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SMART-1 in orbit around the Moon

The European Space Agency's (ESA) SMART-1 probe completed its first orbit around the Moon on 19 November 2004.

Artist's impression of SMART-1 approaching the Moon
Artist's impression of SMART-1 approaching the Moon. Image: ESA
The probe is now gradually descending into a lower orbit. By mid-January 2005 it will orbit the Moon at altitudes between 300 kilometres above the lunar south pole and 3000 kilometres above the lunar north pole and will begin its detailed scientific observations.

SMART-1 was launched successfully on 27 September 2003 on board an Ariane 5 launch vehicle departing from the Kourou spaceport in French Guiana. Its arrival at the Moon is a milestone for ESA.


The mission

The main purpose of the first part of the SMART-1 mission is to demonstrate new spacecraft technologies, particularly the electric propulsion system. Travelling through space using electric propulsion may be the key to future space exploration within our solar system.

Propulsion

Artist's impression of SMART-1's ion engine
Artist's impression of SMART-1's ion engineImage: AOES Medialab, ESA
Electric or ion propulsion uses solar panels to convert sunlight into electrical energy. Inside the drive system, a negatively charged cathode emits electrons. These are attracted into a positively charged anode chamber. The electrons are trapped inside this chamber using a magnetic field. Atoms of xenon gas are then added to the chamber where they collide with the electrons. This dislodges additional electrons from the xenon atoms, converting them to form ions. These positively charged particles are in turn repelled by the anode and are expelled from the spacecraft as an ion beam. The ejection of the ions creates a gentle thrust and drives the spaceship forward.

Ion propulsion is cheaper, lighter, easier and quicker to adjust than conventional chemical rockets. The engines can operate at high efficiency for long periods and drive spacecraft to high speeds, cutting down journey times to more distant targets (although not the Moon – chemical engines can send probes there within a few days).

The engine has performed so well that the spacecraft arrived at the Moon two months ahead of schedule and has used less fuel than predicted. The extra fuel has meant mission controllers are able to move the probe to a lower orbit, greatly improving its scientific observations.

SMART-1 is the first ESA spacecraft to use this propulsion system but several NASA probes including Stardust already employ ion engines.

Navigation

SMART-1 has demonstrated new techniques which it is hoped will ultimately lead to spacecraft navigating independently using on-board systems rather than ground control teams. Tests have shown that navigation software will be able to use images of celestial bodies taken by on-board cameras to determine position and speed.

Communication

SMART-1 also carried out deep-space communication tests using high-frequency radio frequencies, which are better suited to transfer large volumes of data future missions will demand. The spacecraft also tested the feasibility of communication using an Earth-based laser.


Lunar science with SMART-1

The Moon as seen by SMART-1's at a distance of about 60,000km
SMART-1 will examine the Moon with a suite of instruments. For example, the Asteroid Moon micro-Imager Experiment (AMIE) camera will be used to study the Moon’s topography. It has a field view 5° wide and observes in visible and near-infrared light.

The Moon as seen by SMART-1 from a distance of about 60,000km
Image: ESA/Space-X, Space Exploration Institute

AMIE will view regions from different angles and under different lighting conditions to better understand the evolution of the lunar surface.

SMART-1's most complex task is to peer into the dark regions around the poles, looking for signs of water ice and the possibility of frozen carbon dioxide and carbon monoxide. No light falls directly on to the areas that are to be targeted but rays reflected from nearby crater rims may light the ice sufficiently for it to be detected in the infrared.

Many astronomers believe the Moon coalesced out of the debris from a collision between the early Earth and an asteroid the size of Mars. Compared with the Earth, the Moon should have a lower proportion of iron and the heavy elements found in the terrestrial core and a higher proportion of the lighter elements found in the Earth's crust.

Atoms of different elements on the lunar surface absorb X-rays from the Sun. The X-rays are re-emitted with a characteristic signature of the element that absorbed them. The Demonstration of Compact Imaging X-ray Spectrometer (D-CIXS) instrument on the probe detects these re-emitted X-rays and measures the elemental composition of the lunar crust.


The future

ESA has a commitment to develop small, low cost space probes under the auspices of the Small Missions for Advanced Research in Technology (SMART) programme.

SMART-1 should give an impetus to ESA's programme to explore the solar system and observe the wider universe – projects ranging from gravitational wave detectors to probes that will land on Mercury.


Robert Massey and Heather Campbell, July 2003
Updated November 2004


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