On 27 August 2003, Mars is less than 56 million kilometres away — approaching
closer to our planet than it has done in over 60,000 years.

About the same time as this closest approach, Mars Express passes the halfway
mark of its journey, in terms of distance along its trajectory. On 1 September
2003, as it hurtles through space at 10,800 kilometres per hour, the spacecraft
will have covered over 242 million kilometres, half of the total of 485 million
kilometres needed to arrive at Mars. Note that the distance travelled is not
the
same as the distance between the Earth and Mars.

At first glance, you might think that this time would be better than three
months ago to launch a spacecraft to Mars, given the advantage of a shorter
route. However, planning interplanetary journeys is more complicated than just
taking account of the distances.

Playing catch up

This is because the planets are also moving in their respective orbits. If you
want your spacecraft to rendezvous with another object in space, you must
carefully time its launch so that the orbits overlap at some point in the
future.

Imagine the Solar System as an athletics racetrack. If you were watching a
400-metres race from the centre of the track and wanted to intercept one of the
runners taking part, you could simply chase the runner you want to catch. If
you
are fast enough, you might eventually catch up but only after using a lot of
energy and travelling a long way.

A much better way to catch your athlete is simply to walk across the centre to
the other side of the circular track. It is a much shorter distance and you use
a lot less energy and time getting there. You calculate your walk so that you
arrive at the other side of the track at the same time as they do. Too early
and
you are waiting for them. Too late and you have missed them completely — you
would have to wait one lap until they came around again.

In spaceflight, straight-line paths do not exist for the same reason. All
planets move in long, curved paths around the Sun in either circular or
elliptical orbits. To reach Mars, the Mars Express probe is now on a
trajectory,
an arc-shaped path, which gradually makes its way outwards to intersect the
orbit of Mars about six months after launch.

It is no coincidence then that the fleet of spaceprobes now travelling to Mars
was launched within weeks of each other in 2003. Even with the relatively close
positions of the planets, which would cut journey times, scientists still
needed
to calculate the shortest trajectory between the orbits of Earth and Mars. It
makes sense that this occurs around the time of closest approach. Once the
shortest path has been established, you can then work back to get the actual
launch dates.

All in a row

Though Mars will be closest to Earth on 27 August 2003, ‘opposition’ comes on
28
August 2003. Astronomers call the moment when the Sun, Earth, and Mars form a
straight line opposition. Since we are closer to the Sun than Mars, this is
also
when we are overtaking Mars in our respective orbits.

At a distance of exactly 55,758,006 kilometres from Earth, the Red Planet will
be brighter than Jupiter and all the stars in the night sky, outshone only by
Venus and the Moon. You will not see anything gigantic in the sky, but you will
be able to see Mars’s distinctive red-orange colour. Amateur astronomers with
good-sized telescopes will be able to see some of the planet’s features, such
as
the polar ice cap, dark surface features, and perhaps even storm clouds.

Interesting as this closest approach is, professional astronomers will be
looking forward to even better things. Probes that orbit the planet have
studied
Mars and some spacecraft have even landed on it. Later this year, our knowledge
will increase dramatically when the flotilla of spacecraft, including Europe’s
Mars Express, approaches the Red Planet.

Mars Express will help to answer the questions of whether there has been water,
and possibly life, on Mars. It will be mapping the Martian subsurface, surface,
atmosphere, and ionosphere from orbit and by conducting observations and
experiments on the surface.

More about …

* Mars Express overview
http://www.esa.int/esaSC/120379_index_0_m.html

* Types of orbit
http://www.esa.int/esaSC/SEMU4QS1VED_index_0.html
* Mars Express mini site
http://www.esa.int/export/SPECIALS/Mars_Express/index.html

Related articles

* What is a ‘launch window’?
http://www.esa.int/esaSC/SEMO49YO4HD_index_0.html

* Interplanetary trajectories
http://www.esa.int/esaSC/SEM8USS1VED_index_0.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaSC/SEMOU1ZO4HD_index_1.html]
About the same time as Earth and Mars make their closest approach in more than
60,000 years, ESA’s Mars Express passes the halfway mark of its journey, as
regards distance.

Credits: ESA 2003. Illustration by Medialab.

[Image 2:
http://www.esa.int/export/esaSC/SEMOU1ZO4HD_index_1.html#subhead1]
Mars Express, launched on 2 June 2003, at 23h45 (local time) on board a
Soyuz-Fregat rocket from the Baikonur Cosmodrome in Kazakhstan.

Credits: ESA/STARSEM-S. CORVAJA 2003

[Image 3:
http://www.esa.int/export/esaSC/SEMOU1ZO4HD_index_1.html#subhead2]
Taking advantage of Mars’s closest approach to Earth in eight years,
astronomers
using NASA/ESA Hubble Space Telescope took the then sharpest views of the Red
Planet in 1999. The telescope’s Wide Field and Planetary Camera 2 snapped this
image Mars when it was only 87 million kilometres from Earth. From this
distance
the telescope could see features as small as 19 kilometres wide. If you were
travelling with Mars Express, this is the view you might have a few days before
arrival at a distance of several thousand kilometres. This image is centred on
the dark feature known as Syrtis Major, first seen by the astronomer Christian
Huygens in the 17th century. Clearly visible are the icy north and south poles,
and along the right limb, late afternoon clouds have formed around the volcano
Elysium.

Quote: Five days before entering final orbit, Mars Express will be on a
collision course with the planet. – John Reddy

Credits: NASA