Posts Tagged ‘radio telescopes’

Planets Weighed Using Pulsar Flashes

23 Aug

The rotating corpses of massive stars can help scientists weigh the planets in the solar system. By carefully timing radio blips from spinning stellar leftovers called pulsars, astronomers have measured the masses of all the planets from Mercury to Saturn, plus all their moons and rings.

Until now, the only way to figure out the mass of a planet was to send a spacecraft past it. The spacecraft’s orbit is determined by the gravitational oomph of the planet (plus whatever moons lay within the spacecraft’s orbit), which in turn depends on the planet’s mass. The new method is the first to let astronomers weigh planets from the comfort of Earthbound observatories.

“That’s what’s remarkable about this technique,” said space technologist William Folkner of NASA’s Jet Propulsion Laboratory, a co-author of a study in the upcoming issue of Astrophysical Journal. “I can’t think of any other way to measure masses of planets from the Earth.” 

The new method relies on the clock-like regularity of a class of neutron stars called pulsars, the rapidly spinning remains of massive stars that died in supernova explosions. Pulsars shoot tight beams of radio waves into space that sweep across the sky like a lighthouse, so from Earth they appear to blink or pulse.

Because the Earth is always moving around the sun, the time it takes for these radio blips to reach us is always changing. To get rid of this effect, astronomers calculate when the pulse should reach the solar system’s center of mass, or barycenter — the point around which all the mass in the solar system moves. But because the planets’ arrangement around the sun is constantly changing, the barycenter moves around with respect to the sun, too.

To pin down the center of mass at a given time, astronomers use a special table of where all the planets are, called an ephemeris, plus values for the masses of the planets taken from previous space missions. If the masses are slightly wrong, then a regular, repeating pattern of timing errors appears in the pulsar data. For instance, if Jupiter’s mass is a bit off, then an error appears every twelve years, once for every time Jupiter orbits the sun. Correcting the value for Jupiter’s mass makes the error disappear.

“You can see that 12 year wiggle in timing of neutron stars,” Folkner said. “That tells you how far the sun is from the solar system barycenter, which tells you what the mass of Jupiter is.”

An international team of scientists used three different radio telescopes, the 1000-foot-wide Arecibo telescope in Puerto Rico, the 210-foot Parkes telescope in Australia and the 328-foot Effelsberg telescope in Germany to time the blips from four different pulsars over a period of 5 to 22 years. They then used computer models to use the pulsars’ times to calculate the masses of Mercury, Venus, Mars, Jupiter and Saturn.

The masses the team found are not as accurate as the best measurements from spacecraft flybys, but they’re close. The measurement for Jupiter, for instance, was found to be 0.0009547921 times the mass of the sun. This value is more accurate than the mass determined from the Pioneer and Voyager spacecraft, and less accurate than, but consistent with, the value from the later Galileo spacecraft, which includes more decimal places.

“Our error bars are larger than those of these spacecraft measurements,” said study co-author Andrea Lommen of Franklin & Marshall College. “We are admitting freely that you should still use the mass of Jupiter measured from the spacecraft, but it’s comforting to know that our measurement agrees with that.”

The new method is also the first that can measure the masses of everything in a planetary system, including moons and rings.

“Spacecraft flybys don’t tell us the mass of everything in the Jupiter system, only the parts inside the spacecraft orbit,” Folkner said. “With this pulsar timing mechanism, we’re sensitive to the entire system, including the moons that are outside the orbit of any spacecraft that have flown by.”

The technique is actually a stepping stone to studying something even more exotic: ripples in space-time called gravitational waves that were predicted by Einstein but have never been observed. The timing of pulsar blips should change slightly whenever a gravitational wave goes by, but in order to see these changes, astronomers have to subtract out all the other noise that could alter the pulsar’s clock.

This study is “a graphic demonstration that you really have to understand the solar system really well if you’re going to be able to confidently detect gravitational radiation,” commented astronomer Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey, who was not involved in the new work. “If they can continue to develop these techniques to the point where they can detect gravitational waves, that will be a dramatically important event.”

Image: The sun, Earth and Jupiter orbit a common center of mass. David Champion, MPIfR

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Citizen Scientists Make First Deep Space Discovery With Einstein@Home

12 Aug

While your computer is running idle, it could be finding new pulsars and black holes in deep space.

Three volunteers running the distributed computing program Einstein@Home have discovered a new pulsar in the data from the Arecibo Observatory radio telescope. Their computers, one in Iowa (owned by two people) and one in Germany, downloaded and processed the data that found the pulsar, which is in the Milky Way, approximately 17,000 light years from Earth in constellation Vulpecula.

“The way that we found the pulsar using distributed computing with volunteers is a new paradigm that we’re going to make better use of in astronomy as time goes on,” said astronomer Jim Cordes of Cornell University. “This really has legs.”

About 250,000 volunteers run Einstein@Home, on average donating about 250 teraflops of computing power — equivalent to a quarter of the capacity of the largest supercomputer in the world, says program developer David Anderson of University of California at Berkeley’s Space Sciences Laboratory, co-author of the Aug. 12 discovery announcement in Science.

Einstein@Home has been searching for gravitational waves in the data from the US LIGO Observatory since 2005, and since March 2009 has dedicated one-third of its power to searching for radio pulsars and black holes in the Arecibo data. As of this week, it will start dedicating half of its processing power to data from Arecibo, the world’s largest and most sensitive radio telescope, physicist Bruce Allen of the Max Plank Institute for Gravitational Physics in Germany and co-author of the study announced a press conference Aug. 12.

The new pulsar, dubbed PSR J2007+2722, is a neutron star rotating 41 times per second. Pulsars are birthed when stars five to 10 times as massive as our sun explode into a supernova and then collapse into stars composed almost entirely of neutrons.

The data from Arecibo was processed on the computer in Iowa June 11, and then also processed on a computer in Germany June 14 for validation. The finding was part of a larger search that returned results on July 10, which was the first time a human being was aware of the discovery.

Aerial view of the Arecibo Observatory radio telescope.

The person who looked at the results notified Greenbank Observatory in West Virginia, which immediately pointed their telescope at the new pulsar to verify it. Within hours, Arecibo Observatory in Puerto Rico also pointed their telescope at it.

“This is the first time I’ve worked closely with radio astronomers making a discovery,” said Allen. “It was like watching 5-year-olds tearing Christmas presents. Or like watching someone throw chunks of meat at starving sharks.”

Pulsars are named after the pulsing signals they send to Earth. The pulse comes from the spin and the magnetic field of the neutron star being on two different axes, which acts like an electric generator and creates a beamed signal that rotates like a lighthouse. Cordes says theoretical predictions are that only about 20 percent of the pulsars in the galaxy are detectable on Earth because the beam needs to point directly at us to be detected.

Often, pulsars have a companion star or neutron star that was originally born in the same cloud of gas. But this new pulsar doesn’t and is likely a disrupted recycled pulsar. This means the pulsar once had a companion star that it sucked matter from as the star swelled up into a red giant, which caused the pulsar to cycle faster (recycle). The red giant star then exploded into a supernova and blasted the pulsar away, leaving it alone in space (disrupted).

The new pulsar is one of around 2000 pulsars that have been discovered using radio telescopes in the past 43 years, said Cordes. He estimates there are 20,000 pulsars in the Milky Way that could be detected.

“I see this as a long-term effort where we’re going to find really interesting objects,” said Cordes. “We’d like to find a pulsar orbiting a black hole, or a pulsar orbiting another neutron star so that we can test some of Einstein’s predictions of the general theory of relativity”

You can become part of the effort by downloading BOINC. The program has been used to create 70 different distributed computing projects (almost every one in existence except Folding@Home), and you can decide what fraction of your spare computing power you want to dedicate to each of the 70 projects.

In case you need more incentive, Cordes announced that a second pulsar has been already been discovered in the last month by Einstein@Home users in the United Kingdom and Russia. He’s keeping details to himself for now.

“We have a very large data set,” Cordes added at the press conference. “We just need to cull through it, and Einstein@Home lets us use a much finer comb.”


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Images: 1) Screen shot of Einstein@Home/B. Knispel, Albert Einstein Institute. 2) Copyright Cornell University.

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