The Heat Death of Herschel

Most of the astronomers who read this blog will have heard the news that the Herschel Space Observatory is running out of the Helium that it has been using to keep it cool enough (~1.4K) to be sensitive to the far-infra-red radiation emitted by very distant objects.

There’s a gallery of wonderful images obtained by Herschel since it was launched in 2009 at the news item linked to above, but my favourite is one of the least photogenic:


Many of these fuzzy blobs correspond to immensely distant galaxies; what we see is starlight from very young stars absorbed by vast amounts of cosmic dust and then re-radiated in the infra-red. Understanding these sources is decidedly non-trivial and it will take many years to get all the information out that is hidden in images like this.

Anyway, one thing worth pointing out here is that what is going on now with Herschel is not some kind of failure. Quite the contrary, in fact. The original mission lifetime was planned to be three years, and Herschel has now been operating for nine months longer than that. The supply of Helium was always going to be the limiting factor as the spacecraft operates at the second Lagrange point of the Earth-Sun system, which is almost a million miles away and thus too far to be replenished. When the Helium does run out, Herschel will rapidly heat up to the point where its detectors are swamped. It will then be blind.

I was at this point going to make a cheap joke to the effect that after years on its own in the dark preoccupied with images of heavenly bodies, it was entirely predictable that Herschel would go blind. But I decided not to. I’ll save that kind of off-colour remark for Twitter…

ps. Coincidentally, on this day (March 13th) in 1781,  William Herschel  discovered the planet Uranus. The telescope is named in Herschel’s honour because he was also the first person to demonstrate the existence of infra-red radiation.

7 Responses to “The Heat Death of Herschel”

  1. daviud walter Says:

    sorry this is not about the above post. I’ve been following ur post for a while now and i like it a lot so thanks for all u write about. I wanted to ask you a question as a friend of mine and I have an endless loop argument roughly 2 times a year. What would u see if u could ride on a photon of light. i mean would u see black? Would time be still? But once you got off you would be in a different place so would that “appear” as instantaneous b/c the time by our watch wouldn’t have moved? anything u can help with would be great!

    thank you

    • Bryn Jones Says:

      I’m no relativist, but this is my opinion for what it’s worth.

      A fundamental postulate of special relativity is that any observer will always measure the speed of a photon in a vacuum to be the speed of light, c = 3 X 10^8 m/s. So we could never get to the situation where we could ride alongside a photon: whatever we do, we would always measure the velocity of a photon to be c.

      Let us imagine that we start at rest in some normal frame of reference, such as moving with the Earth. We see a beam a light passing us and try to accelerate from rest in the original frame of reference to travel alongside the photons in the beam. We go faster and faster but we could never accelerate to exactly the speed of light (in the original frame of reference). The best we could hope to do would be to accelerate so as to travel just short of the speed of light, at 0.9999c or 0.99999c or whatever, measured relative to our previous frame of rest. But the fundmental principle of special relativity means that we would still measure the velocity of the photon to be c.

      In practice, if we did try to accelerate from rest in some conventional frame of reference (such as the reference frame of the Earth, the Sun, or the Galaxy) to ride alongside a photon, we would see the photons from the rest of the Universe Doppler shifted. When we reached 0.9999c, or 0.99999c, or whatever, relative to our original frame of reference, we would see stars and galaxies in one direction rushing to us at nearly the speed of light, and in the other direction stars and galaxies receding from us at nearly the speed of light. So in one direction the light from the Universe would be extremely bright and Doppler shifted to very short wavelengths (X rays, gamma rays) and the other direction would be dark with a little long-wavelength radiation (infrared and radio). We would see events in the rest of the Universe, including the Earth, passing very slowly, but the passage of time for us would seem normal.

      • No. There is the absolute reference frames as Mach pointed out a century ago. Light travels at 300,000 km / sec with respect to any absolute reference frame. Human would experience like a photon only if he or she were massless!!

  2. Bryn Jones Says:

    And if Peter had written that the picture above shows a Herschel image of the region around the Galactic Centre, I’d have believed it.

  3. Mark McCaughrean Says:

    And just to confirm that the end is nigh, Herschel was actually pushed off L2 today and officially started into its heliocentric orbit.

    Observing continues until the LHe runs out and a much bigger manoeuvre will take place in May, followed by a final one, but as Göran Pilbratt, the Herschel Project Scientist ended his email this evening: “Feels strange!”

    • telescoper Says:

      Why is it being moved? Is there a commitment to return L2 to its original state when the observatory is no longer needed?

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