memory to store new data.
“So we were doing multiple things on the processor on the spacecraft at the same time,” he said. The combination of compressing data and storing the command load was too much for the computer. “The computer was trying to do these two things at the same time, and the two were more than the processor could handle at one time.”
The spacecraft, Fountain said, worked exactly as planned, switching to its backup computer and going into safe mode. The backup computer started transmitting on schedule, allowing engineers on the ground to quickly diagnose the problem. “We realized what was happening,” he said, “and we put a plan in place to recover.”
The spacecraft exited safe mode on July 5, but the project team decided to hold off on resuming science observations until July 7, when the spacecraft will begin carrying out the uploaded series of commands for Pluto flyby. That sequence of commands runs through July 16, two days after the spacecraft’s closest approach to Pluto.
“That was a command decision which I made, and which the team was in complete agreement with, at the beginning of the recovery operation,” said Alan Stern, New Horizons principal investigator, on the decision to wait until July 7 to resume collecting science data. “It’s much more important to focus on getting ready for the flyby than to collect science eight or nine million miles from the target.”
That decision means losing about 30 observations planned between the time of the computer malfunction July 4 and when the spacecraft starts executing the new sequence of commands July 7. That includes 16 images by the spacecraft’s Long Range Reconnaissance Imager camera and four color images by another camera known as Ralph.
Those lost observations, Stern said, account for about six percent of the overall observations New Horizons planned to take between July 4 and 16. However, he added that since the observations were taken while the spacecraft was still millions of kilometers away, they were less significant scientifically.
“Our assessment is that the weighted loss is far less than one percent,” he said. “We can say there is zero impact to the ‘Group One,’ or highest priority science.” Stern characterized the overall science lost because of the computer problem as a “speed bump in terms of the total return that we expect from this flyby.”
Fountain said the specific problem that caused this safe mode won’t happen again: there are no plans to simultaneously upload commands and compress data through the July 14 flyby. “I’m quite confident that this kind of event will not happen,” he said.
The spacecraft will also execute the series of commands for the flyby in what Fountain called “encounter mode,” so that any problems like the one July 4 will not cause a safe mode. Instead, the computer will request help from ground controllers while continuing to carry out commands. Controllers can also send what he called a “slam” command to the spacecraft, forcing the computer to return to the series of commands in its memory for the flyby.
Stern said the spacecraft’s encounter mode has been tested several times earlier in the mission in preparation for the flyby, including a full nine-day rehearsal in 2013. “I’m not worried at all about going into encounter mode tomorrow,” he said.

Mars Rover Curiosity Dealing with Wheel Damage

NASA’s Mars rover Curiosity faces ongoing wheel wear and tear as it continues its trek across the rock-strewn red planet.

The car-size Curiosity rover has been on duty since landing on Mars in August 2012. Curiosity has six aluminum wheels, each with its own individual motor. The rover has a top speed on flat, hard ground of a little over 4 centimeters per second.

But dealing with the rocky Martian landscape has become somewhat of an unanticipated wheel of misfortune for the Curiosity crew. Back here on Earth, mission engineers are watching the wheels turn, keeping an eye on the dings and cracks that have begun to appear.

Grousing About Grousers

“The bottom line is that we are monitoring the wheels all the time,” said Jim Erickson, Curiosity project manager at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

Each of Curiosity’s wheels is about 50 centimeters in diameter and 40 centimeters wide. The wheels have so-called grousers that form something akin to a tread pattern. The skin of a rover wheel is just 0.75 millimeters thick, with the protruding grousers providing structural strength.

Erickson said that, to date, no grouser has been broken — and that’s a good thing. “You can break one. It looks bad, but not horrible. We aren’t there yet,” he said.

Special wheel tests have been performed at JPL. Even with two-thirds of the inner part of the wheel gone, driving on that outer one-third of the wheel appears doable, Erickson said.

Uncertain Wheel Life

Curiosity’s two front wheels began accumulating damage early in the mission.That wear and tear continues, and now the rover’s two middle wheels are showing major damage, Erickson said.

But “the rear wheels are still almost pristine,” he said.

To help cope with the wheel situation, Curiosity engineers are looking at software changes on the vehicle, “to try and make things a little bit better,” Erickson said. “They’ve had some good tests, but it’s not ready for prime time yet.”

The software could provide situational awareness to the wheels, Erickson said, matching wheel drive with electrical current, depending on what terrain the rover faces.

There remain uncertainties about how much overall wheel life is left on Curiosity, Erickson said. One helpful remedy is to carefully guide the robot through less-damaging terrain, he said.

Right Balance

Team members spend significant amounts of time planning out Curiosity’s routes, particularly making use of NASA’s Mars Reconnaissance Orbiter and its high-resolution imaging science experiment (HiRISE) camera system.

Finding the right balance between wheel protection and data collection is also on the mind of Curiosity project scientist Ashwin Vasavada, also of JPL.

“Curiosity’s engineering and science teams have spent over a year understanding how the rover’s design and driving algorithms — and Mars’ terrain — led to more wheel damage than was expected,” Vasavada said. “We’ve also developed a wheel test bed to better predict how the wheels will degrade over time, under certain conditions.”

In addition, Vasavada said that Curiosity teams have mapped out a network of routes up Mount Sharp — the 5.5 kilometer mountain whose foothills the rover is exploring — that vary in their scientific value and also in risk to the robot’s wheels.

“This allows the project as a whole to find the right balance between our scientific progress and factors like wheel wear, slopes and navigability,” Vasavada said. “It all looks quite optimistic and manageable at this point.”

Erickson agrees.

From all of the simulation testing, “the wheel assessment is that we haven’t used up 50 percent of the wheels as yet … and we’ve been driving for three years. I guess I’m neither optimistic nor pessimistic,” Erickson said. “I am more resigned to the fact that we have a consumable.”