Comments on: Lunar Array for Radio Cosmology

By: Charlie

Charlie — Fri, 12 Jun 2009 18:33:32 +0000

Oops, sorry, VLA is in Socorro, NM

By: Charlie

Charlie — Fri, 12 Jun 2009 18:28:21 +0000

Long-baseline radio astronomy on and near the earth is a mature field. They have well-developed methods of recording the data from the individual telescopes, storing it, and analyzing it later. A key element is the use of a local oscillator that can be locked to cesium clocks or equivalent; this allows each station to be stand-alone, working independently during data taking, and only synchronizing a few times around the time of observation. Later, in an office somewhere, the analyst performs interferometric analysis of the data from data tapes, CDs, or whatever. This entails synchronizing the time sequences from each telescope and fitting a source model to those observations. This is almost certainly how it would be done from the moon.

Though we’re familiar with pictures of the VLA in Arizona, where all the radio dishes are within sight of each other, an earth-spanning array of radio telescopes, known as a VLBI array, has been successfully used for several decades. And VLBA (adding a space antenna) could be used with some modest investment. Those telescopes don’t have to see each other; they just need to observe the same object simultaneously and have good stable synchronized clocks.

This is unlike optical interferometry, where pairwise beam combination and detection must occur immediately and with great care and handling of the beams. Optical interferometry from space and the moon is more of a challenge as a result.

By: hyakutake1957

hyakutake1957 — Wed, 05 Mar 2008 19:21:14 +0000

Hi Ed,
I don’t think they know yet but I agree with you. I would think it would be your option #1. NASA has awarded a grant of $500,000 to MIT and a second team at the Naval Research Laboratory to develop the proposal which would have the details of how this would be set up. I didn’t see a timeline when this would be complete, probably after a year or so.

By: Ed

Ed — Wed, 05 Mar 2008 16:20:54 +0000

I’m not even sufficiently familiar with the use of weblogs to distinguish questions from answers, so I can’t tell if what I wish to ask has been answered already. I can think of two ways to sum the individual signal components received by all the elements of the radiotelescope array: 1. at each element, convert the received (analog) signal to a digital signal, have that signal modulate a high-frequency carrier, and transmit it through space to an orbiting relay station that sums the signals and re-transmits that summed signal to Earth or re-transmits all the components for summing on Earth, or 2. sum the individual components while they are still in the analog domain, for example by relaying them over long optical fiber links to a central station on the surface where the summed analog signal is converted to the digital domain and relayed to orbit and then to Earth as in scenario 1. This second scenario seems messier because of the optical fiber links, but does not require as much signal processing equipment at the individual telescope elements, which must receive some “synch”-ing oscillation signal to preserve the received signal phases through the frequency down-conversion and A/D conversion processes. Are either of these scenarios close to what is being envisioned?

By: hyakutake1957

hyakutake1957 — Thu, 28 Feb 2008 06:55:57 +0000

Hey James,
Good point about the grain of dust. I’m skeptical that even if we could travel at the speed of light that unless there is some kind of force field in front of the craft to scoop the cosmic dust out of the way, like an old train engine could scoop animals off the track, there is no way a spacecraft could survive. The dust is sparse in space but when you travel that fast you are compressing space ahead of you and a little dust over a large area could add up to a brick wall!

By: James

James — Thu, 28 Feb 2008 06:40:49 +0000

That is interesting. I was watching a series documentary on the History channel that comes on every Tuesday called The Universe. During an episode about space travel, one high ranking NASA officer stated that after the Appollo disaster, manned flights outside the orbit of the Earth were basically cancelled indefinately. Even if they wanted to send a person on another world, they don’t have the funding anymore. Plus, the reusable spacraft, such as the Endeavor are aging, and aren’t that reliable anymore. So, that is probably why we are sending out probes to Mars, and these robots to the far side of the Moon, instead of having people do it.

“1 grain of dust,” stated the officer, “traveling at supersonic speeds in space, can hit an astronaught in a way that is comperable to being hit by a dumptruck at 65 miles an hour.”

They even showed holes and dents and cracked windows as the result of debris hitting the spaceshuttles while in orbit, you could probably google them if you want to find them.

By: hyakutake1957

hyakutake1957 — Fri, 22 Feb 2008 22:18:29 +0000

A very good question Marc!

I assume that the spacecraft that will be dropping these telescopes down to the surface of the moon will remain in orbit around the moon and serve as the relay for transmitting the information back to Earth.

Bill

By: Marc Linenberger

Marc Linenberger — Fri, 22 Feb 2008 22:00:51 +0000

I listened to Ms. Hewitt talking about LARC on NPR today and had a question I was hoping someone would ask but didn’t. Since the array will be on “dark” side of the moon how do plan to transmit data from the telescope since it won’t be in direct sight of the earth?

Thanks!

Marc