The instant I saw the avalanche image from the HiRISE camera on-board MRO orbiting Mars, I knew I would have a contender for my Top Ten Astronomy Pictures of 2008.
But then they released this one at the same time:
Yeah, that’s us. That’s home. We were 192 million kilometers (115 million miles) from Mars when HiRISE turned around and took this picture. Right away I could tell that was South America’s west coastline… which is incredible. I also was just starting to wonder about the Moon in the image when I read that it had been brightened artificially to make it easier to see; in general the Earth is 3-4 times more reflective than the Moon, so it’s a lot brighter.
The Mars-Earth-Sun angle was just about 90 degrees when this was taken, which is why the Earth and Moon are half-full. Note that in reality, the Moon is about 30 Earth-diameters away from the Earth, so we’re seeing some perspective here. The Moon was a day before third quarter when this was taken, so it was actually a bit closer to Mars than the Earth was when HiRISE snapped this picture.