Herschel Space Telescope Closes Its Eyes on the Universe

ESA’s Herschel space observatory set against a background image of the Vela C star-forming region. Copyright ESA/PACS & SPIRE Consortia, T. Hill, F. Motte, Laboratoire AIM Paris-Saclay, CEA/IRFU – CNRS/INSU – Uni. Paris Diderot, HOBYS Key Programme Consortium.

Sadly – though as expected – the most powerful far-infrared orbital telescope put in orbit has ended mission. The Herschel space observatory has now run out of liquid helium coolant, ending more than three years of pioneering observations of the cool Universe.

The spacecraft needs to be at temperatures as low as 0.3 Kelvin, or minus 459 degrees Fahrenheit to make its observations, and mission scientists and engineers knew since Herschel’s launch on May 14, 2009 that the 2,300 liters of liquid helium would slowly evaporate away.

The Herschel team sent out a notice that the helium was finally exhausted today, noted at the beginning of the spacecraft’s daily communication session with its ground station in Western Australia. The data showed a clear rise in temperatures measured in all of Herschel’s instruments.

“Herschel has exceeded all expectations, providing us with an incredible treasure trove of data that that will keep astronomers busy for many years to come,” said Alvaro Giménez Cañete, ESA’s Director of Science and Robotic Exploration.

The Herschel telescope will be parked indefinitely in a heliocentric orbit, as a way of “disposing” of the spacecraft. It should be stable for 100s of years, but perhaps scientists will figure out another use for it in the future. One original idea for disposing of the spacecraft was to have it impact the Moon, a la the LCROSS mission that slammed into the Moon in 2009, and it would kick up volatiles at one of the lunar poles for observation by another spacecraft, such as the Lunar Reconnaissance Orbiter. But that idea has been nixed in favor of parking Herschel in a heliocentric orbit.

What has Herschel done in its three years of observations? It has made over 35,000 scientific observations, amassing more than 25,000 hours’ worth of science data from about 600 different observing programs. A further 2,000 hours of calibration observations also contribute to the rich dataset, which is based at ESA’s European Space Astronomy Centre, near Madrid in Spain.

But there will be more news the future from Herschel’s observations, as scientists comb through the data. The Herschel team said today that the telescope’s data is expected to provide even more discoveries than have been made during the lifetime of the Herschel mission.

“Herschel’s ground-breaking scientific haul is in no little part down to the excellent work done by European industry, institutions and academia in developing, building and operating the observatory and its instruments,” saids Thomas Passvogel, ESA’s Herschel Program Manager.

“Herschel has offered us a new view of the hitherto hidden Universe, pointing us to a previously unseen process of star birth and galaxy formation, and allowing us to trace water through the Universe from molecular clouds to newborn stars and their planet-forming discs and belts of comets,” said Göran Pilbratt, ESA’s Herschel Project Scientist.

Source: ESA

Infrared Spectroscopy

Silicates in Alien Asteroids. Credit: NASA/JPL/Caltech

Infrared spectroscopy is spectroscopy in the infrared (IR) region of the electromagnetic spectrum. It is a vital part of infrared astronomy, just as it is in visual, or optical, astronomy (and has been since lines were discovered in the spectrum of the Sun, in 1802, though it was a couple of decades before Fraunhofer began to study them systematically).

For the most part, the techniques used in IR spectroscopy, in astronomy, are the same or very similar to those used in the visual waveband; confusingly, then, IR spectroscopy is part of both infrared astronomy and optical astronomy! These techniques involve use of mirrors, lenses, dispersive media such as prisms or gratings, and ‘quantum’ detectors (silicon-based CCDs in the visual waveband, HgCdTe – or InSb or PbSe – arrays in IR); at the long-wavelength end – where the IR overlaps with the submillimeter or terahertz region – there are somewhat different techniques.

As infrared astronomy has a much longer ground-based history than a space-based one, the terms used relate to the windows in the Earth’s atmosphere where lower absorption spectroscopy makes astronomy feasible … so there is the near-IR (NIR), from the end of the visual (~0.7 &#181m) to ~3 &#181m, the mid (to ~30 &#181m), and the far-IR (FIR, to 0.2 mm).

As with spectroscopy in the visual and UV wavebands, IR spectroscopy in astronomy involves detection of both absorption (mostly) and emission (rather less common) lines due to atomic transitions (the hydrogen Paschen, Brackett, Pfund, and Humphreys series are all in the IR, mostly NIR). However, lines and bands due to molecules are found in the spectra of nearly all objects, across the entire IR … and the reason why space-based observatories are needed to study water and carbon dioxide (to take just two examples) in astronomical objects. One of the most important class of molecules (of interest to astronomers) is PAHs – polycyclic aromatic hydrocarbons – whose transitions are most prominent in the mid-IR (see the Spitzer webpage Understanding Polycyclic Aromatic Hydrocarbons for more details).

Looking for more info on how astronomers do IR spectroscopy? Caltech has a brief introduction to IR spectroscopy. The ESO’s Very Large Telescope (VLT) has several dedicated instruments, including VISIR (which is both an imager and spectrometer, working in the mid-IR); CIRPASS, a NIR integrated field unit spectrograph on Gemini; Spitzer’s IRS (a mid-IR spectrograph); and LWS on the ESA’s Infrared Space Observatory (a FIR spectrometer).

Universe Today stories related to IR spectroscopy include Infrared Sensor Could Be Useful on Earth Too, Search for Origins Programs Shortlisted, and Jovian Moon Was Probably Captured.

Infrared spectroscopy is covered in the Astronomy Cast episode Infrared Astronomy.