You may have read that the Curiosity Mars rover is communicating with Earth mostly by relaying data through one of the spacecraft orbiting Mars, and wondered why that is so. Even if you haven’t wondered, I have! Hence this post.
There are multiple reasons, of course, but the most important one is energy conservation. Even with the nuclear power pack that Curiosity carries, there are at most around 125 watts of electrical power available to drive the rover and to operate its scientific instruments. And even though the rover has a high-gain (meaning directional) antenna that can be aimed at Earth, it doesn’t have enough power to transmit data at a very high rate. If we devoted enough power to “yelling” loudly enough to be heard on Earth, we would have correspondingly less power available for other tasks. Earth, after all, is currently over 158 million miles (255 million kilometers) away! But the main orbiting relay station, the Mars Reconnaissance Orbiter (MRO), can pass over the orbiter as closely as 200 miles (320 km). Curiosity can send data at rates up to 2 megabits per second to the MRO without having to consume so much power. For comparison, when I just now tested my cable modem’s upload speed, it clocked in at 1.46 megabits per second.
And the MRO has lots of power and a honking big high-gain antenna: 2000 watts of power (solar cell panels) and a nearly 10-foot (3-meter) diameter antenna. It therefore is the better tool for sending the data back to Earth. A second orbiter can also be used; Mars Odyssey has less power and a smaller antenna, but is still better than direct communication with Earth by the rover.
The other main reason for the satellite relay is the time during which the Earth is in view. Mars rotates on its axis in roughly the same time as the Earth: 24 hours and 37 minutes for Mars. The rover would be unable to communicate with Earth for half of that time, but the orbiters have Earth in view for about 16 hours a day.
The images sent so far are mostly “thumbnails”—low-resolution versions of the higher-resolution images to be uploaded later. These are exactly analogous to the icons you see on your computer desktop when you open a folder full of digital images from your beach vacation. Clicking on the icon on your computer opens up the full-resolution image. Here is one such thumbnail image, showing the surface and one of the rover’s wheels:
This color image is 144 by 144 pixels, 45.7 kilobits. Given that each satellite pass over the rover allows communication for about eight minutes, the rover could upload as many as 21,000 of these images in one pass! Of course, it hasn’t taken anywhere near that many images yet, and there are far more data to be uploaded besides just images. A single full-resolution color image is 1200 by 1200 pixels, roughly 3.2 megabits. You do the math!
The presence of these orbiters at Mars has allowed us to “offload” some of Curiosity’s communication tasks to other spacecraft whose primary missions have long been completed. A smaller antenna means more room for scientific instrumentation. In the next post, I’ll talk about how those instruments may lead us toward the discovery of life on the red planet.