Tracking spacecraft from Earth
Posted by Emily Lakdawalla
2008/12/02 03:56 CST
Shortly after Chandrayaan-1 launched I received an email from a skeptical Indian saying "how can we really be sure that the spacecraft is as successful as the government is telling us it is?" I don't believe that the question was from the point of view of the nutty conspiracy theorists that tend to haunt public discussion of space exploration (e.g. the "we didn't land on the Moon" people) -- it seemed more like the healthy sort of doubt any citizen should have about the claims of his government, particularly in the context of such a high-stakes announcement as the first successful journey for that country beyond Earth orbit.
Governments do lie, and it's certainly not uncommon for governments to hype good news and hide bad news. But in this particular case, I can reassure the questioner that Chandrayaan-1 definitely launched successfully, entered lunar orbit when ISRO said it did, and has been returning data from the Moon. These are actually two separate issues -- whether the spacecraft really is in orbit and traveling where it's supposed to be, and whether it's returning data as expected.
We know where spacecraft are in space because all deep-space spacecraft must communicate with Earth using radio. (Laser communications are in development but are in an experimental stage at this moment in time.) The way that radio transmissions work -- speaking at the basic level at which I understand things -- is that the transmitter broadcasts a signal, called the carrier signal, at a very specific radio frequency, and that the data is sent back through modulations of that signal's amplitude, frequency, or phase. You need to know a lot about how the spacecraft's computer is encoding its data in order to be able to "decode" the data carried within the radio signal, but the carrier signal is basically a beacon shouting away from the spacecraft, seen from Earth as something like a faint star twinkling in the radio sky. Anybody with a sufficiently sensitive radio antenna can see that "star," even though they may not be able to understand what the star's twinkling means in terms of the data being returned to Earth.
Who has sufficiently sensitive radio antennas? For starters, every space agency. NASA has its Deep Space Network in California, Spain, and Australia. ESA has its own deep space antennas in Spain and Australia. India built its own for Chandrayaan-1, and China and Russia have their own as well. When a spacecraft sends a signal to Earth, any radio antenna belonging to any country that's pointed in the right direction can at least pick up the carrier signal, which allows them to pinpoint the spacecraft's location and speed in the sky. Because the force of gravity is in strict control of the motion of spacecraft, it would be impossible for one government to fake a signal, making an Earth orbiting satellite appear to be broadcasting from the Moon; the direction and speed of spacecraft motion, measured using the Doppler shift of the signal, would be wrong, not matching what equations of mechanics would tell you about the spacecraft's predicted motion.
Space agencies aren't the only people with sensitive radio antennas, though. There are a lot of radio scientists who use other dishes to study the sky for scientific purposes, and they routinely pick up signals from distant spacecraft (in fact, I'm told that the SETI Institute often looks for the most distant still-broadcasting spacecraft -- Voyager 1 -- when testing their detection equipment on the Allen Telescope Array). SETI Institute software engineer Tom Kilsdonk also pointed me to an amateur group in Germany who have actually managed to track Voyager 1, using a 20-meter dish. This feat is quite a bit out of the range of possibility for most amateurs, but spacecraft nearer to Earth are often detected by individuals who track deep space probes with private dishes; Kilsdonk pointed me to a newsgroup where many such individuals exchange information.
Governments have to be in close cooperation for one to easily be able to decode the data carried within a spacecraft's radio signal. But it's actually quite normal for the deep space communications networks of all (well, most) of the different space agencies to cooperate to support planetary missions, particularly during critical moments, such as when a spacecraft enters orbit. For example, the NASA Deep Space Network station in Canberra, Australia has been working with India for years to prepare to support that nation's first mission to the Moon, and is now in regular contract with Chandrayaan-1.
So the spacecraft is certainly where ISRO says it is. How about that data? I can't tell you anything about the quantity of data being returned -- like most space agencies, ISRO only releases tidbits in the beginning, with more coming later as the scientists have time to study it and prepare publications about it. What I can tell you, though, is that Chandrayaan-1's science team is truly international. India built five of the eleven instruments itself, but offered space on the bus for six other research groups to build and support their own instruments. Those groups are based in Bulgaria, Germany, Sweden, the United Kingdom, and the United States. To fake the scientific success of such an international mission would require collusion of all these different groups in all these different countries, and that just strains credulity.
So I'm going to continue to believe that Chandrayaan-1 is as successful as ISRO says it is -- just holding on to a little bit of skepticism about the possibility of any accomplishments being perhaps a little more hyped than they might deserve, something that every space agency does -- unless someone provides me with evidence to the contrary.
Or read more blog entries about: