MESSENGER
Depiction of the Messenger spacecraft hovering over Mercury's surface, displayed in enhanced colourNASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Nasa's Messenger probe orbiting Mercury since 2011 will soon crash into the planet's surface, but not before it gathers crucial scientific data in a "hover campaign" designed to capture and send back low altitude observations until the very last moment.

With all scientific payload in good health and returning measurements to Earth, engineers will devised a series of orbit correction manoeuvres (OCM) on Messenger, over the past five weeks, to delay the crash.

The force of solar gravity continues to perturb the spacecraft in a manner that drives the probe towards the planet's surface with each closest approach. For the last few weeks Messenger's altitude at closest approach has remained between 13 and 17kms.

With propellant in its fuel tanks drying up, the spacecraft is being pulled faster by the planet's gravity.

However, by optimising the trajectory design and applying OCMs, the team has managed to delay the inevitable crash.

Experiments carried out with the Magnetometer (MAG) and the Neutron Spectrometer (NS) during this "saving throw" are expected to yield significant results.

They will measure crustal magnetic anaomalies and search for water ice in northern latitudes.

On 18 March, the team conducted the first of the OCMs to allow Messenger to hover possibly until as late as 30 April.

The probe was in an orbit with a closest approach of 11.6kms above the surface of Mercury. With a velocity change of 3.07 metres per second (6.87 miles per hour), the spacecraft's four largest mono-propellant thrusters nudged the spacecraft to an orbit with a closest-approach altitude of 34.5kms.

This maneuver also increased the spacecraft's speed relative to Mercury at the maximum distance from the planet, adding about 1.1 minutes to the spacecraft's eight-hour, 16.5-minute orbit period.

The next manoeuvre, on 2 April, will again raise the spacecraft's minimum altitude.

"We decided on a strategy that includes five manoeuvres in as many weeks to keep the spacecraft within a tight altitude range of 5 to 39 kilometers above the surface of Mercury at closest approach," said APL's Jim McAdams, Messenger's mission design lead engineer.

At periapsis altitude during the hover campaign being 30kms or less throughout the mission, the team hopes to map half the planet with the "magnetic magnifying glass" covering regions never seen at such low altitudes.

The Messenger spacecraft was launched on 3 August 2004, and was inserted into Mercury's orbit on 18 March 2011, to begin its primary mission — a yearlong study of its target planet.

Its first extended mission began on 18 March 2012, and ended a year later.

Extended mission XM2

Messenger is now on a second extended mission, termed XM2-Prime (XM2).

"With MAG, we will look for crustal magnetic anomalies," said deputy project scientist Haje Korth, of The Johns Hopkins University Applied Physics Laboratory (APL), in Laurel. "For instance, we have seen hints of crustal magnetization at higher altitudes (~70 kilometers) over the northern rise in Mercury's northern smooth plains. We will revisit this region at lower altitudes during XM2'. There may be other regions where such signals can be observed, and we will be looking for them," he added.

"With NS, scientists will hone in on shadowed craters at northern high latitudes to search for water ice," Korth said. "We have found such evidence previously in the mission, but we hope to find more at low altitudes and spatially resolve the distribution within individual craters if we are lucky.

"Establishing the presence of crustal magnetic anomalies on Mercury would be a huge result, because it would extend the known temporal baseline for Mercury's internal magnetic field by eight orders of magnitude," he said.

Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a Nasa-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun.

The Johns Hopkins University Applied Physics Laboratory built and operates the Messenger spacecraft.

There have been only two missions to Mercury with the first being Nasa's Mariner 10, launched in 1973 and and was in operation until 1975, when fuel was depleted and communication lost.

Messenger is the second probe sent to the tiny planet.

With temperatures ranging between extremes of 400C on the sun facing side to -170C on the other, and a lack of atmosphere, Mercury with a magnetic field and a dense composition is largely a mystery.