Rico's rants: Space for the day

Space.com has an article by Robert Nayeye about another merger:

In a historic first, researchers across the world detected gravitational waves and light coming from the merging of two dense neutron stars (computerized image, above). This is the fifth detection of gravitational waves, and according to the 16 October 2017 announcement, the corresponding electromagnetic discoveries have ushered in the new field of "multi-messenger astrophysics." On 17 August 2017, Mother Nature delivered a gift to astronomers as precious as anything they could have imagined: gravitational waves from two neutron stars spiraling inward and merging, followed moments later by a burst of gamma rays from the same patch of sky. This cosmic double whammy was officially announced today after nearly two months of rumors. It proves a long-standing theory for an enigmatic class of cosmic cataclysms while heralding a revolutionary new era of multi-messenger astronomy. The sequence of events started at 0841 Eastern time when a train of gravitational waves started rolling through the Virgo detector near Pisa, Italy. The same waves rumbled through the LIGO detector in Livingston, Louisiana, just twenty-two milliseconds later, then the twin LIGO detector in Hanford, Washington, three milliseconds after that. The LIGO and Virgo instruments detected a crescendo of waves for a whopping hundred seconds, much longer than previous detections. The duration, amplitude, and frequency of the waves had all the characteristics that theorists have expected for a binary system consisting of two neutron stars on a death spiral, ending with coalescence. Neutron stars are ultra-dense objects that form from the core collapse of massive stars when they go supernova. These two neutron stars had masses of about one and a half and just over one solar masses, respectively. About one to two percent of that mass was likely ejected into space during the merger, which presumably resulted in a black hole of nearly three solar masses, although the LIGO data does not prove that a black hole formed. If a black hole indeed formed, it’s the lightest black hole yet known. “This discovery is amazing,” says LIGO team member Chad Hanna of Penn State University. “We have all been hoping for a neutron star merger for a long time, and we knew it would come eventually. But it was pretty remarkable to have it come so early.” The joint LIGO/Virgo detection by itself was a momentous discovery: the first direct detection of gravitational waves from merging neutron stars. It follows closely on the heels of the 3 October 2017 announcement that LIGO founding fathers Rainer Weiss, Kip Thorne, and Barry Barish earned the 2017 Nobel Prize in Physics for their pivotal roles in detecting the first gravitational waves, ripples in the fabric of space-time first predicted in 1915 by Albert Einstein in his general theory of relativity. But what really sets this new detection apart is the fact that the gravitational waves were accompanied by a bright source of light. All four prior LIGO discoveries resulted from the inspiralling and merger of binary black holes, which produced no detectable light that could reveal further information about the events. This was expected: black holes are essentially regions where space-time has collapsed around itself, so their mergers don’t involve any matter that can emit light. In contrast, neutron stars are city-sized objects consisting of highly compressed matter, so their mergers are messy, violent affairs. And that’s exactly what was seen. Just less than two seconds after the 17 August 2017 merger, NASA’s Fermi Gamma-ray Space Telescope and the European INTEGRAL satellite picked up a gamma-ray burst (GRB) lasting nearly two seconds from the same general direction of sky. Both the Fermi and LIGO teams quickly alerted astronomers around the world to search for an afterglow. Various ground- and space-based telescopes swung into action. Within ten to eleven hours after the merger, the Chilean-based four-meter Blanco Telescope, with its wide-field Dark Energy Camera, and the one-meter Swope Telescope had both independently pinpointed the optical afterglow in the elliptical galaxy NGC 4993, in the southern constellation Hydra. At a distance of over a hundred million light-years, this was one of the closest GRBs ever observed. The Hubble Space Telescope, the Chandra X-ray Observatory, the Very Large Telescope, the Very Large Array, and numerous other telescopes have studied the afterglow across the electromagnetic spectrum as part of a major international observing campaign. In all, the afterglow has been observed by seventy telescopes. “All of these observations give us a much more complete picture of the neutron star merger and its aftermath than we would have had with gravitational waves alone, or with light alone,” says LIGO team member Amber Stuver of Villanova University, emphasizing the importance of multi-messenger astronomy. Astronomers not only saw gravitational waves and gamma rays from the event, but also light in other portions of the electromagnetic spectrum - such as optical and near-infrared light, gathered by the Swope and Magellan telescopes. Since the 1990s, theorists have predicted that some or most short-duration GRBs, those lasting two seconds or less, originate from colliding neutron stars. This discovery proves beyond doubt that they were correct. According to theory, intense tidal forces rip material off both neutron stars moments before they merge. The merger produces a black hole, which ravenously gobbles up most of the surrounding material. The black hole’s rotational energy channels some of this matter into two oppositely directed jets traveling at near-light speed. As faster jet material slams into slower material, it generates shocks that emit a stupendously energetic burst of gamma rays that lasts a few seconds, just what Fermi and INTEGRAL observed. Short GRBs are among the universe’s most powerful explosions, unleashing almost as much energy in a few seconds as the Sun will radiate in its entire ten-billion-year lifetime. But they are significantly less energetic than their long-duration cousins, which are thought to originate from exploding massive stars. The fact that satellites observed the GRB almost simultaneously with the neutron star merger strongly supports Einstein’s prediction that gravitational waves travel at the speed of light. Theory predicts that the jet produces its gamma rays a few seconds after the neutron stars merge, so the 1.7-second delay between the merger and the GRB matches the predicted sequence of events if both the gravitational waves and gamma rays were traveling at light speed. According to Julie McEnery, the Fermi Project Scientist at NASA’s Goddard Space Flight Center, “The gamma rays and gravitational waves traveled a hundred and thirty million years across space and arrived at Earth within two seconds of each other. Einstein passes another test.”

“That delay is about what we expected for the general gamma-ray burst model,” says Penn State astrophysicist Peter Mészáros, one of the originators of GRB theory. “The jet takes a certain time to expand before it’s able to produce the shocks that produce the gamma-rays. The time delay is in the ballpark of what we had estimated. That was a nice confirmation of the basic GRB model.”
The consistency between the LIGO, GRB, and traditional astronomical observations, in conjunction with theoretical predictions, removes any lingering doubts that LIGO has indeed been detecting gravitational waves for the past two years. As Stuver says, “We have all this redundant data coming in that is telling us that this is not an accident.”
LIGO’s five announced gravitational-wave detections all beautifully match predictions based on general relativity. In the four merging black hole binaries, the black holes contained 8 to 36 solar masses, and the final inspirals and mergers produced gravitational waves in LIGO’s frequency band for just 1/4 to 1.5 seconds. Because neutron stars have much lower masses than black holes, the final inspiral and merger in the August 17 event unfolded over a much longer timescale, producing waves in LIGO’s frequency range for about 100 seconds.
In those hundred seconds, astronomers saw about fifteen hundred orbits of the binary neutron star pair; astronomers think the system was about eleven billion years old. The neutron stars were just two hundred miles apart when astronomers first heard them, says astrophysicist Vicky Kalogera of Northwestern University.
Although the sequence of events played out in a manner very close to theoretical predictions, the gravitational-wave signal was a bit weaker than expected for neutron stars colliding at a distance of more than a hundred million light-years. The short GRB itself was also relatively low in energy for its class. The combined observations suggest we saw the neutron stars orbiting each other at an angle, and that we saw the GRB jet at an angle rather than looking down the barrel. These offset viewing angles reduced the observed intensity of the gravitational waves and the gamma rays.
Further evidence for an off-axis viewing angle comes from the fact that it took nine days for the kilonova to show in X-rays, when Chandra saw it. This indicates that the jet was not pointing directly toward Earth. This is the first time astronomers have observed an off-axis jet in X-rays, while the X-ray emission itself also supports the notion that a black hole formed in the merger. There were no radio detections until the Very Large Array saw the afterglow sixteen days after the merger. This delay also indicates that we were seeing the jet off-axis. The radio source is still bright and will be the last to fade away.
These mergers and their aftereffects are called “kilo-novae.” Astronomers will continue to monitor the afterglow until it fades away, which will reveal how the ejected material interacts with its surrounding environment. This will help astronomers learn about the vital role that kilo-novae play in cosmic chemistry by producing heavy elements, such as gold and platinum. According to Marica Branchesi of the University of Urbino in Italy, a “large amount of these heavy elements form in this kind of merger.” It is also the decay of radioactive elements produced in the collision between the jets and the surrounding material that powers the afterglow.
Kilo-novae can explain the cosmic abundance of gold and platinum without any need for supernovae to produce these very heavy elements. Those elements will eventually combine with other material to form stars and planets. Edo Berger of the Harvard-Smithsonian Center for Astrophysics explained that astronomers saw "direct fingerprints" of these heavy elements in the data from the Chilean telescopes. He added that the total mass of heavy elements created by the kilonova was sixteen thousand times the mass of Earth, including ten times the mass of Earth in gold and platinum alone. In reference to that gold, Andy Howell of the Las Cumbres Observatory and the University of California at Santa Barbara said that "we really did see a pot of gold at the end of a kilonova rainbow."
The ability to detect gravitational waves from neutron star mergers was the result of decades of brilliant theoretical research, combined with the painstaking work of dedicated teams of thousands of scientists and engineers. Still, this detection owed a little bit to Lady Luck. The twin LIGO detectors caught the merger just eight days before they were shut down for several months of scheduled upgrades. And Virgo was turned on just sixteen days prior to the merger. Having all three detectors online was crucial to triangulating the gravitational source to a small enough area of sky (about 28 square degrees) that conventional telescopes could spot the afterglow.
In addition, Mother Nature delivered a neutron star merger well within LIGO’s and Virgo’s detection range. Previously, LIGO team members thought they might have to double LIGO’s sensitivity before they could see deep enough into space to catch gravitational waves from merging neutron stars.
“There’s always luck involved with detecting any gravitational wave, because we have no way of predicting when any of them are going to come by us,” says Stuver. “So yes, this was a gift. Honestly, this is something better than I could have ever hoped for. And I didn’t expect the observation of the first neutron star merger to come along with observations of light. We are really starting the multi-messenger astronomy era.”
The event's optical afterglow initially shone at seventeenth magnitude; bright enough to allow amateur astronomers with large telescopes to detect it. Although the bright blue afterglow faded and reddened quickly, this still opens the door for amateurs to play a part in future gravitational-wave events.
For his part, Hanna wonders if astronomers have been underestimating the rate of neutron star mergers. If so, we can expect many more LIGO detections in the coming years that are accompanied by GRBs. Moreover, some short GRBs might result from mergers between a black hole and a neutron star, and LIGO and Virgo could play critical roles in determining if the GRBs from these events differ from those of neutron star mergers. “I’m hoping that rather than being lucky, we’re seeing a representative sample, an indication of what is to come,” says Hanna.
David Shoemaker of MIT says that LIGO will take a year off for upgrades, increasing the detectors' sensitivity by a factor of two, boosting the volume of space they can search by a factor of eight.
For now, Hanna and his colleagues on the LIGO and Virgo teams will marvel at their latest discovery. “How would you ever imagine that we’re measuring the dynamics of these tiny little compact objects smashing together over a hundred million light-years away?” he asks. “The fact that LIGO is doing this now, and with high confidence, is pretty incredible.”

Rico says the universe continues to exceed our imagination.

Space.com has another article about Elon Musk's BFR (which, knowing him, has to stand for Big Fucking Rocket)...

SpaceX Chief Executive Elon Musk disclosed additional technical details about his proposed BFR launch system on 14 October 2017, although he offered no additional information about the costs and financing of the vehicle.

Musk, in a freewheeling "Ask Me Anything" discussion on Reddit on 14 October 2017, held on just two hours' notice, answered a variety of questions about the BFR design he unveiled on 29 September 2017 at the International Astronautical Congress (IAC) in Adelaide, Australia. That design is a revised, and slightly scaled down, version of the vehicle he announced at the same conference one year earlier in Mexico.

Musk, in that discussion, said that spaceship portion of the BFR, which would transport people on point-to-point suborbital flights or on missions to the Moon or Mars, will be tested on Earth first in a series of short hops.

"Will be starting with a full-scale ship doing short hops of a few hundred kilometers altitude and lateral distance," he wrote. "Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines."

That is similar to what SpaceX President Gwynne Shotwell said at the 5 October 2016 meeting of the National Space Council, when asked about development of the BFR system. "That system is being designed also to do Earth hops, and those are some of the first tests that you'll actually see with the Falcon spaceship," she said.

The spaceship itself, Musk said, is capable of reaching orbit without the assistance of a booster, but with only a "low payload" that he did not specify. "Earth is the wrong planet for single stage to orbit. No problem on Mars."

Musk also addressed changes in the Raptor engine, the large engine powered by methane and liquid oxygen that will power both the booster and the spaceship elements of the BFR system. The original Raptor design, announced last year, was capable of producing nearly seven hundred thousand pounds of thrust. The revised design announced at last month's IAC produces only about four hundred thousand pounds of thrust.

"The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk," Musk wrote when asked about that reduction in thrust. The reduction in thrust also allows for the use of multiple engines, giving the vehicle an engine-out capability for landings.

He added that, since the presentation last month, SpaceX has revised the design of the BFR spaceship to add a "medium area ratio" Raptor engine to its original complement of two engines with sea-level nozzles and four with vacuum nozzles. That additional engine helps enable that engine-out capability, he said, and will "allow landings with higher payload mass for the Earth-to-Earth transport function."

Musk was optimistic about scaling up the Raptor engine from its current developmental model to the full-scale one. "Thrust scaling is the easy part. Very simple to scale the Raptor to 170 tons," he wrote. "The flight engine design is much lighter and tighter, and is extremely focused on reliability." He added the goal is to achieve "passenger airline levels of safety" with the engine, required if the vehicle is to serve point-to-point transportation markets.

The discussion focused primarily on the technical aspects of the BFR system. Musk did not address any financial questions, including the cost of developing the vehicle and how the company will finance it. "I think we've figured out how to pay for it," Musk said in that speech, but didn't discuss specifics beyond retiring the company's existing Falcon and Dragon vehicles so that those resources can be applied to BFR.

Musk did reiterate that the company's long-term goal with the BFR system is the settlement of Mars, but that the actual establishment of a base was something that would be handled largely by other companies and organizations. "Our goal is get you there and ensure the basic infrastructure for propellant production and survival is in place," he said, comparing the BFR to the transcontinental railways of the nineteenth century. "A vast amount of industry will need to be built on Mars by many other companies and millions of people."

Asked about an animation from his IAC talk that showed a time-lapse development of a Martian city, Musk cautioned that was not intended to be a specific, detailed design for such a settlement. "Wouldn't read too much into that illustration," he wrote.

Throughout the discussion, Musk engaged in lighthearted banter with questioners. Before going into the technical explanation of the reduced thrust of the Raptor engine, he wrote simply, "We chickened out."

Later, asked why the BFR spaceship now had four landing legs, versus the three in the design released last year, he responded, "Because four improves stability in rough terrain."