Slashdot Mirror
Spacecraft Buzzes By Mercury
Riding with Robots writes "The robotic spacecraft MESSENGER is making its second fly-by of the first planet today, skimming just 200 kilometers above the surface. The fly-by will reveal portions of the planet that have never been seen before, but the main purpose of the maneuver is to prepare for an orbital insertion in 2011. The mission site offers extensive information, along with the first pictures that are already arriving on Earth, with many more expected in the coming hours and days."
Larry Niven's first published short story was titled "The Coldest Place" (collected in 3 Books of Known Space ), based on the idea that the regions of Mercury not hit by the sun would be the coldest place in the solar system. The story was infamous out of date by the time it hit print, as some studies of Mercury had shown that it never got that cold. Nonetheless, reading the story as a child awoke a certain interest of that planet which never gets as much attention as the sexier Mars or Venus or the gas giants. I look forward to following this mission.
Are you prepared for insertion?
Do you even lift?
These aren't the 'roids you're looking for.
Typical slashdotters long dream of insertion, but are never actually ready.
"I don't have to think. I only have to do it. The results are always perfect, but that's old news." - Meat Puppets
Because it is energetically tough to get to Mercury they are trying to get into with as little fuel expenditure as possible, to send as much payload as possible. Since there is no atmosphere, aerobraking is not possible, and thus they are using gravity assists to help reduce the orbital insertion delta-v to a manageable number. Each flyby speeds up the spacecraft a little, to better match Mercury's orbital velocity, and they decided on 3 of these to get the performance they wanted. There is a synodic period (the orbital beat period) between each such opportunity, so it takes a while to complete three flyby gravity assists.
The mission FAQ has more information on this.
That's because they didn't want to have to rename the company to "Whores Galactic".
Moving around in space is all about changing your velocity. There are a number of ways to effect that change - gravitational slingshot, aerobraking, big sails, thrusters ... Each has advantages and disadvantages. For example, direct thrust may provide the most direct path to your objective, but the fuel requirement may be impractical. The mission designers have chosen a method of getting MESSENGER (about 1000kg of payload) to it's objective with enough fuel on-board to perform it's mission. Many variables have been considered - launch vehicle requirements, time to arrival, duration of mission, required consumables, etc. It's a horribly complex optimization.
The most efficient time/location to make orbital adjustments is apogee or perigee. If you enter into a highly eliptical orbit and wish to circularize at a much lower altitude using only a fractional-Newton thruster, yeah, it'll take a while. MESSENGER has a 650N main thruster, but only about 600kg of propellant. That equates to "not a lot" of thruster time. The main engine has a Specific Impulse (Isp) of 318 seconds. On Earth, you'd get about 318 seconds (5+ minutes) of operation. That gravitational element doesn't really apply out in space, so the available thrust-time will be longer. The NASA PDF indicates that the final orbital insertion burn will consume 30% of the propellant, and will last about 14 minutes. Extrapolating, that indicates that MESSENGER has about 42 minutes of propellant on board.
There's also a nice explanation of the orbital maneuvers on the JHUAPL website, and also a nice PDF showing the orbital insertion cost plots.
When they eventually build a hotel on Mercury, I want an ice machine that works and doesn't keep running out of ice. So how big would such an ice machine have to be on Mercury? Would they have to charge $3 for a soda? I hate those tacky signs that say "No Filling Ice Chests."
The main engine has a Specific Impulse (Isp) of 318 seconds. [spaceref.com] On Earth, you'd get about 318 seconds (5+ minutes) of operation.
No. Specific impulse, despite being measured in seconds, has nothing to do with how long the rocket can fire. That obviously depends on how much propellant you carry.
Take another look at that Wikipedia article you linked on specific impulse.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....