In November 2011, our modern WALL-E was launched, the Mars Science Laboratory “Curiosity” (MSL), to the red planet, landing 10 months later with the revolutionary system Skycrane and becoming the most successful achievement of the joint effort of NASA and the International Community.
Mars is the fourth planet of our solar system, with half the radius of earth and an orbital period of 687 days. Needless to say our neighbor is extremely far, and without space highways, travel all the way there is a hard mission. Mars orbital period doubles the terrestrial one, this way in a mars revolution two perigees occur (moment when the planetary distance has the minimum value), being those the optimal moments to take a look. Because if we’re going to send a very expensive robot to look for little martians, at least we should have a communication channel with him.
Since 1887 when Heinrich Rudolf Hertz achieved the first radio communication in history, we have been using electromagnetic waves to send information through the atmosphere and space. These waves spread in the vacuum at the speed light and apparently following a rectilinear light path (it actually bends a little because of the effects of bigs amounts of mass near the path light, according to the General Relativity), so reaching Mars at that speed doesn’t seem complicated. But in fact it is more complex that it might seem.
- Planets orbit: planets describe an ellipse being the sun one of the focus, and since the revolution period of each planet is different, distances between them change considerably along their orbits. Earth-Mars minimum distance is 56 million km (perigee), and E-M maximum distance is 399 million km (apogee). Mars relative motion regarding Earth makes significant the Relativistic Doppler Effect in transmission.
- There are transmission problems in communicating with a distant objective, since the power of the radiated signal decreases based on the distance according to the expression 1/r^2, so the broadcast power needs to be huge in order to make the reception possible.
- The radiated signal is contaminated with electromagnetic noise from the sun, giant gaseous planets like Jupiter, human radio broadcast…
As we can see, space is an inhospitable and dangerous place for our brave bits. Now that we know what we’re facing, how do we communicate with our bold interplanetary adventurer?
The MSL has two communication systems:
- X-band communication (about 10Ghz) used for direct communication with Earth (Direct From Earth (DFT) or Direct To Earth (DTE)). The MSL has two antennas for this type of communication, one is a low-gain antenna (LGA), and the other one is a high-gain antenna (HGA). The transmission window is open for a few hours a day due to the need of a direct vision with Earth. These communications are slower because of distance between Earth and Mars (225 million km average) and the presence of the atmosphere:
- DTE communications are the slowest because of the low transmission gain available on board (15 W). The HGA transmit at a few hundred bps, and the LGA at a few tens bps.
- DFE communications are faster since we use huge antennas up to 70 meters high with great directivity located on Earth, which transmit with higher gain.
- Short-range UHF communication (about 1 GHz), used for communications with satellites orbiting Mars. These satellites have an 8 minutes transmission window twice a day with the Curiosity. Transmission rates of the UHF line are about 8 kbps.
Now we know the tools that the Curiosity uses for communicating, however, to achieve a better data transmission rate, two satellites orbiting Mars are used, which because of their trajectory are able to communicate with Earth 16 hours a day.
- Mars Reconnaissance Orbiter: This satellite communicates with DSN antennas in X-band and Ka-band, with transmission rates about 0.5-4 Mb/s. Currently, the MRO has already delivered more than 50 Terabytes of data, more than all the other interplanetary satellites together!
- Mars Odyssey: It works in X-band for communication from Earth, and UHF for communication with the rover Odyssey and any other rover in the Mars surface, like the Curiosity. It works from 128kbs to 256kbs.
Now that we know the whole family, we are ready to see a picture of an amazing red sunrise in our neighbour planet, but, how do we receive this information?
For commands and work data of the MSL, X-band communication is used. The rover sends data for about 10 minutes per connection. Then the data takes 14 minutes average to reach Earth, where it is received by NASA Deep Space Net (DSN). If we consider 14 additional minutes of response, we’re talking about 38 minutes only for sending a command to the rover.
For information like photos or experimental data, UHF communication is used, which considering the transmission rate of the system can delay a few days the reception of a single photo.
To sum up, this is how our little WALL-E talks to us.
I can only finish this article mentioning three special dates:
- 17th December 1903 – Wright Brothers: First flight with an airplane heavier than air.
- 20th July 1969 – Neil Armstrong: First man on the moon.
- 6th August 2012 – Curiosity lands on Mars.
Rubén Suárez
“Any sufficiently advanced technology is indistinguishable from magic”
—Arthur C. Clarke