NASA Set To Launch Probe To Mercury

antispam_ben writes "CNN is reporting the upcoming Messenger mission to Mercury is set to launch August 2. The spacecraft uses a combination of technologies (insulation, Peltier devices, careful design and orbit, always keeping the shield side toward the Sun) to keep its electronics at room temperature."

Comments

Re:room temp?

[julesh]

There are various definitions of room temperature. The one most often used is 20 degrees C (Err... about 75 degrees F, I think).

Go Messenger!

[hpulley]

While most other planets have been well studied, Mercury has not even had half its surface mapped! Messenger has non-visual light detectors including a laser altimiter which will let it map the whole planet, counteracting its slow rate of rotation. I hope the launch goes well and look forward to the data return. Kudos to NASA for doing some good science on what is considered a less sexy target than some others which seem to hog all the research money.

Another mission to Mercury to be in 2012

[zzabur]

For more information, see ESA BepiColombo page.

Re:Can't we have just one place?

[JPelorat]

It's just a marker that we went there, not that we own it. Sheesh, settle down.

Re:Explanation

[HarveyBirdman]

You can launch a giant ship with a giant fuel tank that cost 800 billion dollars, or you can launch a small, reasonably priced craft and use the gravitation of the planets to do your work for you.

NASA can explain it better: http://messenger.jhuapl.edu/the_mission/mission_de sign.html

Re:Explanation

[HarveyBirdman]

It's not the shove. It's the slowing down part. The gravity assists decelerate the probe until the onboard propellant has a hope of establishing a stable orbit around Mercury. It's a way of tapping the vast potential energy represented by a planetary sized mass.

Remember, the probe is moving further into the Solar System, so it needs to *decelerate* from Earth-normal angular momentum.

Re:Very interesting.

[confused one]

The Mars rover heater is a faulty switch, causing the heater to be always on. It's so damn cold on Mars you need heaters on all the joints to keep the lubrication from freezing solid. passive thermal management wouldn't work because it assumes you've got a heat source to draw from. The most passive approach you could apply to the Mars rover would be (would have been) to use radio-thermal heaters at each of the joints.

Wouldn't work well on Venus either. You need a sink into which to pump the heat. Given the 800F surface temperature, you'd have to do an awful lot of work to pump to an acceptable temperature in the electronics bays. I'm not saying it's impossible, just hard.

the Mercury mission will work because they're putting a big insulative blanket between the electronics and the sun, to provide shade; and, they're pumping the heat from the electronics bays to the cold side (facing away from the Sun) of the craft where it's -200F

Re:peltzer device?!

[carnivore302]

Actually, it's a Peltier device. Peltier devices, also known as thermoelectric (TE) modules, are small solid-state devices that function as heat pumps. A "typical" unit is a few millimeters thick by a few millimeters to a few centimeters square. It is a sandwich formed by two ceramic plates with an array of small Bismuth Telluride cubes ("couples") in between. When a DC current is applied heat is moved from one side of the device to the other - where it must be removed with a heatsink. The "cold" side is commonly used to cool an electronic device such as a microprocessor or a photodetector. If the current is reversed the device makes an excellent heater.


Click on the Mystery Futures Link!

Re:Explanation

[merlin_jim]

Can someone explain why such a convoluted and time consuming route is required?

It's all about delta-v... how much can you change your velocity?

Earth orbits the sun at a specific velocity.

Mercury orbits the sun at a much smaller velocity.

But in order to fly straight there, you have to counteract all of the orbital velocity you have at earth, then either free fall or thrust to the new location, and then build up the orbital velocity of Mercury to make orbit. That's a lot of delta v, and a lot of working fluid to put into your thrusters. In fact, even if we felt like paying that fuel bill, we don't really have the technology to build a probe large enough to carry all that fuel, or to get that fuel out of Earth's gravity well in the first place.

So instead what we do is figure out a low-delta v way to launch it, bringing it into the inner solar system and slowing it down on the way. The key to this is slingshot maneuvers - using the gravity wells peppered throughout the solar system to change the direction of velocity without having to spend delta-v on it.

That and the craft makes use of a little-known feature of relativity; the more energy in your fuel, the heavier it is; if you burn the fuel you have deep in a gravity well, it is quite a bit more effective than it would be in space. This is related to the law that predicts you cannot travel at the speed of light; as you go faster, your intertial mass rises, in such a way that it would take an infinite amount of thrust to reach the speed of light.

Sure your craft has more inertial mass, too, but you'll be slowing down as you exit the gravity well, leaving your fuel behind you, and that's where the mathematical magic happens.

Ownership of space

[philbert26]

It's just a marker that we went there, not that we own it. Sheesh, settle down.

Correct. According to article II of the Outer Space treaty (signed by the USA): "Outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means."

Re:Anyone else reminded of Brin's Sundiver

[Ignignot]

But what about a laser powered by heat? Can it happen without having to reach the ionization temperature of the lasing medium? Anyone have any insight?

Now I am not a laser scientist (IANALS) but I am an electrical engineer. Almost all lasers are powered by heat, in a roundabout way. Power generators usually use a heat differential to produce a circular motion which is turned into electricity. Electricity goes to your laser and makes it go. So yes, a laser can be powered by heat. I don't think it can be done directly, and attaching a steam power plant to a satellite would be a little bizzare.

I assume that your goal here isn't to produce electricity because solar panels do a good job without all this fuss. Instead I assume the laser is to keep the spacecraft cool. Now stay with me while I describe heat transfer. There are two ways that an object can stay cool - either by bleeding heat into surrounding medium (none or very very little in space) or radiate it away. The radiation emitted is directly related to what is absorbed. For example, white absorbs little and emits little. Black absorbs a lot and radiates a lot. So to keep your spacecraft cool you could just paint the side towards the sun white (or mirror) and paint the side away from the sun black. Or you can do what NASA did and put a heat shield in front of the spacecraft with very little connection between the two. Sure it'll get hot, but the small connection means that there won't be much heat transfer between the heat sensitive electronics and the very hot shield. Its all about effeciency.

NASA Website for Messenger

[tpdei]

For those that care, here's the link for the NASA site on Messenger. http://www.nasa.gov/mission_pages/messenger/main/i ndex.html

Re:hey

[Bill Hayden]

as long as its room temperature in there, why not toss a few people/monkeys/whatever in with it?

That would be because it's going to take several years to arrive.

Re:Careful design and orbit?

[Gilthalas]

The planet does indeed spin - Mercury rotates on its axis 1.5 times per solar orbit (see http://www.solarviews.com/eng/mercury.htm). Because of this 3:2 resonance, a Mercury solar day (sunrise to sunrise) is equivalent to 176 Earth days.

So what this means is that for every Earth year Messenger is orbit, 4 Mercury Years will pass, which consists of 2 Mercury Solar Days (see http://messenger.jhuapl.edu/the_mission/mission_de sign.html.

This gives the spacecraft many passes over the light and dark side of the planet, so much that they can spend one (Mercury) day doing global mapping and the second (Mercury) day doing targeted science investigations.

In terms of heat - the highly elliptical, near polar orbit is designed so that the heat shield always faces the sun, giving the instruments a nice room temperature setting on the other side of the shield. There is the possibility of heat from the surface, but the instruments are designed to take that into account.

Heat limited the lifetime of Venus landers

[Engineer-Poet]

(Note, previous response is wrong.)

The Venera landers were able to make it down to the surface, and IIRC one or two of them actually sent back pictures for a while. Their lifespans were very strictly limited by their insulation; as heat soaked in there was no way to pump it out again, and it did not take long before the electronics were too hot to function.

Re:Heat limited the lifetime of Venus landers

[canavan]

Of those that actually reached the surface, almost all worked until their batteries ran out or the Spacecraft bus which was used as a relay left radio range, whichever happened first. If I'm not mistaken, the early ones were cracked by pressure and not cooked - the immense pressure on the surface wasn't known until Venera 7. There's a very interesting website on the russian missions to Venus. And at least Venera 9, 10 13 and 14 returned pictures from the surface, that's 4.

Re:I'll take that bet

[M1FCJ]

this is a common misconception. Mercury is not in tidal-lock with sun. It rotates slowly and for three Mercury years, it has two Mercury days but it does rotate and sunny side is not always the same side.

This has interesting side effects like Sun popping out from East, moving towards west, halting, moving backwards down again and then raising for a second time before moving across the sky.