Astronomy Down Under

Broadband… in Mars

6 Oct at 11:15 by Wilson

Launched on 12 August and scheduled to arrive to its destination on 10 March 2006, the Mars Reconnaissance Orbiter will bring something very important to Mars: high-bandwidth communications.

The MRO will spend its first six months in Mars slowly adjusting its orbital path, doing several aero-braking manoeuvres over more than 500 orbits until it settles on a path 300 km above the surface. From there, its high-resolution cameras will be able to photograph features that are less than one meter wide. And, when I say high resolution, I mean it: the main optical instrument, named HiRISE (High Resolution Imaging Science Experiment), is actually a 20-inch telescope coupled to a 800-megapixel digital camera. As an example, the photo on the left is the Earth’s Moon seen by that camera from a distance of 10 million kilometres.

This will, of course, generate huge amounts of data to be sent to Earth. Due to the large distances involved and the difficulty of sending high-powered equipment to space, exploration vehicles have always returned data to Earth somewhat slowly. One of the more recent missions, Europe’s Mars Express (which is used to relay part of the data generated by the NASA surface rovers Spirit and Opportunity), sends data at no more than 200 kilobits per second (and as little as 30 kilobits per second, depending on conditions). The previous rover mission, Pathfinder, sent data at 1 to 2 kilobits per second, and the even older Voyagers managed up to 7.2 kilobits per second (as they didn’t have to land, they had larger antennas and heavier transmitters).

Clearly this would be less than appropriate for a spacecraft that can generate the equivalent of a 1,000 megapixel image every three seconds (and that is not counting the other instruments on board, plus occasional data relayed from the surface rovers — current and future). For this reason, MRO was fitted with a telecommunications system that includes a very large antenna (3 meters across), a fast computer and a high-powered (100 watt) transmitter which will allow it to send data at up to 6 megabits per second. Considering that this data will be sent over more than 100 million kilometres, that’s very impressive.

Together with this, the craft has also a second transmitter that will be used to test communications with Earth over a different range of frequencies (32 Ghz - Ka Band - instead of the usual 8 - X Band); if the test is successful, the new band can be used in future missions to increase even more the data rate of transmissions. The reason the higher frequencies aren’t used already is that transmissions in that range are more “fragile”, and are especially sensitive to moisture in Earth’s atmosphere (that is, if it rains, you’re out of luck). Better equipment and higher power may be enough to overcome these problems, and that’s what will be tested.

During its full mission (which lasts until 2010), MRO is expected to send 34 terabits of data to Earth; that is more than the amount of data returned by all previous JPL missions put together. It is expected to increase our knowledge of the red planet by orders of magnitude, and open the way to future scientific missions already being developed.