MSL Landing Timeline: What To Expect Tonight

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."

Comments

Re:Not for any definition of "real time" that I kn

[bcrowell]

That IS indeed real time. Relativity tells us nothing can have an effect here in less time. I don't know if you're trolling or just ignorant, but by your definition you can never look at the stars, galaxies or nebulae in the sky in real time either because they're all at varying distances and we're seeing light that originated anything from about 4 to several million years ago. With telescopes you can go back billions.

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. 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.