Telescope a million miles away brings Space closer to home

At 8 am (NZT) on January 25, the James Webb Space Telescope finally made it to its final destination, 1.5 million kilometres away from Earth, at the top of a metaphorical “hill” that astronomers call L2.

This odd denomination stands for Lagrangian point 2 and essentially means that it is a special location where the gravitational pull from the Earth is perfectly balanced by the gravitational pull from the Sun. Launching the telescope, known as JWST, was the space equivalent of rolling a ball up a mound with just enough speed to make it to the top and stop: too much and you’ll roll over to the other side and end up in a wrong orbit which could be unstable and science operations would be compromised.

But it worked. Twenty-six years of planning and countless hours of human brainpower have resulted in a very smooth launch and deployment – if you ignore the 15-year delay. Delays in launching space telescopes are not unusual: The launch of the Hubble Space Telescope (HST) was seven years late. Parallels between JWST and HST are rampant in the media, but these machines are two distinct wonders of science and engineering that are not comparable. The following will illustrate my point.

The key distinction between JWST and HST is the colour (wavelengths) of the light they excel at capturing. A critical feature of space telescopes is to observe light in wavelengths that are tricky or impossible to see from the ground. Optical wavelengths (the colours of the rainbow) are easily visible from the ground, that’s why our eyes have evolved to be so good at seeing them.

Hubble’s specialty is to observe light in those visible wavelengths as well as light that is bluer (ultraviolet) and blocked very well by the ozone layer – unless you’re in Aotearoa. James Webb, on the other hand, is optimised to observe infrared wavelengths (redder than visible light). That is why its mirror is coated in gold rather than aluminium, as is usually the case: gold is better at reflecting the infrared.

This chase for infrared photons is also the reason why JWST’s home is one million miles away from ours; infrared radiation is what we commonly think of as heat, and so we went and put it far, far away in the freezing vacuum of space, and gave it some protection (five sun shields) to keep it in the dark.

But why so much effort to get a few (very) red photons? JWST has four science goals with distinct themes:

Planetary systems and the origins of life

We can’t travel to Earth-like planets across the galaxy, which makes it hard to check for aliens … but not impossible. By looking through the atmosphere of planets outside of our solar system we can determine their compositions and doing so in infrared could reveal the presence of biomarkers which are chemicals that are only present in large quantities when life is actively producing them. This science objective will also be looking closer to home, with plans to check out Mars, some icy moons and other objects in the solar system.

The End of the Dark Ages

The Universe started with a bang, which you might picture as a big bright explosion, but it was anything but. For millions of years after the Big Bang, the cosmos was mostly dark - all the pretty nebulae we like to see in pictures need a source of light to make them glow. That light came from the first stars in the Universe, which were very different to the ones we know today. Much larger (maybe 1000 times the mass of the Sun), they are predicted to have exploded as extreme versions of the supernovae we know and love. JWST will be able to see the light from these explosions which will have travelled for more than 10 billion years across our expanding Universe.

Birth of stars and protoplanetary systems

Another big unknown in modern astrophysics is the formation of stars. We have a good working model, but the details are fuzzy, because protostars (young stars) are enshrouded in the gas and dust they are born from. Infrared is much better at travelling across dusty regions of space than bluer colours and so we expect JWST to shed some light on these mysterious objects.

Assembly of galaxies

If the birth of stars is difficult to observe, so is the birth of galaxies. We think that small galaxies formed early in the Universe and then merged to create large ones like our Milky Way. JWST’s design will allow us to look back so far in time that we will be able to study the very first galaxies.

These gigantic advances in space science are expected to come through over the next five years, which is the nominal length of JWST’s mission. But it’s highly likely that our new space telescope will stay on top of its “hill” for another 10 or 20 years, looking back at the origins of cosmos and life as we know it.