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Relays in orbit

Space and time · level 6, lesson 4 of 6

A question to start

Perseverance has a radio that can talk straight to Earth. So why does almost all its data go up to a spacecraft in orbit first?

The idea

Sending data across hundreds of millions of kilometres needs a big antenna and a lot of power. A rover has neither to spare. And as Mars turns, the rover's side faces away from Earth for about 12 hours each sol.

So rovers talk to orbiters passing a few hundred kilometres overhead, at up to 2 million bits a second. The orbiters have bigger radios, more solar power and more time in view of Earth, so they pass the data on to the giant dishes of NASA's Deep Space Network.

The Mars Relay Network is four orbiters: NASA's Mars Odyssey and Mars Reconnaissance Orbiter, and ESA's Mars Express and Trace Gas Orbiter. The Trace Gas Orbiter, on average, carries the most relay data each day. Every picture taken on the surface of Mars since 2004 has come home this way.

orbiterroverdish on Earthmillions of km

Worked examples

1. One pass. An orbiter is in view for 10 minutes at 2 million bits a second. \(600 \times 2{,}000{,}000 = 1.2\) billion bits, which is 150 million bytes, or about 150 MB.

2. The extra delay is tiny. An orbiter 400 km up adds \(400 \div 300{,}000 \approx 0.0013\) s to the trip. The real wait is for the orbiter to come overhead.

3. Why not straight to Earth? The rover's side of Mars faces away from Earth for about 12 hours a sol, and a direct link needs far more power for each bit.

Try it

  1. Give two reasons rovers send data through orbiters.
  2. How fast can a rover send data to an orbiter?
  3. Which orbiter carries the most relay data on average?
  4. An orbiter pass lasts 8 minutes at 2 million bits a second. How many MB is that? (8 bits = 1 byte.)
Answers
  1. Any two of: the rover has little power and a small antenna; it cannot see Earth for about half of each sol; orbiters have bigger radios and more contact with Earth.
  2. Up to 2 million bits a second.
  3. ESA's Trace Gas Orbiter.
  4. \(480 \times 2{,}000{,}000 = 960\) million bits, which is 120 MB.

Common mistake

"Relays make the delay longer." They add only a fraction of a second of travel. What they change is when data can be sent: only while an orbiter is overhead.

Where it leads