A brief discussion on the spitzer space telescope

                                             

                                              

Space telescopes have always been a great deal for astronomical observations in the last 3 decades. We have achieved miraculous observational data that helped us find the mysteries that had been hiding from our eyes all these years. One must be wondering, why exactly do we need to launch a telescope to space whereas astronomers around the globe have found equally amazing information from the ground-based telescopes? To be fair and honest, ‘equally amazing’ is slightly exaggerated, and here’s why. Our atmosphere has always kept us safe from many unwanted harmful rays throughout all these years. However, the atmospheric phenomena are very random and mostly unpredictable. Have you ever noticed the twinkling of stars? Why exactly does it happen? Due to the atmospheric turbulence, the refractive index of the atmosphere in a particular region may vary spontaneously, and hence for a given incident of light coming deep from space, the light suffers multiple refractions and there’s a brief angular displacement between the consecutive images that are formed at our eyes. This, although a magnificent thing to witness, significantly affects the resolution of the image. We cannot properly resolve the object to obtain a crisp image. Have you ever looked through the telescope and observed that the image is vibrating or is not particularly fixed at a point? Yes, exactly that’s where the problem arises. The atmospheric distortions can be caused due to many factors. Different temperatures, pressure, humidity, varying wind speed, etc. Wait, can you not visualize how this atmospheric distortion affects your astronomical observations? Don’t worry, I got you.

                                                            This is what mars look like, through a telescope under atmospheric distortion. It’s not regular! It feels as if it’s vibrating. It is due to multiple images overlapping each other as each time we receive an image, it is slightly refracted more or less than the previous image that we’ve received.  And this is what it would look like under a steady atmosphere through a normal telescope. Keep in mind that we do have a good set of images of mars taken from ground-based observatories.

                                                                              Crispy! Isn’t it?

 

Not only atmospheric distortion but the atmosphere also brings up some extra difficulties for infrared observations. For telescopes that see radiations of the infrared region, while observing them, the atmosphere and the telescope itself may start glowing and this is some serious set of unnecessary noises which we don’t need but still, we get them. The Atmosphere can also block some infrared radiations from reaching the telescope and to avoid this, we can set up the telescope for a longer exposure to gain as much information as possible in a single go, but then again, a long exposure time can result in poor resolution of images. Even the presence of water droplets can absorb some infrared radiation and they might not even reach the telescope. Fuh! So many demerits right there!

In space, there’s nothing to block the incoming radiation and of course, there’s no atmosphere for us to care about anymore. Therefore, a space telescope receives all kinds of emissions without any interference from external factors. So, yes! We do need space telescopes.

The unknown world of electromagnetic radiations:

Let’s move on to something very interesting now. You’ve seen those amazing beautiful works of art obtained by the Hubble space telescope, right? The great pillars of creation, so many beautiful galaxies, etc. The images that we are seeing from the images obtained by the Hubble space telescope are in the visible range of electromagnetic radiations. We can only see the radiations belonging to the visible range and any radiation outside of this range is invisible to human eyes. Ever since the big bang happened, space has been cooling down. Apart from stars and galaxies, the universe is filled with gas and clouds of dust, at a relatively low temperature and emits radiations in the infrared region. So, we can say that most of the stuff in space is invisible to us. If somehow, we can capture and process these infrared radiations and all other radiations… just imagine!

 

The fact, that we do have telescopes operating at a different range of the electromagnetic spectrum is even cooler. E.g.: The Chandra space telescope observes the X-ray emissions, the Hubble space telescope observes the visible range radiations, recently launched James Webb space telescope is going to observe the universe from the infrared point of view. But before the James Webb space telescope, there was one other space telescope that observed the universe from an infrared point of view and that is the spitzer space telescope.

The spitzer space telescope, named after the astronomer Lyman Spitzer who had promoted the idea of a space telescope in the 1940s, is an infrared-based space telescope launched by NASA in the year 2003. For a space telescope that was launched to specifically work in the IR region, it cannot be placed anywhere close to the earth in a near-earth orbit. It is because the emission of radiation from the earth can seriously affect our observational data from a different source. Not only that, but the radiation from the sun can also possess a good deal of problems for our observation. You see, the space is filled with infrared radiation. To protect the telescope from all these radiations from the sun, a sun shield was attached to the telescope that shadowed it from direct contact with solar radiation. One of the spitzer’s most important driving requirements was to cool it down below the temperature of zodiacal dust surrounding it and from the cooler interstellar dust. For this purpose, the spitzer was cooled both radiatively and cryogenically, i.e., by involving liquid helium, which is the coldest liquid in the cryogenic world, to cool down further to a much lesser temperature for the telescope to work perfectly fine and with the highest resolution possible. Spitzer works at the range of 3.4 micrometers to 160 micrometers and to achieve this, the instruments onboard have to be cooled down accordingly to a suitable temperature, as per the laws of blackbody radiations. The instruments onboard worked individually that is, one at a time. Hence, it was possible to adjust the temperature accordingly for a particular instrument to work efficiently. The cryostat containing liquid helium was connected directly to the instruments at the bottom to cool them down and liquid hydrogen was also made to flow through the telescope to carry the heat energy produced by the instruments and cool them down and maintain a constant temperature of 5.5K. When launched, the telescope was at ambient temperature and had to be cooled down to a much lower temperature. It did that by rapidly radiating its heat to the vast cold emptiness of outer space. However further cooling down of the instruments was necessary for their proper functioning and as well as to reject the interference of their heat with the data. For that cryostat containing liquid helium was used. But there are still some interesting mechanisms used to make this possible.

 

1.      A system of thermal shields, including solar panels, covered the telescope and prevented the light and heat from the sun, enter the telescope. The back half-cylinder of the telescope was covered with a high emissivity black coating that radiates to cold space, any heat that comes through.

2.       The telescope was also launched on earth trailing heliocentric orbit, which keeps the observatory far from the heat of the earth. In this orbit, the shade providing solar panel is always directed toward the sun which uses solar energy to power the telescope, and the black coating was directed towards the cold outer space, always, radiating the extra heat to the outer space.

3.       The primary mirror, secondary mirror, and the metering tower that connects them are all fabricated from hot isostatically pressed beryllium to maintain their alignment and minimize stress on cooling the telescope from room temperature to the operating temperature of 5K.

With this unique configuration, the outer body of the telescope cooled down entirely, in a passive manner (i.e., a design choice to reduce heat gain and increase heat loss), to around 34K temperature. This resulted in receiving less than 1mW of unwanted heat that diffused inwards from outer space. So, very few amounts of heat reached the cryostat and affected the concentration of liquid helium i.e., evaporated it. Therefore, the cooling down process was much more efficient. The main heat load on the helium was due to the heating of electronics and heater circuits that dissipated heat while taking the data. The liquid helium absorbed this heat and boiled off to cool down the components. The operation was successful for 5 years although it was intended to work for 2.5 years until the liquid helium got exhausted and the instruments could not be kept at 5K temperature anymore. However, some of the instruments still worked perfectly fine at 28.7K temperature and the spitzer continued to work up to the year 2020 when it was finally decommissioned as the alignment of the satellite was not suitable for further operation.

This was all about how the spitzer space telescope worked. But we can never know about the importance of something if we do not witness its glory. The spitzer space telescope was much more than a success story. Let me show you some of its best works.

1.       Saturn is a very valuable jewel of the solar system because of its rings as its most distinct and extraordinary feature. The rings are so beautiful to look at but what if I told you that the rings 

We are seeing is not the end of the limit. Spitzer saw the final and the largest ring of the Saturn via the infrared radiation it was emitting.  And trust me when I say this, it’s huge! Just look at it

It was right in our plain sight but still, we couldn’t see it because this ring is pretty cold and is invisible to our eyes due to its infrared emissions. The final and largest ring was being speculated due to the abnormal surface of Saturn’s moon Iapetus. Iapetus is tidally locked in its orbit around Saturn where one side of it is always facing toward Saturn and the other is facing toward the sun. Its surface is mostly covered in water ice but a mysterious dark color on its surface revealed that it might be swiping out some invisible material as it orbits around Saturn. And there we are, we finally know the source, the final ring of Saturn.

2.       Spitzer detects infrared radiation and it was the first time, that astronomers made the weather map of an exoplanet by looking at the data obtained by spitzer looking at it.

3.       Spitzer has helped find a lot of far away blackholes and not only black holes, supermassive black holes surrounded by hot gases and other materials, also known as Quasars, lurking at the center of the galaxies. Spitzer has even located black holes as far as 13 billion light-years away.

4.       Spitzer has also captured some very small asteroids.

5.       Spitzer has mapped an unprecedented map of the Milkyway galaxy, revealing a much deeper view of the core of the galaxies as infrared radiations can pass through thick gas clouds. 

6.      And last but not the least, the data from its observation revealed that there’s a 7 planet system, known as the TRAPPIST-1 system, around the star known as TRAPPIST-1, among which, 3 of the planets are earthlike and are in the habitable zone of the star.

It’s amazing how much we can achieve by just transforming our perception and our interest to look at the universe from a different angle. Since the spitzer space telescope has finally retired now, its successor, the James Webb space telescope, the most advanced space telescope, is going to continue its legacy as well as make its history to be remembered by mankind.

 

Until next time! Thank you for reading.

by- Suvam Tripathy