New Images from Galileo

deglr6328 writes "New images of Jupiter's Moon Io, along with a false color image of the first isolated ammonia ice cloud discovered on Jupiter; were released yesterday on the Galileo Probe homepage. The probe is currently transmitting back (at it's maximum transmission rate of about 120 Bits per second!) data on previously recorded observations of Ganymede, Jupiter's main ring, Europa and the Jovian Aurorae. Magnetometer measurements that are now being taken in a 100 day survey of Jupiter's huge magnetosphere will be used in a joint investigation with the Cassini spacecraft, which will be making it's closest approach to Jupiter in about two months. After a broken antenna, 5 years in the Jovian environment, 2 extended missions and a total dosage of nearly 4 times the designed radiation exposure, Galileo is still doing spectacular science."

Comments

They're using the Low-Gain Antenna

[RAK]

The reason for the low bitrate is the fact that they are not using the main antenna, which failed to open long ago. Instead the engineers ingeniously reprogrammed the craft to use a low-gain antenna to transmit scientific data. The high-gain antenna was meant to transmit 134,400 bits per second (about one imaging frame per minute). Many software tricks had to be applied, as well as the use of better receiving equipment on earth, for the mission to continue as planned. Prior to the software upgrades, the low-gain antenna had a bitrate of only 8-16 bits per second! See http://www.jpl.nasa.gov/galileo/hg a_f act.html.

Data Rate

[Corgha]

Alright, before I hear any more complaints about the data rate, keep in mind that Galileo is 50 light minutes from Earth. It's one thing to get 100Mbps over twisted pair copper between machines a kilometer apart, but quite another to do it by radio over about 900,000,000 times that distance (by some quick back of the envelope calculations) with a broken antenna.

Bottom line: don't mess with NASA -- chances are one of their engineers wrote your network driver.

Corgha

Re:About the radiation

[tesserae]

Ummmm... you're just talking about the radiation in Earth's upper atmosphere; the radiation environment near Jupiter is a completely different matter.

Remember the Van Allen radiation belts around Earth? They're composed of energetic charged particles (protons and electrons), excited to high energies by the magnetosphere, and are held in their spiraling paths by the terrestrial magnetic field. Before they were discovered (in the late 50's, by James Van Allen), people like Werner Von Braun had planned to put manned satellites in a two-hour orbit -- about 1075 miles up. This turned out to be in the lower Van Allen belt, and the radiation hazard was far too great for safety, so manned spaceflight now is generally in orbits below 250 or 300 miles.

But it's Jupiter we're talking about, and the jovian magnetic field is much stronger than Earth's; the jovian equivalent of the Van Allen belts are millions of times more energetic than the terrestrial belts. Just to give you an idea of what this means to people, compare the exposure at Jupiter (say, in the neighborhood of Io) to that around Earth.

When the Apollo astronauts went to the moon, they had to penetrate the Van Allen belts twice, going out and coming back; in doing so, they received about 2 rem radiation dose. This isn't too much: the U.S. limits radiation workers to about 25 times this, each year, based on the cancer risk. When you talk acute doses -- say, you do a pass by Io, which is in the middle of the jovian belts -- the whole-body exposure which is 50% fatal within 30 days (when untreated) is around 250-300 rad (under these circumstances, 1 rad ~ 1 rem).

Jupiter's radiation belts are millions of times stronger than Earth's, so if an astronaut spent the same time in them (about 3 hours total on a lunar mission -- but Jupiter's much larger, and so your speed would have to be hugely greater to make the transit in that time) they'd get an exposure of millions of rem. In other words, they'd be dead almost immediately -- an exposure of mere seconds would probably be lethal.

Galileo is radiation-hardened, since it was intended to survive in this environment; however, it's been there for almost three times its design limit, and it must be getting pretty fried by now. As a matter of fact, last year when it made its first really close flyby of Io, there was concern that the radiation would corrupt the computer memory and cause it to go into safe mode, or blind the camera's CCD. When that didn't happen, everyone was relieved -- and they promptly did it again!

The radiation environment is severe enough that they actually expect the spacecraft to be physically destroyed in a few hundred million years, if they left it in Jupiter orbit.

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120 bits per second?

[e_n_d_o]

You'd think for a few hundred million dollars they'd find something better than an AOL account?

As the probe approaches Jupiter. . .

[kfg]

Something wonderful will happen Dave.