How to Land a Spacecraft on Mercury

Mercury remains a puzzle to scientists. NASA’s MESSENGER orbiter revealed many mysteries surrounding Mercury, such as its abnormally large core and abundance of volatile substances not expected.

European Space Agency’s bepiColombo probe will soon take an intimate look at Mercury, yet its primary mission isn’t taking stunning images.

Mercury’s surface

Mercury’s surface is covered in thousands of craters and large basins, as well as smooth plains resembling “mare” regions of the Moon, smooth plains that resemble lunar mare regions, ridges and scarps that range in height from several meters up to over one kilometer high, as well as scarps stretching for hundreds of kilometers long.

Mercury’s soil interacts with particles from the Sun, expelled into space as particles interact and release molecules and atoms called exosphere. It contains non-metal volatile species like water, carbon dioxide, nitrogen, argon and xenon; in addition to helium and trace amounts of hydrogen and oxygen.

MESSENGER’s g-ray and neutron detectors employed maps of elemental ratios to pinpoint regions with varied composition on Earth (Nittler et al., 2011). These areas were called geochemical terranes; ratios of moderately volatile potassium to refractory thorium provide sensitive indicators of silicate melting conditions on Mars, as well as mineral assemblages like olivine-pyroxenes, Na-rich plagioclases, Mg-Ca-Fe-sulfides, and FeO-poor pyroxenes.

Mercury’s atmosphere

Mercury is a small planet with an atmosphere composed of hydrogen, helium and oxygen as well as trace amounts of sodium, calcium potassium and magnesium blown off by solar winds. Since Mercury lacks enough gravity to hold onto an atmosphere thick enough for holding air molecules back, those gases which remain are often dispersed into space by solar winds. Its gravity cannot support such an atmosphere so only small quantities remain.

Mercury’s surface is mostly heavily cratered, which allows planetary scientists to estimate its age. However, some regions feature smooth plains without many craters; these could possibly contain ancient volcanic lavas that formed them.

Mariner 10’s images of Mercury showed bright streaks known as “crater rays,” emanating from many craters. These “rays” consist of fine bits of crushed rock which reflect sunlight to illuminate them further and make the craters appear brighter. Scientists suspect these crater rays were created by an impact that created a large hole beneath the surface and dislodged material beneath.

Mercury’s magnetic field

Mercury interacts directly with solar wind currents, producing currents that influence its magnetosphere and creating variations that affect large-scale external current systems, including those at the magnetopause and nightside southern tail lobe. These magnetic field variations occur over seconds to 88 days and affect large-scale current systems such as those located near magnetopause or nightside southern tail lobe.

Before Mariner 10, only 45% of Mercury had ever been photographed from spacecraft. With MESSENGER’s ability to fly low over Mercury’s surface at low altitude, scientists were able to study it extensively – including sections which contain thermoremanently magnetized crustal regions.

The MAG instrument was created to measure magnetic fields produced by currents on Mercury and their variations. Initial observations show that residual signatures in Bpar and Bperp components correlate with magnetopause shape, providing data which scientists can use to estimate Mercury’s internal magnetic field size as well as track its development over time. This will aid them in understanding its evolution within its core.

Mercury’s gravity assist

As spacecraft move towards Mercury, their trajectory is affected by gravity’s pull of the Sun, accelerating their travel towards Mercury and forcing them to burn more fuel in order to overcome this acceleration – something which is both costly and impractical when considering that Mercury does not possess thick atmosphere like Earth or Venus that provides aerobraking capability.

BepiColombo uses multiple gravity assist flybys to alter its trajectory, carefully calculated in advance so as to pass Mercury at exactly the right distance, angle, and velocity.

BepiColombo comprises two satellites – ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetosphere Orbiter (MMO, or Mio). MPO will study planetary surface features and composition while Mio will explore Mercury’s magnetosphere and near solar environment.

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