Countdown to Mercury

Artist's impression of MESSENGER spacecraft in orbit at Mercury

Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury.

  • Mercury is the smallest, hottest and densest planet in the Solar System
  • MESSENGER spacecraft has travelled 7.6 billion kilometres to reach it
  • Will study the planet’s surface, thin atmosphere and geologic history

NASA’S MESSENGER SPACECRAFT is scheduled to slide into orbit around the closest planet to the Sun on March 18 (Sydney time zone). The mission is an effort to study Mercury’s geologic history, magnetic field, surface composition and other mysteries.

The findings are expected to broaden our understanding of rocky planets, more and more of which are being discovered in other Solar Systems.

At 11:45am on March 18, Sydney time (8:45pm US EDT on March 17) the MESSENGER spacecraft will execute a 15-minute braking manoeuvre that will place it into orbit around Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet.

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

MESSENGER image of Mercury

It might look like the Moon, but Mercury is very different. It is the smallest, hottest and densest planet in the Solar System.

Mercury is an extreme among the rocky planets in our Solar System—it is the smallest, the densest (after correcting for self-compression) and the one with the oldest surface and largest daily variations in surface temperature and the least explored.

Understanding this “end member” among the terrestrial planets is crucial to developing a better understanding of how the planets in our Solar System formed and evolved.

“Now that so many new planets are being discovered around stars in other Solar Systems, we need to know the effects of space weathering on rocky surfaces so we can accurately interpret telescopic and other remote sensing data we obtain from other rocky or dusty worlds,” says Ann Sprague, a research scientist at the University of Arizona’s Lunar and Planetary Laboratory.

The heat is on

When MESSENGER streaked into the early morning sky over Cape Canaveral on August 3, 2004, very little was known about Mercury. No spacecraft had approached the planet since the Mariner 10 space probe performed three fly-by manoeuvres over the course of 1974 and 1975, imaging the planet’s surface. However, Mariner 10 sent back photos of only one side of the planet, leaving the other shrouded in mystery.

One of the mysteries scientists are hoping to solve with the MESSENGER mission surrounds Mercury’s magnetic field. At a diameter only slightly larger than that of the moon (about 4,800 kilometres), Mercury should have solidified to the core. However, the presence of a magnetic field suggests the planet’s innards are partially molten.

Artist's impression of a rupes (cliff) on Mercury

Artist's impression of a rupes (cliff) on Mercury. These giant cliffs are believed to have formed when Mercury¹s interior cooled and the entire planet shrank slightly as a result.

During its long journey toward Mercury, MESSENGER passed the planet several times, filling in the imaging gaps left by Mariner 10.

Now, the entire planet with the exception of about five percent has been observed. MESSENGER will focus its cameras on getting the best possible images of the remaining portions, mostly in the polar regions.

One of the great challenges MESSENGER will face is the intense heat due to Mercury’s proximity to the Sun. At the planet’s equator, surface temperatures become hot enough to melt lead. The heat reflected from the planet’s surface is so intense that the spacecraft’s instruments need to be shielded against the glare.

Follow the live webcast of MESSENGER’s arrival from 10:55am Sydney time on Friday, March 18 (7:55pm US EDT on March 17): MESSENGER arrival webcast

Adapted from information issued by the University of Arizona. Images courtesy NASA / JHU APL / CIW. Rupes artwork: Michael Carroll/Alien Volcanoes by Lopes and Carroll, The Johns Hopkins University Press, 2008.

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