In a previous post I talked about the Large Synoptic Survey Telescope (LSST), a large telescope being built in Chile which will spend 90% of its time surveying most of the sky a total of 1000 times over a 10 year period (in the remaining 10% it will revisit areas of specific interest). In this post I’ll talk about the scientific objectives of the LSST and the threat posed by large constellations of Low Earth Orbit satellites to it meeting them.
The Rubin Observatory Buildings in which the LSST is situated under construction (June 2020) -image credit Rubin Observatory. Note the clear blue sky!
The summarised scientific objectives are given below. If you want any more detail, I have put a more technical paper on the Explaining Science references page.
Objective 1 Cosmological studies
Weak gravitational lensing
Any massive object, such as a galaxy cluster, bends the path of light passing near it. Occasionally this effect produces the multiple images, rings and arcs associated with strong gravitational lensing.
An example of strong gravitational lensing. When a background source, a massive object and the observer are in perfect alignment, the light of the background source is bent around the massive object by an equal amount in all directions. This creates a circle around the massive object known as an Einstein ring -image credit NASA
Most gravitational lensing is however, weak lensing which is harder to detect. When weak lensing occurs, there are no multiple images, rings or arcs. Instead the shape and distribution of distant galaxies is changed slightly and subtly when light from them passes a massive object on its way to Earth. Weak lensing is a statistical phenomenon which can be detected on large sky surveys.
Baryon Acoustic oscillations.
There is an unevenness in the distribution of galaxies caused by oscillations in the early Universe. This means that galaxies are more likely to be found at a distance of roughly 500 million light years from each other than if the distances were truly random. This is such a large topic I will write a post about it in the future.
Photometry of Type Ia supernovae
When a white dwarf approaches the Chandrasekar limit (1.44 solar masses) it can reach a point where runaway nuclear fusion occurs. This results in a Type Ia supernova (SN) which releases an enormous amount of energy in a short time, totally destroying the star and blasting a cloud of plasma into space. Because the detonation of the white dwarf occurs at a particular mass, all type 1a SN have a similar maximum brightness – about 5 billion times brighter than the Sun, although this varies a little.
The American astronomer Philips in 1993 derived a formula to accurately estimate the maximum brightness of a Type 1a SN based upon the decline in apparent brightness 15 days after the peak. Once the absolute brightness of the Type 1a SN is known its distance can be calculated from its apparent brightness.
Objective 2 Map and measure the Milky Way
The LSST’s accurate collection of data will enable us to vastly improve our knowledge of our Milky Way galaxy. The telescope will be able to observe most of the sky and will make around one thousand observations of each surveyed area of sky. When stitched together in time, this set of observations will yield the motions of millions of Milky Way stars. Because of its higher sensitivity, this set of observations will yield a map having over 1000 times as many objects as previous surveys – cataloging the colours and brightnesses of billions of new stars.
Objective 3 study transient optical events
By imaging the entire night sky repeatedly, with high sensitivity, the LSST will reveal new information about objects which suddenly change in brightness. These so called transient optical events include novae, supernovae, objects associated with gamma-ray bursts, and the change in brightness of quasars.
A nova the sudden increase in brightness of a star, which then slowly fades over several weeks or many months – image credit Wikimedia Commons
Within a minute of each change in the sky, the Rubin Observatory will generate an alert (which is a notification that the change has taken place), allowing the astronomical community to respond. It is possible that the LSST might discover entirely new classes of transient events.
Objective 4 Catalogue Solar System Objects
The LSST will be able to measure the properties of several million moving objects (10 to 100 times more objects than are currently available). The properties it will measure includes plotting their orbits, measuring their colour (i.e. the amount of sunlight reflected at different wavelengths) and variability information. This will include Near Earth Objects (NEO), some of which could come very close to Earth and cause serious damage if a collision took place.
The unique depth and coverage of the LSST survey may lead to some totally unexpected discoveries. For example gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies. They are the brightest and most energetic electromagnetic events known to occur in the Universe. A typical GRB releases around 1044 Joules of energy in a few seconds which roughly same as the Sun will in its entire 10-billion year lifetime.
They were first detected in 1967 by US military satellites which had been designed to detect gamma rays emitted by secret nuclear weapons tests by the Soviet Union carried out in space. Any such tests, would have been in violation of test ban treaties and would have emitted short bursts of gamma rays. When it became clear that the GRBs were not man-made, the discovery was later declassified and published in 1973.
Artist impression of a Gamma Ray Burst – image credit NASA
Role of citizen science
The data generated by the LSST will provide opportunities for discovery by volunteers taking part in citizen science projects. These projects ask an online crowd to sort through data in order to add context and search for the unexpected. The LSST project has partnered with Zooniverse (https://www.zooniverse.org/) which is now ten years old and has nearly two million registered volunteers, having grown from its original, Galaxy Zoo project, which asked volunteers to classify galaxies by shape. Other Zooniverse projects have included Planet Hunters, working with data from NASA’s Kepler and TESS satellites and Planet 4, which has surveyed the Martian polar regions looking for seasonal changes.
A key citizen science activity for the LSST involves the search for transients, taking advantage of the survey’s unprecedented and repeated coverage of the sky.
Potential Impact of large satellite constellations
Many of you will have seen the recent images showing long ‘trains’ composed of as many as sixty SpaceX Starlink satellites crossing the sky.
A ‘train’ of SpaceX Starlink satellites just after their launch. In the days after launch these trains break up as the satellites position themselves into their final orbits
These satellites are being launched to deliver broadband to every corner of the globe. 540 have already been sent into orbit at the time of writing this post.
The purpose of the satellites is to provide worldwide fast Internet coverage. Traditional satellite internet operators have a small number of satellites in geostationary orbit, 35 800 km above the Earth’s surface. This limits speed, coverage and bandwidth. It takes around 0.12 seconds for radio-waves to travel between the satellite and Earth. SpaceX’s plan is to surround the Earth with a large number of satellites in Low Earth Orbit (LEO). These would link together forming a ‘floating internet backbone’, providing a faster alternative to the fibre-optic cables that span the globe.
By the year 2025 there may be 12 000 satellites in orbit and, according to a recent report on space.com, SpaceX have sought permission to put an additional 30 000 Starlink satellites into orbit.
And SpaceX aren’t the only operator.
- In 2019 Amazon announced project Kuiper – a constellation of 3000 satellites.
- The Canadian company Telesat are planning to launch a satellite broadband service in 2022.
- The British company OneWeb also planned a global internet satellite broadband service, although these plans were put on hold when the company filed for bankruptcy in March 2020
To give a sense of perspective on how rapid the growth in the number of LEO satellites is, in December 2019 there were only 2200 operational satellites orbiting the Earth, of which 1418 were in LEO, which is defined as below 2000 km, (Union of Concerned Scientists 2019).
There has been much written in the press recently about the impact of these satellite constellations on astronomy. Compared to astronomical objects and also satellites in higher orbits, LEO Satellites move rapidly across the sky and are in general brighter than other satellites because they are closer to Earth. In raw astronomical images they are sometimes captured as they move across a telescope’s field of view. Whenever this happens, they appear as bright streaks, looking a little like meteor trails. However, Image processing software used by astronomers (including amateurs) can identify satellite tracks and strip them out of the final images.
This might present more of an issue if the number of LEO satellites were to increase twenty-fold in the next ten years. Rather than an image occasionally containing a single satellite trail, a typical image would contain multiple trails as numerous satellites crossed the field of view. There also might be potential problems with wide field surveys such as the LSST which with its large field of view and sensitive detectors would capture many satellites in its images.
On 19 May 2020, the Rubin Observatory made a statement; a small part of this is given below.
‘The Vera C. Rubin Observatory science community is concerned about the increasing deployment of communications satellite constellations which, if unchecked, could jeopardize the discoveries anticipated from the Rubin Observatory when science operations begin in 2022. Because Rubin Observatory is uniquely impacted by these satellite constellations, its science team is taking an active role in pursuing mitigation strategies to reduce the impact of the satellites on Rubin Observatory science.’
However, despite various reports to the contrary, the most accurate thing to say is that although there will be some impact of these satellite constellations on the LSST, at the moment the detail is unknown. The Rubin Observatory team are working with SpaceX to find ways to lessen the impact of the satellite trails. Efforts such as designing fainter satellites, improving image processing algorithms so they can better deal with satellite streaks and improving the telescope observation schedule based on knowledge of where satellites will be, may well provide additional mitigation.
It will be a case of watching and waiting to see what happens over the next five years or so !
Union of Concerned Scientists (2019) UCS Satellite Database, Available at: https://www.ucsusa.org/resources/satellite-database (Accessed: 1 August 2020).
A video containing some of the material in this post can be viewed on the Explaining Science YouTube channel.