Wednesday, March 26, 2008

A Backpacker’s Guide to Mercury

Even though the planet is not very far from Earth, Mercury will not be an easy place to view through a telescope – even visit – due to its closeness to our Sun. Will this be a problem to future travel agencies?

By: Vanessa Uy

Our Solar System’s smallest and innermost planet, Mercury, has been - for ages – proved notoriously difficult for Earth – based astronomical observation. This piece of rock, with a diameter of 3,030 miles (4,800 km.) is hardly bigger than our Moon (2,160 miles). Mercury is also not that far from Earth, sometimes coming in as near as 48,337,000 miles and it is fairly bright when viewed from ground level.

The main reason that makes the planet Mercury so difficult to Earth-based observers is the planet’s closeness to our Sun. Thus the angle between Mercury (which appears as a fairly bright star when viewed by our naked eye at ground level) and our Sun is always less than that between the two hands of a watch at 1 o’clock. This “quirk of geometry” makes itself known every time you try to observe the planet Mercury at daytime, the Sun’s blazing light complicates optical – based observation of the planet, and when nighttime comes, the planet disappears from view almost as quickly as our Sun does. Mercury can be seen alone only when it is low above the horizon, just before sunrise or soon after sunset. Observations at such low angles is seldom satisfactory because of the great distance that the planet Mercury’s light must travel through the Earth’s murky and turbulent lower atmosphere.

Despite the handicaps of Earth-based observations, astronomers were able to measure the rate of rotation of the planet Mercury via Doppler radar. In 1965 the great radio telescope at Arecibo, Puerto Rico, measured the rotation of the planet Mercury by the Doppler shifts of wavelength in radar echoes from its surface. But more sophisticated – and therefore reliable - surface observations of the planet Mercury necessitates the use of unmanned interplanetary probes.

Via Earth – based optical telescopes, planet Mercury always appeared as a nearly featureless blob. Then came the Mariner X (Mariner 10) flybys whose first ever close-up photographs of the planet Mercury’s surface produced an astonished double take among the astronomical community back in 1974. The volumes of data gathered by the Mariner X space probe has the astronomical community concluding back then that the planet Mercury is like our Moon on the outside, but it may well be like the planet Earth on the inside. Like our Moon, the surface of Mercury is pocked with craters and lava-filled basins. But Mariner X also detected an Earth-like magnetic field.

Scientists knew that planetary magnetism was produced by a “dynamo effect” – the rapid rotation of iron-cored planets like the Earth. But planet Mercury rotates far too slow – once in every 58.6 Earth days – for the “dynamo effect” to work. So back in 1974, scientists postulated that a large iron core could also produce magnetism in a slowly rotating body.

The discovery of scarps or cliffs – via the Mariner X’s close-up photographs – towering some two miles high and snake for hundreds of miles through Mercury’s crated regions. These findings made scientists think back in 1974 that these scarps are wrinkles that formed some 4 billion years ago when the planet’s core began to shrink. Which made the planet’s surface crack. Despite the wealth of data collected by the Mariner X spacecraft, the many mysteries surrounding the phenomena that occurs on the planet Mercury necessitates the use of more sophisticated space probes with more advanced instruments in upcoming planetary exploration programs.

Then came the M-Ercury Surface, Space E-Nvironment, G-Eochemistry and Ranging (MESSENGER) probe. The NASA space craft was launched in August 3, 2004 to further study the planet Mercury from orbit to augment the data collected from the Mariner X program that ended back in March 1975. The current MESSENGER mission is the first to visit the planet Mercury in over 30 years. The MESSENGER spacecraft is fitted with the latest generation of scientific instruments that allows it to study from orbit not only the chemical composition of Mercury’s surface. But also the planet’s environment, geologic history, the nature of the magnetic field, the size and state of the core, the volatile inventory at the poles and the nature of Mercury’s exosphere and magnetosphere over a nominal orbital mission of one Earth year.

The current MESSENGER spacecraft has vastly improved optics for improved scanning capability. The cameras supplied to MESSENGER are capable of resolving surface features that are only 18 meters (59 feet) across. A vast improvement compared to the 1.6 kilometers (0.99 miles) resolution of Mariner X. MESSENGER will also be able to image the entire planet as opposed to the previous Mariner X mission which was only able to observe one hemisphere that was lit during the spacecraft’s flyby.

After being launched from a Boeing Delta II rocket, the MESSENGER spacecraft’s travel to the planet Mercury required an extremely large velocity change, or delta-v (known colloquially to aerospace types as “delta vee”), to perform a Hohmann-transfer because Mercury lies deeper in the Sun’s gravity well. A spacecraft travelling to Mercury is greatly accelerated as it falls toward the Sun’s gravity well, so most of the fuel expenditure is used to slow it down to perform a Hohmann-transfer so that the spacecraft can enter Mercury’s orbit.

As MESSENGER’s voyage to the planet Mercury requires extensive use of gravity assists to lower the spacecraft’s fuel expenditure. But this will greatly prolong the time of the trip. And to save rocket fuel even further because there are still no existing refilling stations for hydrazine and nitrogen tetroxide in the spacecraft’s flight path en route to Mercury. The thrust used for insertion into orbit around Mercury will be minimized, resulting in a notably elliptical orbit. Besides the advantage of saving its own propellants, such an orbit allows the MESSENGER spacecraft to measure solar wind and magnetic field strength at various distances from Mercury. Despite of the notably elliptical orbit, the improved instrumentation of MESSENGER can still allow close-up measurements and photographs of Mercury’s surface. As of January 14, 2008, MESSENGER mapped another 30% of Mercury’s surface in addition to the photos taken by Mariner X back in 1974 to 1975. Full orbital insertion of the MESSENGER spacecraft into Mercury will happen in March 18, 2011.

As a follow-up to the MESSENGER mission, the European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit the planet Mercury with two space probes: one to map the planet and the other one to study the planet’s magnetosphere. The original plan to include a lander has been shelved due to budget constraints and of its dubious scientific value. A Russian Soyuz rocket will launch the “bus” carrying the two probes in 2013 from E.S.A. ’s Guyana Space Center to take advantage of fuel savings when launching from an equatorial location. As with the MESSENGER spacecraft, the BepiColombo “bus” will make close approaches to other planets en route to Mercury for orbit-changing / Hohmann-transfer gravitational assists. The BepiColombo “bus” will first fly past our Moon then to the planet Venus and making several approaches to the planet Mercury before entering orbit.

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