False alarm: Here's why people thought Opportunity phoned home last week
Last week there were brief rumors, apparently started at the NASASpaceFlight.com user forums, that NASA's Opportunity rover had risen from its dusty slumber to phone home. Opportunity has been asleep since a planet-wide dust storm blotted out the Sun a couple months ago, keeping the rover's solar panels from getting enough sunlight. NASA hasn't heard from the spacecraft since June 10.
The cause of the rumors was NASA's Deep Space Network Now dashboard, which shows the spacecraft that Deep Space Network (DSN) dishes are talking to at any given time. Opportunity's call sign is MER-1, and at some point, MER-1 was apparently on the dashboard.
First of all, just because a spacecraft is on the DSN board doesn't mean it's communicating with Earth. Time on the DSN is booked in discrete slots, and the board simply shows which missions have dishes reserved at any given time. Part of a scheduled communication slot might be used for setup, or "cleaning up," said Doug Ellison, a NASA engineer who works with Mars rovers at the Jet Propulsion Laboratory, in a posting at unmannedspaceflight.com. Other times, a dish might just be sitting there, waiting to hear from a spacecraft.
Opportunity has likely suffered three different fault conditions: a low-power fault, a mission clock fault (it doesn't know what time it is), and an uploss fault (it's been out of contact with Earth for so long, it thinks maybe its communications system is damaged). Whenever the dust finally clears enough to give the rover some power, it will use clues like the brightness of the Sun to roughly figure out what time it is, and try to signal Earth during pre-determined "fault communication windows" when the DSN is listening. During those times, you'll see MER-1 up on the DSN Now board. Additionally, NASA is sending the rover commands three times per week asking it to beep if it's awake.
Apparently, not only was MER-1 on the DSN Now board last week, its dish also showed a little radio wave animation indicating Opportunity was phoning home! Ordinarily, this might be a good sign, except that in this case, it wasn't Opportunity phoning home—it was the Mars Reconnaissance Orbiter.
When the DSN is listening for Opportunity, it's aimed at Mars. And there's a lot of stuff in orbit around Mars. Every spacecraft uses a unique frequency, but some of those frequencies are fairly close together. Ellison told me Opportunity's frequency is 8435.37037 megahertz, and Mars Reconnaissance Orbiter’s is 8439.444445 megahertz! (Check out this cool document on DSN frequency allocations, and this deep dive into DSN performance, for more information.)
Depending on which direction a Mars orbiter is moving with respect to Earth, that frequency will shift a little bit, and in some cases, overlap with Opportunity's frequency. This is because of the Doppler effect—the same phenomenon that causes an ambulance siren to sound higher-pitched when it's coming towards you, and lower-pitched as it moves away from you.
When the DSN saw Mars Reconnaissance Orbiter’s signal coming in on the same frequency, it briefly locked on, following the signal until it shifted back out of Opportunity's expected range. This was further confirmed by the reported data rate of the signal: 6 megabits per second, which is what Mars Reconnaissance Orbiter uses. Opportunity's data rate is much, much slower; Ellison said that even using its high-gain antenna, the rover never talks faster than 28,800 bits per second, and due to its fault conditions, it will initially contact Earth at a mere 10 bits per second.
So, a false alarm. No Opportunity for now.
The good news is that NASA says the dust storm is dissipating! The amount of dust in Mars' atmosphere is measured with a figure called a tau—the higher the tau, the more dust in the air. Opportunity needs a tau below about 2.0 to charge its batteries. Right before NASA lost contact, Opportunity measured a tau of about 10.8. Recently, the tau dropped to an estimated 2.1, before going back up to 2.5.
When NASA does hear from Opportunity, it could take several weeks to fully recover the mission. Engineers will try to learn more about the rover's health and see if anything is permanently damaged, before deciding when and how to proceed with normal operations again.
For more about Opportunity and the dust storm, head over to the Mars Exploration Rovers Update by A.J.S. Rayl, which has more comprehensive coverage of the mission than anywhere else!