Something went wrong on the Ingenuity Mars Helicopter's sixth flight. Not to worry, though: the copter is fine. The story of what went wrong and why it's okay now reminds us once again just how impressively smart space engineers have to be and usually are.
Mission, Interrupted
An image taken by the helicopter during its sixth missionCredit: NASA / JPL-Caltech
The helicopter was sent aloft to take stereo images of a region of interest. The plan was for it to ascend to a height of ten meters and then travel at a speed of four meters per second for 150 meters to the southwest, capturing images as it flew. Next, it was to travel 15 meters to the south with its camera facing westward, and then finally 50 meters to the northeast where it was to land.
At the end of the mission's first leg, however, telemetry revealed that the helicopter had begun adjusting its velocity and repeatedly tilting backward and forward. It kept on with this strange behavior before successfully landing at the end of the mission's third leg.
How the helicopter knows where it is
Credit: NASA / JPL-Caltech
Here's how things normally work.
The helicopter's navigation system has two parts. The first is an onboard inertial measurement unit (IMU). This device keeps track of the helicopter's acceleration and rotation. It monitors these aspects of its motion 500 times per second, allowing the craft to estimate where it is, how fast it's traveling, and its attitude. (IMUs also feature prominently in the navigation systems of autonomous cars back here on Earth.)
However, this is just an estimate, and since small errors build up over time, the IMU alone is not enough to keep the helicopter on course. A second system confirms the IMU estimate or alerts the craft that something has gone wrong.
This system involves a downward-pointing camera that takes time-stamped images of the ground beneath the helicopter during most of a flight. It fires each image directly to the craft's navigation system, where:
- The copter makes note of the timestamp to know when the image was captured.
- An algorithm predicts what the image should be based on the last image it received and the time that's elapsed since that first image was taken. (The system recognizes colors and topographical features such as sand ripples and rocks.)
- The algorithm examines the newest image for the predicted features.
- If it doesn't see what it expects — in other words, there's some kind of discontinuity — it corrects its IMU estimates of the craft's position, velocity, and altitude and makes adjustments accordingly.
This all happens incredibly quickly — the down-facing camera takes 30 images per second.
What went wrong
Apparently, for unknown reasons, about 54 seconds into the flight, a glitch occurred in the system responsible for transferring the down-facing images to the navigation system, and one single image got lost along the way. This had the effect of throwing the timestamp of all of the subsequent images off.
For the rest of the flight, the Ingenuity Mars Helicopter was unsure where it was. Its weird behavior was a frantic — not really, it's a machine — attempt to respond as the discrepancy compounded over time.
Anticipating such surprises, the designers built into the algorithm a stability margin that allows the craft to remain relatively stable even if it encounters a significant number of errors, as happened here. As chief pilot of the craft Håvard Grip puts it: "This built-in margin was not fully needed in Ingenuity's previous flights, because the vehicle's behavior was in-family with our expectations, but this margin came to the rescue in Flight Six."
The system also had one final trick up its sleeve that allowed the confused craft to land safely. When a craft is close to the Martian surface, either landing or taking off, a lot of dust gets kicked up. Concerned that flying dust would create problems for the algorithm, the craft is programmed to ignore the images once the craft's altitude is one meter or less.
In this case, that meant that the helicopter set aside the confused image system during landing, relying solely on its IMU. We'll give Grip the final word:
"In a very real sense, Ingenuity muscled through the situation, and while the flight uncovered a timing vulnerability that will now have to be addressed, it also confirmed the robustness of the system in multiple ways."
Now it seems that, yep, it was too good to be true. Scientists at Dresden University of Technology (TU Dresden) appear to have conclusively proven that the EmDrive does not, in fact, produce any thrust. They provide some compelling evidence that small indications of thrust in previous research were simply false positives produced by outside forces.
How the EmDrive is supposed to work
Credit: AndSus/Adobe Stock
In the EmDrive, says
the company that owns rights to the invention, "Thrust is produced by the amplification of the radiation pressure of an electromagnetic wave propagated through a resonant waveguide assembly." In simpler words, trapped microwaves bounce around a specially shaped enclosed container, producing thrust that pushes the whole thing forward.
They also assert that while the EmDrive is not exactly on speaking terms with Newton's Third Law, the company says it's perfectly in line with the second one:
"This relies on Newton's Second Law where force is defined as the rate of change of momentum. Thus, an electromagnetic (EM) wave, traveling at the speed of light has a certain momentum which it will transfer to a reflector, resulting in a tiny force."
Interest in the EmDrive has been understandable considering what it was supposed to do. Speaking to Popular Mechanics last year, Mike McCulloch, the leader of DARPA's EmDrive investigation, describes how the engine could "transform space travel and see craft lifting silently off from launchpads and reaching beyond the solar system." He mentioned his excitement at being able to get from here to Proxima Centauri — 4.2465 light years away — in just 90 human years.
It doesn't work. Yes it does. No, it doesn't.
NASA Eagleworks' EmDriveCredit: NASA/Wikimedia Commons
DARPA, part of the U.S. Department of Defense, is only one of the organizations investigating the claims made for the EmDrive. In 2018 the agency invested $1.3 million to study the device in research that will be wrapping up this May barring any significant last-minute breakthroughs.
Teams from all over the world have been testing Shawyer's idea since it was introduced and releasing often contradictory test results. This may have to do with the fact that teams detecting any EmDrive thrust at all have reported vanishingly small amounts of it, measured in milliNewtons (mN). A mN equals about 0.00022 pounds of force.
As Paul Sutter wrote in an op-ed for Space.com:
"Ever since the introduction of the EmDrive concept in 2001, every few years a group claims to have measured a net force coming from its device. But these researchers are measuring an incredibly tiny effect: a force so small it couldn't even budge a piece of paper. This leads to significant statistical uncertainty and measurement error."
For a sense of how minuscule these results are, consider that the possible thrust force reported by NASA in 2014 of 30-50 micro-Newtons is roughly equivalent to the weight of a big ant. Chinese researchers have claimed detection of 720 mN in their tests. That would be 72 grams of thrust. An iPhone 11 with a case weights 219 grams.
Too small to stand out against background noise
These tiny amounts of EmDrive thrust lie at the heart of what the TU Dresden researchers are saying: The effects are simply too small to rule out effects that don't really come from the EmDrives at all. The researchers have just published three papers. The title of one "High-Accuracy Thrust Measurements of the EmDrive and Elimination of False-Positive Effects" tells the story. The other two studies are here and here.
When the UT Dresden team turned on their EmDrive based on NASA's EmDrive, they, too witnessed tiny amounts of apparent thrust.
However, says Martin Tajmar of UT Dresden to German media outlet GreWi, they soon realized what was going on: "When power flows into the EmDrive, the engine warms up. This also causes the fastening elements on the scale to warp, causing the scale to move to a new zero point. We were able to prevent that in an improved structure."
Putting the kibosh on other researchers' results, the authors of the studies write:
"Using a geometry and operating conditions close to the model by White et al. that reported positive results published in the peer-reviewed literature, we found no thrust values within a wide frequency band including several resonance frequencies. Our data limits any anomalous thrust to below the force equivalent from classical radiation for a given amount of power. This provides strong limits to all proposed theories and rules out previous test results by more than three orders of magnitude."
This would seem to be the definitive end of the EmDrive story.
From time to time, asteroids whizzing past Earth get trapped by our gravitational pull, falling into orbit around the planet. These rocks only stay for a while, eventually escaping and continuing on their journey to who-knows-where. While they're here, they're considered "minimoons."
Astronomers have detected an object that's likely to become our next minimoon. But it's either not an asteroid or it's an odd one. Really, scientists suspect it's man-made tech returning home after many years out in cold, lonely space.
Minimoons
Scientists have confirmed just two prior minimoons. One was 2006 RH120, which orbited us from September 2006 to June 2007. The other was 2020 CD3, which got stuck in the 2015–2016 timeframe, and is believed to gotten away in May 2020.
2020 SO, the new kid on the block, is expected to arrive in October 2020 and pop out of orbit in May 2021.
Asteroid 2020 SO may get captured by Earth from Oct 2020 - May 2021. Current nominal trajectory shows shows capture… https://t.co/F5utxRvN6Z— Tony Dunn (@Tony Dunn)1600621989.0
Identifying 2020 SO
The first clue 2020 SO isn't your ordinary asteroid is its exceptionally low velocity. It's traveling much more slowly that a typical asteroid — their average rate of travel is 18 kilometers (58,000 feet) per second. Even moon rocks sent careening into Earth orbit by impacts on the lunar surface outpace pokey 2020 SO.
For another thing, 2020 SO has an orbital path very similar to Earth's, lasting about one Earth year. It's also just slightly less circular than our own orbit, from which it's barely tilted off-axis.
So, what is it? NASA estimates that the object has dimensions very reminiscent of a discarded Centaur rocket stage from the Surveyor 2 mission that landed an unmanned craft on the moon. Back in the day, rocket stages were jettisoned as craft were aimed toward their desired position. This stuff, if released high enough, remains in space. It appears that this Centaur rocket, launched in September 1966, is now making its way back homeward, at least for a little bit.
When 2020 SO arrives at its closest point in December, the rocket is expected to be about 50,000 kilometers from Earth. Its next closest approach is much further: 220,000 kilometers, in February 2021.
Centaur rocket stage
Credit: NASA/Wikimedia
What we may be able to learn
Earthly space programs being as young as they are, scientists would love to know what's happened to our rocket during a half century in space.
While 2020 SO won't get close enough to drop into our atmosphere, its slow progress has scientists hopeful that they'll still get some kind of a decent look at it.
Spectroscopy may be able to reveal what the rocket's surface is like now — has any of its paint survived, for example? Of course, being out in space, it's likely to have been hit by lots of dust and micrometeorites, so the current state of its surfaces is also of interest. Experts are curious to know how reflective the rocket is at this point, valuable information that can help planners of future long-term missions anticipate how well a craft out in space for extended periods will remain able to reflect sunlight.
How to take a picture of a giant ball of fire
Situated 77,000,000 kilometers (48,000,000 miles) from Earth, roughly halfway to the Sun, the Solar Orbiter's cameras have taken high-quality images from a closer vantage point than any camera ever. More importantly, they can take pictures in ultraviolet light, which is highly filtered by Earth's atmosphere and challenging to do as well without being in space.
The images, seen below, are stunning.
The arrow points to a "nanoflare" approximately 700 km across.
SOLAR ORBITER/EUI TEAM (ESA & NASA)
These images show the sun's appearance at a wavelength of 17 nanometers, which is in the extreme ultraviolet region of the electromagnetic spectrum. Images at this wavelength reveal the upper atmosphere of the sun, the corona, with a temperature of around one million degrees. (quoted from https://phys.org/news/2020-07-close-ups-sun.html)
Credit: Solar Orbiter/EUI Team (ESA & NASA); CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL
They also help to answer a few questions about how the Sun works while raising new ones as the mission continues.
If you look at some of these images, the top image with an arrow stands out; you will notice small white smears. These are nanoflares, also called "campfires." They are 700-kilometer-wide relatives of solar flares burning at temperatures of one million degrees, nearly 200 times hotter than the photosphere below them. One hypothesis maintains that a vast number of these could be part of the mechanism that keeps the corona, the Sun's outer atmosphere, hotter than its surface.
ESA project scientist Daniel Müller explained this notion to the BBC:
"The Sun has a relatively cool surface of about 5,500 degrees and is surrounded by a super-hot atmosphere of more than a million degrees. [...]There's a theory put forward by the great US physicist Eugene Parker, who conjectured that if you should have a vast number of tiny flares this might account for an omnipresent heating mechanism that could make the corona hot."
While larger nanoflares can be seen from Earth, the images this spacecraft provided suggest they can be smaller than previously known and arise more frequently than supposed. At this moment, scientists aren't sure why they exist or what mechanisms they interact with.
All of these images are from tests to see how the equipment operates in outer space. The probe's primary mission will take place when it reaches a point a mere 48,000,000 kilometers from the Sun. Those future pictures and data will be all the more impressive, as the Sun is currently moving out of a quiet phase and will be more active.
However, it'll take two years to get into position, so we have plenty of time to get acquainted with the images it has already shared.
