New concept may enhance Earth-Mars communication
October 16, 2009
This is an end-on view of an alternative Mars/Earth communication relay architecture option, looking into the Ecliptic plane. Credit: Credits: ESA/University Strathclyde/University Glasgow
Direct communication between Earth and Mars can be strongly disturbed and even blocked by the Sun for weeks at a time, cutting off any future human mission to the Red Planet. An ESA engineer working with engineers in the UK may have found a solution using a new type of orbit combined with continuous-thrust ion propulsion.
The European researchers studied a possible solution to a crucial problem affecting future human missions to Mars: how to ensure reliable radio communication even when Mars and Earth line up at opposite sides of the Sun, which then blocks any signal between mission controllers on Earth and astronauts on the red surface. The natural alignment, known as a conjunction, happens approximately every 780 days, and would seriously degrade and even block transmission of voice, data and video signals.
The research findings were released this week at the 60th International Astronautical Congress (IAC), the world's biggest space event, being held in Daejeon, South Korea.
According to the paper, "Non-Keplerian Orbits Using Low Thrust, High ISP Propulsion Systems," an innovative solution to the Mars communication problem may be found by placing a pair of communication relay satellites into a very special type of orbit near Mars: a so-called 'B-orbit' (in contrast to an 'A-orbit', based on natural orbital laws).
However, to counter the effects of gravity and remain in place, they would have to be equipped with cutting-edge electric ion propulsion.
The ion thrusters, powered by solar electricity and using tiny amounts of xenon gas as propellant, would hold the satellites in a B-orbit in full view of both Mars and Earth. The satellites could then relay radio signals throughout the Mars-Earth conjunction season, ensuring that astronauts at Mars were never out of touch with Earth.
Johannes Kepler (December 27, 1571 - November 15, 1630) was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. This image is a copy of the 1610 original in the Benedictine Monastery, Krems. Credit: Credits: Artist unknown
François Bosquillon de Frescheville, based at ESA's European Space Operations Centre, Darmstadt, is co-author of the paper together with five engineers at the Universities of Strathclyde and Glasgow, Scotland. He agreed to answer questions on the results being presented by his colleagues at IAC.Q1. What is special about the orbital positions described in your paper?
Satellites usually follow Keplerian orbits named after Johannes Kepler, who helped discover over 400 years ago the basic mathematical equations that describe orbital motions.
Once it is launched, a satellite in unpowered free flight will essentially 'glide' through our Solar System following the troughs and crests of gravitational forces exerted on it by the Sun, the planets and other bodies, much like a surfer glides over wave tops and troughs as she surfs toward a beach. In fact, an unpowered satellite can't do anything but follow these swells of gravitational potential, which constrain its trajectory.
Q2. But if a satellite could generate continuous thrust, it could skip across these gravitational peaks and troughs?
Yes - it could jump, as you say, into another class of orbits - the B-orbits, or non-Keplerian orbits. But you have to provide some on-board means of generating a continuous thrust, pushing in a certain direction against residual gravity. Then, an entirely new set of orbital trajectories become available.
Q3. Why not simply use the thrusters that most satellites already have, like those on Mars Express or Venus Express?
Traditional thrusters use a lot of fuel, so we only fire them for short periods to kick the satellite into a new, free-flight orbit. It is prohibitively expensive in terms of weight to equip a satellite with continuous thrust capability.
But a solar electric propulsion system uses electricity generated from sunlight to emit chemical ions, giving a tiny thrust - about the same force that you feel if you blow on your hand - but over time, it's enough to move almost anything. ESA's SMART-1 got to the Moon in 2004 after 16 months using ion propulsion; its thruster only generated 0.2 millimetres per second per second of acceleration, but that's sufficient!
The trick is to find possible orbital trajectories in our Solar System where such a tiny amount of thrust, applied perpendicular to the direction of the satellite's motion, can usefully keep it in a certain location, supporting scientific observations or communications, for example.
Q4. And that's when you considered the Mars radio communication problem?
Yes. It has been known for some time that, due to the natural orbital motions of the Sun, Earth and Mars, any communication relay satellite that orbits Mars in a traditional, unpowered Keplerian orbit will, at some point, be blocked by the Sun. So it will never enable continuous communications between Mars and Earth for 100% of the time. That's not good for any astronauts on Mars.
What we have shown is that if you can provide continuous thrust, a pair of spacecraft could 'hover', respectively, over a point leading, and under a point trailing, the Mars orbit, and provide continuous radio communications between Earth and Mars. You would need two relay spacecraft to cover both halves of Mars.
You would get, in effect, full-time communications to almost anywhere on the Red Planet's surface. When the Earth-Mars conjunction season is over, the spacecraft could stop thrusting, save fuel and take up regular, unpowered or near-Keplerian orbits until the following conjunction approaches, and then take up their relay positions again for the next conjunction.
We found that a pair of relay satellites would only have to switch on their thrusters for about 90 days out of every 2.13-year period, and this solution would only increase the one-way signal travel time by one minute, so it could be effective.
This is an artist's impression of Mars Express. The spacecraft left Earth for Mars on 2 June 2003. It reached its destination after a six-month journey, and has been investigating the planet since early 2004. Credit: Credits: ESA - D. Ducros
Q5. Could such a double-spacecraft, 'continuous-thrust' mission be launched today?Well, most of the technologies are in place or are very close to being ready.
However, our research was only the first step in understanding the complex details of such a mission. A lot more work must be done to understand in detail how the satellites have to apply the thrust - for example, taking into account the natural eccentricity of the Martian orbit. Also, failure scenarios must be studied, to have a back-up plan in case one of the ion thrusters failed. In addition, as part of our research, we catalogued other possible mission profiles.
One example would be to use continuous thrust to create a fixed, virtual 'truss' between two spacecraft perpendicular to their flight direction. It would be like having the two spacecraft connected by fixed bar or rod; this could be useful for certain applications.
Another example would be to hover near one of the Earth-Sun system Lagrange points. NASA studied just such a mission profile, called GeoStorm, back in the 1990s with a view to stationing a satellite closer to the Sun than the L1 Lagrange point so as to provide improved early warning of magnetic storms caused by solar coronal mass ejections. Such a mission would have used a solar wind sail for its thrust, but it could also be done using ion propulsion, which can offer control advantages compared to solar sails; this must be studied further.
There's still lots to be done, but this research will help pave the way for future robotic missions to places we've never been or for a human mission to Mars.
Source: European Space Agency (news : web)
Mar 06, 2006 |
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Actually, I just thought of a more scalable solution yet. Park them at EARTH's Lagrange points, which would permit year-round communication to EVERY planet in the solar system (with the possible exception of Mercury).
--edit- I forgot to say that both of these solutions require exactly zero propulsion, once parked.
Although solar Lagrange points do not provide the shortest transmission distance, wouldn't they be able to maintain higher bandwidth during the solar eclipse, than direct (through the corona) transmissions that would probably have to reduce transmission speed (increase the error-correction) in order to overcome the signal disruptions?
If the solar disruptions reduces the bandwidth, it reduces the advantage of direct line of sight.
However, the Lagrange point satellites could use less thrust to remain in position and may have access to higher frequency transmissions (upto laser optical?) through 'empty space'
Due to the finite speed limit of light, no satellite anywhere could provide "real-time coverage" between Earth and Mars, although you do have a point about longer transmission times. And L4 and L5 do not "require some thrust due to perturbations from other planets." You're confusing them with L1 and L2.
Even a sufficiently large circular orbit around Mars would suffice (although you'd have to tailor the orbit to ease satellite-to-Mars comm).
Re-read the definition of L4 and L5 Lagrange points. They are the most stable of the Lagrange points. No power needed. Also, there's no reason you need the satellite to be stationary. It just needs to be in roughly the same position, so you know where to point your antennae.
There would still be frequent periods of no communication with Earth, because both Mars and the Sun would occlude it. Stable placement at L4 or L5 would guarantee that at least one of the satellites can see any other solar system object.
But the L4 and L5 points are only stable in an idealized two-body system, like the Earth-Sun system. When you consider the entire solar system's influence, the L4 and L5 points will experience small perturbations, which push the satellite away without orbital corrections.
Note that all Lagrange points are positions of unstable equilibrium! Satellites currently orbiting L1 require orbital corrections from time to time, and L1, L2, and L3 are only unstable in one dimension, whereas L4 and L5 are unstable in two.
Also, how do you bring your satellite to a standstill in relation to Earth (in order to be placed at L4/L5)? Another costly burn.
Just playing devil's advocate ;)
That's not true. L4, L5 and the Sun make up an equilateral triangle, so they are *half* that distance.
Besides, you would only need to use the L4/L5 units when Mars is actually occulted. That is the original problem trying to be solved.