Get Adobe Flash player
ChemCam’s first study target: Coronation Rock on Mars. CREDIT: nASA/Jpl-CAlTECH/MSSS/lAnl Los Alamos in Space Two los Alamos experiments described in recent editions of 1663 just arrived at their extraterrestrial destinations in August. The ChemCam instrument aboard the Mars Sci- ence laboratory’s Curiosity rover landed on Mars on August 5 (See “Shooting Rocks on Mars” in the november 2010 issue of 1663), and the HOpE spectrometers aboard the Radiation Belt Storm probes (RBSp) mission was launched for the van Allen radiation belt surrounding Earth on August 30 (see “The Stuff That DREAM Is Made Of” in the June 2012 issue of 1663). ChemCam is alive and well on Mars, and the RBSp satellites have arrived in their proper orbit and begun a 60-day period of instrument and satellite subsystem testing before beginning full sci- ence operations. Since its arrival on Mars, ChemCam has been drilling into rocks with a laser, vapor- izing small portions of them to create a bit of glowing plasma. A telescope on Curiosity’s mast captures the glow and sends it through a fiber-optic cable to a spectrometer, which resolves the light into different wavelengths to reveal what chemical elements are pres- ent in the rock. This information will help determine key aspects of Martian history, such as timescales for the presence of liquid water and the overall habitability, past and present, of Mars’s surface. According to los Alamos scientist and ChemCam team leader Roger Wiens, both the Curiosity rover and the ChemCam instrument are working perfectly. 24 The Helium Oxygen proton Electron, or HOpE, spectrometers on the two RBSp satellites are part of a suite of instruments that los Alamos is using to understand the acceleration, global distribution, and vari- ability of radiation belt particles. The new observations will help researchers to better model and predict space weather—and protect satellites from the worst of it. HOpE measures its namesake ions and electrons, which initiate the processes that control the radiation belt structure and dynamics. According to los Alamos team lead geoffrey Reeves, if all goes as planned, HOpE will be producing continuous science data by Hal- loween. — Craig Tyler Structurally Sound How do you know there isn’t a danger- ous, microscopic crack hidden inside a bridge’s support beam when you drive across or in an aircraft wing when you fly? Fortunately, structures like these can be tested for internal damage with acous- tic waves, with material defects causing detectable changes in the waves. But unfortunately, this technique normally requires physically attaching to the surface of the structure a transducer that produces ultrasound (higher pitched sound than can be perceived by human hearing) and gathers data from its immediate vicinity only. It is economically impractical to cover and test every structure with such transducers, and in some cases it is impossible to use them: objects being tested may contain hazardous materials or may be too small to attach a transducer. The natural alternative to attaching an acoustic source to a structure is to broadcast a sound near the structure. Then the source could be moved around to scan for defects everywhere. But this approach has been unsuccessful because noncontact 1663 los alamos science and technology magazine october 2012 sources do not induce waves with sufficient amplitude in the material being tested, so the telltale sound of internal damage goes unheard. When a sound wave encounters a discontinuity inside the material, such as a crack, it naturally “echoes” with the original frequency plus several harmonic frequencies, and these harmonic frequen- cies indicate internal damage. But existing noncontact acoustic wave sources simply do not cause the material to produce these harmonics at a detectable level—until now. pierre-Yves le Bas, T. J. Ulrich, and Brian Anderson of the laboratory’s geophysics group recently discovered how to generate and detect high-amplitude sound waves in a solid material without physical contact. First, they broadcast a pulse of ultrasonic waves with a chosen frequency into a cavity as “loudly” as their equipment allows without distortion. This original pulse is only a few tenths of a millisecond long, but it bounces around inside the cavity in a complex man- ner before escaping the cavity through a small hole centered over the material being tested. The bouncing waves escape in suc- cession, causing the material’s surface to vibrate in a complex pattern of fluctuating amplitudes lasting several milliseconds. This long-duration vibration still lacks sufficient overall amplitude to reveal the internal structure of the material, but the los Alamos team uses it for a different purpose. With a laser vibrometer, the team records a signal detailing the vibration of the material’s surface. They then program their acoustic source to broadcast the exact same signal, but reversed in time. (For example, if the observed signal was “loud then quiet,” they would broadcast “quiet then loud,” but a real signal would be more complex.) This causes the cavity to produce the exact time-reverse of its original motion: Instead of the cavity converting a brief pulse into a long, fluctuating, low-amplitude signal that causes a ringing of the material’s sur- face, it does the opposite. It concentrates the longer and more complicated signal into a short, intense burst. Each time the team broadcasts, they amplify their source