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For 13 months, the European Space Agency probe SMART-1 (Small Missions for Advanced Research in Technology-1) struggled to reach the Moon, using its extremely low-thrust but high-efficiency ion drive to gently boost its orbit higher and higher. Once it finally achieved lunar orbit, it observed our natural satellite for nearly two years, taking images and surveying its minerals.
Its low orbit and fuel consumption guaranteed how the mission would end: crashing into the Moon. Scientists and engineers, knowing this, planned its demise with careful scrutiny. They aimed the probe to impact on the lunar near side, so it could be seen from Earth, but also to occur in a spot that was just on the Moon’s night side, where the Sun had not yet risen. That way the impact flash would be bright enough to see, but any lunar material ejected upward might get lit by the Sun as it rose above the surface, allowing astronomers to observe it. Even in death, the probe would yield important science.
Everything went as planned. The probe’s orbit brought it down to the surface, where it impacted in Lacus Excellentiae (the Lake of Excellence) at 05:42 UTC on September 3, 2006. The energy of the collision was high — a couple of hundred kilos of spacecraft slamming into the Moon at 2 kilometers per second makes quite a bang — and the flash of light was detected by ground-based telescopes 400,000 km away.
But … where exactly did it hit? From our distance, the impact site couldn’t be determined precisely. And we didn’t have any other spacecraft at the Moon to witness the event or view the location after.
At least, not until the Lunar Reconnaissance Orbiter. LRO entered lunar orbit nearly three years after SMART-1 left it, and has mapped the Moon many times, in some spots with resolution smaller than a meter. Canadian astronomer Phil Stooke pored over LRO images of the impact area, and now, after all these years, he found the exact location of SMART-1’s gravesite.
This image shows a gouge in the lunar surface about 4 meters wide and 20 meters long (a little bit wider and longer than an 18-wheeler truck). The location is at 34.262° W, 46.193° S, very close to the coordinates measured from Earth where the flash was seen (as you’d expect). In the image north is up, and SMART-1 was moving toward the bottom of the image, in a roughly north-to-south direction. You can see a sideways splash of material outward from the initial point of impact, and in this second image (taken when the Sun was higher, eliminating shadows but highlighting different brightness features) you can also see the spreading fan of material radiating outward from the end of the gouge:
I have to admit, my initial reaction was surprise at how short the scar is! The impact angle was very low (about 1° relative to the surface, so it dropped something like 15 meters over the last kilometer of travel) so I thought the gouge would be longer. But our instincts are not trustworthy when it comes to ultra-high-speed impacts; the way the energy is dissipated is different than in low-speed impacts. For example, as the MythBusters showed, high-speed bullets are stopped in less than a meter of water! The energy of motion of a very fast projectile is very efficiently transferred into the ground at impact, so a lot of material is displaced (blown out) and the projectile slows extremely rapidly.
Finding the impact site is pretty helpful for scientists. Over the next few years we’ll be sending more sophisticated probes to the Moon, and they’ll be able to investigate this area in more detail. How did the material get excavated? What does the splash pattern look like at the centimeter-scale level? What materials were uncovered? How did the spacecraft itself disintegrate?
These are all useful things to find out as we continue to explore our nearest cosmic neighbor. And I rather like the lesson from this: It shows that even now, long after its demise, SMART-1 still has much to tell us about the Moon.