Which came first, the chicken or the egg? Or, in the case of the Universe,
was it massive black holes or galaxies?

To answer this question by studying black holes in the early Universe
requires an extremely sensitive X-ray telescope — one that is even more
sensitive than the European Space Agency’s XMM-Newton Observatory, which
has been operational since 1999.

Even though XMM-Newton has an unprecedented capacity for collecting X-rays,
it is not sensitive enough to pick out the individual sources of the most
distant X-ray emissions. To do this, ESA, together with industry,
universities and research institutes in Europe, Japan and the United States
has started to study a revolutionary new mission called XEUS — the X-ray
Evolving Universe Spectroscopy mission.

The scheme is a scientific by-product of ESA’s participation in the
International Space Station (ISS), which opens the way for researchers in
Europe to use the ISS for scientific research. As a result, senior European
space scientists have advised the agency to think about a major X-ray
astronomy facility associated with the ISS.

XEUS, a mission in search of the first objects ever created, is likely to
become an important component in ESA’s expanding role in the ISS.

An overview of this exciting X-ray observatory will be presented at the UK
National Astronomy Meeting in Cambridge on Thursday 5 April by Dr. Arvind
Parmar (European Space Agency).


One of the current hot topics in astronomy is how, and when, the first
galaxies formed. Many astronomers now believe that most galaxies have
massive black holes at their centres. It is also possible that the
presence of these black holes may be necessary to spark the formation
of the first stars and thus the complexity that led to intelligent life.

Black holes cannot be observed directly, but by studying the X-rays
produced by material falling into a black hole, astronomers can measure
their mass and distance, and even how fast they are rotating. They do this
by spreading the X-rays into a spectrum (rather like the colours of the
rainbow). This allows X-rays characteristic of iron atoms with a sharply
defined energy to be identified.

When seen from afar, this energy is altered by the conditions near the
black hole as the material swirls rapidly around in an accretion disk.
At any moment, some of the iron atoms are rushing away from us, so their
apparent energy is decreased, while others are moving towards us, and
their apparent energy is increased due to the well-known Doppler effect.

Another effect, known as “gravitational red-shift” causes the X-rays to
lose energy as they climb out of the region of very strong gravity near
a black hole.

This is not the end of the story of the iron signature. For very distant
black holes, the expansion of the Universe shifts the whole pattern into
the low-energy part of the spectrum.

So by studying the shape and energy of the iron emission from some of the
first black holes, astronomers hope to learn more about the black holes
themselves, as well as the composition and properties of the material
falling into them.


XEUS is a radically different approach compared to previous X-ray
observatories. It will consist of two spacecraft — one carrying a 4.5 m
diameter X-ray mirror, and the other a detector — separated by the 50 m
focal length of the optics. The detector spacecraft will use solar-electric
propulsion to continuously follow the focal spot of the optics.

A single Ariane 5 launch will place XEUS-1 into a 600 km orbit. In this
precursor mission, XEUS-1 will prepare for the sensitive observations of
the fully-grown XEUS. After 4 years the mirror spacecraft will dock to
the ISS to experience the growth required in order to study the distant

Using the European Robotic Arm on the space station, additional mirror
segments will be added to the mirror spacecraft, building the world’s
largest and most powerful X-ray telescope. With an eventual diameter of
10 m, the effective area of the optics will be increased by a factor of 5,
reaching 30 sq. m — comparable in size to the largest ground-based optical

The servicing of XEUS at the ISS will be based on the currently foreseen
capabilities of the space station and will depend heavily on robotics and
the presence of astronauts. Having completed its mission, the detector
spacecraft is de-orbited, to be replaced later by the XEUS-2 detector
spacecraft, which is equipped with the latest instrument technology.

Due to the phased approach of the mission concept, the development and
investment costs are spread over a longer time than for a normal space
project. An ambitious mission with powerful challenges for both the
scientific community and industry, XEUS promises to become a truly global
mission with international collaboration at a significant level, involving
many of the partners that have teamed up to build the ISS.

Peering deeper into space than any previous X-ray mission, XEUS-2 will
look back in time to when the Universe was only a few percent of its
current age, observing and measuring the properties of the first massive
objects to develop after the Big Bang.


Dr. Arvind Parmar

Astrophysics Division Space Science Dept.

ESTEC 2200

AG Noordwijk

The Netherlands

Phone: +31-71-5654532

Fax: +31-71-5654690

E-mail: aparmar@astro.estec.esa.nl