NASA's PACE satellite is being used to track autumn colour across the whole planet, a job that until recently was done on foot.
The spacecraft carries the Ocean Color Instrument, a hyperspectral sensor with 286 spectral bands that observes the oceans and land vegetation daily. It is, the agency says, the first satellite able to measure all three of the relevant leaf pigment groups globally every day or two.
Those groups are the anthocyanins, which produce reds, the carotenoids, responsible for yellows and oranges, and the chlorophylls, which are green. Scientists compare the red, orange and yellow signals against the green: values near one mean little change so far, and higher values mean more red pigment relative to green.
Leaves turn because chlorophyll has to be continually resynthesised. Shorter days and cooler temperatures slow that production, and the pigments underneath become visible.
Maps covering 29 September to 4 October and 5 to 11 October this year show the pattern on the move. Redness peaked in New England, northern Canada and northern Russia in the first period, then spread south into the Mid-Atlantic, Mongolia and northern China in the second, while fading in New England and upstate New York.
Not all of the red is autumn. Crops, harvested fields and cloud can each produce a false signal, and the tundra reds in Alaska, Canada and Russia come largely from blueberries, bilberries, sedges, mosses and grasses rather than from trees.
A Landsat 9 image from 3 October shows red and orange foliage around Ripton, Vermont, where Robert Frost kept a summer home and farm and taught at Middlebury's Bread Loaf School of English. Sugar and red maples dominate many of the highest-redness areas, among them the Adirondacks, the Green Mountains and the White Mountains.
"The time-series data allow us to determine peak redness and yellowness anywhere on Earth," said Karl Huemmrich, a vegetation scientist at the University of Maryland, Baltimore County, and a member of the PACE science team.
The activity itself is old. Japan's momijigari, or red leaf hunting, goes back more than a thousand years, and American foliage excursions are recorded from the nineteenth century; the agency counts Frederic Church and Emily Dickinson among those who noticed.
NASA data scientist Skye Caplan posts weekly foliage maps as part of GLOBE's North America Phenology Campaign and asks members of the public to send in photographs from the ground to validate the satellite data. One such photograph, from Meredith, New Hampshire, was taken on 11 October. Caplan noted that drought and insect damage can change pigments too, and that the team wants to know whether PACE can detect early warning of drought, insects and plant disease.
It is the quoted claim that repays attention, because it is a very strong one and appears to be true.
Peak redness and peak yellowness can now be determined anywhere on Earth. Momijigari, by contrast, requires a person to travel somewhere and look at a tree. The thousand-year-old version of the activity had a severe coverage problem and produced no global product at all.
The Department of Unnecessary Research notes that the modern arrangement has fixed this and then reversed the order of operations. The satellite determines where peak colour is. Members of the public are then asked to go there, photograph the leaves, and submit the photographs so that the determination can be validated.
Our analysts accept the scientific necessity of ground truth. They observe that under this design the leaf hunters are no longer hunting leaves. They are confirming a finding.
Red leaf hunting has run for more than a thousand years without supervision. It now has a quality assurance process, and the trees are the samples.