Putting a radio beacon on an asteroid
The asteroid named "the
Beast" is approximately a thousand feet wide, and is big enough to release
2,000 Megatons of energy if it collides with the earth. That could wipe out a
whole metropolitan city. Fortunately, it did not do that during its latest pass
on June 8, 2014. It is not expected to come dangerously close any time soon.
Officially, its name is 2014 HQ124. For further information, here are a couple
of URLs.
http://www.jpl.nasa.gov/news/news.php?release=2014-178
http://www.jpl.nasa.gov/news/news.php?release=2014-178
This time, the Beast passed
within 750,000 miles of the earth. Isn’t time that a technology is developed to
attach a transponder to such asteroids so that they can be tracked accurately
as far away as a hundred million miles? A good solution would be to have the
transponder orbit the asteroid in a fairly tight orbit. It would be necessary
for the transponder to orient itself in space and point an antenna towards the
earth for long distance communication. Would the development of such an orbiting
transponder be a good project for university students?
Why a transponder? Why not a simple beacon? A transponder allows fairly accurate distance measurement; but that may not be essential. A beacon that transmits at fixed intervals may be sufficient to let good tracking of the orbit to be done. I think that a major challenge is in designing a system to locate the direction of the earth and to keep a directional antenna pointed at it. Are there powerful signals sent out 24/7 from the earth that can be utilized to sense the direction of the earth from the asteroid? The Wikipedia gives a list of radio time signal stations around the world, including a 2000 KW transmitter working at 162 KHz in France and a 2.5 KW transmitter working at 20 MHz in the US. Obviously, the 20 MHz transmitter is attractive because one can design a highly directional antenna to work with it, but how do you design a simple mechanism to locate it in the first place and to re-acquire the signal if it is lost for a short time for some reason? One option seems to be to use a Sun seeker and to use rotation with an axis perpendicular to the plane of the solar orbit to acquire the earth signal. Assuming that the asteroid's orbit and that of the earth are not highly inclined to each other, the directional antenna sweeping the plane of the orbit can locate the earth. The satellite can then de-spin itself and point the antenna to wards the earth. The system should be able to work with a few different radio sources spread over the world.
Another challenge would be in
increasing the design life of the transponder to make it worth the effort of
launching it. Yet another challenge would be in having the transponder adjust
its orbit over the years, so that it does not crash, or get pulled away by the
earth or the moon during its future passes. Both challenges would require that
one does not depend upon rocket fuel being carried on board, because it would
run out rather soon. Considering the very low gravitational force involved, ionic propulsion is a possibility for making necessary changes to the orbit. Or, who knows, may be a good solution is to launch a one ton
tank of fuel, with a 20 Kg transponder tacked on outside!
Srinivasan Ramani
2 comments:
FM and TV transmitters usually operate at a higher power level and frequency. This makes for better transmission to space and for use with smaller antennas. But their radiation pattern, close to the horizon, probably makes them less detectable from space. Besides they do not offer stable operation over the years as a station might shut down or get sold. All this indicates that the earth-seeker on the spacecraft should be capable of looking for multiple signals instead of depending on a single one.
Srinivasan Ramani
What do you get out of branding an asteroid with a beacon? A beacon transmitting at a precise frequency would give us information through the Doppler shift of the received frequency. This will indicate quite accurately the moment-to-moment radial velocity, making tracking far more accurate. Automating the tracking of a large number of potentially dangerous objects would become easier and less subject to errors.
Srinivasan Ramani
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