MSL Landing Timeline: What To Expect Tonight 140
An anonymous reader writes "When the Curiosity rover lands on Mars later tonight, it'll be executing a complex series of maneuvers. JPL will be relying on the Mars Odyssey orbiter to relay telemetry back to Earth in time-delayed real-time, and if all goes well, we'll be getting confirmation on the success (or failure) of each entry, descent, and landing phase, outlined in detail here."
"Time Delayed Real Time" (Score:3, Interesting)
"Time Delayed Real Time"
More like "Real Time as constrained by the Speed of Light", it's not like NASA is doing what NBC is doing with the olympics... :rolleyes:
Why the skycrane? (Score:2, Interesting)
Re:crazy (Score:4, Interesting)
Geezer Butler tells a completely different story. [archive.org]
It had nothing to do with me. In fact, I was the one who thought it was really corny. We had Sharon Osbourne's dad, Don Arden, managing us. He came up with the idea of having the stage set be Stonehenge. He wrote the dimensions down and gave it to our tour manager. He wrote it down in meters but he meant to write it down in feet. The people who made it saw fifteen meters in stead of fifteen feet. It was 45 feet high and it wouldn't fit on any stage anywhere so we just had to leave it the storage area. It cost a fortune to make but there was not a building on earth that you could fit it into.
Re:Not for any definition of "real time" that I kn (Score:4, Interesting)
Then by your definition NOTHING in space is real time. You look up into the sky and everything you're seeing is sending it's light to you from the past.
Duh. Real time is a bit subjective, but it basically is a threshold of control over a system. The strict definition is that a system is real time, if it meets strict time constraints imposed on it (I gloss over some important nuance). So in a sense, one can have real time systems with say, years of lag, for really generous time constraints.
For me, I have an informal definition of real time, namely, a control methodology which wouldn't change, if communication lag were instantaneous.
For example, the human body wouldn't move differently even if human nerves were transmitting signals instantaneously. Walking, running, and such still are the best means for moving. You might be able to try other movement forms (such as cartwheeling), but these wouldn't give you an advantage in normal operation over the usual means of movement.
So in this sense, the human body and its normal means of moving about are "real time". Movement of the MSL and other rovers remotely controlled from Earth have less optimal movement schemes (there's a lot of need to evaluate terrain obstacles, for example, resulting in a lot of move-then-stop operation) than if someone were controlling them from nearby on the Martian surface. So these systems are not real time in my sense.
Now suppose instead of the MSL, one were piloting this fine piece of gear [wikipedia.org], one of the largest excavators in the world. Suddenly that 15 minute lag time is not so significant and the machine probably wouldn't operate all that differently, if the operator was sitting in a cockpit directly rather on distant Earth.
Since I mentioned systems with extremely long lag still qualifying as real time, consider this example. One could set up vast streams of gargantuan slow moving space vehicles carrying basic crude, bulk resources (water, organic compounds, metals, etc) between different planetary systems, say moving at the speed of the Voyager spacecraft. It might take 50,000 years to make the trip and adjustments in trajectory would be very minimal and glacial. Would it matter if one had instantaneous communication? Not really. The years of communication lag have no real effect on the system.
Re:Not for any definition of "real time" that I kn (Score:5, Interesting)
You're both rude and wrong. GP is correct.
"Relativity tells us nothing can have an effect here in less time." True, but that doesn't mean that it's real time. Here are a few examples that show that it's completely ridiculous to call it real time:
The cosmic microwave background is the glow of the hot early universe, from shortly after the Big Bang. No cosmologist would refer to this as seeing the Big Bang "in real time."
It's possible for a ray of light to travel in a circular orbit around a black hole. That means that it would theoretically be possible for me to face in a certain direction, stick out my tongue, and then turn around 180 degrees, look through a telescope, and, some time later, see myself sticking my tongue out at myself. I'm obviously not seeing myself "in real time."
As a third example, there are distant galaxies whose light hasn't gotten to us yet. I don't think anyone would argue that we are seeing them "in real time" -- we haven't even seen them yet.
It sounds like you're misinterpreting something you heard about the nature of simultaneity in relativity. You can define simultaneity in relativity. You simply have to keep in mind that it's relative, not absolute.
In special relativity, the standard way to do this is Einstein synchronization [wikipedia.org]. The relative motions of the bodies in the solar system, as well as all space probes launched so far, is at velocities much less than c, so it doesn't even matter very much whether you talk about doing your Einstein synchronization in the frame of the earth, of mars, or whatever. This is the sense in which the information from Mars is 15 minutes behind "real time." (There are also gravitational time dilations, and they're also quite small.)
Since you brought up astronomy and cosmological look-back times, it's worth addressing that as well. To describe cosmological scales, you need general relativity, and in general relativity Einstein synchronization doesn't work. However, there is a natural notion of clock synchronization in cosmology that is defined as follows. At any spot in the universe, define a frame of reference that is at rest with respect to the cosmic microwave background (or the local flow of galaxies, which amounts to the same thing). Define a time coordinate as measured by a clock that is at rest in that frame. This is what cosmologists mean when they state the age of the universe as so many billions of years. This time coordinate is also the only reasonable definition of "in real time" for use in cosmology.
Next time, please try being more polite and/or getting your facts right.