My first Magazine Sky & Telescope - 04.2019

NEWS NOTES GRAVITATIONAL WAVES LIGO & Virgo Discover Four More Black Hole Collisions Laser Interferometer Gravitational- wave Observatory (LIGO) and the Virgo interferometer in Italy found four new events, bringing the total number of gravitational-wave events detected so far to 11. The tally includes the most distant and most powerful black hole merger yet discovered. LIGO and Virgo announced the new events, designated GW170729, GW170809, GW170818, and GW170823, at a gravitational-wave conference on December 1, 2018. The ﬁrst of these became the most massive merger detected to date — and the most distant too, as the signal had traveled 5 billion years to Earth. To create the sig- nal, two black holes weighing in at 34 and 51 solar masses had coalesced into an 80-solar-mass monster, unleash- ing the energy equivalent of ﬁve solar masses as gravitational waves. The ﬁnds help characterize the origin of this population of binary black holes and the frequency with which they merge. The collaboration has used the detections to date to show that between 10 and 100 binary black holes merge per year in a volume 3 billion light-years on a side. This number appears to grow with increasing distance, which would be expected since stars formed more rapidly in the past than now. However, while the black holes are all of stellar mass and have likely supernova origins, GW170729 might pose a challenge to stellar evolution models, which have trouble producing hefty black holes. Then again, uncer- tainties in the mass measurements of the progenitors mean the signal is still consistent with current models. In addition to the four new statisti- cally signiﬁcant ﬁnds, the LIGO/Virgo Collaboration also published a list of 14 “marginal event candidates.” The col- laboration will release all data from the second observing run in February 2019. Meanwhile, all three gravitational-wave ASTEROIDS NASA’S OSIRIS-REX arrived at aster- Osiris-REX Arrives, Finds Hints of Ancient Water oid 101955 Bennu on December 3rd and kept pace with the asteroid for several weeks before entering into orbit on December 31st. Now, the spacecraft is mapping Bennu from about 730 meters (2,400 feet) above its surface. Even before it entered into orbit, Osiris-REX’s preliminary surveys had led to the detection of hydrated minerals on Bennu’s surface, suggest- ing that the asteroid’s larger parent body once hosted water. Amy Simon (NASA Goddard) announced the results December 10th at a meeting of the American Geophysical Union in Washington, D.C. Two spectrometers onboard Osiris- REX picked up the presence of hydrox- yls, molecules that contain oxygen and hydrogen atoms bonded together. The mission team suspects that these p Bennu, as seen by Osiris-REX 10 A PR I L 2 019 • SK Y & TELESCOPE p Illustration of a black hole duo detectors are being upgraded for a new simultaneous observing run beginning in April 2019. Given the instruments’ improved sensitivity, scientists expect to ﬁnd at least a few events per month. ■ GOVERT SCHILLING To learn more about what LIGO and Virgo are teaching us about black hole spin, visit https://is.gd/LIGOspins. molecules are locked up in clay miner- als, created in interactions with liquid water or water vapor. Bennu itself is too small to have ever hosted liquid water. However, researchers think that the roughly 500-meter-wide (0.3-mile-wide) space rock is actually a chunk knocked off a much larger asteroid long ago. The ﬁ nd suggests that water existed at some point on Bennu’s home world. Most of the next year is dedicated to mapping and scanning Bennu’s surface, surveying for the perfect spot from which to grab some mate- rial. Then, around July 2020, the real excitement begins, as Osiris-REX moves in for a series of sampling maneuvers. The spacecraft will return the sample to Earth, with a planned homecoming in 2023. ■ DAVID DICKINSON & CHRISTOPHER CROCKETT M N I A RE-ANALYSIS OF DATA from the

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