Asteroids help scientists measure distant stars

Look up at the sky on a clear night, and you’ll see lots of stars. Sometimes they seem almost within reach or at least a short rocket ride. But the closest star to Earth—not counting our sun—is more than four light years away, at a distance of 25 trillion miles.

Asteroids help scientists to measure the diameters of faraway stars

When an asteroid passes in front of a star, the resulting diffraction pattern (here greatly exaggerated) can reveal the star’s angular size. Credit: DESY, Lucid Berlin

There are more than 100 billion stars in our Milky Way Galaxy, and, while we have learned much about them, there are relatively few whose size has been directly measured because they are so far away. A star’s size is a key piece of information that unlocks many other mysteries about it. Several methods have been used to measure star sizes, yet each has its limitations.

But now an international team, including researchers from the University of Delaware, has discovered a new way to determine the size of stars. Their method draws on the unique capabilities of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in Arizona—and asteroids that pass by at just the right place and time.

Using the technique, a collaboration of 23 universities and research institutes, led by Tarek Hassan of Deutsches Elektronen-Synchrotron (DESY) and Michael Daniel of the Smithsonian Astrophysical Observatory, has revealed the diameters of a giant star 2,674 light-years away, and a sun-like star at a distance of 700 light-years—the smallest star measured in the night sky to date. The research was reported on Monday, April 15 in the journal Nature Astronomy.

Sizing up a star

“Knowing the size of a star is of overall importance,” said Jamie Holder, associate professor in UD’s Department of Physics and Astronomy and a co-author of the study. “How big and how hot a star is tells you how it was born, how long it will shine, and how it will eventually die.”

Yet almost any star in the sky is too far away to be measured accurately by even the best optical telescopes.

“You just can’t resolve the point-like image of a star,” Holder said. “It will look fuzzy through your telescope.”

To overcome this limitation, scientists use an optical phenomenon called diffraction to measure a star’s diameter. When an object passes in front of a star, an event called an “occultation,” the shadow and surrounding pattern of light waves can be used to calculate the star’s size.

In this pilot study, the object passing in front of the star was an asteroid—a bit of space rubble likely leftover from when the planets were formed about 4.6 billion years ago.

Asteroids travel at an average speed of 15 miles per second, which added to the team’s challenge. Normally, the VERITAS telescopes watch for the faint bluish blip that high-energy cosmic particles and gamma rays produce when they race through Earth’s atmosphere. While the telescopes do not produce the best optical images, they are extremely sensitive to fast variations of light, including starlight, thanks to their huge mirrored surface, segmented in hexagons like a fly’s eye. Holder was involved in the construction and commissioning of the telescopes in 2006, and all of the light sensor modules for the four telescopes were assembled at UD.

UD doctoral student makes pioneering observations

Using the four large VERITAS telescopes on Feb. 22, 2018, the team could clearly detect the diffraction pattern of the star TYC 5517-227-1 as the 60-kilometer (37 mile) asteroid Imprinetta passed by. UD doctoral student Tyler Williamson was there for the observation.

“It was our first time performing this kind of measurement, so we made sure to give ourselves plenty of time to get set up and follow the procedure exactly,” said Williamson, who was one of three scientists on the shift that night. “The occultation itself takes only a few seconds, but we point the telescope at the star for about 15 minutes or so to get an estimate of what it looks like before and after the event. If you want to detect a shadow, you need to know what the object looks like without anything blocking it.”

Usually, when the crew takes data, a computer gives them a real-time view of what they are collecting as it comes in. But there was no way for them to see this occultation occur. They simply had to point the telescope and wait.

“Nobody was sure the occultation would even be visible from our location in the first place,” he said. “The most recent estimate we had going into the night was that there was about a 50 percent chance that the shadow would be cast over our observatory—the asteroid is small, and there were uncertainties in size and trajectory, making it impossible to say for sure where the shadow would fall.”

The crew took the data, emailed it to the principal investigators on the project, and called it a night.

“I remember waking up the following afternoon to an email from the PIs with a nice plot showing a clear detection of the shadow,” Williamson said. “We were all very excited, and, as observers, we were quite happy to be a part of the result.”

Sizing up a starry night
UD Professor Jamie Holder (left) and doctoral student Tyler Williamson are part of an international team that has developed a new method for measuring the size of stars. The technique hinges on the unique capabilities of the VERITAS telescopes in the Arizona desert (shown in the background) and on asteroids passing by at the right place and time. Credit: Evan Krape and NASA

The VERITAS telescopes allowed the team to take 300 snapshots every second. From these data, the brightness profile of the diffraction pattern could be reconstructed with high accuracy, resulting in an angular, or apparent, diameter of the star of 0.125 milliarcseconds. Together with its distance of 2,674 light-years, the scientists determined that the star’s true diameter is 11 times that of our sun, categorizing it as a red giant star.

According to Holder, this star is about 200 million times farther away from us than the sun, but it’s still well within our Milky Way Galaxy, which is 100,000 light years across.

The researchers repeated the feat three months later on May 22, 2018, when asteroid Penelope with a diameter of 88 kilometers occulted the star TYC 278-748-1. The measurements resulted in an angular size of 0.094 milliarcseconds and a true diameter of 2.17 times that of our sun—the smallest star ever measured directly.

But “small” is relative. “This star is a G dwarf, twice as big as our sun and about 700 times farther away from us than our closest star,” Holder said.

While the new technique delivers a ten times better resolution than the standard method astronomers have been using, based on lunar occultation, and is twice as sharp as size measurements using interferometric techniques, Holder said the team is working to refine it for even greater accuracy.

“Asteroids pass by Earth every day,” Holder said. “VERITAS is gearing up to increase its observations and extend its observation range, building data on a whole new population of stars.”

Original article here

Scientist anticipated “snowman” asteroid appearance

On Jan. 2, the New Horizons spacecraft made the most distant flyby ever attempted, successfully returning images of the Kuiper Belt object Ultima Thule. While the world is agog at the so-called “snowman” shape of this icy asteroid, the concept is nothing new to PSI scientist and artist, Bill Hartmann. The figure shows paintings that Hartmann made from 1978 to 1996, to illustrate the possible outcome of very low-velocity collisions of distant asteroids. These are compared with the first released color image of Ultima Thule. The story goes back 50 years.

In 1969, University of Arizona astronomers at the Lunar and Planetary Lab (“LPL”), Larry Dunlap and Tom Gehrels, noticed that as the asteroid 624 Hektor, far beyond the main asteroid belt in the region of Jupiter showed extreme changes in brightness as it rotated. In the late 1970s, Hartmann (having recently founded PSI) and Dale Cruikshank (then at the University of Hawaii), observing at 14,000-foot Mauna Kea Observatory, proved that the brightness change was not caused by one side having brighter materials, but rather by a very unusual elongated shape.

Hartmann became intrigued with how such bodies might have formed in the primordial solar system by low-velocity collisions of asteroidal bodies, from which the planets were growing. These still-theoretical bodies were called “compound binary” asteroids – “binary” meaning two bodies, and “compound” indicating that they were touching each other, instead orbiting around each other. PSI’s Stu Weidenschilling published a paper on how the shapes of the two halves of the compound binary might have their shapes distorted, depending on their bulk strengths and the rotation rate of the object.

Hartmann’s 1978 painting showed the compound binary concept with grey colors as found on the Moon. No such bodies had been seen at close range, but Hartmann wanted to depict them. “My astronomical paintings are not just flights of fancy, but a serious attempt to make something both beautiful and realistic out of what we humans have learned about other worlds,” Hartmann said. By 1980, Cruikshank and Hartmann had shown that many bodies in the outermost solar system had a dark, reddish brown color, and his 1980 and 1996 paintings added Hartmann’s estimate of how this color might look.

Ultima Thule was not only the first obvious example of a compound binary structure, but also looked strikingly like Hartmann’s 1996 visualization from 22 years ago. Hartmann happily notes that his 1978 and 1996 paintings show bright material in the “contact zone” where the two bodies collided, and sure enough, the New Horizon spacecraft photo also show bright material there.

“We live in an era where scientific findings are being criticized, but if we can predict phenomena we see on other worlds, we must know something about what we are doing,” Hartmann said


Read the original article here.


Japanese mini-rovers send back their first images as they hop around an asteroid

Two mini-rovers have sent their first pictures back from the surface of the asteroid Ryugu, a day after they were dropped off by Japan’s Hayabusa 2 spacecraft.

The pictures are blurry because they were taken while the rovers were falling and hopping around the half-mile-wide asteroid, more than 180 million miles from Earth.

The rovers are built to spring up from the surface repeatedly and hop as far as 50 feet at a time. A follow-up image from Rover-1A showed the asteroid’s sun-illuminated surface from above during a hop. Yoshimitsu said that picture allowed him to “confirm the effectiveness of this movement mechanism on the small celestial body and see the result of many years of research.”

Hayabusa 2 project team spokesman Takashi Kubota seconded Yoshimitsu’s sentiment, saying that seeing the pictures taken in mid-hop “allowed me to relax as a dream of many years came true.”

“I felt awed by what we had achieved in Japan,” Kubota said. “This is just a real charm of deep space exploration.”

The pictures are reminiscent of the attention-grabbing views sent back from the surface of a comet by a European-built lander during the Rosetta mission.

When Makoto Yoshikawa looked at the rover pictures, he saw redemption. Yoshikawa served as project scientist for the original Hayabusa mission to Itokawa, and is now the project mission manager for Hayabusa 2’s rendezvous with Ryugu.

“I was so moved to see these small rovers successfully explore an asteroid surface, because we could not achieve this at the time of Hayabusa, 13 years ago,” he said.

The Hayabusa 2 mission team is continuing to acquire data for analysis, and further pictures are likely to be distributed first via the mission’s Twitter account.

A larger MASCOT rover, contributed by the German and French space agencies, is due to be dropped toward the surface next month. Next year, Hayabusa 2 will dispatch yet another mini-rover with its MINERVA-II-2 deployable carrier. The main spacecraft will also descend to the asteroid’s surface to collect samples for return to Earth in late 2020.

As fuzzy as they are, the photos represent a huge victory for the $260 million Hayabusa 2 mission, which was launched nearly four years ago to get an unprecedented look at the surface of an asteroid.

The Japan Aerospace Exploration Agency first tried to put a rover on the surface of an asteroid more than a decade ago, during a mission to a space rock called Itokawa. That part of the mission fizzled, however, when the MINERVA rover carrier missed the mark and sailed off into interplanetary space. Hayabusa 2, in contrast, dropped its MINERVA-II-1 carrier right on target. The carrier deployed two 7-inch-wide, disk-shaped rovers that touched down on Ryugu’s rock-strewn terrain.

It took a while to get the pictures back to Earth because they had to be uploaded from the rovers to the mothership — and then relayed back to Earth for processing.

The first snapshot, taken during Rover-1A’s descent, shows Hayabusa 2 as a bright smudge in the black sky above and the surface of Ryugu as a bright smear below. Hayabusa’s solar panels, which account for most of the spacecraft’s 18-foot width, can be made out as fuzzy blue rectangles.

“Although I was disappointed with the blurred image that first came from the rover, it was good to be able to capture this shot as it was recorded by the rover, as the Hayabusa2 spacecraft is shown,” JAXA’s Tetsuo Yoshimitsu, a team leader for MINERVA-II-1, said today in a statement.

Original article please finde here.



Kick Asteroid Campaign

Kick Asteroid: Planetary Defenders, Earth Needs You!

The Planetary Society is excited to partner with space artist and designer, Thomas Romer, and backers around the world to create Kick Asteroid—a colorful graphic poster that will illustrate the effect of past catastrophic impacts, and methods to deflect future asteroid threats. Compelling and scientifically accurate art will be created for posters and other “merch” that backers can use in their everyday lives to spread the word about planetary defense.

Kick Asteroid preliminary poster design


Join the movement and be a Planetary Defender!

There’s no time to spare… ! And with your help, the Society is gearing up to do its part. There are many things we can be doing to help protect against asteroid threats and we want to give you awesome artwork that helps you spread the word.

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