C
CalliArcale
Guest
Today, the MESSENGER probe is five years old! (Bearing in mind, of course, that spacecraft usually have a very long gestation period before launch.) On August 3, 2004, it blasted off from Cape Canaveral Air Station's LC17B aboard a Delta rocket. Boosted out of Earth orbit by a PAM-D upper stage, the MESSENGER probe was released to begin its long and roundabout journey to Mercury. Today, it is still en route; it isn't scheduled to arrive into Mercury orbit until 2011.
There is a reason for this, and it's rather interesting. It's intuitively obvious why a spacecraft going to, say, Saturn might take a while. It's a very long ways away. But interplanetary spaceflight is about more than just distance. It's also about energy. The whole exercise is basically a complicated ballistics exercise taking place over a very long period of time. For MESSENGER to be captured by Mercury's gravity and become the first Mercury orbiter, it will need to be close enough to Mercury's velocity vector that it's easy for Mercury to grab it. Too fast, and Mercury will only slow it a bit. Even just getting close to Mercury is challenging; its deep within the Sun's gravity well, and you need to shed most of the orbital energy you get by launching off of the Earth in order to get there. When Mariner 10 was launched towards Mercury in the 70s, they had to do something no unmanned probe had done before -- steal orbital motion from a planet. They launched Mariner 10 towards Venus, aiming it so that Venus' gravity wouldn't *quite* capture it but would slingshot it onto the Mercury encounter trajectory.
MESSENGER has to work harder, because it isn't going to just fly by Mercury a couple of times. It's going to go into orbit and map Mercury over the course of a year. (This will be tricky; Mercury has a strange resonance between its rotation and its orbit; it rotates exactly three times for every two orbits. So much of Mercury is in darkness for long periods of time.) To this end, MESSENGER encountered Earth once and Venus twice, and has already encountered Mercury twice as well. It will encounter Mercury again late next month, setting the stage for orbital insertion in 2011.
So, happy birthday MESSENGER!
There is a reason for this, and it's rather interesting. It's intuitively obvious why a spacecraft going to, say, Saturn might take a while. It's a very long ways away. But interplanetary spaceflight is about more than just distance. It's also about energy. The whole exercise is basically a complicated ballistics exercise taking place over a very long period of time. For MESSENGER to be captured by Mercury's gravity and become the first Mercury orbiter, it will need to be close enough to Mercury's velocity vector that it's easy for Mercury to grab it. Too fast, and Mercury will only slow it a bit. Even just getting close to Mercury is challenging; its deep within the Sun's gravity well, and you need to shed most of the orbital energy you get by launching off of the Earth in order to get there. When Mariner 10 was launched towards Mercury in the 70s, they had to do something no unmanned probe had done before -- steal orbital motion from a planet. They launched Mariner 10 towards Venus, aiming it so that Venus' gravity wouldn't *quite* capture it but would slingshot it onto the Mercury encounter trajectory.
MESSENGER has to work harder, because it isn't going to just fly by Mercury a couple of times. It's going to go into orbit and map Mercury over the course of a year. (This will be tricky; Mercury has a strange resonance between its rotation and its orbit; it rotates exactly three times for every two orbits. So much of Mercury is in darkness for long periods of time.) To this end, MESSENGER encountered Earth once and Venus twice, and has already encountered Mercury twice as well. It will encounter Mercury again late next month, setting the stage for orbital insertion in 2011.
So, happy birthday MESSENGER!