On Sept. 3, 2006, a European satellite named SMART-1 crashed into the moon! This wasn't actually as disastrous as it sounds, because the spacecraft did it on purpose.
SMART-1 launched in 2003 and was the first European spacecraft to go to the moon. It was also the first mission of a program called Small Missions for Advanced Research in Technology, which was a series of low-cost spacecraft that the European Space Agency launched to test new spaceflight technologies.
SMART-1's primary objective was to test something called Solar Electric Primary Propulsion, or a solar-powered electric engine. While it orbited the moon for almost two years, it looked for water ice and other chemicals on the lunar surface.
When SMART-1 slammed into the moon, that was also done for science. Astronomers back on Earth watched the impact with their telescopes, hoping that it would give them some insight into the mechanics of meteor impacts.
Why it mattered
At its core, ESA's SMART-1 moon probe was a technology testbed for spacecraft technologies, that doesn't mean the mission was without science merit.
The spacecraft carried a suite of instruments to map the surface of the moon, study the mineral composition of the lunar surface and seek out infrared signatures for signs of water ice. Scientists hoped the probe would help build a geologic history of the moon to better understand its violent origins in a cosmic impact.
SMART-1 accomplished all this and more. The mission was expected to last just six months at the moon, but the probe persisted for about three years.
ESA initially expected SMART-1 to crash into the moon in September 2005, but mission managers devised a series of engine maneuvers that added a full year onto the mission. An even later maneuver, which completely exhausted SMART-1's xenon supply, extended the probe's life by a few more weeks, ESA officials have said.
But on Sept. 3, 2006, the moon's gravity pulled SMART-1 out of its lunar orbit. The probe crashed into the moon at a relatively slow 4,473 mph (7,200 kph) -- Leonid meteoroid impacts on the moon are more than of 156,585 mph (252,000 kph), for example -- in a region known as the "Lake of Excellence." No spacecraft in lunar orbit saw its demise, but a telescope on Earth did record the flash from the lunar impact.
It would take another 11 years before NASA's Lunar Reconnaisance Orbiter as able to spot the gouge in the lunar surface caused by the SMART-1 impact. That probe's photo showed that SMART-1 created a crater just over 13 feet (4 meters) wide and a gouge over 65 feet (20 m) long when it slammed into the moon. You can see the photo below.
Still, demonstrating the solar electric ion engine on SMART-1, which used inert xenon gas as a propellant, was the primary goal of the mission.
At the time, SMART-1 was only the second spacecraft to test the solar electric engine (NASA's Deep Space 1 was the first). ESA was hoping the mission could serve lead to a new era of interplanetary exploration by using a lower-cost and more fuel-efficent way of reaching other planets. The mission, ESA said at the time, could pave the way for missions to Mercury, asteroids and more.
Today, those hopes have been realized, quite literally.
The BepiColombo mission to Mercury, a joint effort by the European Space Agency and Japan to send two orbiters to the planet, is beginning its arrival phase today - Sept. 3, 2026 - by releasing the joined orbiter ahead of orbital entry. The Mercury Transfer Module that carried the two probes toward Mercury is powered by a solar electric xenon propulsion system.
NASA's Psyche asteroid spacecraft, Dawn asteroid mission and DART asteroid impact mission also used similar propulsion systems.
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On this day in space! Sept. 2, 2016: Rosetta spacecraft finds long-lost Philae lander on Comet 67P after 2-year search
