SMART-1: Chips Off the Terrestrial Block

In just over two weeks the European Space Agency’s (ESA) lunar probe, SMART-1, begins its journey to the Moon–the first such trip for Europe. Due to be launched from Kourou in French Guiana on 3rd September (12.04 a.m. 4th September BST) SMART-1 will be powered by an ion engine, a technology which Europe will be testing for the first time as the main spacecraft propulsion. Onboard will be D-CIXS, an X-ray spectrometer which will provide information on what the Moon is made of.

Computer modeling of half an hour after Giant Impact
Half an hour after the Giant Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others.
Copyright William K. Hartmann

SMART-1 represents a new breed of spacecraft. It is ESA’s first Small Mission for Advanced Research in Technology – designed to demonstrate innovative and key technologies for future deep space science missions. As well as the ion propulsion mechanism SMART-1 will test miniaturized spacecraft equipment and instruments, a navigation system which in the long term will allow spacecraft to navigate autonomously through the solar system. Also slated for the mission is a space communication technique whereby SMART-1 will establish a link with the Earth using a laser beam.

Once it has arrived at the Moon (expected to be in January 2005), SMART-1 will perform a scientific study of the Moon’s composition, particularly any hints of its water-ice history if deposited there by colliding comets. The spacecraft will search for signs of water-ice in craters near the Moon’s poles, as well as provide data on the still uncertain origin of the Moon and reconstruct its evolution by mapping and the surface distribution of minerals and key chemical elements.

Commenting on the mission Prof. Ian Halliday, Chief Executive of PPARC said," This mission to our only natural satellite is a masterpiece of miniaturization…" Halliday added, "SMART-1 is packed with innovative technology that promises to revolutionize our future exploration of neighboring planets whilst answering some fundamental questions about the Moon – how did the Moon form and how did it evolve?"

D-CIXS, a compact X-ray Spectrometer, will make the first ever global X-ray map of the Moon’s surface.

Principal Investigator Professor Manuel Grande from the CCLRC Rutherford Appleton Laboratory near Oxford explains how D-CIXS works: "When the Sun shines on the Moon, its surface fluoresces and D-CIXS will measure the resulting X-rays to determine many of the elements found on its surface. This will provide us with vital clues which will help understand the origins of our Moon."

Weighing just 4.5 kilograms and the size of a toaster, one of the challenges for the D-CIXS team has been to fit all the necessary components into the instrument. This has been achieved through clever miniaturization and the development of new technology such as novel X-ray detectors – based on new swept charge devices (similar to the established charged couple devices found in much of today’s technology) and microfabricated collimators with walls no thicker than a human hair.

Lord Sainsbury, Minister for Science and Innovation at the Department of Trade and Industry said: "SMART-1 is an unprecedented opportunity to provide the most comprehensive study ever of the surface of the Moon."

Recent Lunar Timelines

- Japanese Hiten, Lunar Flyby and Orbiter
– Michael Rampino and Richard Strothers propose Earth could be periodically struck by comets dislodged from orbits when the solar system passes through galactic plane
- US Dept. Defense/NASA Clementine mission, Lunar Orbiter/Attempted Asteroid Flyby
- First commercial lunar mission, AsiaSat 3/HGS-1 , Lunar Flyby
Lunar Prospector launches and enters lunar orbit
Lunar Prospector tries to detect water on the Moon (polar impact)
– Lunar soil samples and computer models by Robin Canup and Erik Asphaug support impact origin of moon
SMART 1, to launch lunar orbiter and test solar-powered ion drive for deep space missions
– Japanese Lunar-A, Lunar Mapping Orbiter and Penetrator, to fire two bullets 3 meters into the lunar soil near Apollo 12 and 14 sites
– Japanese SELENE Lunar Orbiter and Lander, to probe the origin and evolution of the moon

What’s Next

Computer simulations have been used to model whether the Moon is a chip off of the terrestrial block–a theory called the ‘impact-origin’ hypothesis for how our moon-earth combination might have formed.

Computer modeling of 5 hours after impact
Five Hours after impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others.
Copyright William K. Hartmann

The Earth-Moon system is unusual in several respects. The Moon has an abnormally low density compared to the terrestrial planets (Mercury, Venus, Earth, and Mars), indicating that it lacks high-density iron. While the Earth’s iron core is about 30 percent of the planet’s total mass, the Moon’s core constitutes only a few percent of its total mass. In addition, the angular momentum of the Earth-Moon system is quite large. It implies that the terrestrial day was only about five hours long when the Moon first formed close to the Earth. These characteristics provide strong constraints for giant impact models that try to explain the Moon’s formation.

Lunar Clementine mission shows the South Pole of the Moon. The permanently shadowed region center shows evidence of meteor cratering and ice never exposed to direct sunlight.
Credit: NASA/DOD Clementine

Previous models have shown at least two classes of impacts capable of producing an iron-poor Moon, but both are problematic. One model involves an impact with twice the angular momentum of the Earth-Moon system. This model requires that a later event (such as a second large impact) altered the Earth’s spin after the Moon’s formation.

The second model proposes that the Moon-forming impact occurred when Earth was about half of its present mass. This model requires that the Earth accumulated the second half of its mass after the Moon formed. However, if the Moon also accumulated its proportionate share of material during this period, it would have gained too much iron-rich material — more than can be reconciled with the Moon today.

Simulations performed by Southwest Research Institute (SwRI) and University of California at Santa Cruz (UCSC) researchers show that a single impact by a Mars-sized object in the late stages of Earth’s formation could account for an iron-depleted Moon and the masses and angular momentum of the Earth-Moon system. This is the first model that can simultaneously explain these characteristics without requiring that the Earth-Moon system be substantially modified after the lunar forming impact.

Missions like SMART-1 and further modeling of lunar formation are important to the overall understanding of the origin of the terrestrial planets. Because the Moon appears to have formed by an impact event, studying such gigantic impacts may tell us something about the formation of Earth-like planets throughout the Galaxy.

Related Web Pages

Lunar Prospector
Impact origin of moon
Review of Theories of Moon-Forming Impact (Planetary Science Institute)