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Solar conjunction

Space and time · level 6, lesson 5 of 6

A question to start

Every couple of years, NASA stops sending commands to every spacecraft at Mars for a week or two. Nothing is broken. What is going on?

The idea

About every 26 months, Earth and Mars are on opposite sides of the Sun. This is solar conjunction. Seen from Earth, Mars slips behind the Sun's glare.

The Sun throws out hot, electrically charged gas from its outer layer, the corona, and it reaches far into space. When a radio signal passes close to the Sun, this gas can scramble it. A scrambled command could make a spacecraft do something nobody intended.

So engineers declare a command moratorium: no commands are sent while Mars is close to the Sun in the sky. Before it starts they give each spacecraft a simple to-do list. Rovers stop driving. Orbiters keep collecting data, and send it home properly once conjunction ends.

In 2019 the moratorium ran from 28 August to 7 September.

Earththe Sun and its coronaMarssignal

Worked examples

1. How far? At conjunction, using circular orbits, Mars is about \(1 + 1.52 = 2.52\) au away, about 377 million km.

2. The delay then. \(377{,}000{,}000 \div 300{,}000 \approx 1{,}257\) s, about 21 minutes one way. Long, but that is not the problem; the corona is.

3. How often? NASA says "every two years". The exact gap is the synodic period of Mars, about 780 days, nearly 26 months.

Try it

  1. What is solar conjunction?
  2. Why is it risky to send commands then?
  3. What do rovers do during the moratorium?
  4. If conjunction happened in January of one year, about when is the next one?
Answers
  1. When Earth and Mars are on opposite sides of the Sun.
  2. Gas from the Sun's corona can scramble radio signals, so a command could arrive corrupted.
  3. They stop driving and carry out a simple list of tasks sent beforehand.
  4. About 26 months later: around March, two years on.

Common mistake

"Conjunction is when Mars is closest." It is when Mars is farthest, on the far side of the Sun. Closest is opposition.

Where it leads