Australia's very own Little Eye in the Sky - A Deep Dive into SkyHopper

In 2016, Melbourne University coordinated an international collaboration of scientists to conceive what may well be one of the most ambitious Australian satellite projects ever conceived. 

The idea for the project is to build Australia’s first space-based science observatory, designed to catch light from distant sources across the Universe, helping contribute to global science collaborations in resolving some of our deepest questions about the Universe to date. 

SkyHopper is a small CubeSat with the goal to observe a suite of distant events in the Infrared (IR) frequency band that are observable for only a short time. These events cannot be observed from the ground-based telescopes because there is simply too much background interference from water in the atmosphere, which in particular blocks out lots of the infrared bands - which are the target wavelengths for the space-based telescope.

The project consortium is made up of talented scientists from an all-star cast of Australian institutions, as well as sixteen world-leading institutions around the globe. Led by the University of Melbourne, it also includes the Australian Astronomical Observatory, Australian National University, Curtin University, Macquarie University, Swinburne University, University of New South Wales, University of Southern Queensland, and the University of Western Australia, all of which have all significantly contributed to both the engineering and science aspects of this project. 

Once achieving orbit, the CubeSat has a list of ambitious goals it hopes to achieve, which all revolve around the Infrared part of the electromagnetic spectrum. This is much like the soon-to-be-launched James Webb Space Telescope, which is also designed to search across infrared wavelengths. 

One of SkyHopper’s primary goals will be to identify the IR afterglow from Gamma-Ray Bursts (GRBs) that occurred in the first billion years after the Big Bang. The IR afterglows of GRBs are generated by high-energy flashes of gamma-radiation from exploding stars - but because these events occur so long in the past, the wavelengths of gamma radiation are stretched by the expanding Universe - so by the time they reach us here on Earth, they have been shifted towards the IR portion of the spectrum. The more distant the burst, the further back in time it can be seen happening. The information gained will provide knowledge on how stars and galaxies formed in the early Universe.

Another goal is searching for potentially habitable exoplanets as they cross between us and their red dwarf star hosts along our line of sight. Red dwarf stars are favoured targets because they are much cooler than other stars, so their habitable zone (the distance from a star a planet must orbit for liquid water to exist on it) is much closer to them. This means that there is a greater chance of the orbits of the exoplanets aligning such that they cross between us and the observed star.

Much like the Cosmic Microwave Background radiation left an imprint on the Universe when it was much younger age, as did the Cosmic Infrared Background Radiation - which is why it is a target science goal for SkyHopper. The light emitted by vast numbers of stars in galaxies too faint and too distant to be observed by current telescopes contributes to the Cosmic Infrared Background. Existing space-based telescopes (or even future versions like JWST) have large apertures, and so get great detail in a smaller field of view. But to study something as big as the Universe’s background radiation, smaller apertures produce better results - which makes SkyHopper ideal for these kinds of analysis. Studying this phenomenon could tell scientists about how stars and galaxies formed in the early Universe.

Transient events like Fast Radio Bursts (FRBs) also make the target list for SkyHopper. Like gamma-ray bursts, FRBs are millisecond-quick emissions of radio waves. Unlike gamma-ray bursts, astronomers currently have very little idea where they come from, though a few leading theories are starting to emerge. At one point, even aliens were considered. By studying the IR afterglows of these bursts, astronomers may be able to shed some light on their origin.

But how does SkyHopper hope to observe these events? Well, in short, it will be built to be very stable with an excellent mirror and top-shelf detector. But this is a deep dive, and you aren’t here to see me sum up the technical specifications of this remarkable satellite in a sentence. 

For starters, this satellite will be incredibly stable. So stable that it will have less than four arcseconds, or about one nine-hundredth of a degree, deviation from where it is supposed to be looking. To put how small that is in perspective, take your pinky finger and stretch it to arm's length in front of you. The width of your pinky finger is approximately one degree when measuring angles in the sky. Now imagine dividing your pinky up into nine hundred even sections. One of those divisions is the furthest SkyHopper will drift from its target. What will allow it to be so stable is its four miniature star trackers. These pieces of equipment monitor the telescope’s position in relation to three separate stars to make sure it does not drift off from its desired position in any axis. 

The telescope itself is a rectangular three-mirror anastigmat - which means that the mirrors are designed to eliminate all forms of distortion that can occur when collecting light through a telescope. These three mirrors focus the light into what is known as a Kester Prism. This type of prism is actually four prisms bonded together just designed to split the incoming light into four separate wavelengths that will be sent into four separate detectors tuned to those wavelengths. This set-up will help SkyHopper observe the Infrared afterglows of distant gamma-ray bursts, as well as boost the efficiency of the telescope as it will image objects in four different wavelengths rather than just one.

“Since GRB afterglows exhibit a strong variability, with the signal strength decaying rapidly over time, simultaneous imaging in multiple broad-band filters (“colours”) greatly improves the reliability of the photometric redshift measurement (which gives the distance to the afterglow).” SkyHopper Principal Investigator Prof. Michele Trenti said in an article about the prism in 2018. 

The telescope assembly will be protected by efficient baffling which will protect the instruments from stray rays from the Earth, Moon and Sun. The last thing you want when observing faint and distant cosmic phenomena is light from the Earth, Moon, and Sun infiltrating your detector and ruining your data. 

SkyHopper will be monitored through a 24/7 satellite phone connection and will be able to rotate at a rate of three degrees per second. This means that it could be able to react to and observe transient events like Fast Radio Bursts or Gamma-Ray Bursts 1,000 times faster than Hubble.

SkyHopper represents the potential for Australia to make a significant contribution to the world of astronomy and could solidify Australia as a leader in the exploration of the deepest depths of the Universe.

SkyHoppper is currently funded for the initial design stage and is scheduled to launch in 2024.