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 Journey
Technological challenges of the mission, including the spacecraft's trajectory and orbit

1. When, and from where, did MESSENGER launch?

The spacecraft launched on August 3, 2004, aboard a Boeing Delta II rocket from Cape Canaveral Air Force Station, Florida.

2. How long will it take MESSENGER to get to Mercury?

MESSENGER will travel more than six and a half years before it begins to orbit Mercury in March 2011. This journey includes a flyby of Earth (in August 2005), two flybys of Venus (October 2006 and June 2007) and three flybys of Mercury (January 2008, October 2008 and September 2009). Visit the Mission Design section for details on MESSENGER’s journey.

3. What is a flyby - and why do it?

During a flyby a spacecraft flies near the planet but does not land on it or enter an orbit around it. Flybys are done because:

  • The spacecraft doesn’t have enough fuel to go into orbit or make a safe landing (like Mariner 10’s flybys of Mercury); or
  • The spacecraft will continue to other destinations (such as the Voyager flybys of Jupiter, Saturn, Uranus, Neptune); or
  • The spacecraft needs a “gravity assist” from the planet, changing its trajectory without using much fuel (such as the New Horizons flyby of Jupiter in February 2007).

4. Will MESSENGER return any science during its flybys of Venus and Mercury?

Absolutely. The Venus flybys provided important opportunities to calibrate MESSENGER’s instruments on the way to Mercury and to make new scientific observations of Earth’s “sister planet.” The team imaged the upper cloud layers at visible and near-infrared wavelengths for comparison with ongoing observations by the European Space Agency’s Venus Express mission, and laser ranging to the cloud tops was attempted. Magnetic field and charged particle observations  permitted searches for solar wind pick-up ions, and ultraviolet (UV)-visible spectrometry observations are being used to check for changes in the composition of the upper atmosphere.

Flying past Mercury will allow MESSENGER to image most of the hemisphere Mariner 10 was not able to view (because it was in darkness during each of the three Mariner 10 flybys), and even the hemisphere viewed by Mariner 10 will be imaged at higher resolution. MESSENGER will also map nearly the entire planet in color and measure the composition of the surface, atmosphere, and magnetosphere. These data will help the MESSENGER team plan the orbital mission.

5. Why take such a long and complex route to Mercury?

Although it is not that difficult to fly a spacecraft by Mercury, the task of placing a spacecraft into orbit around the planet is a significant challenge. When designing the spacecraft's journey to Mercury orbit, mission planners must consider limits on technology and cost; the spacecraft’s speed when leaving Earth orbit; the help along the way from flybys of Venus and Mercury; the type of fuel on the spacecraft; and a time limit for completing the trip. It is much more difficult to leave Earth and orbit another planet than to fly by the planet, because the spacecraft must make large changes to the shape, size, and tilt of its orbit around the Sun. The goal is to lower MESSENGER’s speed relative to Mercury's orbit speed around the Sun and set up a maneuver in which the spacecraft can be “captured” into orbit around the planet.

6. What is a gravity assist?

A gravity assist changes a spacecraft's orbit around the Sun by having it fly by a planet. During the flyby, linear momentum is transferred from the planet to the passing spacecraft. In effect, the spacecraft either takes from (if it needs to accelerate) or gives to (if it needs to decelerate) the planet a small amount of linear momentum.

Because the effect of the added/reduced linear momentum depends on the mass of the object, this process has a negligible effect on the planet's orbital velocity around the Sun. On the other hand, the spacecraft, being much smaller, receives a great boost (or brake) on the way to its next destination.

Additional factors that affect the acceleration (or deceleration) and direction change of the spacecraft trajectory are the closest approach distance between the spacecraft, the mass of the planet, and the speed of the spacecraft relative to the planet. The greatest change in the spacecraft's speed and direction would occur for a slow-moving spacecraft approaching just above the surface or cloud tops of a very massive planet. The least change in the spacecraft's speed and direction would occur for a fast-moving spacecraft approaching a great distance from a very small planet.

Click here for specific information about gravity assists during the MESSENGER mission.

7. Why have there been so few missions to Mercury?

The energy required to send a spacecraft so close to the Sun is significant, and an orbital mission only raises the requirement. Using the gravity assist of inner planet flybys reduces the amount of fuel needed onboard, making the mission possible with current technology. (In fact, the precise trajectories available for a Mercury orbiter were not known until the mid 1980s.)

Once the spacecraft arrives it meets a hostile environment, so it must be designed to withstand high radiation and temperatures. MESSENGER is the first spacecraft for which the technologies needed for a Mercury orbital mission were available and combined in an effective manner.

8. How close to Mercury will MESSENGER come during orbit?

MESSENGER’s highly elliptical orbit will bring it as close as 200 kilometers (about 125 miles) from the planet’s surface, and as far out as 15,193 kilometers (9,420 miles). The inclination – the angle between the orbital plane of MESSENGER and the equatorial plane of Mercury – is 80°, and the lowest point of the orbit is reached at the latitude 60° North on Mercury’s surface.

MESSENGER will orbit Mercury twice every 24 hours. For eight hours of the second 12-hour orbit, the spacecraft will be oriented for sending data to Earth. The MESSENGER team designed the orbit to optimize the scientific yield of the mission and data transfer to Earth, while addressing thermal environment concerns.

9. How long will MESSENGER take to gather data on Mercury's entire surface?

The answer depends on the scientific instrument. MESSENGER will have imaged most of the surface after the three Mercury flybys. It will have flown over every part of Mercury (except a small circular area at each pole, which must be viewed obliquely) after three months in orbit, and the team will have viewed – in sunlight – the entire planet after six months in orbit. Stereo imaging, as well as our best global models of surface chemistry, internal magnetic field geometry, and the planetary gravity field, will come only after the full Earth year in orbit.

10. Why isn't MESSENGER a manned mission?

The space environment near Mercury is hostile to living organisms due to its proximity to the Sun. Even unmanned, the spacecraft electronics and scientific instruments must be shielded against heat and radiation. Protecting a manned crew in such a hostile environment would require extensive shielding. Furthermore, a manned mission would require the return of the spacecraft to Earth. All of these considerations would raise the fuel requirements to levels that would make the mission currently impossible.

11. Will MESSENGER return to Earth?

No. The spacecraft will eventually impact Mercury's surface, several years after the mission officially ends and spacecraft operators lose ability to maintain MESSENGER's minimum altitude within the range adopted for the mission's orbital phase.

12. How far -- and how fast -- will MESSENGER travel?

Our intrepid spacecraft will have traveled 4.9 billion miles (7.9 billion kilometers) from its launch in summer 2004 until insertion into Mercury orbit in March 2011. During the yearlong Mercury orbital phase of the mission, the probe will travel 22.7 million miles (39.9 million kilometers) around Mercury.

In the course of its journey to Mercury orbit insertion, MESSENGER’s average speed will be approximately 84,500 miles per hour (nearly 38 kilometers per second) relative to the Sun. MESSENGER's maximum speed relative to the Sun will be about 140,870 mph (nearly 63 km/s).

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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