Short circuits on Mars and mapping asteroids

CuriosityFollowing my last Curiosity update, which noted that other than for one potential drilling / sampling target, work was wrapping-up for the Mars Science Laboratory in the “Pahrump Hills” location on the lower slopes of “Mount Sharp”, the decision was taken to indeed gather one more sample.

The selected target had been dubbed “Telegraph Peak”, and sits towards the top end of “Pahrump Hills”. It was selected because Alpha Particle X-ray Spectrometer (APXS) measurements carried out by the rover during its 5-month “walkabout” in “Pahrump Hills” revealed the rocks in the area to be relatively enriched in silicon when compared to the corresponding amounts of aluminium and magnesium, which is somewhat different to rocks sample prior to the rover arrival at the basal slops of “Mount Sharp”. This enrichment has also shown to increase the further up the slopes of “Pahrump Hills” the rover climbed, which is of interest to the science team.

“When you graph the ratios of silica to magnesium and silica to aluminium, ‘Telegraph Peak’ is toward the end of the range we’ve seen,” Curiosity co-investigator Doug Ming explains. “It’s what you would expect if there has been some acidic leaching. We want to see what minerals are present where we found this chemistry.”

Sampling took place on February 24th, 2015 (PDT) or Sol 908 for the rover on Mars. For the first time in Curiosity’s time on Mars, it was carried out with no preliminary “mini-drill” operation. Instead, the science team judged that analysis of the rock by APXS indicted it was of a very similar nature to the previous two sample drilling sites in “Pahrump Hills”, and the new lower percussion drilling capabilities the rover now has were judged as sufficiently safe enough to go ahead with a direct sample gathering operation.

How the drill works: On the left, a view of the drill mechanism mounted on the rover's turret, with the drill bit centre bottom. On the right a cutaway showing the sample collection mechanism in the drill bit

How the drill works: On the left, a view of the drill mechanism mounted on the rover’s turret, with the drill bit centre bottom. On the right a cutaway showing the sample collection mechanism in the drill bit

As I’ve covered previously in these pages, obtaining a sample for analysis is a multi-part operation. First the rock is drilled, and a core sample forced up through the drill bit into a one of two sample collection chambers at the top of the drill mechanism. From here, the sample is “shaken” through a feed to another device in the rover’s robot arm turret called CHIMRA – the Collection and Handling for In-Situ Martian Rock Analysis system, used to separate the tailings through a series of sieves, ready for different sizes of sample grains to be passed through the the rover’s on-board laboratory systems.

Both of these operations require the use of the drill’s percussive system to vibrate the turret, forcing material both from the drill’s sample collection chamber and through CHIMRA. However, on February 27th, during the initial operation to move the sample tailings from the drill chamber to CHIMRA, Curiosity’s on-board fault protect system identified a transient short circuit within the robot arm’s electronics. The immediately resulted in all arm-related activities being shut down, and the arm and turret locked into position ready for diagnostic operations to commence.

A transient short can occur for a number of reasons, and can pass without significant problems. However, it may also indicate a potential issue which might require some measure of action, such as a change in operating procedures or a restriction on how a mechanism is used, in order to avoid the issue becoming a serious problem in the future. To this end, following the fault report, mission engineers started diagnosing the problem, with almost all rover operations halted while they did so.

A monochrome image from Curiosity’s Navigation Camera (Navcam) shows the position in which the rover held its arm for several days after a transient short circuit triggered on-board fault-protection programming to halt arm activities on February 27th, 2015 PDT, the 911th Sol of the rover’s work on Mars.

On Thursday, March 5th, as a part of the investigative process, the rover was commanded to carry out a series of vibration tests of the kind performed while forcing the transfer of samples from the drill to CHIMRA. The vibrations were carried out with the robot arm and turret in the same orientation and position which caused the initial triggering of the fault protection system, and in the third of 180 repeat motions, a similar transient short occurred, lasting less than one one-hundredth of a second, enough to trigger the rover’s fault protection systems, and confirming there does appear to be some kind of electrical issue.

Tests are now under-way to determine whether or not the short will occur with the turret in different orientations, and may be followed by additional tests to see if it occurs with the arm in different positions. If no shorting occurs with either a change in the orientation or position of the turret / arm, then the most obvious step in preventing any recurrence of the issue is to avoid placing the turret / arm in the same orientation for sample transfer operations during future drilling activities.

It is hoped that the tests can be completed in the course of the next week. If they show that operations can be resumed safely, it is anticipated that the sample transfer operations will be completed, and Curiosity will then be ready to resume its climb up “Mount Sharp”, leaving “Pahrump Hills” via a narrow valley the science team have dubbed “Artist’s Drive”.

Dawn over Ceres

An artist’s impression of the NASA Dawn space probe, carrying a range of NASA / ESA instruments, as it approaches Ceres

On March 6th, 2015, the NASA / ESA Dawn deep-space mission entered an initial orbit around its final destination: the dwarf planet / asteroid Ceres, the largest body in the asteroid belt laying between the orbits of Mars and Jupiter.

Launched in 2007, and costing US $446 million, Dawn, like Europe’s Rosetta mission currently in operation around the comet 67P/C-G as it continues its “fall” through the solar system towards the Sun, is intended to answer fundamental questions about the origins of the solar system and the initial formation of the planets. It has been a “two stage” mission, initially rendezvousing with the dwarf planet Vesta, also in the asteroid belt, which it studied for 14 months following its arrival in orbit in July 2011, prior to breaking orbit to manoeuvre itself for a rendezvous with Ceres.

As such, Dawn has clocked up a number of “firsts”: it is the first exploratory mission to use ion propulsion to enter orbit around a planetary body, it is the first to orbit a dwarf planet, the first to orbit two separate extraterrestrial bodies, and the first to visit Ceres.

At 950 kilometres (590 miles) in diameter, Ceres accounts for one-third of the mass of the entire asteroid belt and is just 2.5 times smaller than Pluto, and occupies the indefinable boundary between being regarded as an asteroid or a dwarf planet, with opinions sharply divided as to which it actually is.Such is its size and mass, however, that it is the only body in the asteroid belt to be unambiguously rounded due to its own gravity, and appears to be differentiated into a rocky core and icy mantle in keeping with several of the moons in the solar system.

In fact, when first discovered in 1801, Ceres was considered to be the “missing” planet many astronomers in the 1700s believed sat between the orbits of Mars and Jupiter. So powerful was this belief that for almost 50 years following its discovery, it was recorded as a planet, complete with its own planetary symbol. It was only after more an more objects were discovered within the region between Mars and Jupiter that Ceres and its bothers and sisters were re-classified as “asteroids” (meaning “star-like”), a term coined by astronomer William Herschel.

Ceres as seen by the Dawn spacecraft, 19 February 2015. The bright spots appear to be sunlight reflecting off of objects – quite possibly high peaks – within the centre of a crater, although conspiracy theorists are already convinced they are the result of illuminated artificial structures, and claims that NASA and ESA will “cover up” images of Ceres as the mission progresses  (click to enlarge)

There is still a way to go before the main science mission around Ceres will commence. While captured by the planetoid’s gravity, Dawn will continue to use its ion propulsion system during March and April to slow itself and manoeuvre itself into a more circular orbit around Ceres.  Once there, it will commence a comprehensive mapping of the surface of the little world, and probe beneath its surface.

However, the imaging mission may be in part impacted by the failure of two reaction wheels on the space vehicle while it was mapping Vesta. These wheels, which are operated electrically, are used by the craft to reorient itself without using its limited supplies of on-board manoeuvring propellants, thus allowing it to keep its imaging systems correctly oriented for photographic operations. With two of these wheels now inoperative, mission planners having been working on alternative means by which the imaging mission can be accomplished as fully as possible in the 2.5 years Dawn has been transiting from Vesta to Ceres.

Just how much of an impact the loss of the two rotation wheels will have on the mapping mission – and longer-term goals for Dawn – remains to be seen. It had been hoped that once the vehicle had completed its primary mission around Ceres, which is due to continue through 2015 and 2016, it would be able to travel on to rendezvous with another asteroid, Pallas, in 2018.

As it stands now, it looks very likely that the craft will have to use all of its limited reserves of manoeuvring hydrazine in order to assist with correctly orienting itself to image Ceres. If this is the case, and with only one functional rotation wheel remaining, sending the vehicle on to Pallas is unlikely to be of any scientific benefit. Instead, it seems more likely that Dawn will be left in orbit around Ceres and shut down once it is no longer able to carry out any useful scientific activities.

All images and drawing (other than additional annotation on the latter) courtesy of NASA / JPL.

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