Some more questions concerning Herschel...
The Herschel Space Observatory, a.k.a. Herschel, was the largest space telescope at the time of its launch and still is. With a collecting area twice as large as the Hubble Space Telescope’s, and operating in the far-infrared part of the spectrum, Herschel could look through clouds of gases and dust in the farthest reaches universe and pick up even really faint signals from distant stars and nebulae.
To do this, it depends on three instruments – PACS, SPIRE and HIFI – that are cooled to fractions of degrees above absolute zero, much colder than anything you could find in the solar system. At these temperatures, the instruments are at their most sensitive. The frigidity it achieved using liquid helium, a superfluid coolant that constantly boils off as it removes heat from the instruments. By the end of this month (March, 2013), all the helium will have boiled off, leaving PACS, SPIRE and HIFI effectively blind.
I wrote an article in The Hindu on March 28, a run-of-the-mill news piece that had to leave out some interesting bits of information I’d gathered from the lead scientist, mission manager, and project scientist I’d spoken to. I’ve attached my questions and their answers that contain said bits of information. I think they’re important because
Here are the answers (My questions are in bold).
Herschel was foreseen to run out of helium in early 2013. Considering its unique position in space, why wasn’t a “warm” experiment installed on-board?
MATT GRIFFIN – Lead Scientist, SPIRE Instrument, Herschel Space Observatory
Herschel was designed to operate in the far infrared part of the spectrum – wavelengths typically hundreds of time longer than the wavelengths of visible light. For the far infrared, extreme cooling is always required. For a telescope operating at shorter wavelengths (about ten times longer in wavelength than visible light) a “warm mission” is feasible. This could have been done with Herschel, but it would have required that the surface of the telescope be made far more precise and smooth. That would have made it very much more expensive, leaving less money available for the rest of the spacecraft and the instruments.
Any space mission must be built within a certain budget, and it is usually best to design it to be as effective as possible for a certain wavelength range. Herschel actually covers a very wide range – from 55 to nearly 700 microns in wavelength. That’s more than a factor of ten, which is very impressive. To make a warm mission possible would have meant making the telescope good enough to work at ten times shorter wavelength, and adding a fourth instrument.
Herschel was and is the only space telescope observing in the submillimeter and far infrared part of the spectrum. After it goes blind, are there any plans to replace it with a more comprehensive telescope? Or how far do you think the loss of data because of its close-of-ops will be offset by upcoming ground-based telescopes such as the ALMA?
GORAN PILBRATT – Project Scientist, European Space Research and Technology Centre, Noordwijk
There are currently no concrete ESA (or anywhere else) plans for a replacement or follow-up mission. What many people hope for is the Japanese SPICA mission, which may fly beyond 2020 with an ESA telescope and a European instrument called SAFARI, both based on Herschel experience. Time will tell. Of course the NASA JWST will be important to almost every astronomer which it finally flies. In the meantime ALMA and also the flying SOFIA observatory are of interest. There is also a lot of follow-up observing to be done with many different ground-based telescopes based on Herschel data. This is already happening.
LEO METCALFE – Mission Manager, ESA Centre, Madrid
After the anticipated exhaustion of the Liquid Helium cryogen which keeps the Herschel instruments cold, scientific observations with Herschel will cease. However, the data gathered during the 4-years of operations, stored in the Herschel Science Archive (HSA) at ESAC, will remain available to the worldwide astronomical community for the foreseeable future. Until the end of 2017 ESA, for much of the time in collaboration with the instrument teams and NASA Herschel Science Centre, will actively support users in the exploitation of the data.
That said, there is no comparable mission in the currently approved ESA programme considering launches into the early 2020s. The Japanese Aerospace Exploration Agency (JAXA) mission SPICA is of comparable size to Herschel and will operate out to wavelengths a little over 210 microns – in the far-infrared, but only barely reaching what would generally be termed the sub-millimetre region. It may be launched before 2020.
Because of absorption of infrared radiation by the Earth’s atmosphere, ground based telescopes have limited capacity to compete with orbital systems over much of the Herschel wavelength range.
However, the Atacama Large Millimeter Array (ALMA) in the Chilean Andes overlaps in its wavelength coverage with the sub-millimeter parts of the Herschel range, but a typical map size for Alma might be on the order of, say, 10 arcseconds (the full Moon spans about 1800 arcesconds, to give some scale), while a typical Herschel map might cover an area 10 arcminutes (600 arcseconds) on a side. Instead of large area coverage, ALMA provides extremely high spatial resolutions (down to small fractions of an arcsecond), far finer than Herschel could achieve.
So ALMA is well suited to the detailed follow-up of Herschel observations of single high-interest sources, rather than providing comparable coverage to Herschel.
There must be a lot of data left to be analysed that was gathered by Herschel. While creating a legacy archive, will you also be following some threads of investigation over others?
MATT GRIFFIN – Lead Scientist, SPIRE Instrument, Herschel Space Observatory
Although Herschel’s life was limited, it was designed to make observations very quickly and efficiently, and it has collected a huge amount of data. It will be very important during the next few years, in what we call the Post-Operations period, to process all the data in the best and the most uniform way, and to make it available in an easy-to-use archive for future astronomers.
This means that the real scientific power of Herschel is still to be realised, as its results will be used for many years in the future. Only a small fraction of the data from Herschel has so far been fully investigated.
It is clear that when the data are fully explored, and when Herschel’s observations are followed up with other telescopes, a great deal more will be learned. This is especially true for the large surveys that Herschel has done – surveys of many thousands of distant galaxies, and surveys of clouds of gas and dust in our own galaxy in which stars are forming. In the coming years, although Herschel will no longer operate, its scientific project will continue – to understand the birth processes of stars and galaxies.
When did you start working with the Herschel mission? How has your experience been with it? What does the team that worked on Herschel move on to after this?
LEO METCALFE – Mission Manager, ESA Centre, Madrid
In 1982 the Far Infrared and Sub-millimetre Telescope (FIRST) was proposed to ESA. This mission concept evolved and eventually was named Herschel, in honour of the great German/English astronomer William Herschel.
The build-up of the ESA team for Herschel started in earnest in the early 2000s. I came on board as Herschel Science Operations Manager in 2007, with the main task of integrating and training the ESA Science Operations Team and the wider Science Ground Segment (SGA – which includes the external-to-ESA Instrument Control Centres) to be a smoothly functioning system in time for launch, which took place in May 2009.
So my experience of Herschel began with the recruitment of many of the operational team members and the integration of the Science Ground Segment (SGS) focussed on the pre-launch end-to-end testing of the entire observatory system, with data flowing from the spacecraft then on the ground in the test facility at ESTEC in the Netherlands, and continued through a series of pre-flight simulations which put the SGS through all the procedures they would need to follow during operations.
As a result we “hit the ground running” after launch, and the operations of the SGS have been smooth throughout the mission. Those operations have spanned the Launch and Early Orbit (LEOP) Phase, the in-flight Commissioning Phase, the Performance Verification, Science Demonstration, and Routine Operations Phases of the mission, and have included the recovery from the early failure of the prime chain of the HIFI instrument, and the handling of various lesser contingencies caused by ionising radiation induced corruptions of on-board instrument memory, among others.
It has been a fast paced and exciting mission which in the end has returned data from almost 35000 individual science observations. It’s going to be hard to adjust to not having an active spacecraft up there.
Concerning what happens to the team(s) that have worked on Herschel: The ESA team that supervised the construction of the Spacecraft already moved on to other missions soon after Herschel was launched.
The Science Operations Team at the Herschel Science Centre at HSC/ESAC in Spain, together with the Instrument Control Centres (ICCs) formed by the teams that built the scientific instruments (distributed through the ESA member states) and the Mission Operations Centre at ESOC in Germany, have been responsible for the operation of the Spacecraft and its instruments in flight. Those teams will now run down.
A fraction of the people will continue to work in the Herschel project through its almost 5-year Post-operations Phase mentioned already above, while the remainder have or will seek positions with upcoming missions like Rosetta, Gaia, BepiColombo, Solar Orbiter, Euclid … or in some cases may move on to other phases of their careers outside the space sector.
We are talking about people who are highly experienced software engineers, or PhD physicists or astronomers. Generally they are highly employable.
EOM
