On to Mercury!
Published on Jun 6, 2007 at 7:35 am.
8 Comments.
Filed under planets, space exploration.
NASA’s MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) spacecraft passed by Venus yesterday on its way to Mercury. MESSENGER was launched almost three years ago, and it is taking a rather circuitous route to Mercury, passing Earth once and Venus twice along the way. MESSENGER will be only the second spacecraft to every visit Mercury, the first being Mariner 10, which flew past Mercury three times in 1974 and 1975.
Since MESSENGER was passing Venus a couple times, it has instruments on board that can study that planet, too. A spectrometer will measure the composition of Venus’ atmosphere, and will later be used to measure the tenuous gas around Mercury (it is hard to call something as thin as is around Mercury an “atmosphere”, though that is what it is). Cameras that will be used on Mercury were also trained on Venus. But, MESSENGER isn’t alone looking at Venus. The European Space Agency’s Venus Express is currently in orbit around Venus studying that planet. It has been a very long time since two spacecraft have studied Venus at the same time. Now, MESSENGER is moving on to Mercury, passing by that planet January 14, 2008. It will pass Mercury again October 6, 2008, and again on September 29, 2009. MESSENGER will again approach Mercury in March 2011, but this time it will go into orbit around Mercury on March 18, 2011.
So, why such a strange trip to Mercury? Why not just fly straight to Mercury? The answer has to do with how orbits work. You see, when the spacecraft is launched from Earth, it is initially moving at the same speed around the Sun as Earth. In order to get to Mercury, then the spacecraft needs to slow down its orbit. Then, the Sun’s gravity will pull it inward. If the spacecraft were to speed up, then it would sling farther out from the Sun. But, Mercury is much closer to the Sun than Earth. This means that the spacecraft must slow down a lot to get to Mercury. Unfortunately, that takes a big rocket. But, there is another way to do the same thing. When Mariner 10 was launched towards Mercury in 1973, it also flew past Venus. Mariner 10 was did not use a big enough rocket to slow it down enough to get to Mercury. Instead, it only slowed down enough to get to Venus. But, Mariner 10 passed in front of Venus, and Venus’ gravity pulled on the spacecraft, slowing it down further so that it would fall closer to the Sun — to Mercury. This is sometimes called the “Slingshot Effect,” though the preferred name today is “Gravity Assist.” This is what you call it when a planet’s gravity does what your rocket cannot.
Mariner 10 made the trip with one gravity assist from Venus. However, MESSENGER is a much larger spacecraft, and it has a more complicated mission. MESSENGER would have had to use a larger rocket to get to Mercury with one gravity assist. Also, the plan is to have MESSENGER eventually go into orbit around Mercury. Mariner 10 only passed by. A path that will lead to orbital insertion is much more demanding than a flyby. Also, NASA has gotten quite good at this whole gravity assist technique, and it has become fairly commonplace. So, this turns out to be the most economical way of getting a spacecraft to Mercury. The main disadvantage is that it takes a long time. The spacecraft was launched in 2004, but it won’t reach Mercury until 2008, three and a half years later.
-Astroprof
Images courtesy of Carnegie Institute of Washington and NASA
A Ler…-- Rastos de Luz on June 6, 2007 at 11:30 am: 1
[…] “On to Mercury!“, no Astroprof’s Page. Ainda sobre este tema, “…and MESSENGER flyby was successful“, no The Planetary Society Blog; “Geoengineering Comes with Huge Risks“, no Universe Today. […]
jhm on January 11, 2008 at 11:00 am: 2
I’m trying to understand the aqua line (from Earth flyby to Venus flyby (I). Does Messenger really do two loops before encountering Venus? Wouldn’t it be a waste of an orbit?
Astroprof on January 11, 2008 at 1:03 pm: 3
It flies past Venus once, adjusting the orbit some, and then again, adjusting it more. The orbit needs to be fine tuned, rather than a single big change, because the goal is to gradually approach Mercury eventually and go into orbit instead of just flying past like Mariner 10.
jhm on January 11, 2008 at 6:13 pm: 4
That’s what I thought. However, if you look at the graphic (thanks a lot for this, by the way), there is the aqua orbit (which includes DSM 1), but it appears to go on one more orbit before turning into an olive line which proceeds to complete a third orbit before it visits Venus again. From here it turns peach and falls to Mercury.
I’m sorry if I’m being obtuse here, and I thank you for the reply.
Astroprof on January 13, 2008 at 12:12 pm: 5
Oh, I think that I see what you mean, now. Remember that the positions of the planets are drawn arbitrarily. They are moving around as the spacecraft is moving around, too. So, an orbit may appear to loop around twice needlessly, when really it is waiting for everything to be in the right place.
Astroprof’s Page » Mercury, finally. on January 13, 2008 at 12:49 pm: 6
[…] The MESSENGER spacecraft is now nearly to Mercury. It will pass that planet tomorrow, Monday, January 13, 2008, at about 1:05pm, Central Standard Time. I have published two other postings about MESSENGER (here and here). I am not going to restate all of that, so you can go back and read them if you wish. But, I will summarize the key points. […]
moonbase on January 9, 2010 at 5:30 pm: 7
moonbase…?
moonbase on January 9, 2010 at 5:30 pm: 8
moonbase…???