Last week, NASA's Messenger spacecraft ended its 11-year mission by crashing into Mercury. Of course, Messenger was doomed anyway, but sometimes a mission's entire point is to smash one thing into a bigger thing and watch the explosion.
Messenger isn't the first (nor the last) craft to end in a dramatic smash. We've crashed vessels into moons, comets, and more — all in the hope of learning something new from the wreckage. Here are some of the highlights in space exploration collisions.
Apollo 13 Third Stage (1970): Seismometer Calibration
The third stage of the Saturn V rocket that launched the ill-fated Apollo 13 mission did, unlike the mission itself, reach the moon. The rocket stage left a 30m wide crater on the lunar surface, and its impact provided calibration data for a seismic instrument network on the Moon.
Galileo (2003): Protecting Jupiter's Moons
Of course, sometimes a spacecraft is going to crash no matter what, and scientists just want to minimise the damage. For instance, NASA deliberately plunged its Galileo orbiter into Jupiter's atmosphere in September 2003.
Leaving the spacecraft to crash on its own meant risking an impact with one of Jupiter's moons — Europa, Io, Ganymede or Callisto. Since Europa and Ganymede have salty oceans and are considered likely homes for alien life, if it exists, NASA wasn't willing to risk contaminating those environments.
Deep Impact (2005): Geology of a Comet
In 2005, NASA slammed an 370kg probe into Comet Tempel 1 at 36,693km/h, in order to learn more about how the Solar System formed.
This was the climax of the aptly named Deep Impact mission, one of the most famous crashes in history. The impactor was fairly small, just 1m wide and 1m long, but the weight of its onboard navigation system and thrusters had been supplemented with 113kg of "cratering mass", layers of copper plating at the front end. Deep Impact was built for just one purpose: to hit really hard.
When a spacecraft crashes into a planet, a moon or a comet, it sends up a spray of debris: dust, rock, ice, liquid and gas. Thanks to the physics of impact cratering, most of that material is being excavated from below the surface as the impactor digs itself a crater. Crashing a spacecraft basically scoops out a big sample of the local geology and throws it into space, where it's easy to take spectrographs and identify its chemical makeup.
The Deep Impact fly-by spacecraft dropped the impactor off in the comet's path on July 2, then got the heck out of there. For the next 24 hours, the impact used its star-tracking system, navigation algorithms, and small thrusters to hold itself on course. Deep Impact essentially flung itself into the path of the 14km long, 5km wide comet to be run over.
It hit with as much force as 4.35 tonnes of TNT, digging a crater several hundred meters across and sending a plume of ice, dust and gas out into space. The fly-by spacecraft, along with the Spitzer Space Telescope and other observatories, trained infrared cameras and spectrometers on the impact plume. Spitzer's spectrometer discovered a mix of carbon, clay minerals, water vapour and ice, and both crystalline and glassy silica. That mixture pointed to conditions in the early Solar System capable of rapidly heating and cooling material and then mixing hot and cold materials in the nebula of gas and dust surrounding the sun. It's a much more complex picture of the early Solar System than scientists had previously.
SMART-1 (2006): A Great View
When the ESA's SMART-1 lunar orbiter ran out of fuel in 2006, the agency timed the crash to happen on the Earth-facing side of the moon, for the benefit of observers on Earth who wanted to see the crash. (Who says science is no fun?) On its way down, SMART-1 provided some low-angle images of parts of the lunar landscape which had previously only been seen from the top down.
LCROSS (2009): Water on the Moon
Then, on October 9, 2009, the 16km deep lunar crater Cabeus became the site of a double crash.
NASA's Lunar Crater Observation and Sensing Satellite, or LCROSS, came to the moon looking for water, but it did not come in peace. LCROSS carried a Centaur upper stage rocket, and after several orbits around the Earth, it launched the Centaur into the moon.
The impact, deep in the shadows of Cabeus Crater, kicked up a plume of debris well above the ten-mile-high walls of the crater. Most of the material in the plume had rested in the cold shadow of the crater's walls for several billion years. According to spectrographs from the LCROSS spacecraft, observing the impact from orbit, Cabeus crater contained water ice, as well as volatile chemicals that scientists count among the building blocks of life: ammonia, carbon dioxide, carbon monoxide, hydrogen, methane, and sodium. The crater also held mercury and silver. Scientists say those materials were probably deposited by an ancient comet impact, possibly even the impact that formed the crater.
Before the dust settled, LCROSS plunged down into the plume, analysing its contents and sending data back to Earth until it joined its Centaur rocket stage at the bottom of the crater.
Messenger (2015): Space Weathering
After last week's crash, Messenger's remains will sit in Mercury's newest crater, slowly decaying under the constant onslaught of cosmic rays, solar wind and tiny meteorites. For scientists, this is good news. They will learn even more about a process called space weathering.
We already know that space weathering happens faster on Mercury, with its thin atmosphere and proximity to the sun's powerful radiation, but they still don't know exactly how fast it happens. In 2017, the European Space Agency plans to launch a new mission to Mercury, called Bepi Columbo, and when it arrives at the innermost planet in 2024, it will look for Messenger. Images of a relatively fresh crater, with a known impact date, will help scientists better understand how weathering works in space.