Hello Space Fans and welcome to another edition of Space Fan News. In this episode Russia
successfully launches Spekra-RG, a dual space telescope designed to map the sky in X-rays.
This mission, in collaboration with Germany’s eRosita System, marks Russia’s re-entry
into space astronomy and promises to put them in the forefront of x-ray astronomy for quite some time
OK here we go… On July 13th Spektr-RG was
launched from a powerful Proton rocket from the Baikonur Cosmodrome in Kazakhstan at 12:31
GMT and heralded Russia’s first major space telescope since the Soviet era. I did a very
quick search of the history of space telescopes and found that the Soviet Union had launched
a couple of Gamma Ray observatories in 1965, worked with the French to launch Gamma, another
gamma ray observatory which launched in 1990 and lasted till 1992.
Russia then re-entered the space astronomy business in 2011 with the launch of Spektr-R,
a 10m radio telescope that looked at the structure and dynamics of radio sources both inside
and outside of our galaxy. That observatory stopped responding to commands earlier this
year in January 2019. Russia’s re-entry into space astronomy is
very strong and promises to be amazing because with the launch of Spektra-RG an x-ray space
observatory, if everything goes well once it reaches its operating orbit at L2, then
this mission will provide us an all-sky map of the universe in X-rays at energies up to
30 kilo-electron volts (I’ll explain more about that in a minute). This map will hopefully
shed some light on the nature of dark matter and dark energy.
The Spektra-RG Observatory consists of two instruments: the first one is the Astronomical
Roentgen Telescope (ART), an x-ray concentrator that consists of seven identical mirror modules
coupled with seven CdTe strip focal-plane detectors.
The mirror modules were developed at NASA’s Marshall Space Flight Center (MSFC.) Each
module has ~65 sq. cm effective area and an on-axis angular resolution of 30 arcseconds.
The second instrument is called eRosita and was developed Max Planck Institute for Extraterrestrial
Physics in Garching, Germany. This will be the primary instrument of the mission and
has seven identical Wolter-1 mirror modules. Each module contains 54 nested mirror shells
in order to meet the required sensitivity. Observing x-rays through a telescope is not
a trivial thing. In addition to needing to be up in space where we can see them, anyone
who’d had an x-ray in a doctors office knows that they are hard to reflect. X-rays tend
to go through things, not bounce off them, so designing a telescope to focus them is
a tough technical challenge. So far, the best way that’s been found to
focus x-rays is to move them a little bit when the angle of reflections are really shallow.
By making a tube with special material, an incoming xray can be reflected a tiny amount
at shallow angles. This is called grazing incident reflectors and basically just concentrate
the x-rays more than focus them. Both the Russian ART and Germany’s eRosita are grazing-incidence
x-ray telescopes. The ART instrument has seven telescopes all
lined up with each other and each telescope contains 28 concentrically nested shells,
each made from a nickel-cobalt alloy. At the focus of each telescope are special semi-conductor
detectors, known as Double-sided Silicon Strip Detectors, DSSDs, made of cadmium telluride
(CdTe) to record the x-ray photons when they hit.
As I said before: NASA’s Marshall Space Flight Center made the seven mirror models
and Russia constructed the detectors, onboard computer and data memory system, an avionics
unit for temperature control and a star tracker for the telescope's guidance.
eRosita also has seven mirror modules but each one has 54 concentric shells to reflect
the x-rays. The detectors uses on these modules are based on a design that came out of ESA’s
XMM-Newton Mission. OK so back to electron volts: why do x-ray
telescopes use these units? Because x-rays are high energy photons, they have very, very
short wavelengths, on order of about a few nanometers (that’s 10 to the minus 9 meters).
A billionth of a meter and for reasons I’m not entirely sure of, x-ray and other high-energy
scientists tend to use the electron volt as their unit. The electron volt is a unit of
energy and it’s the amount of work it takes to accelerate an electron through one volt.
Luckily, there’s a handy relationship between energy and wavelengths of electromagnetic
photons and so you can figure out for a given energy in electron volts what the wavelength
or frequency is we’re looking at. And it turns out that hard x-rays, the stuff that
the SRG mission is looking at, are in the 5 to 10 thousand electron volt range. Longer
wavelength soft x-rays are below 5 kilo electron volts.
Because I wanted to know and I thought you might too, I looked up what energies x-ray
machines in the doctors office use and I found that a typical package scanner uses 140-160
keV, which is about 5 times more powerful than what ART will see and about 14 times
more powerful than what eRosita will look at.
OK so that’s what the instruments are onboard the SRG observatory and after it’s launch
on Saturday, it is now on it’s way to everyone’s observatory spot in space - the Earth-Sun
L2 point. It will take about 3 months to get there so in the meantime mission planners
will be checking out the instruments and start calibrating everything.
The total mission lifetime is planned to be seven and a half years. The first four years
will be spent performing eight all-sky surveys. The three years after that are scheduled for
observations of selected galaxy clusters and other objects of interest.
What do they expect to see? As the observatory scans the sky, they are going to map about
100,000 galaxy clusters out to about redshift z>1 and look at the hot gas in between the
clusters to form a complete all-sky map of x-rays. In fact, the astronomers on the mission
say that they will actually see all clusters of galaxies above a certain mass contained
in the universe. All of them. All these clusters, and their masses, and
looking at how they evolve over time will give a good indicator of where all the dark
matter is in the universe and how it clumps together.
This map will give not only the large scale structure of the universe as it exists today,
right now, but also show how the cosmos evolved over time and that will give the ability to
see how the universe changed which will show the influence of dark energy over the structure
of everything. The x-ray survey is also expected to unveil
up to three million hidden black holes obscured in distant galaxies and it will also show
what’s inside our own galaxy and study in detail the physics of galactic X-ray source
populations, like pre-main sequence stars, supernova remnants and X-ray binaries.
With the launch of Spektr-RG, Russian and Germany move to the forefront of x-ray astronomy
with the most important astrophysics venture in post-Soviet Russia. The discoveries heralded
by this mission promise to be nothing short of amazing and as they come it, I’ll be
sure to keep you posted. Before I leave you, let me just talk a little
about the currently flagship x-ray observatory in space: Chandra. The Chandra x-ray telescope
was launched on July 23, 1999, which means we’re coming up on 20 years of operation
with this venerable scope. Chandra is a 1.2 meter telescope that has about a half-arcsecond
resolution up to 10 kilo electro volts. It has four telescope modules compared with eRosita’s
and ART’s seven and 58 nested grazing-incidence shells. The big difference here though is
that Chandra is not a survey telescope. It is a point and shoot, meaning it looks at
areas of the sky one at a time and then moves to another area. SRG will scan the whole sky
in x-rays 8 times and will see more x-ray objects than have been seen in the entire
50 years history of x-ray astronomy. One more thing about Chandra’s twenty years
in space before I go. While Chandra is not at the L2 point like many of the other space
telescopes being built today, it flies 200 times higher than Hubble. That’s almost
⅓ of the way to the moon! The elliptical orbit it is in means that the shuttle couldn’t
repair it either. Why do I bring this up? Because many of you are getting your panties
all bunched up about not being able to repair JWST when it launches… well, whenever it
launches… currently March 2021… we’ll see. But my point is, just remember, between
the Chandra x-ray observatory - which has never been repaired and been up there 20 years
- and the SOHO Solar Observatory with is at the L1 point and been up there 24 years since
1995, we’ve got experience with long running space telescope missions. Just wanted to put
that out there because people forget. So before I forget: Happy 20 years chandra!
You’re doing great for a mission designed to only last five years.
Well that’s it for this episode Space Fans, thanks to Deep Astronomy Patreon Patrons for
keeping SFN going and to OPT Telescopes, a world leader in telescopes and accessories
for both amateur and professional astronomers. Thanks to all of you for watching and as always,
Keep Looking Up!.