vendredi 23 février 2018

Crew Goes into Weekend Preparing to Split Up on Tuesday









ISS - Expedition 54 Mission patch.

February 23, 2018


Image above: Flying over North Pacific Ocean seen by EarthCam on ISS, speed: 27'619 Km/h, altitude: 407,41 Km, image captured by Roland Berga (on Earth in Switzerland) from International Space Station (ISS) using ISS-HD Live application with EarthCam's from ISS on February 23, 2018 at 18:48 UTC.

Three Expedition 54 crew members are going into the weekend packing up and preparing to return to Earth on Tuesday. Commander Alexander Misurkin will lead fellow crew members Joe Acaba and Mark Vande Hei back to Earth inside the Soyuz MS-06 spacecraft Tuesday for a landing in south central Kazakhstan at 9:31 p.m. EST.

NASA TV will broadcast live all of the departure activities on Monday and Tuesday. The Change of Command Ceremony begins Monday at 2:40 p.m. when Misurkin hands over station control to cosmonaut Anton Shkaplerov. The new commander will stay behind with Flight Engineers Scott Tingle of NASA and Norishige Kanai of the Japan Aerospace Exploration Agency and become Expedition 55 when their crewmates undock the next day.


Image above: (Clockwise from bottom) Expedition 54 Commander Alexander Misurkin of Roscosmos; NASA astronauts Mark Vande Hei and Joe Acaba; Roscosmos cosmonaut Anton Shkaplerov; Astronaut Norishige Kanai of the Japan Aerospace Exploration Agency; NASA astronaut Scott Tingle. Image Credit: NASA.

The departing trio will say farewell Tuesday and close the Soyuz hatch at 2:15 p.m. They will undock from the Poisk module at 6:08 p.m. signifying the start of Expedition 55 and the end of Expedition 54. Next, the Soyuz engines will fire one last time at 8:38 p.m. sending the crew back into Earth’s atmosphere for a parachuted landing in Kazakhstan at 9:31 p.m.

The trio will have spent 168 days in space, orbiting Earth 2,688 times, conducted dozens of science experiments and seen the departure and arrival of eight different space ships. The departing crew members will also go home as experienced spacewalkers. Misurkin and Acaba each conducted one spacewalk and Vande Hei conducted four spacewalks during their five-and-half month stay in space.

Related links:

Expedition 54: https://www.nasa.gov/mission_pages/station/expeditions/expedition54/index.html

Expedition 55: https://www.nasa.gov/mission_pages/station/expeditions/expedition55/index.html

Space Station Research and Technology: https://www.nasa.gov/mission_pages/station/research/index.html

International Space Station (ISS): https://www.nasa.gov/mission_pages/station/main/index.html

Images (mentioned), Text, Credits: NASA/Mark Garcia/Orbiter.ch Aerospace/Roland Berga.

Best regards, Orbiter.ch

On Second Thought, the Moon's Water May Be Widespread and Immobile












NASA - Lunar Reconnaissance Orbiter (LRO) patch.

Feb. 23, 2018

A new analysis of data from two lunar missions finds evidence that the Moon’s water is widely distributed across the surface and is not confined to a particular region or type of terrain. The water appears to be present day and night, though it’s not necessarily easily accessible.

The findings could help researchers understand the origin of the Moon’s water and how easy it would be to use as a resource. If the Moon has enough water, and if it’s reasonably convenient to access, future explorers might be able to use it as drinking water or to convert it into hydrogen and oxygen for rocket fuel or oxygen to breathe.

“We find that it doesn’t matter what time of day or which latitude we look at, the signal indicating water always seems to be present,” said Joshua Bandfield, a senior research scientist with the Space Science Institute in Boulder, Colorado, and lead author of the new study published in Nature Geoscience. “The presence of water doesn’t appear to depend on the composition of the surface, and the water sticks around.”


Image above: If the Moon has enough water, and if it's reasonably convenient to access, future explorers might be able to use it as a resource. Image Credits: NASA's Goddard Space Flight Center.

The results contradict some earlier studies, which had suggested that more water was detected at the Moon’s polar latitudes and that the strength of the water signal waxes and wanes according to the lunar day (29.5 Earth days). Taking these together, some researchers proposed that water molecules can “hop” across the lunar surface until they enter cold traps in the dark reaches of craters near the north and south poles. In planetary science, a cold trap is a region that’s so cold, the water vapor and other volatiles which come into contact with the surface will remain stable for an extended period of time, perhaps up to several billion years.

The debates continue because of the subtleties of how the detection has been achieved so far. The main evidence has come from remote-sensing instruments that measured the strength of sunlight reflected off the lunar surface. When water is present, instruments like these pick up a spectral fingerprint at wavelengths near 3 micrometers, which lies beyond visible light and in the realm of infrared radiation.

But the surface of the Moon also can get hot enough to “glow,” or emit its own light, in the infrared region of the spectrum. The challenge is to disentangle this mixture of reflected and emitted light. To tease the two apart, researchers need to have very accurate temperature information.

Bandfield and colleagues came up with a new way to incorporate temperature information, creating a detailed model from measurements made by the Diviner instrument on NASA’s Lunar Reconnaissance Orbiter, or LRO. The team applied this temperature model to data gathered earlier by the Moon Mineralogy Mapper, a visible and infrared spectrometer that NASA’s Jet Propulsion Laboratory in Pasadena, California, provided for India’s Chandrayaan-1 orbiter.

The new finding of widespread and relatively immobile water suggests that it may be present primarily as OH, a more reactive relative of H2O that is made of one oxygen atom and one hydrogen atom. OH, also called hydroxyl, doesn’t stay on its own for long, preferring to attack molecules or attach itself chemically to them. Hydroxyl would therefore have to be extracted from minerals in order to be used.

The research also suggests that any H2O present on the Moon isn’t loosely attached to the surface.

“By putting some limits on how mobile the water or the OH on the surface is, we can help constrain how much water could reach the cold traps in the polar regions,” said Michael Poston of the Southwest Research Institute in San Antonio, Texas.

Sorting out what happens on the Moon could also help researchers understand the sources of water and its long-term storage on other rocky bodies throughout the solar system.

Lunar Reconnaissance Orbiter or LRO. Image Credit: NASA

The researchers are still discussing what the findings tell them about the source of the Moon’s water. The results point toward OH and/or H2O being created by the solar wind hitting the lunar surface, though the team didn’t rule out that OH and/or H2O could come from the Moon itself, slowly released from deep inside minerals where it has been locked since the Moon was formed.

“Some of these scientific problems are very, very difficult, and it’s only by drawing on multiple resources from different missions that are we able to hone in on an answer,” said LRO project scientist John Keller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

LRO is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, for the Science Mission Directorate at NASA Headquarters in Washington, D.C. JPL designed, built and manages the Diviner instrument.

Read the paper in Nature Geoscience: http://dx.doi.org/10.1038/s41561-018-0065-0

LRO (Lunar Reconnaissance Orbiter): http://www.nasa.gov/mission_pages/LRO/main/index.html

Images (mentioned), Text, Credits: NASA/Karl Hille/Goddard Space Flight Center, by Elizabeth Zubritsky.

Greetings, Orbiter.ch

NASA’s SDO Reveals How Magnetic Cage on the Sun Stopped Solar Eruption












NASA - Solar Dynamics Observatory (SDO) patch.

Feb. 23, 2018

A dramatic magnetic power struggle at the Sun’s surface lies at the heart of solar eruptions, new research using NASA data shows. The work highlights the role of the Sun’s magnetic landscape, or topology, in the development of solar eruptions that can trigger space weather events around Earth.

Solar Dynamics Observatory, or SDO. Image Credit: NASA

The scientists, led by Tahar Amari, an astrophysicist at the Center for Theoretical Physics at the École Polytechnique in Palaiseau Cedex, France, considered solar flares, which are intense bursts of radiation and light. Many strong solar flares are followed by a coronal mass ejection, or CME, a massive, bubble-shaped eruption of solar material and magnetic field, but some are not — what differentiates the two situations is not clearly understood. 

Using data from NASA’s Solar Dynamics Observatory, or SDO, the scientists examined an October 2014 Jupiter-sized sunspot group, an area of complex magnetic fields, often the site of solar activity. This was the biggest group in the past two solar cycles and a highly active region. Though conditions seemed ripe for an eruption, the region never produced a major CME on its journey across the Sun. It did, however, emit a powerful X-class flare, the most intense class of flares. What determines, the scientists wondered, whether a flare is associated with a CME?


Animation above: On Oct. 24, 2014, NASA’s SDO observed an X-class solar flare erupt from a Jupiter-sized sunspot group. Animation Credits: Tahar Amari et al./Center for Theoretical Physics/École Polytechnique/NASA Goddard/Joy Ng.

The team of scientists included SDO’s observations of magnetic fields at the Sun’s surface in powerful models that calculate the magnetic field of the Sun’s corona, or upper atmosphere, and examined how it evolved in the time just before the flare. The model reveals a battle between two key magnetic structures: a twisted magnetic rope — known to be associated with the onset of CMEs — and a dense cage of magnetic fields overlying the rope.

The scientists found that this magnetic cage physically prevented a CME from erupting that day. Just hours before the flare, the sunspot’s natural rotation contorted the magnetic rope and it grew increasingly twisted and unstable, like a tightly coiled rubber band. But the rope never erupted from the surface: Their model demonstrates it didn’t have enough energy to break through the cage. It was, however, volatile enough that it lashed through part of the cage, triggering the strong solar flare.

By changing the conditions of the cage in their model, the scientists found that if the cage were weaker that day, a major CME would have erupted on Oct. 24, 2014. The group is interested in further developing their model to study how the conflict between the magnetic cage and rope plays out in other eruptions. Their findings are summarized in a paper published in Nature on Feb. 8, 2018.

“We were able to follow the evolution of an active region, predict how likely it was to erupt, and calculate the maximum amount of energy the eruption can release,” Amari said. “This is a practical method that could become important in space weather forecasting as computational capabilities increase.”


Image above: In this series of images, the magnetic rope, in blue, grows increasingly twisted and unstable. But it never erupts from the Sun’s surface: The model demonstrates the rope didn’t have enough energy to break through the magnetic cage, in yellow. Image Credits: Tahar Amari et al./Center for Theoretical Physics/École Polytechnique/NASA Goddard/Joy Ng.

Related:

Nature Feb. 8, 2018: https://www.nature.com/articles/nature24671

NASA Watches the Sun Put a Stop to Its Own Eruption: https://www.nasa.gov/feature/goddard/2017/nasa-watches-the-sun-put-a-stop-to-its-own-eruption

Two Weeks in the Life of a Sunspot: https://www.nasa.gov/feature/goddard/2017/two-weeks-in-the-life-of-a-sunspot/

NASA’s Solar Dynamics Observatory (SDO): http://nasa.gov/sdo

Images (mentioned), Text, Credits: NASA/Rob Garner/Goddard Space Flight Center, by Lina Tran.

Greetings, Orbiter.ch

Time-lapse Sequence of Jupiter's South Pole












NASA - JUNO Mission logo.

February 23, 2018


This series of images captures cloud patterns near Jupiter's south pole, looking up towards the planet's equator.

NASA's Juno spacecraft took the color-enhanced time-lapse sequence of images during its eleventh close flyby of the gas giant planet on Feb. 7 between 7:21 a.m. and 8:01 a.m. PST (10:21 a.m. and 11:01 a.m. EST). At the time, the spacecraft was between 85,292 to 124,856 miles (137,264 to 200,937 kilometers) from the tops of the clouds of the planet with the images centered on latitudes from 84.1 to 75.5 degrees south.

At first glance, the series might appear to be the same image repeated. But closer inspection reveals slight changes, which are most easily noticed by comparing the far left image with the far right image.

Directly, the images show Jupiter. But, through slight variations in the images, they indirectly capture the motion of the Juno spacecraft itself, once again swinging around a giant planet hundreds of millions of miles from Earth.

Citizen scientist Gerald Eichstädt processed this image using data from the JunoCam imager.

JunoCam's raw images are available at http://www.missionjuno.swri.edu/junocam for the public to peruse and process into image products.

More information about Juno is online at http://www.nasa.gov/juno and http://missionjuno.swri.edu.

Juno spacecraft orbiting Jupiter

NASA's Jet Propulsion Laboratory manages the Juno mission for the principal investigator, Scott Bolton, of Southwest Research Institute in San Antonio. Juno is part of NASA's New Frontiers Program, which is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama, for NASA's Science Mission Directorate. Lockheed Martin Space Systems, Denver, built the spacecraft. Caltech in Pasadena, California, manages JPL for NASA.

Image, Animation, Text, Credits: NASA/Gerald Eichstädt.

Greetings, Orbiter.ch

Swarm trio becomes a quartet








ESA - SWARM Mission logo.

February 23, 2018

With the aim of making the best possible use of existing satellites, ESA and Canada have made a deal that turns Swarm into a four-satellite mission to shed even more light on space weather and features such as the aurora borealis.

In orbit since 2013, ESA’s three identical Swarm satellites have been returning a wealth of information about how our magnetic field is generated and how it protects us from dangerous electrically charged atomic particles in the solar wind.

Aurora from above

Canada’s Cassiope satellite carries three instrument packages, one of which is e-POP.  It delivers information on space weather which complements that provided by Swarm. Therefore, the mission teams began looking into how they could work together to make the most of the two missions.

To make life easier, it also just so happens that Cassiope’s orbit is ideal to improve Swarm’s readings.

And now, thanks to this international cooperation and formalised through ESA’s Third Party Mission programme, e-POP has effectively become a fourth element of the Swarm mission. It joins Swarm’s Alpha, Bravo and Charlie satellites as Echo.

Josef Aschbacher, ESA’s Director of Earth Observation Programmes, noted, “This is a textbook example of how virtual constellations and collaborative initiatives can be realised, even deep into the missions’ exploitation phases.

“We embrace the opportunity to include e-POP in the Swarm mission, especially because it is clear that the more data we get, the better the picture we have of complex space weather dynamics.

Cassiope carries e-POP

“ESA is looking forward to seeing the fruits of this collaboration and the improved return on investment for both Europe and Canada.”

Andrew Yau from the University of Calgary added, “Swarm and e-POP have several unique measurement capabilities that are highly complementary.

“By integrating e-POP into the Swarm constellation, the international scientific community will be able to pursue a host of new scientific investigations into magnetosphere–ionosphere coupling, including Earth’s magnetic field and related current systems, upper-atmospheric dynamics and aurora dynamics.”

Birkeland currents

John Manuel from the Canadian Space Agency noted, “We are pleased to see e-POP join ESA’s three Swarm satellites in their quest to unravel the mysteries of Earth's magnetic field.

“Together, they will further improve our understanding of Earth's magnetic field and role it plays in shielding Canada and the world from the effects of space weather.”

Giuseppe Ottavianelli, Third-Party Mission Manager at ESA concluded, “I am pleased that the e-POP ensemble is now formally integrated into our Swarm constellation.

The force that protects our planet

“This milestone achievement confirms the essential role of ESA’s Earthnet programme, enabling synergies across missions, fostering international cooperation, and supporting data access.”

While e-POP changes its name to Echo as part of the Swarm mission, it will also continue to provide information for its original science investigations.

Related links:

Swarm: http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm

Swarm technical info & data: https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/swarm

CASSIOPE: https://epop.phys.ucalgary.ca/cassiope/

e-POP: https://epop.phys.ucalgary.ca/

University of Calgary: http://www.ucalgary.ca/

Canadian Space Agency: http://www.asc-csa.gc.ca/eng/

ESA Third Party Missions: https://earth.esa.int/web/guest/missions/3rd-party-missions/overview

Images, Videos, Text, Credits: ESA/AOES Medialab/Canadian Space Agency/University of Calgary.

Best regards, Orbiter.ch

jeudi 22 février 2018

Astronauts Open BEAM and Prepare for Crew Departure









ISS - Expedition 54 Mission patch.

February 22, 2018


Image above: Sunrise over South China Sea seen by EarthCam on ISS, speed: 27'613 Km/h, altitude: 405,33 Km, image captured by Roland Berga (on Earth in Switzerland) from International Space Station (ISS) using ISS-HD Live application with EarthCam's from ISS on February 22, 2018 at 22:35 UTC.

Three Expedition 54 crew members continued preparing for their return to Earth next week. A pair of astronauts also opened up BEAM today to stow a robotic hand and to check for contaminants.

Commander Alexander Misurkin joined his Soyuz MS-06 crewmates Joe Acaba and Mark Vande Hei and reviewed their procedures for next week’s descent into Earth’s atmosphere. The trio also familiarized themselves with the sensations they will experience flying through the atmosphere and feeling gravity for the first time after 168 days in space.

Misurkin will hand over command of the International Space Station to cosmonaut Anton Shkaplerov on Monday at 2:40 p.m. EST. Misurkin, Vande Hei and Acaba will then close the hatch to their Soyuz spacecraft Tuesday at 2:15 p.m. and undock from the Poisk module 6:08 p.m. The trio will then parachute to a landing in Kazakhstan at 9:32 p.m. NASA TV will cover all the landing activities live.


Image above: Expedition 53-54 crew members (from left) Joe Acaba, Alexander Misurkin and Mark Vande Hei pose for a portrait inside the Japanese Kibo Laboratory module. Image Credit: NASA.

Flight Engineers Scott Tingle and Norishige Kanai will stay behind on the station with Shkaplerov as commander officially becoming the Expedition 55 crew when their crew mates undock next week. They will be joined March 23 by new Expedition 55-56 crew members Oleg Artemyev, Ricky Arnold and Drew Feustel. The trio will launch March 21 and were in Red Square in Moscow today for traditional ceremonial activities.

Today, Tingle and Kanai opened up the Bigelow Expandable Activity Module (BEAM) and stowed a degraded robotic hand, or Latching End Effector (LEE), that was attached to the Canadarm2. The LEE was returned inside the station after last week’s robotics maintenance spacewalk. The duo also sampled BEAM’s air and surfaces for microbes.

Related links:

NASA TV: https://www.nasa.gov/multimedia/nasatv/index.html

BEAM: https://www.nasa.gov/content/bigelow-expandable-activity-module

Expedition 54: https://www.nasa.gov/mission_pages/station/expeditions/expedition54/index.html

Space Station Research and Technology: https://www.nasa.gov/mission_pages/station/research/index.html

International Space Station (ISS): https://www.nasa.gov/mission_pages/station/main/index.html

Images (mentioned), Text, Credits: NASA/Mark Garcia/Orbiter.ch Aerospace/Roland Berga.

Greetings, Orbiter.ch

Improved Hubble Yardstick Gives Fresh Evidence for New Physics in the Universe











NASA - Hubble Space Telescope patch.

Feb. 22, 2018

Astronomers have used NASA's Hubble Space Telescope to make the most precise measurements of the expansion rate of the universe since it was first calculated nearly a century ago. Intriguingly, the results are forcing astronomers to consider that they may be seeing evidence of something unexpected at work in the universe.

That's because the latest Hubble finding confirms a nagging discrepancy showing the universe to be expanding faster now than was expected from its trajectory seen shortly after the big bang. Researchers suggest that there may be new physics to explain the inconsistency.

"The community is really grappling with understanding the meaning of this discrepancy," said lead researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, both in Baltimore, Maryland.


Image above: This illustration shows three steps astronomers used to measure the universe's expansion rate (Hubble constant) to an unprecedented accuracy, reducing the total uncertainty to 2.3 percent. The measurements streamline and strengthen the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near to and far from Earth. The latest Hubble study extends the number of Cepheid variable stars analyzed to distances of up to 10 times farther across our galaxy than previous Hubble results. Image Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU).

Riess's team, which includes Stefano Casertano, also of STScI and Johns Hopkins, has been using Hubble over the past six years to refine the measurements of the distances to galaxies, using their stars as milepost markers. Those measurements are used to calculate how fast the universe expands with time, a value known as the Hubble constant. The team’s new study extends the number of stars analyzed to distances up to 10 times farther into space than previous Hubble results.

But Riess's value reinforces the disparity with the expected value derived from observations of the early universe's expansion, 378,000 years after the big bang — the violent event that created the universe roughly 13.8 billion years ago. Those measurements were made by the European Space Agency's Planck satellite, which maps the cosmic microwave background, a relic of the big bang. The difference between the two values is about 9 percent. The new Hubble measurements help reduce the chance that the discrepancy in the values is a coincidence to 1 in 5,000.

Planck's result predicted that the Hubble constant value should now be 67 kilometers per second per megaparsec (3.3 million light-years), and could be no higher than 69 kilometers per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it is moving 67 kilometers per second faster. But Riess's team measured a value of 73 kilometers per second per megaparsec, indicating galaxies are moving at a faster rate than implied by observations of the early universe.

The Hubble data are so precise that astronomers cannot dismiss the gap between the two results as errors in any single measurement or method. "Both results have been tested multiple ways, so barring a series of unrelated mistakes," Riess explained, "it is increasingly likely that this is not a bug but a feature of the universe."

Explaining a Vexing Discrepancy

Riess outlined a few possible explanations for the mismatch, all related to the 95 percent of the universe that is shrouded in darkness. One possibility is that dark energy, already known to be accelerating the cosmos, may be shoving galaxies away from each other with even greater — or growing — strength. This means that the acceleration itself might not have a constant value in the universe but changes over time in the universe. Riess shared a Nobel Prize for the 1998 discovery of the accelerating universe.

Another idea is that the universe contains a new subatomic particle that travels close to the speed of light. Such speedy particles are collectively called "dark radiation" and include previously-known particles like neutrinos, which are created in nuclear reactions and radioactive decays. Unlike a normal neutrino, which interacts by a subatomic force, this new particle would be affected only by gravity and is dubbed a "sterile neutrino."

Yet another attractive possibility is that dark matter (an invisible form of matter not made up of protons, neutrons, and electrons) interacts more strongly with normal matter or radiation than previously assumed.

Any of these scenarios would change the contents of the early universe, leading to inconsistencies in theoretical models. These inconsistencies would result in an incorrect value for the Hubble constant, inferred from observations of the young cosmos. This value would then be at odds with the number derived from the Hubble observations.

Riess and his colleagues don't have any answers yet to this vexing problem, but his team will continue to work on fine-tuning the universe's expansion rate. So far, Riess's team, called the Supernova H0 for the Equation of State (SH0ES), has decreased the uncertainty to 2.3 percent. Before Hubble was launched in 1990, estimates of the Hubble constant varied by a factor of two. One of Hubble's key goals was to help astronomers reduce the value of this uncertainty to within an error of only 10 percent. Since 2005, the group has been on a quest to refine the accuracy of the Hubble constant to a precision that allows for a better understanding of the universe's behavior.

Building a Strong Distance Ladder

The team has been successful in refining the Hubble constant value by streamlining and strengthening the construction of the cosmic distance ladder, which the astronomers use to measure accurate distances to galaxies near to and far from Earth. The researchers have compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They then have used the apparent outward velocity of galaxies at each distance to calculate the Hubble constant.

But the Hubble constant's value is only as precise as the accuracy of the measurements. Astronomers cannot use a tape measure to gauge the distances between galaxies. Instead, they have selected special classes of stars and supernovae as cosmic yardsticks or milepost markers to precisely measure galactic distances.

Among the most reliable for shorter distances are Cepheid variables, pulsating stars that brighten and dim at rates that correspond to their intrinsic brightness. Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.


Image above: These Hubble Space Telescope images showcase two of the 19 galaxies analyzed in a project to improve the precision of the universe's expansion rate, a value known as the Hubble constant. The color-composite images show NGC 3972 (left) and NGC 1015 (right), located 65 million light-years and 118 million light-years, respectively, from Earth. The yellow circles in each galaxy represent the locations of pulsating stars called Cepheid variables. Image Credits: NASA, ESA, A. Riess (STScI/JHU).

Astronomer Henrietta Leavitt was the first to recognize the utility of Cepheid variables to gauge distances in 1913. But the first step is to measure the distances to Cepheids independent of their brightness, using a basic tool of geometry called parallax. Parallax is the apparent shift of an object's position due to a change in an observer's point of view. This technique was invented by the ancient Greeks who used it to measure the distance from Earth to the Moon.

The latest Hubble result is based on measurements of the parallax of eight newly analyzed Cepheids in our Milky Way galaxy. These stars are about 10 times farther away than any studied previously, residing between 6,000 light-years and 12,000 light-years from Earth, making them more challenging to measure. They pulsate at longer intervals, just like the Cepheids observed by Hubble in distant galaxies containing another reliable yardstick, exploding stars called Type Ia supernovae. This type of supernova flares with uniform brightness and is brilliant enough to be seen from relatively farther away. Previous Hubble observations studied 10 faster-blinking Cepheids located 300 light-years to 1,600 light-years from Earth.

Scanning the Stars

To measure parallax with Hubble, the team had to gauge the apparent tiny wobble of the Cepheids due to Earth's motion around the Sun. These wobbles are the size of just 1/100 of a single pixel on the telescope's camera, which is roughly the apparent size of a grain of sand seen 100 miles away.

Therefore, to ensure the accuracy of the measurements, the astronomers developed a clever method that was not envisioned when Hubble was launched. The researchers invented a scanning technique in which the telescope measured a star's position a thousand times a minute every six months for four years.

The team calibrated the true brightness of the eight slowly pulsating stars and cross-correlated them with their more distant blinking cousins to tighten the inaccuracies in their distance ladder. The researchers then compared the brightness of the Cepheids and supernovae in those galaxies with better confidence, so they could more accurately measure the stars' true brightness, and therefore calculate distances to hundreds of supernovae in far-flung galaxies with more precision.

Another advantage to this study is that the team used the same instrument, Hubble's Wide Field Camera 3, to calibrate the luminosities of both the nearby Cepheids and those in other galaxies, eliminating the systematic errors that are almost unavoidably introduced by comparing those measurements from different telescopes.

Hubble Space Telescope (HST). Animation Credits: NASA/ESA

"Ordinarily, if every six months you try to measure the change in position of one star relative to another at these distances, you are limited by your ability to figure out exactly where the star is," Casertano explained. Using the new technique, Hubble slowly slews across a stellar target, and captures the image as a streak of light. "This method allows for repeated opportunities to measure the extremely tiny displacements due to parallax," Riess added. "You're measuring the separation between two stars, not just in one place on the camera, but over and over thousands of times, reducing the errors in measurement."

The team's goal is to further reduce the uncertainty by using data from Hubble and the European Space Agency's Gaia space observatory, which will measure the positions and distances of stars with unprecedented precision. "This precision is what it will take to diagnose the cause of this discrepancy," Casertano said.

The team's results have been accepted for publication by The Astrophysical Journal:
http://iopscience.iop.org/journal/0004-637X

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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

For more about Hubble, visit:

http://hubblesite.org/
http://www.nasa.gov/hubble
http://www.spacetelescope.org/

For additional imagery to this story, visit: https://media.stsci.edu/news_release/news/2018-12

Images (mentioned), Animation (mentioned), Text, Credits: NASA/Karl Hille/Space Telescope Science Institute/Donna Weaver/Ray Villard.

Best regards, Orbiter.ch