Supersonic shock waves detected at the edge of the
Solar System – a new study by European scientists clarifies
conditions at our Earth’s outermost shield against interstellar
charged particles.

The local interstellar cloud
Our Solar System entered an
interstellar cloud 10,000 years
ago. Today it is speeding
through this nebulosity at
Mach 2 behind a supersonic
shock wave – in much the
same way that a Concorde
crosses the Atlantic at
supersonic speed. Since its
formation 4.6 billion years ago
our Solar System has encountered numerous interstellar clouds,
knots, filaments, shells and bubbles of different sizes and contents
on its path through the Milky Way. For more than 80 years
astronomers have been attracted by these past and future
encounters, have tried to understand the physics behind them in
order to decipher the dynamic interplay between the interstellar
material and the Solar System.

There is some chance that the Solar System will cross small dense
clouds that have diameters up to 100 times the distance from the
Earth to the Sun. These encounters may increase the number of
interstellar charged particles bombarding Earth, with the risk of
altering the climate here. Our interstellar environment may thus be
important for the short and long-term prospects for life on Earth.
Even though there is still some work to be done before it will be
possible to construct a ‘Galactic weather forecast’, it is clear that for
the past 200,000 years we have been in a favourable environment
that has not altered our climate significantly. Recent studies by a
group of European scientists of the conditions at the outermost
edge of the Solar System using the NASA/ESA Hubble Space
Telescope and Voyager have shown some surprising results.

The heliosphere

Charged particles from the Sun spiral out into space and form the
solar wind. The solar wind particles follow the lines of the solar
magnetic field and fill a region of space called the Heliosphere that
encloses the Solar System. The solar particles at the edge of the
heliosphere form a barrier to deflect other incoming charged particles
and so partially protect the inner Solar System from the surrounding
interstellar medium. The motion of the Solar System through the
dust, gas and nebulosity that make up the interstellar medium give
the heliosphere a comet-like shape with a head and a tail. At the
leading edge of the heliosphere, atoms and ions from the interstellar
medium slow down as they approach the head, forming a shock
wave, known as the interstellar bow shock. As the leader of the
group of scientists, French astrophysicist from the Institut
d’Astrophysique de Paris, Lotfi Ben Jaffel, explains: “The bow
shock has been predicted for more than 30 years, but its existence
has so far been questionable. Now it seems that we have proof”.

The observations

Recent analysis of observations made in the far ultraviolet with
Hubble’s Goddard High Resolution Spectrograph (GHRS) has been
carried out by the international group of scientists. By combining
measurements from the Hubble Space Telescope with Voyager
measurements, the scientists have not only located the interstellar
bow shock, but have also discovered that the nose of the
heliosphere points 12° away from the direction from which the local
cloud is approaching. In this way the group has been able to
determine the direction of the interstellar magnetic field which causes
this 12° tilt.

By observing regions free of bright stars and galaxies, the team were
able to detect a feeble ultraviolet glow called the Fermi glow, which
arises when incoming light from stars and the Sun passes through
the violent transition region between the heliosphere and the
surrounding interstellar medium. By studying this faint glow and
combining the data with intensity measurements from Voyager, Lotfi
Ben Jaffel and his team have been able to deduce the direction of
the interstellar magnetic field based on the observed inclination of
the heliosphere. This discovery is highly significant as Ben Jaffel
argues: “For many years it has been thought that the charged
particles from the interstellar medium were hitting the heliosphere
head-on. Now we see that these ions are deflected by the
interstellar magnetic field. Only by understanding the processes at
the boundary of the Solar System can we realise what influence the
interstellar medium may have on our planet”.

The next step – an interstellar probe

It has been a long-standing dream for the scientists to make direct
measurements of both the heliosphere and the interstellar medium
with a probe. This dream may well come true. Scientists are currently
investigating the different particles of interstellar origin that have
reached the inner heliosphere using the ESA/NASA solar explorers
Ulysses and SOHO. In the long-term, NASA is working on plans to
send a probe to investigate the boundary between the Solar System
and the interstellar medium. This so-called ‘Interstellar Probe’ will fly
into the region of the bow shock closest to Earth and try to clarify the
complex interactions occurring at this boundary. The scientists are
excited at the prospects: “Such a probe will explore the nature of
the interstellar medium and help predict the long-term influence of
charged particles from the Milky Way on our weather and climate”.
They add: “The new results from Hubble and Voyager will
undoubtedly influence the design of the ‘Interstellar Probe’ and
help pinpoint the regions of greatest scientific interest”.

Notes for editors

The Hubble Space Telescope is a project of international
co-operation between NASA and ESA.

These results will be published in the May 2000 issue of
Astrophysical Journal.


Lars Lindberg Christensen

Space Telescope-European Coordinating Facility, Garching,


Phone: +49-89-3200-6306


Lotfi Ben Jaffel

Institut d’Astrophysique de Paris (CNRS-INSU), France

Phone: +33-1-4432-8076


Ben-Jaffel’s collaborators are Romana Ratkiewicz (Space Research

Center, Warsaw, Poland), Olivia Puyoo (IAP), C. Emerich (IAS, IAP),

M.L. Loucif (Observatoire d’Alger, IAP), and Jay Holberg (LPL,

University of Arizona, Tucson).