Smart Way to the Moon

ion engine
SMART ion engine.
Credit: ESA

SMART-1 is successfully making its first orbit of the Moon, a significant milestone for the first of Europe’s Small Missions for Advanced Research in Technology (SMART) spacecraft.

A complex package of tests on new technologies was successfully performed during the cruise to the Moon, while the spacecraft was getting ready for the scientific investigations which will come next. These technologies pave the way for future planetary missions.

SMART-1 reached its closest point to the lunar surface so far – its first ‘perilune’ – at an altitude of about 5000 kilometers at 18:48 Central European Time (CET, or 12:48 ET US) on 15 November.

smart solar
Electrical solar energy for SMART probe.
Credit: ESA

Just hours before that, at 06:24 CET, SMART-1’s solar-electric propulsion system (or ‘ion engine’) was started up and is now being fired for the delicate maneuver that will stabilise the spacecraft in lunar orbit.

During this crucial phase, the engine will run almost continuously for the next four days, and then for a series of shorter burns, allowing SMART-1 to reach its final operational orbit by making ever-decreasing loops around the Moon. By about mid-January, SMART-1 will be orbiting the Moon at altitudes between 300 kilometers (over the lunar south pole) and 3000 kilometers (over the lunar north pole), beginning its scientific observations.

The main purpose of the first part of the SMART-1 mission, concluding with the arrival at the Moon, was to demonstrate new spacecraft technologies. In particular, the solar-electric propulsion system was tested over a long spiralling trip to the Moon of more than 84 million kilometers. This is a distance comparable to an interplanetary cruise.

Clementine
Lunar Clementine mission shows the South Pole of the Moon. The permanently shadowed region center showed earlier evidence of meteor cratering and ice never exposed to direct sunlight, but Arecibo radar reveals dust.
Credit: NASA/DOD Clementine

For the first time ever, gravity-assist maneuvers, which use the gravitational pull of the approaching Moon, were performed by an electrically propelled spacecraft. The success of this test is important to the prospects for future interplanetary missions using ion engines.

SMART-1 has demonstrated new techniques for eventually achieving autonomous spacecraft navigation. The OBAN experiment tested navigation software on ground computers to determine the exact position and velocity of the spacecraft using images of celestial objects taken by the AMIE camera on SMART-1 as references. Once used on board future spacecraft, the technique demonstrated by OBAN will allow spacecraft to know where they are in space and how fast they are moving, limiting the need for intervention by ground control teams.

venus_occultation
Moon occulting Venus, the morning star, taken by the lunar probe, Clementine.
Credit: NASA/DOD/
Clementine

SMART-1 also carried out deep-space communication tests, with the KaTE and RSIS experiments, consisting of testing radio transmissions at very high frequencies compared to traditional radio frequencies. Such transmissions will allow the transfer of ever-increasing volumes of scientific data from future spacecraft. With the Laser Link experiment, SMART-1 tested the feasibility of pointing a laser beam from Earth at a spacecraft moving at deep-space distances for future communication purposes.

During the cruise, to prepare for the lunar science phase, SMART-1 made preliminary tests on four miniaturized instruments, which are being used for the first time in space: the AMIE camera, which has already imaged Earth, the Moon and two total lunar eclipses from space, the D-CIXS and XSM X-ray instruments, and the SIR infrared spectrometer.

In all, SMART-1 clocked up 332 orbits around Earth. It fired its engine 289 times during the cruise phase, operating for a total of about 3700 hours. Only 59 kilograms of xenon propellant were used (out of 82 kilograms). Overall, the engine performed extremely well, enabling the spacecraft to reach the Moon two months earlier than expected.

The extra fuel available also allowed the mission designers to significantly reduce the altitude of the final orbit around the Moon. This closer approach to the surface will be even more favourable for the science observations that start in January. The extra fuel will also be used to boost the spacecraft back into a stable orbit, after six months of operations around the Moon, in June, if the scientific mission is extended.

SMART-1 will be looking at the darker parts of the Moon’s south pole for the first time. It will be mapping the so-called Peak of Eternal Light, an eerie mountaintop that is permanently bathed in sunlight, while all around are dark craters never touched by the Sun. These craters are believed to harbour water-ice in the lunar soil.

SMART-1 will also help scientists to confirm if ice is present at the lunar poles, where the temperature never rises above -170°C. Any water on the lunar surface would be very helpful in the creation of permanent bases on the Moon.

Among important milestones in moon exploration over the last decade, the successful HITEN (formerly called MUSES-A) Japanese mission was launched in 1990, to perform a sophisticated Earth-Moon circumnavigation, including Earth atmospheric breaking. HITEN was finally directed to impact on 10 April 1993 near Stevinus crater on the southeast part of the lunar side. This hyper-velocity planetary impact could be observed from Earth.

Between February and May 1994, the American Clementine orbiter observed the Moon with visible, and infrared imagers and a laser ranger which mapped practically the entire lunar surface with a 200 m resolution. Its most remarkable discovery was the strong mineralogical and tectonic large-scale inhomogeneity of the Moon. This explains why, even with the Apollo and Luna samples taken from near-side equatorial areas, we do not know the global Moon.

Clementine also discovered the South Pole Aitken basin on the far side, the largest impact basin (2500 km) in the Solar System. Clementine confirmed earlier evidence of permanently shadowed polar craters. A bistatic radar experiment looked at the possible presence of ice in certain polar craters. Clementine’s first objective had been to demonstrate new technologies for the United States Strategic Defense Initiative Organisation but lunar and planetary science benefited greatly.

Lunar Prospector followed, launched in January 1998. The low-cost NASA Discovery mission had been designed to provide answers to long-standing questions about the Moon, its resources, structure and origins. Lunar Prospector’s 18-month mission reaped very valuable scientific data, mapping thorium and potassium radioactive elements and iron. In March 1998 the Lunar Prospector team confirmed the existence of surface hydrogen, with an enhancement at the lunar poles interpreted as water ice. The spacecraft was then deliberately crashed into the Moon’s South Pole, in the hope, a vain one, that this water projected into space would be observed and analyzed from Earth.


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SMART-1: Chips Off the Terrestrial Block
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End of an Era, Dawn of Another?
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