Fingers Crossed

I've been meaning to mention this for a while, but all sorts of news has been all sorts of crazy for the past week and more. On top of that, Interzone has (finally) switched ISP's, but the wifi is still buggy for long stretches at a time. To describe browsing as glacially slow would malign the celerity of glaciers. But in about two hours, the MESSENGER probe will fire its main engines for 15 minutes, allowing itself to be gravitationally captured, and become the first man-made object to orbit Mercury. The Planetary Society Blog has the time line and web resources (as well as the image above at a larger size) to follow it live, if that's your thing.

Even though Mercury is one of the closest planets, it's poorly known and studied. It's too close to the sun, from the earth's point of view, to get detailed telescopic images, and it's so deep in the sun's gravity well that spacecraft from earth have gained enormous speed after "falling" toward it from earth. That speed has to be nearly matched to Mercury's for the probe to orbit the planet. The MESSENGER probe has used a series of planetary flybys, over fifteen orbits of the sun, to bleed off some of its excess speed. Tonight's burn will be the conclusion of one voyage and the beginning of another, if all goes well.

Mercury is an important key to understanding the early evolution of the solar system, and I have little doubt there will be images and discoveries that flabbergast me. I'm pretty excited about this, and I'm keeping my fingers crossed.

Followup, 6:50 PM PDT: Reports coming from NASA say the orbital insertion maneuver has been successful; expect first imagery in a couple of weeks. Congratulations, NASA and MESSENGER!

Followup 2, 7:28 PM PDT: The MESSENGER news service I subscribe to just sent me this report, titled "MESSENGER Begins Historic Orbit around Mercury" Full text of report:

At 9:10 p.m. EDT, engineers in the MESSENGER Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., received the anticipated radiometric signals confirming nominal burn shutdown and successful insertion of the MESSENGER probe into orbit around the planet Mercury.

The spacecraft rotated back to the Earth by 9:45 p.m. EDT, and started transmitting data. Upon review of these data, the engineering and operations teams confirmed that the burn executed nominally with all subsystems reporting a clean burn and no logged errors.

MESSENGER’s main thruster fired for approximately 15 minutes at 8:45 p.m., slowing the spacecraft by 1,929 miles per hour (862 meters per second) and easing it into the planned eccentric orbit about Mercury. The rendezvous took place about 96 million miles (155 million kilometers) from Earth.

“Achieving Mercury orbit was by far the biggest milestone since MESSENGER was launched more than six and a half years ago,” says MESSENGER Project Manager Peter Bedini, of APL. “This accomplishment is the fruit of a tremendous amount of labor on the part of the navigation, guidance-and-control, and mission operations teams, who shepherded the spacecraft through its 4.9-billion-mile [7.9-billion-kilometer] journey.”

For the next several weeks, APL engineers will be focused on ensuring that MESSENGER’s systems are all working well in Mercury’s harsh thermal environment. Starting on March 23, the instruments will be turned on and checked out, and on April 4 the primary science phase of the mission will begin.

“Despite its proximity to Earth, the planet Mercury has for decades been comparatively unexplored,” adds MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “For the first time in history, a scientific observatory is in orbit about our solar system’s innermost planet. Mercury’s secrets, and the implications they hold for the formation and evolution of Earth-like planets, are about to be revealed.”

Mercury Flyby

The MESSENGER (MErcury Surface, Space Environment, Geochemistry and Ranging; Mercury in the Roman pantheon was also the messenger of the gods) probe will make its final flyby of Mercury in about 2 hours. This is a "gravity-assist" maneuver that will allow the spacecraft to be captured by the planet's gravitational field and go into orbit on its next approach, about a year and a half from now. The Planetary Society blog has a nice set of approach pictures, and the official MESSENGER website has some great photos and a countdown clock. Also, there's a nice rundown of the central questions to be addressed by the mission:

Question 1: Why is Mercury so dense?

Mercury's density implies that a metal-rich core occupies at least 60% of the planet's mass, a figure twice as great as for Earth! MESSENGER will acquire compositional and mineralogical information to distinguish among the current theories for why Mercury is so dense.

Question 2: What is the geologic history of Mercury?

Before the MESSENGER mission, only 45% of the surface of Mercury had been photographed by a spacecraft! Using its full suite of instruments, MESSENGER will investigate the geologic history of Mercury in great detail, including the portions of the planet never seen by Mariner 10.

Question 3: What is the nature of Mercury's magnetic field?

Mercury has a global internal magnetic field, as does Earth, but Mars and Venus do not. By characterizing Mercury's magnetic field, MESSENGER will help answer the question of why the inner planets differ in their magnetic histories.

Question 4: What is the structure of Mercury's core?

Through a combination of measurements of Mercury's gravity field and observations by the laser altimeter, MESSENGER will determine the size of Mercury's core and verify that Mercury's outer core is molten.

Question 5: What are the unusual materials at Mercury's poles?

At Mercury's poles, some crater interiors have permanently shadowed areas that contain highly reflective material at radar wavelengths. Could this material be ice, even though Mercury is the closest planet to the Sun? MESSENGER will find out.

Question 6: What volatiles are important at Mercury?

MESSENGER will measure the composition of Mercury's thin exosphere, providing insights into the processes that are responsible for its existence.

Note that most of the questions are equivalent to geology. Even though Mercury is quite close compared to the other planets, it's hard to observe because it's so close to the sun from our perspective. And though intuitively it seems that falling deeper into the gravity well should be easy, there are two issues that make it much more difficult than one might think: first, we're traveling pretty rapidly around the sun. If you want to fall inward, you have to slow your orbital velocity dramatically. Second, on the fall inward, a tremendous amount of gravitational potential energy is converted into kinetic energy; the space craft's velocity, despite its initial decrease, is enormously increased during the fall toward the sun. So if you want to get it to orbit Mercury, you have to slow it down once again when it arrives. Simply put, it's not possible to carry enough fuel (with current technology) to pull this off without some ballistic trickery. MESSENGER has performed 5 "gravity assist" maneuvers already. The last one, today, will allow it to be captured on its next approach.

Followup, 4:21 PM: MESSENGER has successfully completed its maneuver, and will start returning data gathered during the fly-by in a couple of hours. Images will be released at about 10 AM Eastern time (7 Pacific) tomorrow, and will be available here.

Followup, Fri. Sept. 30: New Pictures! I'm fascinated by the pit-floor crater.

I Like Catastrophic Explosions

As long as they're somewhere else. Phil Plait at Bad Astronomy found this great clip of a moon-sized body impacting a Mercury-sized planet, and does a great job of explaining the evidence that led astronomers to conclude that this happened in a system 100 light years away, in the last thousand years or so.