A NASA Discovery mission to conduct the first orbital study
of the innermost planet
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Frequently Asked Questions
The Mission   |  The Journey   |  The Planet   |  The Science

The Science
Questions the mission hopes to answer

1. What are MESSENGER's scientific goals?

MESSENGER is designed to answer six broad scientific questions:

  • Why is Mercury so dense?
  • What is the geologic history of Mercury?
  • What is the nature of Mercury's magnetic field?
  • What is the structure of Mercury's core?
  • What are the unusual materials at Mercury's poles?
  • What volatiles are important at Mercury?

The "Why Mercury?" section of this Web site explains the rationale for these six questions.

2. How much data will MESSENGER send back?

The average data downlink rate while in Mercury orbit will be 15 megabytes (MB) per day, but the rate will vary enormously during the mission because of the large variation in distance between Mercury and Earth over one year. While at Mercury, MESSENGER will transmit data for about 80 hours per week. Depending on Mercury's location, signals from MESSENGER can take anywhere from 4 to 12 minutes to reach Earth.

3. Why has it taken three decades to return to Mercury?

Mariner 10’s flybys of Mercury raised intense scientific interest in a follow-on orbiter mission, but the planetary science community did not know how to achieve Mercury orbit with the spacecraft propulsion systems available in the 1970s. In the mid-1980s, clever mission designers discovered that Mercury orbit could be attained with existing propulsion systems by using multiple flybys of Venus and Mercury for gravity assists.

However, the mid-1980s were not a good time for proposing new planetary missions. A decision by NASA to launch all deep space missions on the Space Shuttle, followed by the Challenger accident in 1986, created a gap in new launches of U.S. planetary missions that lasted longer than 10 years.

It took the launch of the many missions in the queues at NASA and other space agencies, as well as further advances in materials engineering and thermal design, before a Mercury orbiter mission became feasible.

4. Why can't we learn what we need to know about Mercury using Earth-based telescopes, or maybe the Hubble Space Telescope?

Mercury is so far from Earth and so close to the Sun that astronomical observations from the ground or even from near-Earth space orbits are difficult. Its average distance from the Sun is 57 million kilometers (36 million miles), about two-thirds closer than Earth. The planet is only visible from Earth just after sunset or before sunrise, and astronomers have trouble observing it with telescopes through the haze of Earth’s atmosphere. Even the operators of the Hubble Space Telescope won’t view it, lest they risk significant damage to the telescope from looking too close to the Sun.

Ground-based observations have raised tantalizing questions about the planet – such as the discovery of the radar-reflective material, possibly ice, near the polar regions – but they cannot answer these questions. Many of the answers to the scientific questions require measurements at Mercury (such as characterizing the planet’s magnetic field) or long-term observations.

5. Until MESSENGER arrives at Mercury, where can I get pictures of the planet?

MESSENGER Science Team member Mark Robinson has processed and posted a great collection of Mariner 10 images on his Arizona State University Web site. You can also try the National Space Science Photo Data Center.

6. How will MESSENGER shed light on the evolution of the Solar System?

Mercury has the densest material of all the planets in the solar system (Earth is denser, but only because it is larger and gravity tends to compress it more). The problem is that we really do not know why. By studying all of Mercury (more than 50% of the planet's surface has not been seen by spacecraft) and mapping out the elemental composition and the minerals in surface material, we should be able to determine whether Mercury had most of its rocky crust and mantle blown off by a collision with another planet-sized body, whether much of the outer rocky part of the planet was evaporated and driven off by the heat of the early solar nebula of gas and dust, or whether the inner portion of the nebula from which Mercury formed was enriched in iron relative to the outer portions. If we can figure out which of these three ideas answers the question of how Mercury formed, we will know more about the early history of the inner part of the solar system at the time the planets were forming. That will, in turn, tell a lot about the later evolution of the solar system.

7. What will MESSENGER teach us about Earth?

Mercury is an "end member" of the terrestrial planets (Mercury, Venus, Earth, and Mars). Right now, we do not understand why the four are so different – for example, Mars is about half the size of Earth but has almost the same length of day. Venus is about the same size as Earth, but barely spins. Mercury is the smallest and takes 59 Earth days to spin once - but, like the Earth, and unlike Venus and Mars, Mercury has a magnetic field. Earth is the only planet with water (Venus and Mars may both have once had oceans, but the surface water disappeared long ago). To really understand Earth and how it became the way it is today (rather than going the way of Mars or Venus), we need to understand how all of the terrestrial planets formed and, because it is the extreme case, Mercury is the key to that understanding.

If you have a question that has not been answered here, we invite you to submit it to us.
We also welcome questions and comments on the MESSENGER Website. Send a note to the Webmaster or check our Contacts page.


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