A brief follow-up to What will MOM do when Earth isn't looking?

As already premised in the referenced question; terrestrial artifacts on/around Mars encounter a communications blackout every 26 months, or so. This article conveys the blackout may be more of a precautionary note to avoid a corrupt command from throwing a spoke in the wheel.

Is there, today, a relatively error-free means to communicate with orbiters/rovers on the Martian surface whilst Sol occults Earth/Mars?

p.s. The impression from the answer chain indicates it is possible to bounce signals off other craft, and may have been done during the Apollo program. Perhaps other long-term missions too. As a follow-up question - Have there been any instances of a terrestrial robot on/around Mars actually communicating thus? What was/were the relay/s involved, and the circumstances under which it was done?


For a direct opposition, no. You could use a relay probe, or, if you have enormous amounts of transmission power at your disposal, bounce a signal off another planetary body.

But it rarely happens that Mars is actually occluded by the sun, because the orbits are ever so slightly inclined to each other. And if it does, it only lasts a short while.

But the Sun is surrounded by lots of charged particles that it emits, the heliosphere. It extends into interplanetary space, but it's most dense near the sun. This makes it more challenging to get signals through during any opposition. More power and heavy error correction helps, of course.

So it's not a question of "can you get a signal through?" but "how long is the blackout and how large is the antenna?", both on earth, and on the probe.

  • $\begingroup$ Does the Sun emit strong radiation at all wavelengths? $\endgroup$
    – LocalFluff
    Oct 3 '14 at 6:43
  • 1
    $\begingroup$ Compared to a 100 W transmitter further away, yes. Look up black-body radiation. $\endgroup$
    – Mark Adler
    Oct 3 '14 at 8:07

No. Communications between Earth and assets at Mars are suspended for three weeks or so around solar conjunction, depending on the angles.

There is no real danger of corrupt commands as they are checked for errors. What would happen if you tried is no commanding.


Direct communication between two planets while on opposite sides of the sun is nearly impossible since the signal gets swamped out by noise from the sun. In principle, of course, just as today we can bounce signals off communication satellites in orbit around Earth and communicate between opposite sides of the planet, someday we could set up satellites in orbit around the sun to relay signals.

Terminology note: The term for that, when Mars and Earth are on opposite sides of the sun, is "conjunction", and the term for Mars and Earth being close together is "opposition". Personally this makes no sense to me intuitively, but it probably made sense to early astronomers who thought of Earth as being the center of the universe. Since it was astronomers that proved otherwise, by definition astronomy started at a time when everyone thought in geocentric terms. And in all fairness, almost all telescopes are still on Earth or in orbit around Earth, and the problem really is that the sun is too close to Mars from the point of view of the Deep Space Network.

  • $\begingroup$ Uh. Apposition, or Opposition? $\endgroup$
    – Everyone
    Oct 3 '14 at 6:40
  • $\begingroup$ Opposition. "In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place (usually Earth)." $\endgroup$ Oct 4 '14 at 0:26

I like the answers already posted, but I'll add an example (which you've probably heard of) to demonstrate that while it would definitely be impossible, it really wouldn't be a problem.

During the days of the Apollo program, early missions to the moon did not actually land on the moon, but merely orbited it (although many times). The flights, among other things, helped pave the way for the now more-famous landings on the moon. There was one important thing that affected each mission, though. In order for the craft to orbit, it obviously had to go around the back of the moon. This led to a notorious period of radio silence, which was dramatized in Apollo 13. The astronauts (I'm no longer talking about the movie) were able to get along just fine so long as no problems occurred onboard.

We can see that it is impossible to communicate between objects on opposite sides of the Sun for the same reason that made Deke Slayton just a little more nervous during each mission. Clearly, autonomous landers, probes, or other robots on Mars (or anywhere else, for that matter) would be just fine without human intervention for a while.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.